No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Improved karst exploration by VLF-EM-gradient survey: comparison with other geophysical methods
Abstract
The knowledge of size, density and both orientation and vertical distribution of fractures as well as their opening and filling material or overburden thickness is a valuable contribution to estimating hydraulic conductivity and to evaluating the vulnerability and protection strategy of karst aquifers. To obtain some of these parameters and to as certain high-permeability zones, the Very Low Frequency-Electromagnetic Gradient (VLF-EM GRAD) method was applied, together with Radio Frequency-Electromagnetics (RF-EM), Radiomagnetotellurics (RMT), Geoelectrical Tomography and refraction seismics, over a karstic terrain in the Swiss Jura Mountains. In this area, karst springs infiltrate a porous aquifer. This survey investigated the highly permeable karst structures, which provide fast water-infiltration pathways into the karstic flow network. Adye tracer test validated the geophysical results. The results show the efficiency and reliability, particularly of the VLF-EMGRAD method, for high-resolution investigation at shallow depths and for its potential for fast data acquisition over large surfaces at catchment area scale without ground contact.
... At the same time, the development degree of karst will affect the normal construction of water conservancy, railways, highways, bridges and other construction projects. As the scale of engineering buildings in karst areas continues to expand and the number continues to increase, how to improve the detection and exploration technology in karst areas is an important issue faced by exploration units [1][2][3][4]. Therefore, in order to improve the quality and effect of exploration, the exploration personnel should adopt scientific and reasonable methods according to the characteristics and nature of karst. ...
... 3 The vertex of the hyperbola (t 0 , y 0 ) is estimated in the echo curve of the hollow target. 4 Since the target echo curve points should meet the relationship (18), the point set (t i , y i ) can be brought in to calculate the radius of the empty target, namely, ...
In view of the current technical difficulties in the detection of unfavorable geological bodies in karst areas, this paper starts with the fine detection of a borehole wall, near-field of the borehole wall rock and far-field of the borehole wall rock, deeply excavates the correlation characteristics between multi-source borehole data, and proposes detection technology for unfavorable geological bodies in karst development areas based on multi-source borehole data. This is used to establish a multi-source data depth mining model. The correlation characteristics between multi-source data are clarified. The borehole multi-source heterogeneous detection technology is proposed to realize the organic fusion between the detection data of different scales, and effectively estimate the extension range of structural planes and cavities. The joint interpretation method of multi-source drilling data can effectively realize the three-dimensional visualization of unfavorable geological bodies in karst development areas. Through a case analysis, it is shown that this technology is a new and effective exploration method in the engineering geological exploration of karst development areas, and can provide more abundant and reliable exploration data for the fine exploration of geological bodies in the exploration area.
... These three frequencies data were selected for further analysis for all lines. The selection is based on the principle of VLF-EM method and target anomaly as described in several literatures (Bosch and Müller 2001;McNeill and Labson 1993;Bosch and Muller 2005). ...
... DCR method is a noninvasive method for resistivity imaging of subsurface. The method is popular in assessing the stability of embankments, because the method can produce imaging of the causes of embankment failures, such as fractures and seepage (Panthulu et al. 2001;Sjödahl et al. 2009;Bosch and Muller 2005). Thus, in order to ensure the accuracy of VLF-EM data inversion results, the location of the DCR profiles (P79 to P81) is presented in Fig. 1. ...
The earth on LUSI embankment has a high failure potential due to several factors such as: seepage, leakage, vertical and horizontal deformation, cracking and fault (discontinuity), overtopping, mud slide, and swelling. Very low frequency electromagnetic method was carried out at LUSI embankment in order to delineate fracture and potential pathway of seepage occurring through the subsurface structure the embankment body. To reach these objectives, several methods were carried out in the selected area, such as direct current resistivity, total station and Rayleigh wave dispersion to provide information on the mechanical properties LUSI embankment subsurface. This study indicates that seepage and fracture through the LUSI embankment is presented by a set of lines which are possibly caused by deformation in the LUSI area. Furthermore, based on the fracture and seepage positions and mud or fluid flow direction, the unstable LUSI embankment is determined as located around the northwest and northern parts of the area.
... The ABEM WADI receiver requires no physical contact with the transmitter and the ground during VLF survey as it operates on induction mechanism. The optimal configuration of VLF survey as in Figure 4 is to have the geologic strike oriented parallel to the transmitter direction so that a vertical magnetic component is generated for any conductivity contrast by the propagating horizontal and concentric magnetic and orthogonal electrical fields due to induction [8]. Thus, the DMB transmitter located in Germany that is oriented to the north from the site and of 26.9 kHz VLF frequency was chosen for this survey in consideration to the two prevailing fracture sets in the Abakaliki Basin trending northwest and northeast respectively, with subordinates trending north directly. ...
... Field components of VLF Field from transmitter at remote distance[8]. ...
Very low frequency Electromagnetic (VLF-EM) exploration over the Ishiagu area of Abakaliki Basin, Lower Benue
Trough, Nigeria was carried out to determine the Pb-Zn mineralization in the sedimentary bedrock. The conductivity
contrast between the conductive mineralized veins and the host rock as generated by induction mechanism was
used to delineate the potential zones of Pb-Zn mineralization. Results of high in-phase and quadrature readings due
to strong EM induction were detected in the survey area and on the average, the deeper sources response range
from 4.7% to about 7.6%, while the shallower sources response range from 8.8% to about 17.1% and these probably
indicate the presence of Pb-Zn deposits with thick overburden in the northern part. Current density maps show the Pb-
Zn mineralized veins trends in NW-SE direction with their subordinates in N-S direction. The central part of the study
area displays very sharp VLF tipper responses indicating shallow sources, while the northern part displays broad VLF
tipper responses indicating deeper sources. These readings correlate to the depth values over the Pb-Zn mineralized
veins which range from 10 to 13 m in the central part and 16 to 22 m in the northern part of the mapped area. The high
VLF anomalies delineated from Ishiagu area, in the SW part of Abakaliki basin as a rule yielded a high conductivity
contrast result well-suited with the Pb-Zn mineralization and geologic information of the area.
... The ABEM WADI receiver requires no physical contact with the transmitter and the ground during VLF survey as it operates on induction mechanism. The optimal configuration of VLF survey as in Figure 4 is to have the geologic strike oriented parallel to the transmitter direction so that a vertical magnetic component is generated for any conductivity contrast by the propagating horizontal and concentric magnetic and orthogonal electrical fields due to induction [8]. Thus, the DMB transmitter located in Germany that is oriented to the north from the site and of 26.9 kHz VLF frequency was chosen for this survey in consideration to the two prevailing fracture sets in the Abakaliki Basin trending northwest and northeast respectively, with subordinates trending north directly. ...
... Field components of VLF Field from transmitter at remote distance[8]. ...
Very low frequency Electromagnetic (VLF-EM) exploration over the Ishiagu area of Abakaliki Basin, Lower Benue Trough, Nigeria was carried out to determine the Pb-Zn mineralization in the sedimentary bedrock. The conductivity contrast between the conductive mineralized veins and the host rock as generated by induction mechanism was used to delineate the potential zones of Pb-Zn mineralization. Results of high in-phase and quadrature readings due to strong EM induction were detected in the survey area and on the average, the deeper sources response range from 4.7% to about 7.6%, while the shallower sources response range from 8.8% to about 17.1% and these probably indicate the presence of Pb-Zn deposits with thick overburden in the northern part. Current density maps show the Pb- Zn mineralized veins trends in NW-SE direction with their subordinates in N-S direction. The central part of the study area displays very sharp VLF tipper responses indicating shallow sources, while the northern part displays broad VLF tipper responses indicating deeper sources. These readings correlate to the depth values over the Pb-Zn mineralized veins which range from 10 to 13 m in the central part and 16 to 22 m in the northern part of the mapped area. The high VLF anomalies delineated from Ishiagu area, in the SW part of Abakaliki basin as a rule yielded a high conductivity contrast result well-suited with the Pb-Zn mineralization and geologic information of the area.
... The borehole locations are labeled in yellow. 482 RMT soundings have been carried out (Figure 2) during seven days of field work using a scalar prototype device (Bosch & Müller 2005) developed at the Centre of Hydrogelogy Neuchâtel (CHYN, University of Neuchâtel, Switzerland). Four radiotransmitters sending from northwestern directions were used: 207 kHz (130°N), 81 kHz (105°N), 37.5 kHz (130°N) and 19.6 kHz (115°N). ...
... Twenty-one soundings have been carried out in the area along distinct profiles (Figure 2). Results from two NW-SE profiles with a station interval of about 1000 m are displayed inFigure 3 482 RMT soundings have been carried out (Figure 2) during seven days of field work using a scalar prototype device (Bosch & Müller 2005) developed at the Centre of Hydrogelogy Neuchâtel (CHYN, University of Neuchâtel, Switzerland). Four radiotransmitters sending from northwestern directions were used: 207 kHz (130°N), 81 kHz (105°N), 37.5 kHz (130°N) and 19.6 kHz (115°N). ...
... Compressional wave techniques like seismic methods have acceptable results in locating the subsurface cavernous features particularly when it integrated with another techniques (Sheehan et al. 2005;Riddle et al. 2010;Abdelrahman et al. 2023). The electromagnetic technique like ground penetrating radar (GPR), time domain and frequency domain EM are effective in location the karstic features (Ogilvy et al. 1991;Bosch and Müller 2005;Gómez-Ortiz and Martín-Crespo 2012). Obviously, the combination of more than one geophysical technique make the detection of subsurface cavernous features are more accurate (Chalikakis et al. 2011;Golebiowski et al. 2016). ...
The cavernous features are considered one of the geological hazards that are dominant in the carbonate rocks as a results of dissolution effect of groundwater. The dissolution process extends for many years at the subsurface layers and causes catastrophic collapse if migrated to the surface layers. The problem become crucial in the urbanization areas where there are wide varieties of construction activities. Beside the natural weathering process, the human interventions, representing in the wastewater, can accelerate the weathering process of the prevailing cracks. Al Kharj governorate is one of the promising areas close to Riyadh, the capital of Saudi Arabia, that attracts wide aspects of urbanization, agriculture and industrial activities. The study area is located inside an urban neighborhood at the western part of Al Kharj city. The site is surrounded by buildings extensions from three sides and the fourth side was excluded from urbanization activities due to existence of surface cracks along the area especially at the northern direction. Excavations and boring tests reveal that the exposed cracks are extended downward and may be connected to subsurface karstic features in the limestone rocks. Two surface geophysical measurements have been planned to provide clear information about the locations, horizontal and vertical extensions of the cavernous features along the study site. The Time Domain Electromagnetic (TEM) and Electrical Resistivity Tomography (ERT) methods were applied for investigating the subsurface characteristics up to 40m depth with different resolutions. The data have been acquired along profiles path across most of the surface fractured features. In spite of the inverted TEM data representing the subsurface resistivity variations in 1D mode, the dense TEM stations and the direct comparing the results with the available borehole lithology, enabled to trace the subsurface cavernous features laterally and vertically with reasonable resolution. Moreover, the 2D ERT processed data have been compared with the TEM results along some localities for guiding the data interpretation to locate the subsurface cavernous features. The two methods are analyzing the subsurface in two different resolutions, the TEM can explore the deeper parts and the layer successions, whereas the ERT methods can provide the lateral extension tracing of cavernous features. Based on the results 3D fence diagrams, most of the cavernous features that are located along the northern part of the study area related to fractured limestone zones extend vertically to more than 20m in some localities. Data interpretation has been flowed by excavation process for confirming the interpreted anomalies and resulted in discovery of huge cavity system extended laterally and vertically at deeper massive limestone layer.
... Application of different modifications of VLF surveys for underground cave localization can be found in Ogilvy et al (1991), Guérin and Benderitter (1995), Danişman, (2004), Bosch and Müller (2005), Eppelbaum (2007) Apparent resistivity. In most of the resistivity methods, the normal electrical field is taken into account by the introduction of effective parameter ρ k termed apparent specific resistivity. ...
Generally, sinkholes are surface (also named subaerial) manifestations of karst or pseudo-karst. The term karst includes a wide range of surface and subsurface landforms that develop by the dissolution of soluble bedrocks and the development of subsurface drainage (e.g., De Waele et al 2011, Ford and Williams 2007, Frumkin 2013, Benson and Yuhr 2016). On the other hand, some sinkholes were not formed by the natural dissolution of bedrock despite resembling or acting like karst. These sinkholes are termed ‘pseudokarst’ (e.g., Grimes 1975, Eberhard and Sharples 2013, Benson and Yuhr 2016). During the pseudokarst processes, the material is physically removed in the solid state (like suffosion), with effects resulting from changes in phase and the removal of liquid material. They are termed ‘false-karst’ or, sometimes, ‘analogous karst’ (McGee 2012).
Two important principles of karst investigations using geophysical methods deserve to be noted.
1. The geophysical scale of measurements should correspond to the scale of considered objects. It means that the method scale and target scale must coincide (in other words, the void must be of the same scale as the method). It should be considered when planning geophysical surveys (Benson and Yuhr 1993).
2. Any geophysical measurements must be in close correspondence with geological data. Such measurements should help in visualizing the hidden geological peculiarities. Geophysicists must use geological data from outcrops, boreholes, or other features such as ‘benchmarks’.
Geophysical measurements can be very effective if one uses interpolation between such sources of geological information or extrapolation away from them. In every case, the knowledge derived from geophysical measurements must be eventually reconfirmed against the known geological condition (McNeil 1980b).
... Other types of geophysical methods are geoelectric (e.g., ERT) and electromagnetic (e.g., TEM) techniques. ERT has been shown to be useful in identifying aquifers and fluid pathways in karst regions (e.g., Militzer et al., 1979;Bosch and Müller, 2001;Margiotta et al., 2012;Miensopust et al., 2015), especially in combination with seismic methods that deliver structural constraints (e.g., Sandersen and Jørgensen, 2003;Bosch and Müller, 2005;Tanner et al., 2020). The advantage of TEM over ERT is the improved resolution in the near surface down to ca. 300 m depth, but without ERT the information about the electrical resistivity at greater depths of, e.g., 400 to 500 m would be missing, where subsurface dissolution and erosional processes at faults may occur at greater depths. ...
The subsurface dissolution of soluble rocks can affect areas over a long period of time and pose a severe hazard. We show the benefits of a combined approach using P-wave and SH-wave reflection seismics, electrical resistivity tomography, transient electromagnetics, and gravimetry for a better understanding of the dissolution process. The study area, “Esperstedter Ried” in northern Thuringia, Germany, located south of the Kyffhäuser hills, is a large inland salt marsh that developed due to dissolution of soluble rocks at approximately 300 m depth. We were able to locate buried dissolution structures and zones, faults and fractures, and potential fluid pathways, aquifers, and aquitards based on seismic and electromagnetic surveys. Further improvement of the model was accomplished by analyzing gravimetry data that indicates dissolution-induced mass movement, as shown by local minima of the Bouguer anomaly for the Esperstedter Ried. Forward modeling of the gravimetry data, in combination with the seismic results, delivered a cross section through the inland salt marsh from north to south. We conclude that tectonic movements during the Tertiary, which led to the uplift of the Kyffhäuser hills and the formation of faults parallel and perpendicular to the low mountain range, were the initial trigger for subsurface dissolution. The faults and the fractured Triassic and lower Tertiary deposits serve as fluid pathways for groundwater to leach the deep Permian Zechstein deposits, since dissolution and erosional processes are more intense near faults. The artesian-confined saltwater rises towards the surface along the faults and fracture networks, and it formed the inland salt marsh over time. In the past, dissolution of the Zechstein formations formed several, now buried, sagging and collapse structures, and, since the entire region is affected by recent sinkhole development, dissolution is still ongoing. From the results of this study, we suggest that the combined geophysical investigation of areas prone to subsurface dissolution can improve the knowledge of control factors, hazardous areas, and thus local dissolution processes.
... Several geophysical techniques had been used for a long time in mapping and locating superficial objects including pipelines but not limited to magnetics, VLF-EM, Very Low Frequency (VLF) electromagnetic, Time domain electromagnetic; electrical resistivity; Ground Penetrating Radar (GPR) and limited trials of using Frequency Domain electromagnetic (FDEM) (Gay, 1986;Won and Huang 2004;Bosch and Muller, 2005;Babu et al., 2007;Auken et al., 2006;Panissod Birken et al., 2002;Borgioli et al., 2008;Soliman and Wu, 2008;Huang and Won 2000). However the local site conditions, the depth of the buried target and the primary requirements of the survey limited the valid exploration technique to provide; non-destructive technique, low cost survey, speed of data acquisition, rapid data processing steps, sensitive to small background contrast and less affected with the closely background noise sources. ...
... Many investigators have been applied the VLF method for searching and localization of ore deposits (Ginzburg et al., 1981;Gordeyev et al., 1981;Ginzburg, 1982;Bayrak, 2002;Basokur and Candansayar, 2003;Liu et al., 2006;Sandrin and Elming, 2006;Eppelbaum and Khesin, 2012;Sharma et al., 2014;Shendi et al. 2017), different types of geological-geophysical mapping (e.g. Fischer et al., 1983;McNeill and Labson, 1991;Hu, 2001;Pedersen and Oskooi, 2004;Bosch and Müller, 2005;Monteiro et al., 2006;Gürer et al., 2009;Hamdan et al., 2010;Sharma et al., 2014;Abtani et al., 2016;Rajab, 2021) and discovering of archaeological artifacts (e.g., Ogilvi et al., 1991;Darnet et al., 2004;Drahor, 2006;Khalil et al., 2010;Tawfik et al., 2011;Simon et al., 2019). ...
Investigating the electromagnetic (EM) fields from distant VLF military transmitters is one of the most prompt and low-expensive geophysical methods. It currently finds frequent applications in prospecting for various deposits, a search of underground subsurface water, archaeogeophysical studies, and various types of geological mapping. The VLF-radiations propagate in a spherical waveguide formed by the Earth and ionosphere where the energy of radio frequencies is not attenuated even at great distances due to the channel effect. A few dozen VLF transmitters in various countries can be used for geophysical investigations. The different frequencies and angles of registered EM radiation enable obtaining additional preferences by interpretation. The depth of the investigation depends on the host medium's radio wave frequency and averaged resistivity and usually ranges from several tens to several hundred meters (last values – under very favorable conditions). Both the electric and magnetic components of the EM field are used in the investigation by the VLF method. Generally, only the magnetic field (H) is employed. The absence of reliable methods limited the widespread use of the VLF technique to eliminate the EM field time variations, rugged relief influence, and procedures for quantitative interpretation of the VLF anomalies. These problems are successfully solved, and a unified methodological system is developed. A continuous, direct filtering procedure is proposed to eliminate the temporal variations. The correlation technique enables the reduction of the rugged relief influence significantly. It is possible to use the modern interpreting methods elaborated in magnetic prospecting for complex geological-geophysical conditions for a quantitative interpretation. Finally, for revealing hidden objects against the high-intensive geological noise background, an application of non-conventional statistical, informational, and wavelet algorithms is suggested. The main components of the developed system were successfully tested in the Caucasian polymetallic, copper, and gold deposits. An example of the VLF quantitative analysis in an archaeological site is shown. The possibility of VLF-data utilization as precursors of dangerous geodynamic events at a depth is demonstrated.
... In contrast, the VLF method relies on EM signals transmitted by radio stations operated by navies around the globe in the frequency range 15-30 kHz (Pedersen et al. 1994). The radiofrequency EM (RF-EM) method (Bosch and Műller 2005) extends this range to frequencies in the low frequency (LF) range, which includes radio fields transmitted for navigation and communication purposes by naval and commercial radio stations. The digitisation rates of most active-source AEM systems are high enough to analyse VLF and even LF responses. ...
The recording of raw or streamed airborne electromagnetic (AEM) data, as done by CGG during MEGATEM surveys, allows for the extraction of passive EM signals also present, but not normally processed. These include (1) powerline responses, (2) responses in the very low frequency (VLF) range due to radio transmissions and (3) natural-source audio-frequency magnetic (AFMAG) and VLF responses in the frequency range 25 Hz–25 kHz extracted from individual atmospheric electrical discharges (sferics). The latter approach manages to extract good signal in the audio-frequency magnetotelluric (AMT) dead band (1–5 kHz) for one of the discussed data sets. The recording of the three-component AEM data allows for the vector processing of these passive EM responses, including the derivation and modelling of the vertical-to-horizontal magnetic field ratio (tipper) data. Conductivity information can be derived from the tipper data with an apparent conductivity transformation and, more rigorously, with 2D and 3D inversions that take into account the terrain’s topography. The extraction and modelling of passive EM responses is demonstrated on two data sets. A powerline apparent-conductivity grid derived from a MEGATEM survey near Timmins, Canada indicates conductivity structures similar to those in the corresponding active-source EM data. VLF and AFMAG responses derived from South American MEGATEM data show a strong correlation to topography. These data were successfully modelled with 2D and 3D inversions, and the derived shallow conductivity structures confirm and complement the information extracted from the active-source EM data.
... We propose the use of shear wave reflection seismic as a standard method, in combination with other geophysical techniques (e.g. gravimetry to detect cavities and mass movement (Neumann 1977;Butler 1984;Kobe et al. 2019), and electrical resistivity tomography (ERT) and various electromagnetic methods to identify fluid pathways and subsurface erosion areas (Militzer et al. 1979;Bosch & Müller 2005), for further investigation of sinkhole areas. ...
Elastic moduli derived from vertical seismic profiles (VSPs) and 2-D SH-wave reflection seismic profiles are used to characterize mechanical properties of rocks in sinkhole areas. VP and VS were used to calculate the Poisson’s ratio and the dynamic shear modulus. The study shows that 2-D shear wave reflection seismics is suited to depict the heterogeneities of the subsurface induced by subsurface erosion. Low shear wave velocities of ca. 120–350 m s–1 and low shear strength values between 25 and 250 MPa are identified for the subsurface erosion horizon that consists of soluble Permian evapourites and the disturbed overlying deposits. These low values are a result of cavities and fractures induced by dissolution, creating unstable zones. In compliance with the shear modulus the Poisson’s ratio derived from the VSPs shows values of 0.38–0.48 for both the presumed subsurface erosion horizon, and the deposits above. This is a further indicator of reduced underground stability. In the VSPs, anomalies of the shear modulus and the Poisson’s ratio correlate with low electrical resistivities of less than 10 Ωm from borehole logs, indicating high conductivity due to fluid content. Further investigation reveals a conversion of S-to-P wave for the subsurface erosion horizon, which is probably the result of dipping layers and an oriented fracture network. Seismic attribute analysis of the 2-D sections shows strong attenuation of high frequencies and low similarity of adjacent traces, which correlate with the degree of subsurface erosion induced wave disturbance of the underground.
... Les méthodes VLF -R et la Radio magnétotellurique ( Le but consiste à mesurer le gradient de la composante horizontale du champ magnétique résultant à l'aide de l'équation suivante (Bosch andMüller, 2001, 2005 ...
LA MÉTHODE ÉLECTROMAGNÉTIQUE (VLF) ou MÉTHODE ÉLECTROMAGNÉTIQUE B.F
EST UNE MÉTHODE EN CHAMP LOINTAIN
Ce chapitre traite la Prospection géophysique par V.L.F , les Paramètres mesures , Relation entre les champs magnétique primaire et secondaire , Profondeur de pénétration et d’investigation - Différentes versions du VLF : (VLF –R) , (VLF Multifréquences) ,VLF inclinaison, VLF-EM, VLF-gradient
... Most effective geophysical methods for identifying (individual) fractures and/or fracture zones (a dense set of fractures) include VLF-EM (e.g. [14]), VLF-R [15], VLF-EM-gradient [16], RMT [17], EM-34 [18], Electric Resistivity Tomography [19] [20] or geoelectric null-arrays [21] [22]. The resolution of these methods with the exclusion of the geoelectric ones is however smaller than it was required in the given study, where fractures were expected to be in even less than 4 -5 m distance. ...
Position, width and fragmentation level of fracture zones and position, significance and characteristic distance of fractures were aimed to determine in a carbonate aquifer. These are fundamental parameters, e.g. in hydrogeological modelling of aquifers, due to their role in subsurface water movements. The description of small scale fracture systems is however a challenging task. In the test area (Kádárta, Bakony Mts, Hungary), two methods proved to be applicable to get reasonable information about the fractures: Electrical Resistivity
Tomography (ERT) and Pricking-Probe (PriP). PriP is a simple mechanical tool which has been successfully applied in archaeological investigations.
ERT results demonstrated its applicability in this small scale fracture study. PriP proved to be a good verification tool both for fracture zone mapping and detecting fractures, but in certain areas, it produced different results than the ERT. The applicability of this method has therefore to be tested yet, although its problems most probably origin from human activity which reorganises the near-surface debris distribution. In the test site, both methods displayed fracture
zones including a very characteristic one and a number of individual
fractures and determined their characteristic distance and significance. Both methods prove to be able to produce hydrogeologically important parameters even individually, but their simultaneous application is recommended to decrease the possible discrepancies.
... The VLF method relies on EM signals transmitted by VLF stations operated by navies around the globe in the frequency range 15-30 kHz. The radiofrequency EM (RF-EM) method (Bosch and Műller, 2005) extends this range to frequencies in the LF range, which includes radio fields transmitted for navigation and communication purposes by naval and commercial radio stations. The recording of 3 components of the magnetic field data at AFMAG, VLF and LF frequencies allows for the derivation of tipper data and the application of 2D and 3D inversions (Holtham and Oldenburg, 2010;Sattel and Witherly, 2012;Kamm and Pedersen, 2014). ...
The recording of raw or streamed data, as done by CGG during MEGATEM and HELITEM surveys, allows for the extraction of passive EM responses, inadvertently recorded during AEM surveys. These include powerline responses in data sets acquired in the vicinity of strong powerlines, VLF responses in data sets recorded with sufficiently high sampling frequencies and potentially AFMAG responses in the frequency range 25-600 Hz.
The recording of the three-component AEM data allows for the vector processing of these passive EM responses, including the derivation and modelling of the tipper data. Conductivity information can be derived from the tipper data with an apparent conductivity transformation and, more rigorously, with 2D and 3D inversions that take into account the terrain’s topography.
The extraction of passive EM responses is demonstrated on a number of data sets. A powerline apparent-conductivity grid derived from a MEGATEM survey near Timmins, Canada indicates conductivity structures not evident in the corresponding active-source EM data. VLF responses derived from South American MEGATEM and North American HELITEM data show a strong correlation to topography. The former were successfully modelled with 2D and 3D inversions, and the derived shallow conductivity structures confirm and complement the information extracted from the active-source EM data.
... Electromagnetic (EM) methods are being used successfully to solve various problems associated with exploration and environmental geophysics (e.g., Tezkan 1999;Bosch and Müller 2005). In general, all EM methods are based on the principle of the induction phenomenon, which permits them to be used for locating conducting targets in resistive surroundings. ...
... . Les techniques VFL-EM, VLF-Gradient et RMT peuvent aussi se révéler efficaces pour la détection de l'épikarst[Bosch et Müller 2001 ;Bosch et Müller, 2005] et de failles remplies par des argiles[Guérin et Benderitter, 1995]. Les passages préférentiels de l'eau ou concentrations des infiltrations L'ERT, le GPR et le Slingram sont les trois techniques les plus utilisées pour imager les structures susceptibles de concentrer l'eau et pouvant jouer le rôle de passage préférentiel. ...
يمكن للطبقة غير المشبعة٬ في نظم المياه الجوفية ذات الطبيعة الجيرية (الكارست) أن تلعب دورا مهما في حركة و تخزين المياه الجوفية٬ لكن آليات عمل هذه الطبقة تبقى إلى حد ما مبهمة. إن السؤال الذي يبقى مطروحا : هل بإمكان علم الهيدروجيوفيزياء أن يمكننا من تقييم التغيرات في البعدين المكاني و الزمني لمخزون المياه في الطبقة غير المشبعة بغية تطوير فهمنا لآلية عملها؟خلال الفترة الممتدة بين سنتي 2011 – 2012 م٬ تم إستخدام العديد من الأدوات في حقل تجريبي على مستوى المختبر التحت أرضي عديم التشويش بروستال بفرنسا (LSBB de Rustel)٬ والذي يتواجد في حوض التغذية لمنبع الفوكلوز(Fontaine de Vaucluse) بغية دراسة بنية الطبقة غير المشبعة للكارست و كيفية أدائها فيما يتعلق بالمياه الجوفية. إن هذه الأبحاث نفذت بمنهجية مكونة من أربعة مراحل٬ و هن كالآتي: (1) المرحلة الابتدائية و تخص الدراسة الجيولوجية و الهيدروجيولوجية٬ (2) التوصيف الجيوفيزيائي للبنية الجيولوجية٬ (3) التوصيف الهيدروجيوفيزيائي لحركة الطبقة المائية٬ (4) الخلاصة من خلال إنشاء نموذج جيولوجي ثلاثي الأبعاد.إن النتائج المتحصل عليها في الموقع التجريبي أظهرت أهمية الهيدروجيوفيزياء في الوسط الكارستي و أكدت نجاعة المنهجية المقترحة. إن الأبحاث المنجزة٬ مكنت في وقت وجيز٬ من تحديد البنيات الجيولوجية التي يمكن أن تلعب دورا مهما في معادلة التدفقات المائية. من منظور آخر٬ مكنتنا الطريقة المقترحة من قياس التغيرات الموسمية للمخزون المائي في الطبقة غير المشبعة (ZNS) و ذلك على عمق عشرات الأمتار. إن الرصد الجيوفيزيائي الحدثي مكن من تصوير التغيرات المكانية و الزمنية لإعادة التغذية. أخيرا يظهر جليا أن النمذجة الجيولوجية ثلاثية الأبعاد ذات الأسس الجيولوجية٬ الهيدروجيولوجية و الجيوفيزيائية٬ أمر أساسي بالنسبة للمنهجية المقترحة من أجل حوصلة معلوماتنا بخصوص الموقع المدروس٬ تكملة مخطط التشغيل و وضع أسس الأعمال المستقبلية لنماذج حركة المياه خلال الطبقة غير المشبعة لأحواض المياه الجوفية ذات الطبيعة الجيرية (الكارست Karst).المفردات الرئيسية:الهيدروجيوفيزياء (علم جيوفيزياء المياه الجوفية)، الكارست (حوض مائي جوفي ذو طبيعة جيرية)، إعادة تغذية الطّبقة المائيّة، الطّبقة غير المشبّعة (ZNS)، النّمذجة الجيولوجيةGéomodélisation) )، منبع الفوكلوز (ontaine deF Vaucluse)، المختبر التحت أرضي عديم التشويش بروستال بفرنسا (LSBB de Rustel).
... Lorsque l'émetteur est à « l'infini », la profondeur d'investigation dépend de la fréquence du signal, et lorsque l'émetteur est proche du récepteur, la profondeur d'investigation dépend principalement de la distance émetteurrécepteur. Ces méthodes sont utilisées souvent en technique de profilage ou de cartographie et parfois en sondage (Bosch et Müller, 2005). ...
The Magnetic Resonance Sounding (MRS) is non invasive geophysical method which allows estimating the quantity of groundwater and the permeability of porous media. The thesis aims to better understand the factors which control the precision of MRS results and to improve this precision.
Experimental studies including geophysical measurements, pumping tests and measurements on samples were carried out on four sites with various hydrogeological conditions. Concurrently, numerical modelling made it possible to test the sensitivity of MRS to different parameters.
Thus, the precision to estimate the top of aquifers depends on its depth and on the properties of the unsaturated zone. For the case of heterogeneous media, a new 2D/3D methodology of MRS acquisition and interpretation was developed and validated on a karst conduit.
The measurements on samples show that the MRS water content is often underestimated comparing with the water content measured by NMR or classical methods. The quantity of missed water is linked to the magnetic susceptibility of rocks. The MRS water content is generally a good approximation of the effective porosity, but a quantitative relation between these two parameters has still to be found. When additional geological and geophysical data are taken into account to interpret MRS, the precision on the water content is improved.
The MRS transmissivities have values and uncertainties close to those obtained from pumping tests and can be quantitatively used for hydrodynamic modelling.
... Several geophysical techniques had been used for a long time in mapping and locating superficial objects including pipelines but not limited to magnetics, VLF-EM, Very Low Frequency (VLF) electromagnetic, Time domain electromagnetic; electrical resistivity; Ground Penetrating Radar (GPR) and limited trials of using Frequency Domain electromagnetic (FDEM) (Gay, 1986;Won and Huang 2004;Bosch and Muller, 2005;Babu et al., 2007;Auken et al., 2006;Panissod Birken et al., 2002;Borgioli et al., 2008;Soliman and Wu, 2008;Huang and Won 2000). However the local site conditions, the depth of the buried target and the primary requirements of the survey limited the valid exploration technique to provide; non-destructive technique, low cost survey, speed of data acquisition, rapid data processing steps, sensitive to small background contrast and less affected with the closely background noise sources. ...
In this paper the application of multi frequency electromagnetic techniques to locate buried pipelines is described. The survey site has two pipelines of SUMED, one of the world chokepoints. At desert or arid areas, regular geophysical surveys usually are difficult to carry out. EM techniques could be the best among geophysical techniques to be used for this target at these conditions. The EM survey was performed using a GEM-300 multi-frequency electromagnetic profiler. It is of handheld electromagnetic induction-type that measures in-phase and quadrature terrain conductivity without electrodes or direct soil contact. An area of 60 × 15 m was surveyed, that supposed SUMED pipeline existed. Six different frequencies, typically 2025, 2875, 4125, 5875, 8425, 12,025 Hz, have been used simultaneously. The slice maps for in-phase and conductivity distribution at each frequency could help to trace the extension of the pipeline. Two pipelines were traced successfully with 20 m spacing of each others.
... Electromagnetic (EM) methods are being used successfully to solve various problems associated with exploration and environmental geophysics (e.g., Tezkan 1999;Bosch and Müller 2005). In general, all EM methods are based on the principle of the induction phenomenon, which permits them to be used for locating conducting targets in resistive surroundings. ...
The very low-frequency (VLF) electromagnetic method utilizes primary signals (field) transmitted from worldwide distant transmitters located in coastal areas. These transmitters are meant for long distance marine communication. VLF transmitters operate at a low communication frequency band (between 5–30 kHz) and the transmitted signal travels a long distance. Transmitted signals penetrate the Earth’s subsurface and produce electromagnetic induction in the subsurface even several thousands of kilometres away from the transmitters. The VLF method is quite simple and frequently used in the delineation of near-surface conducting structures of various practical applications. Several conducting structures lying along a measured profile with different conductivities can be properly induced at distinct frequencies that yield the maximum response. Therefore, such conductors may not be identified or resolved well using single frequency VLF measurement. A 2D numerical modelling study was carried out over a wide frequency range (1–500 kHz) to find the frequency that produces the maximum response for a given conductor. Results show that a particular frequency (focusing frequency) produces the maximum (peak) response for a conductor. When the measuring frequency either increases or decreases with respect to the focusing frequency, then the peak response always decreases. The focusing frequency remains almost similar with an increase in target depth and host resistivity. An increase in the overburden conductivity shows a decline in the focusing frequency. Two or more targets of different conductivity present in the subsurface yield peak responses at corresponding focusing frequencies. This shows that they will be resolved well at corresponding focusing frequencies. In such circumstances, inversion using single frequency VLF data yields inaccurate results. However, the use of multi-frequency VLF data yields better results. Inversion of multi-frequency VLF data is presented to show the efficacy of the approach. A field measurement is also presented and the effectiveness of multi-frequency VLF measurement is highlighted. Since the numerical modelling study is performed over a broad frequency range covering the VLF and radiomagnetotelluric signal, the focusing study is valid for radiomagnetotelluric applications as well.
... Electrical mapping has proved useful (Guerin and Benderitter, 1995) as well as electrical resistivity tomography (Vouillamoz et al., 2003;Zhou et al., 2000;Zhou et al., 2002). Very-low-frequency (VLF) measurements (Armadillo et al., 1998;Bosch and Müller, 2005) have led to good results in some cases. The capacitive-coupled resistivity method has recently proved suitable for the detection of shallow karst features (Neukum et al., 2010;Shah et al., 2008). ...
A combined approach using two surface geophysical methods (capacity-coupled geoelectrics and ground-penetrating radar) and careful inspection by optical televiewer probes in a network of boreholes (< 30 m) has been undertaken at an experimental karst site located in the immediate vicinity of the Nerja cave (S. Spain), an important show cave developed in Triassic dolomitic marbles. Our approach has proved helpful in characterizing geometrically and speleologically the vadose zone of these highly heterogeneous environments in that we have been able to identify a considerable number of caves and arrive at an idea of their approximate volume and length. Televiewer logging in the borehole network allowed us to identify a number of karst features: horizontal and vertical openings and cavities, as well as the relative degree of speleothem development. We had previously made a systematic record of the CO2 content of the air inside these boreholes and the results of this earlier study together with the present one have allowed us to further our knowledge of the ventilation patterns in the vadose zone of the research site, which suggest that high CO2 contents (up to 60,000 ppm) are directly linked to the degree of karstification. The general geometrical model of karstification in the area suggests a common genesis for the Nerja cave and the numerous cavities revealed during this study. The flows of CO2-rich air and the ventilation patterns in the vadose zone in general seem to be different in the Nerja cave from those in the network of small cavities surrounding it, a circumstance that most likely derives from the fact that the main cave communicates both naturally and artificially with the outside atmosphere. The air-flow regime in the network of boreholes has still to be studied in detail however.
... Henson et al. 1997), very-low-frequency EM gradient surveys (e.g. Bosch and Müller 2005), and electromagnetic methods (e.g. Vogelsang 1987;Doolittle and Collins 1998;Shah et al. 2008). ...
Geometry and connectivity of high-permeability zones determine groundwater flow in karst aquifers. Efficient management of karst aquifers requires regional mapping of preferential flow paths. Remote-sensing technology provides tools to efficiently map the subsurface at such scales. Multi-spectral remote sensing imagery, shuttle radar topography data and frequency-domain airborne electromagnetic (AEM) survey data were used to map karst-aquifer structure on the Yucatan Peninsula, Mexico. Anomalous AEM responses correlated with topographic features and anomalous spectral reflectance of the terrain. One known preferential flow path, the Holbox fracture zone, showed lower bulk electrical resistivity than its surroundings in the AEM surveys. Anomalous structures delineated inland were sealed above by a low-resistivity layer (resistivity: 1–5 Ωm, thickness: 5–6 m). This layer was interpreted as ejecta from the Chicxulub impact (Cretaceous/Paleogene boundary), based on similar resistivity signatures found in borehole logs. Due to limited sensitivity of the AEM survey, the subsurface configuration beneath the low-resistivity layer could not be unambiguously determined. AEM measurements combined with remote-sensing data analysis provide a potentially powerful multi-scale methodology for structural mapping in karst aquifers on the Yucatan Peninsula and beyond.
... Electrical mapping has proved useful (Guerin and Benderitter, 1995) as well as electrical resistivity tomography (Vouillamoz et al., 2003; Zhou et al., 2000; Zhou et al., 2002). Very-low-frequency (VLF) measurements (Armadillo et al., 1998; Bosch and Müller, 2005) have led to good results in some cases. The capacitive-coupled resistivity method has recently proved suitable for the detection of shallow karst features (Neukum et al., 2010; Shah et al., 2008). ...
A combined approach using two surface geophysical methods (capacity-coupled geoelectrics and groundpenetrating
radar) and careful inspection by optical televiewer probes in a network of boreholes (b30 m)
has been undertaken at an experimental karst site located in the immediate vicinity of the Nerja cave (S.
Spain), an important show cave developed in Triassic dolomitic marbles. Our approach has proved helpful
in characterizing geometrically and speleologically the vadose zone of these highly heterogeneous environments
in that we have been able to identify a considerable number of caves and arrive at an idea of their
approximate volume and length. Televiewer logging in the borehole network allowed us to identify a number
of karst features: horizontal and vertical openings and cavities, as well as the relative degree of speleothem
development. We had previously made a systematic record of the CO2 content of the air inside these boreholes
and the results of this earlier study together with the present one have allowed us to further our knowledge
of the ventilation patterns in the vadose zone of the research site, which suggest that high CO2 contents
(up to 60,000 ppm) are directly linked to the degree of karstification. The general geometrical model of
karstification in the area suggests a common genesis for the Nerja cave and the numerous cavities revealed
during this study. The flows of CO2-rich air and the ventilation patterns in the vadose zone in general seem
to be different in the Nerja cave from those in the network of small cavities surrounding it, a circumstance
that most likely derives from the fact that the main cave communicates both naturally and artificially with
the outside atmosphere. The air-flow regime in the network of boreholes has still to be studied in detail
however.
... Hence, a higher lat- eral resolution can be achieved. Furthermore, VLF-Grad is more sensi- tive to near surface structures than VLF at the same frequencies (Bosch, 2002;Bosch and Müller, 2005). The frequency range of the VLF-Grad receiver air-loops are limited to f =10-40 kHz. ...
Common studies on the static electric field distribution over a conductivity anomaly use the self-potential method. However, this method is time consuming and requires nonpolarizable electrodes to be placed in the ground. Moreover, the information gained by this method is restricted to the horizontal variations of the electric field. To overcome the limitation in the self-potential technique, we conducted a field experiment using a non conventional technique to assess the static electric field over a conductivity anomaly. We use two metallic potential probes arranged on an insulated boom with a separation of 126 cm. When placed into the electric field of the free air, a surface charge will be induced on each probe trying to equalize with the potential of the surrounding atmosphere. The use of a plasma source at both probes facilitated continuous and quicker measurement of the electric field in the air. The present study shows first experimental measurements with a modified potential probe technique (MPP) along a 600-meter-long transect to demonstrate the general feasibility of this method for studying the static electric field distribution over shallow conductivity anomalies.
The soils and underlying weathered carbonate rock in karstic regions play an important role in the infiltration, storage, and retention of water and nutrients. Because of significant heterogeneity of the karst, the use of individual geophysical techniques is often not sufficient for unambiguous assessment of the irregular distributions of soils and underlying fractures. In this study, ground penetrating radar (GPR) and electrical resistivity tomography (ERT) are jointly used with additional observations to delineate the shallow subsurface structure in two exposed profiles. The results show that ERT is effective for detecting the soil–rock interfaces, even for irregular terrain and fracture structures, such as a funnel‐shaped doline, as the soils and rocks show a large resistivity contrast. Although ERT may be able to sense the presence of extensive fracturing, it cannot detect individual small aperture fractures. Joint use of different frequencies of the GPR antenna (e.g., 100 and 500 MHz in this study) allowed the detection of most fractures at different depths in the study sites. However, forward modeling of typical weathered rock features illustrates that the GPR data cannot resolve any reflection signals of the vertical fractures, so the features of vertically enlarged fractures filled by soils cannot be seen from the GPR images. Moreover, large uncertainties of resistivity at the interface between soils or fractures and bedrock limit the identification of an irregularly distributed subsurface structure. Despite the limitations of individual techniques, the combination of ERT and GPR enhances the delineation of the soil–bedrock interface and identification of the fracture network, which can allow an enhanced geological interpretation of shallow subsurface features in the karst areas.
In this study, we aimed to determine the boundaries, locations, and depths of some buried archaeological remains of service buildings belonging to Hatipler Palace in Acemhöyük Settlement Mound from the Assyrian Trade Colony Period (2000–1750 BC) using the very low frequency (VLF) method. After geophysical VLF measurements of the mound, the VLF tipper data were filtered, as is common practice, by Karous and Hjelt (K&H) and Fraser filters. The 2-D VLF inversion technique was also applied to the data, which is a relatively modern procedure for VLF. The 2-D resistivity models obtained after the inversion enabled the construction of 2-D and pseudo-3-D subsurface resistivity images. These imaging techniques revealed two main distinguishing zones of mud-brick walls and stone foundations (moderate resistive zones > 13-Ω m) and surrounding materials (relatively low resistivity zones < 8-Ω m). VLF images, additionally, showed boundaries between the moderate and low resistivity zones successfully. Iso-resistivity images for resistivity values greater than 13-Ω m representing walls are also presented in this study. This imaging method clearly exposed building walls from the third level of the mound. Images obtained for the Acemhöyük mound site with the VLF method allowed nondestructive excavation, and the images suggest new excavation locations for Hatipler Palace buildings, which is one of the most prominent buildings at this site. Our results also revealed that presenting the 2-D inverted VLF data as 2-D and pseudo-3-D images is a successful tool for subsurface imaging.
Supply of sustainable water is a priority for urbanization of the country. Managed aquifer recharge (MAR) is recommended to enhance the groundwater resources, but finding the favorable site for MAR is quite challenging, particularly, in urbanized karst terrains because of highly heterogeneous and anisotropic properties of carbonate aquifers. The expansion along Naya Raipur is posing a severe threat to its groundwater resources by altering the hydrological framework in the area. In this work, we characterize the unsaturated zone for finding potential pathways for MAR in karst terrain of central India by using hydro-geophysical techniques. Sub-surface geophysical investigations including electrical sounding and profiling, captured the shallow surface of the area to the depth of around 40 m. The Wenner–Schlumberger and gradient methods could decipher strong lateral and vertical anomalies. The low resistivity in the unsaturated zone, as compared to the high resistivity of limestone bedrock is the main path for infiltration. The low resistivity anomalies in the 2D inverted sections might be water filled conduits or solution channels with uncertain geometry. Hydrogeological heterogeneity based on geophysical data helped to locate the favourable zones for construction of MAR structures. The Chokra Nala in the Telibandha area of Raipur is the favorable zone along which MAR could be more effective in replenishing the groundwater. The Telibandha lake area with the indication of the presence of hidden mafic dyke later intruding the limestone after its deposition, was also inferred from the geophysical data. Characterizing the unsaturated zone for finding potential pathways for MAR in karst terrain using hydro-geophysical techniques.Establishing MAR along the solution channels, conduits and fracture network.3D fracture system in limestone from 2D ERT profiles.Mapping hydrogeological heterogeneity based on geophysical data. Characterizing the unsaturated zone for finding potential pathways for MAR in karst terrain using hydro-geophysical techniques. Establishing MAR along the solution channels, conduits and fracture network. 3D fracture system in limestone from 2D ERT profiles. Mapping hydrogeological heterogeneity based on geophysical data.
Very Low Frequency (VLF) method is the simplest among all electromagnetic induction techniques with a variety of geophysical applications. Various qualitative and quantitative interpretation techniques are developed to interpret VLF data. However, most of the quantitative interpretation techniques using numerical methods are based on rectangular grid based modeling. Such discretization may produce a large error while modeling the surface topography and also, number of the model parameters will increase drastically to model the surface topography. An effort is made to develop an inversion approach based on triangular grid to reduce the problem size by utilizing minimal number of model parameter and model the rugged topography accurately. First, an analytical expression is derived to compute the vertical component of the magnetic field due to a triangular block of an assigned current density in the block. Subsequently, subsurface structure is optimized in terms of apparent current density using preconditioned conjugate gradient method by considering a model including the surface topography. Since, noise reduction in VLF data is also an important step of interpretation, nonlinear empirical mode decomposition technique is used to minimize the noise from the data. Efficacy of the developed interpretation approach is discussed on one synthetic and one field data set. Inversions for the synthetic and field data sets were carried out with and without topographical variation. The study reveals that depth to the top of the subsurface conductor is correlated with the surface topography. Inversion of VLF data measured over topographical variation with the flat surface model resulted in shifting of the depth to the top of the conductor to an incorrect position (shallower depth). Therefore, consideration of topography in the inversion scheme is vital for delineation of subsurface structures accurately. The efficacy of triangular element in depicting the topography accurately and inversion is demonstrated in the present study.
The economic value of mineral deposits today calls for more interest in geophysical exploration over the Ishiagu area of Abakaliki Basin, Nigeria to delineate the Pb-Zn mineralization in the sedimentary bedrock. Recorded as VLF-EM anomalies are the in-phase and quadrature components of the geomagnetic transfer function, tipper due to induction and these reflect the conductivity contrast between the Pb-Zn mineralized veins and the host rock. Results of sharp tipper responses due to strong EM induction were detected in the surveyed area and on the average, the deeper sources responses range from 4.7% to 7.6%, while the shallower sources responses range from 8.8% to 17.1%. Interpretation with the recorded tipper responses was noted to be of much misfit to the dimensionality, trends and geometry of the Pb-Zn mineralization in view at the nearby existing mines. As a geoelectrical conductivity technique, the efficacy and high resolution of the work is revealed by the 2D current density transformation of the tipper responses. Dimensionality analysis of the current density maps bares these prospective Pb-Zn mineralized veins to trend in the NW-SE direction with their subordinates in N-S direction in correlation to the geologic information of the area that describes/indicates the various episodes of deformation with their characteristic structures. The shallow anomaly sources of Pb-Zn mineralized veins as shown by very sharp tipper responses were detected in the central part, while the deeper sources as shown by broad tipper responses were in the northern part of the study area. The dimensions, trends and shallow depth range of the prospective mineralized veins obtained from VLF readings immensely indicate the prominence of Ishiagu Pb-Zn mineralization.
The economic value of mineral deposits today calls for more interest in geophysical exploration over the Ishiagu area of Abakaliki Basin, Nigeria to delineate the Pb-Zn mineralization in the sedimentary bedrock. Recorded as VLF-EM anomalies are the in-phase and quadrature components of the geomagnetic transfer function, tipper due to induction and these reflect the conductivity contrast between the Pb-Zn mineralized veins and the host rock. Results of sharp tipper responses due to strong EM induction were detected in the surveyed area and on the average, the deeper sources responses range from 4.7% to 7.6%, while the shallower sources responses range from 8.8% to 17.1%. Interpretation with the recorded tipper responses was noted to be of much misfit to the dimensionality, trends and geometry of the Pb-Zn mineralization in view at the nearby existing mines. As a geoelectrical conductivity technique, the efficacy and high resolution of the work is revealed by the 2D current density transformation of the tipper responses. Dimensionality analysis of the current density maps bares these prospective Pb-Zn mineralized veins to trend in the NW-SE direction with their subordinates in N-S direction in correlation to the geologic information of the area that describes/indicates the various episodes of deformation with their characteristic structures. The shallow anomaly sources of Pb-Zn mineralized veins as shown by very sharp tipper responses were detected in the central part, while the deeper sources as shown by broad tipper responses were in the northern part of the study area. The dimensions, trends and shallow depth range of the prospective mineralized veins obtained from VLF readings immensely indicate the prominence of Ishiagu Pb-Zn mineralization.
Abstrak Air merupakan kebutuhan paling dasar yang harus dipenuhi oleh setiap makluk hidup termasuk manusia. Pada musim kemarau beberapa kecamatan di Kabupaten Blitar mengalami kekeringan, Kecamatan Panggungrejo termasuk kecamatan yang mengalami kekeringan tersebut. Jika dilihat pada peta hidrogeologi Kecamatan Panggungrejo memiliki akuifer air tanah yang mengalir melalui celahan dengan produktifitas sedang sampai tinggi, Sehingga tidak seharusnya terjadi kekeringan pada musim kemarau. Untuk mengatasi masalah ini diperlukan data yang lengkap tentang potensi, eksploitasi dan konservasi air tanah di kecamatan tersebut. Data potensi air bawah tanah bebas diperoleh dengan analisa Sistem Informasi Geografis (SIG) pada seluruh faktor penentu keberadaan air tanah bebas (geologi, jenis tanah, tutupan lahan, kelerengan, kelurusan dan drainase). Data potensi air tanah melalui rekahan (sungai bawah tanah) didapatkan dengan cara menerapkan metode Very Low Frequency (VLF) pada Formasi Wonosari (Tmwl) dan Formasi Campur Darat (Tmcl). Untuk potensi air bawah tanah melalui celahan serta ruang antar butir digunakan metode Geolistrik tahanan jenis pada Alluvium (Qa), Formasi Nampol (Tmn), Formasi Wuni (Tmw) dan Formasi Mandalika (Tomm). Sebaran potensi air bawah tanah di area penelitian akan lebih bermanfaat dan tepat guna jika dilakukan analisa Sistem Informasi Geografis (SIG) pada faktor pendukung distribusi air pada masyarakat. Dari hasil penelitian ini didapatkan rekomendasi titik pengeboran dan wilayah konservasi air bawah tanah. Kata kunci: Air tanah, SIG, VLF, dan Resistivitas.
Very Low Frequency (VLF) measurements were successfully acquired using an Unmanned Aircraft System (UAS) as the measurement platform at two sites. The first site was characterized by strong anthropogenic anomalies and was located in Switzerland. The second site was a saltwater-freshwater transition zone in Northern Germany. The induction coils were used to measure the three components of the magnetic field of VLF transmitters. The magnetic field sensors and the corresponding data logger were suspended below a 50 kg rotary wing UAS. Prior to the test survey, noise measurements were carried out to establish a proper distance for the logger and the sensor, which were found at 2 m and 4 m from the body of the UAS, respectively. A suspension system was developed to enable stable flight characteristics in combination with the VLF system. The test flights were carried out with low speed (1 m/s) and at low altitude (approximately 3 m) above ground.
A processing algorithm was developed to analyze the time series of the measured magnetic fields. Transfer functions for the vertical magnetic field were derived using both a scalar and a bivariate analysis approach. After rotating the transfer functions into the strike direction of the 2D subsurface anomaly, a two dimensional inversion method was applied for the interpretation of the conductivity structure to a depth of 15 m.
In addition, radiomagnetotelluric (RMT) measurements were carried out on the same UAS-VLF profiles. The RMT data were interpreted by 2D inversion for comparison and are used here to derive the background resistivity, which cannot be achieved by VLF measurements. As a result, good correlation was found between the UAS-VLF and RMT conductivity models.
Managed Aquifer Recharge (MAR) is a technique of intentional storage and treatment of water in aquifers where natural recharge processes have been affected (Dillon, 2005; Dillon et al., 2009). MAR is a cost-effective process that has solved groundwater quantity and quality problems in many of the world’s stressed aquifer systems (Gale & Dillon, 2005). Natural processes of groundwater recharge have been highly influenced and affected by climate change, urbanization and other unmanaged anthropogenic practices. Groundwater is drafted at a rate which cannot be replenished through natural recharge conditions alone. Groundwater quality is constantly deteriorating due to uncontrolled dumping of waste directly in groundwater interacting zones. Therefore, artificial recharge and MAR is of great interest to replenish the heavy losses of groundwater for sustainable management of the aquifers by enhancing natural infiltration and recharge processes through purposeful and managed activities (Dillon et al., 2009). The process of aquifer recharge involves and is influenced by both surface as well as sub-surface characteristics. However,
in the estimation and evaluation of aquifer recharge, often only surface features and not the sub-surface features are considered, as the latter are much more complex and hidden compared to the surface features that are visible and easily mapped. The sub-surface contains both unsaturated and saturated zones and the two have very contrasting properties. Thus, to investigate the sub-surface, geophysical methods are promising tools as they are mostly non-invasive and reveal the physical properties of the sub-surface. In this chapter relevant geophysical techniques applicable for MAR investigations and their application are briefly described.
The groundwater potential mapping have been studied at Wonotirto and Panggurejo district to encounter drought. The location study have 3 kinds of groundwater potential, namely: (1) an unconfined ground water potential, (2) a confined ground water potential and (3) the underground river potential. The unconfined ground water obtained from analysis of Geographic Information Systems (GIS) at the determinants of the presence of ground water (lithology, soil type, slope, land cover, drainage density, lineament density, precipitation). The confined groundwater potential obtained from geo electrical resistivity measurement. The underground river potential obtained from Very Low Frequency (VLF) measurement. The study also have been recommended exploitation locations and cathment (conservation) area using Geographic Information System (GIS) analysis. It is found that the area of high potential unconfined aquifer is 90 km 2 , the confined aquifer have a thickness of between 8 to 89 meters and underground river can be found on the south side of the District Wonotirto and Panggungrejo with a depth of between 25 to 50 meters. To keep the ground water balance, the water recharge area (32.96 km 2) and the conservation area (35.94 km 2) is needed.
The investigation methods currently suggested concerning the recognition and the protection of the karstic aquifers call only seldom upon geophysical measurements. The goal of these research tasks is to show that the combination of various geophysical methods (TDEM, FDEM, RMP) makes it possible to draw from the sub surface, the necessary information to characterize, partly and on various space scales, the geometry of the tanks and the distribution of the zones of high permeability in the karstic mediums.
Three experimental sites, characteristic of karstic areas observed around the Mediterranean basin, were selected: the site of Loussoi (north-eastern of the Peloponnese - Greece), the site of Poumeyssen (Lot - France) and the site of Paralia Andreas Agios (eastern Peloponnese - Greece). Geophysical techniques adapted to the geological context and the hydrogeological questions were selected and used.
New results were obtained in three fields: (1) the geological knowledge of the studied sites, (2) the specific developments of the geophysical techniques and (3) the definition of a methodological approach using geophysics to the study of the karsts.
All the sites studied in this thesis, which are situated in tabular regions, allowed us to observe and analyze from a geophysical point of view a diversity of morphologies associated with karst plateaus and karsts undercover: outflow mounds and suffosion sinkholes in eastern France, buried and dissolution sinkholes in the Causses du Quercy, epikarst and weathering in the Causse du Larzac. The electrical resistivity and seismic refraction tomographies were very useful for characterizing the geometry of such objects. The contribution of 3D seismic refraction has been significant for imaging fracturing and weathering features in the epikarst. Electrical and seismic time-lapse measurements was also realized on a karst aquifer in south France and on two epikarstic areas on the Causse du Larzac. Time-lapse resistivity tomography was not very effective, but time-lapse seismic refraction tomography shows a high potential to identify infiltration and storage areas, particularly in the epikarst. New results were obtained: (1) the understanding of karstic morphologies by analyzing their structure, (2) the structure and functioning of the epikarst, (3) the effectiveness of electrical and seismic methods for structural imaging and water saturation monitoring in karstic systems.
Western Turkey is one of the most spectacular regions of widespread, active continental extension in the world. The most prominent structures of this region are E-W trending Gediz and Buyuk Menderes Grabens. Geothermal activity around city of Manisa in Gediz Graben has been investigated by many researchers and many geothermal boreholes were drilled in order to produce electricity. In the middle of May 2012, a geothermal blowout occurred 150 m away from the deep drilling (depth 1,100 m), in Alkan village of AlaAYehir resort in city of Manisa. After that, four big blowouts took place at the same area and thermal water had been diverted to Alkan stream for almost 4 months. The boron (B) and Fe levels of thermal water are 87 and 11.2 ppm where the background B and Fe values of topsoil vary between 0.2-1.5 ppm and 3.08-5.63 ppm, respectively. The objective of this study was to utilize joint magnetic susceptibility and VLF-R investigations to determine possibly contaminated area with B, related to high Fe level on the topsoil and thermal water in the subterranean layers. It is observed that magnetic susceptibility values vary between 500 and 790 cgs (x10(-6)) in the vicinity of blowouts and 150-340 cgs (x10(-6)) at a distance of 50 m around, where the background average value is 20 cgs (x10(-6)), by using the data collected at 346 stations. Besides, the VLF-R studies were carried out along 18 profiles at three different frequencies. Laterally constrained two-layer inversion was applied to each station; in addition to the inversion of all profiles for each frequency, all lines were stacked, 2-D resistivity maps were obtained, and the contaminated area was determined approximately as 35,000 m(2). The results show not only satisfactory agreement between magnetic susceptibility and VLF-R data, but also provide fast and valued interpretation of geochemical and geophysical properties of the geothermal contaminated area.
In a pilot project, very low frequency (VLF) measurements were performed using a prototype unmanned airborne system (UAS) at a test area near Neuchǎtel, Switzerland. The VLF system operates over an extended frequency range of 1-250 kHz and consists of a three-component magnetic field sensor and a data logger, both deployed on an unmanned helicopter. Various developments had to be done prior to the first flight. The devices went through a weight reduction process, the noise influence of the aircraft on the devices had to be investigated, and an appropriate suspension had to be developed. This first experiment was conducted on a test site with anomalies due to man-made objects: a buried pipeline and a power cable with known orientation and location. The performance of the system in comparison with ground-based VLF data was investigated. First results demonstrate the feasibility of airborne VLF measurements with a UAS to detect buried pipelines and power cables.
Sinkhole formation is an ongoing hazard for urban karst settings, compromising construction foundations, and accelerating groundwater pollution. In Springfield, Missouri, the uppermost exposed unit is the Burlington limestone, a carbonate bedrock which is susceptible to karst formation, including caves and sinkholes. This study site concerns a detention basin near a major highway interchange, in which a new sinkhole is forming. Electric resistivity, very low-frequency electromagnetics, and dye-tracing surveys were performed at this sinkhole to determine its subsurface extent and any relation to surface features. The low resistivity associated with the sinkhole and thin soil contrasts with the high resistivity of the underlying Burlington limestone. This contrast highlights a curvilinear trough of thicker soil, which correlates with a former intermittent stream and represents a stream channel, which was graded and filled during construction of the highway and basin. The newly formed sinkhole lies entirely within this former stream channel. This relationship between the sinkhole and stream provides useful insights into the effects of urbanization on sinkhole formation. Knowledge of these relationships allows urban managers to better understand the risk this sinkhole poses to major highways and public buildings in the area.
The common hydrogeological concepts assume that water enters in karst media by preferential pathways. But it is difficult to identify these pathways, particularly if soil or scree covers the karst features. When and where does water enter in the hydrosystem? How fast? A unique large-scale Electrical Resistivity Tomography (ERT) surface-based time-lapse experiment was carried out during a typical Mediterranean autumn rainy episode (230 mm of rain over 17 days). A total of 120 ERT time-lapse sections were measured over the same profile during and after this event (30 days). The main goal was to evaluate efficiency and limits of the ERT to monitor water infiltration, under natural conditions. Apparent (directly measured) and inverted resistivity’s variation during the rainy event highlights some interesting zones. They could be interpreted as preferential pathways, where water dynamic seems quicker in term of moistening and drainage. Nevertheless, these results have to be interpreted reasonably because ERT does not provide enough precision to determine exact pathways geometry and functioning. In addition, forward modeling provided relevant data treatment limitations mainly for the deeper parts of the sections.
So called Unmanned Airborne Systems (UAS) can be used beneficial for geophysical measuring campaigns. Their advantages reach from faster data acquisition compared to ground measurements to higher versatility (e.g. ground vicinity) to conventional aerial systems. The geophysical Very Low Frequency (VLF) method exploits this advantages especially good. This is because it is well suitable for mapping and therefore extremely powerful in combination with aerial systems. The system presented here is a combination of the unmanned helicopter Scout B1-100 from Aeroscout and the Super High Frequency induction coil Triple (SHFT) and Analog Digital Unit (ADU)-07 data logger from Metronix. The sensor and logger went trough a weight reduction process and had to be deployed on a special suspension developed by Aeroscout. With a payload of 18 kg and an operation time of one hour the system is ideal for mid sized measuring campaigns, i.e. several hundred meters. A proof of concept study was carried out in March 2012 on a test area close to Neuchâtel. Two man-made anomalies, a pipeline and a power cable were investigated to test the practicability of the system. The feasibility of airborne VLF measurements with an UAS to detect buried anomalies is demonstrated by first results.
This paper highlights the efficiency and complementarity of a light
package of geophysical techniques to study the structure of karst
Unsaturated Zone (UZ) in typical Mediterranean environment where soil
cover is thin or absent. Both selected techniques, 2D Ground Penetrating
Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used
in environmental studies and their application is accessible for a lot
of scientists/engineers. However, GPR or ERT alone is not able to
provide an enhanced characterization of geological features in karst
media. In the present study, GPR results supply a near surface high
resolution imaging and thus can provide relevant geological information
such as stratifications and fractures. Despite the quality of the
results GPR's investigation depth remains limited to around 12 m.
Apparent and inverted resistivity provided by ERT surveys shows strong
lateral and vertical variations. These variations can inform about
general geological structuring and feature orientation. ERT is able to
prospect down to 40 m but it's a low resolution integrative technique.
In the study area the investigated limestone is a commonly electrical
resistive formation (more than 2000 Ω.m). However deeper than
5-7 m, the ERT profiles reveal several zones of moderate
resistivity (around 900 Ω.m). In these zones a stratification
change corresponding to slanted bedding is clearly identified by GPR
results. The combination of both GPR and ERT results can allow a
well-established geological interpretation. These moderate resistivity
zones with slanted beddings can explain the presence of a perennial
water flow point 35 m below the surface of the studied site within the
underground gallery of the Low-Noise Underground Laboratory (LSBB).
Very low frequency wave-resistivity (VLF-R) method has been widely used for near surface and archaeological prospection over the last two decades. Shallow buried structures that show resistivity variation with respect to a surrounding medium could be determined with VLF-R. It is also a particularly rapid and cost-effective technique for collecting data on large-scale exploration. VLF-R studies were carried out in the Agora of Magnesia archaeological site (Aydın, Turkey) in order to determine the location and depth of ruins of the temple of Zeus. After performing theoretical studies to test the inversion algorithm, apparent resistivity and phase data were collected with three different frequencies and a laterally constrained two-layer inversion process was applied to each station. In addition to the inversion of all profiles for each frequency, all lines were stacked within a precise resistivity interval to obtain a 3D view of the structure. An excavation site is recommended after achieving the location of the temple.
The karst environment is one of the most challenging in terms of groundwater, engineering and environmental issues. Geophysical methods can provide useful subsurface information in karst regions concern-ing, for instance, hazard estimation or groundwater exploration and vulnerability assessment. However, a karst area remains a very difficult environment for any geophysical exploration; selection of the best-suited geophysical method is not always straightforward, due to the highly variable and unpredictable target charac-teristics. The state of the art is presented, in terms of the contributions made by geophysical methods to karst-system exploration, based on extensive analysis of the published scientific results. This report is an overview and should be used as a preliminary meth-odological approach, rather than a guideline.
Karst aquifers represent important but very vulnerable sources for water supply to a significant part of the Earth's population. For sustainable use of these resources, development of integrated manage-ment tools based on numerical groundwater models is required. In principle karst aquifers are characterized by the presence of two distinct flow domains: the limestone matrix fractures and the conduits. A flow model of karst aquifers requires detailed, spatially distributed information on the hydrologic characteristics of the aquifer and flow paths. Geophysical methods determining the distribution of the electrical resistivities within the subsurface could provide such information. An international scientific research project was initiated to explore the potential of airborne electromag-netic mapping for providing such innovative information for improving groundwater modelling of karst aquifers. The project study area is located in the Sian Ka'an Biosphere Reserve located in Yucatán, Mexico, a coastal wetland of international importance. As a first step ground geoelectric and ground electromagnetic measurements were performed in March 2006 to determine the electri-cal properties of the Sian Ka'an Biosphere Reserve subsurface environment. These results were used for 3D forward modelling to calculate the expected airborne electromagnetic response. Based on these promising results, an airborne pilot survey was performed in 2007 to evaluate the applica-bility of airborne electromagnetic methodology. This survey covers an area of 40 square kilometres above the well-mapped Ox Bel Ha cave system. The results showed that the signature of the cave system could be clearly detected. The pilot survey offered as well the chance to define the limits of current state-of-the-art airborne data acquisition and inversion. The study helped to define the needs for further developments and improvements to establish the frequency domain electromagnetic method as a practical karst exploration method. water supply for a significant portion of the Earth's population (e.g., Darnault 2008). In order to protect karstic groundwater systems and associat-ed ecosystems, it is necessary to develop integrated tools for sustainable management of these resources based on accurate coupled surface-/groundwater models. Although the mathemati-cal formulation of such models is well established, results are mostly unsatisfactory. The reason for this is that both location and geometry of dominant karst features, controlling the flow, are either not known at all or not known with the required
Artificially-created hydrological catchments are characterized by sediment structures from technological construction processes that can potentially be important for modelling of flow and transport and for understanding initial soil and ecosystem development. The subsurface spatial structures of such catchments have not yet been sufficiently explored and described. Our objective was to develop a structure generator programme for modelling the 3-D spatial sediment distribution patterns depending on the technical earth-moving and deposition processes. For the development, the artificially-constructed hydrological catchment "Chicken Creek" located in Lower Lusatia, Germany, served as an example. The structure generator describes 3-D technological sediment distributions at two scales: (i) for a 2-D-vertical cross-section, texture and bulk density distributions are generated within individual spoil cones that result from mass dumping, particle segregation, and compaction and (ii) for the whole catchment area, the spoil cones are horizontally arranged along trajectories of mass dumping controlled by the belt stacker-machine relative to the catchment's clay layer topography. The generated 3-D texture and bulk density distributions are interpolated and visualized as a gridded 3-D-volume body using 3-D computer-aided design software. The generated subsurface sediment distribution for the Chicken Creek catchment was found to correspond to observed patterns although still without any calibration. Spatial aggregation and interpolation in the gridded volume body modified the generated distributions towards more uniform (unimodal) distributions and lower values of the standard deviations. After incorporating variations and pedotransfer approaches, generated sediment distributions can be used for deriving realizations of the 3-D hydraulic catchment structure.
Water and pollution transport takes places in fissures, and fissures may also reduce the stability of the rock body. Therefore, fissures in rock are an important issue, especially in case of multi-directional fissure systems, where the situation is more complicated.
The so-called azimuthal technique, by using the Schlumberger array is a routinely applied method in geoelectric fissure detections. As we demonstrate it by using analogue model measurements, this method has several shortages. In absence of a priori information, the fissure directions could be obtained only in 60% of the total cases with a precision of ±10%. We have also demonstrated that the effectivity of the method becomes even worse, when the covering sediment layer is thicker. At the same time, when we simply rejected the anisotropy paradox, it was found, that the effectivity of the method is increasing with increasing sediment thickness. The original azimuthal technique is based on the generally accepted hypothesis of the so-called geoelectric anisotropy paradox. The analogue modelling results have made clear that the critical point of the method is the application of the anisotropy paradox. The fissure directions can only be determined, if the conditions of the anisotropy paradox are clearly defined. However, having more than one fissure directions, statistically these fissure directions can be correctly determined.
In the context of cover-collapse sinkholes, as in the Tournaisis area, the detection of palaeokarst features allows to delineate, at a local scale, areas that might be subject to future collapse. These areas should therefore be avoided or dealt with in the perspective of earthworks for civil engineering. Before the last decade, karst mapping in this context was often wrongly considered as a cavity detection problem and tackled in this way. A better understanding of these phenomena allows developing more appropriate detection methods. We precise the geological settings and karst context of the Tournaisis. We present the key characteristics of the palaeokarsts and the main contrasts in petrophysical properties expected between alterite (ghost-rock) and bedrock. Given these contrasts and the wide range of overburden thicknesses found in the Tournaisis, we evidence the main advantages and major limitations of potentially effective geophysical investigation methods. We illustrate our purpose by describing some results of electrical resistivity tomography and seismic surveys applied to palaeokarst detection. These results are assessed using numerous geotechnical data and geological knowledge. Finally, we show how these results could be used for geological and geotechnical problem solving in particular trough the design of adapted detection and mapping strategies.
We report herein on a finite element algorithm for 2-D magnetotelluric modelling which solves directly for secondary variations in the field parallel to strike, plus the subsequent vertical and transverse auxiliary fields, for both transverse electric and transverse magnetic modes. The governing Helmholtz equations for the secondary fields along strike are the same as those for total field algorithms with the addition of source terms involving the primary fields and the conductivity difference between the body and the host. Our approach has overcome a difficulty with numerical accuracy at low frequencies observed in total field solutions with 32-bit arithmetic for the transverse magnetic mode especially, but also for the transverse electric mode. Matrix ill-conditioning, which affects total field solutions, increases with the number of element rows with the square of the maximum element aspect ratio and with the inverse of frequency. In the secondary formulation, the field along strike and the auxiliary fields do not need to be extracted in the face of an approximately computed primary field which increasingly dominates the total field solution towards low frequencies. In addition to low-frequency stability, the absolute accuracy of our algorithm is verified by comparison with the TM and the TE mode analytic responses of a segmented overburden model.
A fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer. This technique is based on the smoothness-constrained least-squares method and it produces a two-dimensional subsurface model from the apparent resistivity pseudosection. In the first iteration, a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. For subsequent iterations, a quasi-Newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times, respectively, compared to the conventional least-squares method. Tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly. This technique takes about one minute to invert a single data set on an 80486DX microcomputer.
The term "null array" is introduced for those electrode configurations where the measured potential difference is zero above a homogeneous half-space when using a measuring dipole (MN0)-N-0. Different types of null arrays (three-electrode, Schlumberger, and dipole axial/equatorial null arrays) and their corresponding traditional arrays are studied. It was shown in a field study carried out in a karstified limestone area covered by thin sediments that it is possible to obtain geologically meaningful results with null-array techniques. The main features of the null-array data are as follows. (1) Null-array data appear to be more spatially variable than the classical data. The spatial variability provides information about the presence of karstic fractures in the subsurface; (2) The null-array anomalies caused by nearly vertical karstic fractures in the limestone basement do not decay with depth as quickly as the classical array anomalies. (3) The strike direction,of the fractures is much less ambiguous than that found by using classical arrays. Nevertheless, the depth variation of the basement is more reliably observed in geoelectric anomalies obtained using traditional arrays. Therefore a joint use of classical arrays and their corresponding null methods is recommended, because the combined methods provide more information about the subsurface structure.
A new flow-through field fluorometer sonde has been designed for use in downhole tracer tests in 2’’ boreholes. The instrument is capable of determining the partial concentration of two dye tracers present simultaneously in the water. In addition, turbidity can be measured if the water is free of tracers. Although the sonde is aimed at boreholed hydrological investigations, it can also be used in surface waters.
A simple algorithm for the inversion of electrical tomography data in the form of pseudosections has been developed. This uses the measured apparent resistivity data as the starting model for a two-dimensional finite difference iterative imaging scheme. Differences between the computed and measured pseudosections are used to apply corrections to the model so that over about ten iterations the model provides an acceptable inversion of the observed data. Example surveys across a landfill site, an igneous intrusion covered by sediment and a variety of models show that useful and valid electrical images of the subsurface can be produced. -Author
A system is described for the automatic measurement of electrical resistivity pseudo-sections. This comprises a linear array of up to 32 electrodes connected through a multicore cable to a computer controlled switching module and a resistivity meter. The processing of the measured sections to produce two-dimensional true resistivity images of the subsurface is briefly described. Some account is given of the capabilities and limitations of the technique. This is illustrated by a series of computed constant separation traverses for models of simple subsurface structures. Examples of processed images derived from sections measured in areas of relatively complex geology follow, a comparison being made of the interpretations obtained using an automatic imaging method and a manual iterative approach. It is concluded that with the equipment and software so far developed, in areas of modest subsurface geological complexity where some control is available and where the structures are essentially two-dimensional, then good approximations to the true geoelectric sections can be obtained down to depths of between 100 and 200 m.
Institute of Scientific Investigations of the Earth Magnetism Translated by L. Perlovsky and C. Tarlowski Magnetic and electric fields in the earth should follow certain interrelations as different characteristics of natural electromagnetic fields of the earth. Empirical comparison of a derivative of the East component of magnetic field Hy and the North component of the electric field Ex shows, that between these functions there is a rough proportionality. Between these functions there also is a phase shift. The aim of this note is to present a theoretical con-sideration of the relations between magnetic and electric fields from the point of view of Maxwell equations. Although suggested theory is based on very simplified considerations, it permits establishing and refining the above relations and also allows reaching some conclusions about electrical characteristics of the earth's crust. I use this study to formulate the problem of using natural electric fields to study the electric characteristics of the deep layers of the earth's crust, i.e. the problem of deep electrical prospecting. To establish qualitative relations between magnetic and electric fields, I schematically consider the earth's crust as a layer 0:::; z:::; £ of finite conductivity, on a perfectly conductive basement. I consider the one-dimensional (plane-wave) problem. The components of the magnetic field H x , H y, and H., and also the components of the electric field of the earth Ex and E y, are known on the earth's surface z=O. The component E. at z=O is practically equal to zero because of the very small conductivity of the air. At 0:::; z:::; £ these functions satisfy Maxwell equations. Disregarding the displacement currents, then each of those functions satisfies ~ =..L au (0 <z <£ ..L = 4?rap.) . and the boundary problem without initial conditions u(O,t) = p.(t), p.(£,t) = O. The solution of this problem is very simple. Expressing the boundary value p.(t) as a superposition of harmonic functions and taking separate harmonics gives 11 (t) = A c~s wt , r sm wt where Re J 82 cos wt (sh ~(£-z») u = lm A sh n£ sin wt ' and au I = Re (A 0w coth ITc:0) cos wt . az z = 0 lm J 8"2 J 82 (sm wt In this way I obtain for the harmonic amplitudes H(w) and E(w) of the magnetic and electric fields at frequency w iw l!:: H(w) = E (w) 0w coth ~£, ex Y.)8"2 J8i and analogously for H)w) and Ex(w).
The Centre of Hydrogeology Neuch‚tel (CHYN) investigates and monitors karst aquifers, which are highly vulnerable to contamination. One of our particular research tasks within this scope is the characterization and localization of the 'epikarstic zone' (0-20 m depth) (Mangin 1975; Kiraly 1995). Density, size and aperture of fractures that connect the water reservoir with the surface, are features which characterize the vulnerability of karst aquifers. Very local scale mapping of these types of high-permeability fracture zone over extensive areas is therefore of interest for karst hydrogeology. In order to investigate this zone the very low frequency (VLF, 15-30 kHz) method has been adapted at CHYN for investigation at shallow depths with high lateral resolution. Geophysical surveying using VLF radio signals began in the 1960s as a tool for prospecting conductive minerals (McNeill & Labson 1991; Paal 1965). The CHYN equipment uses VLF and low frequency (LF, 30-300 kHz) radio transmitters, which are located at several positions around the world and are used principally for navigation and submarine communication. Consequently the CHYN has developed a radio frequency- resistivity device (RF-R), also called radio magnetotelluric (RMT, 12-240 kHz), and a continuously measuring radio frequency-electromagnetic (RF-EM, 15- 300 kHz) device (Turberg et al. 1994; Turberg & Barker 1996; Stiefelhagen 1998). In fissured limestone with a resistivity of several hundreds of Wm, the penetration depth for these frequencies is about 10-100 m. However, when investigating small structures at very shallow depths (0-10 m) in the epikarstic zone, these devices have limited use due to their excessive penetration depths and their poor horizontal resolution. In this paper we introduce a method that is based on measuring the vertical gradient of the horizontal magnetic field component of VLF radio signals (VLF-EM-vGRAD(Hy)). According to our knowledge, such a technique has not been previously published.
We simulate the magnetotelluric response to two- dimensional earth topography using finite elements. Linear interpolation of the secondary field parallel to strike over triangular elements allows accurate mod- eling of inclined resistivity boundaries, including topo- graphic surfaces. To avoid discontinuities in field de- rivatives or resistivity, care must be taken that the nodal values of the field parallal to strike used to obtain the auxiliary secondary fields are kept within uniform earth media. The nodal locations may be shifted, but the de- rivatives still are evaluated at the field points of interest. Correct values may be returned at gentle breaks in slope as well as along straight surfaces. The finite-element program is verified by comparison with the analytic transverse magnetic response of a hemicylindrical de- pression and with Rayleigh scattering and transmission surface results for transverse electric and transverse magnetic polarization. Agreement with the other meth- ods generaiiy is exceilent, with the exception of some results of the transmission surface technique (especially the transverse magnetic mode). A result presented shows that placing the H-field sensors horizontally reduces topographic anomalies compared to locating sensors parallel to the slope. Moreover, if earth resistivity in- creases with elevation, the apparent resistivity is rela- tively nonanomalous near the base of the topography.
From Ampere’s Law (for a homogeneous earth) and from Maxwell’s equations using the concept of Hertz vectors (for a multilayered earth), solutions are obtained for the horizontal components of the electric and magnetic fields at the surface due to telluric currents in the earth. The ratio of these horizontal components, together with their relative phases, is diagnostic of the structure and true resistivities of subsurface strata. The ratios of certain other pairs of electromagnetic elements are similarly diagnostic. Normally, a magneto‐telluric sounding is represented by curves of the apparent resistivity and the phase difference at a given station plotted as functions of the period of the various telluric current components. Specific formulae are derived for the resistivities, depths to interfaces, etc. in both the two‐ and three‐layer problems. For two sections which are geometrically similar and whose corresponding resistivities differ only by a linear factor, the phase relationships are the same and the apparent resistivities differ by the same proportionality constant which relates the corresponding true resistivities. This “principle of similitude” greatly simplifies the representation of a master set of curves, such as is given for use in geologic interpretation. In addition to the usual advantages offered by the use of telluric currents (no need for current sources or long cables, greater depths of investigation, etc.), the magneto‐telluric method of prospecting resolves the effects of individual beds better than do conventional resistivity methods. It seems to be an ideal tool for the initial investigation of large sedimentary basins with potential petroleum reserves.
A bstract
The suitability of VLF ground surveys in the investigation of shallow two‐dimensional structures is analyzed. For such structures the polar formalism is derived, necessary in practice since the transmitters are generally not in the structural strike or profile. A simple vertical dike is considered to demonstrate the striking anisotropy which can be expected over such a structure, in particular the high apparent resistivity along the direction of a well‐conducting dike and the low resistivity across it. The theory is then confronted with the practical example of an asymmetrical vertical dike resulting from a strike‐slip fault. Modelling of the survey results is very successful and yields good confirmation of the polar behaviour. VLF ground surveys thus provide a quick and powerful tool for the study of geological accidents within about 100 m of the surface.
Algorithm PW2D is a finite element program for simulation of magnetotelluric responses of two-dimensional earth resistivity structure. Linear interpolation of the unknown field parallel to strike over triangular subdomains is utilized in conjunction with the Galerkin method of basis weighting to derive a system of linear equations which approximates the governing Helmholtz equation. From the field parallel to strike, the auxiliary vertical and transverse fields obtain from a numerical approximation to Maxwell's equations. Both transverse electric (TE) and transverse magnetic (TM) modes of plane-wave excitation can be modeled. The algorithm described herein unlike previous versions of the program solves directly for secondary variations in the field parallel to strike, plus the subsequent auxiliary fields. This approach has circumvented a difficulty with numerical precision at low frequencies observed in total field solutions for the TM mode especially, but also for the TE mode. 14 refs., 18 figs.
An analytic one-dimensional magnetotelluric inversion scheme is described. The main characteristics of this new scheme are its simplicity and its minimal requirements in computer time and storage space. The basic idea of the scheme is that for a given period T only the structure above a certain depth matters. The scheme therefore starts with the shortest periods of the available data set and tries to explain the observed response, specifically the apparent resistivity pa(T) and the phase ø(T), in terms of a two-layer structure. Shifting successively to longer periods, discrete new layers are introduced at progressively greater depth. Some stabilizing features simultaneously keep the inversion process from diverging and hold the necessary number of layers to a minimum. The properties of the scheme are analysed by studying its performances when working on synthetic and real field data. The scheme can handle fairly scattered data and is very stable. It has, in fact, never been observed to diverge. Examples are given of how the scheme might be handled in practice, especially regarding the use of the causal dispersion relations with which it is possible to ensure that the model proposed by the inversion scheme will return a response function in agreement with the initial data.
A simple filter is developed which transforms VLF-EM real magnetic field transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two-dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF-EM resistivity measurements or d.c. resistivity profiling. This low-pass filter has the advantage of reducing high-wavenumber noise in the data; therefore only the major features of the VLF-EM profile are displayed.
The quantities measured in transient electromagnetic (TEM) surveys are usually either magnetic field components or their time derivatives. Alternatively it might be advantageous to measure the spatial derivatives of these quantities. Such gradiometer measurements are expected to have lower noise levels due to the negative interference of ambient noise recorded by the two receiver coils. Error propagation models are used to compare quantitatively the noise sensitivities of conventional and gradiometer TEM data. To achieve this, eigenvalue decomposition is applied on synthetic data to derive the parameter uncertainties of layered-earth models. The results indicate that near-surface gradient measurements give a superior definition of the shallow conductivity structure, provided noise levels are 20–40 times smaller than those recorded by conventional EM instruments. For a fixed-wing towed-bird gradiometer system to be feasible, a noise reduction factor of at least 50–100 is required. One field test showed that noise reduction factors in excess of 60 are achievable with gradiometer measurements. However, other collected data indicate that the effectiveness of noise reduction can be hampered by the spatial variability of noise such as that encountered in built-up areas. Synthetic data calculated for a vertical plate model confirm the limited depth of detection of vertical gradient data but also indicate some spatial derivatives which offer better lateral resolution than conventional EM data. This high sensitivity to the near-surface conductivity structure suggests the application of EM gradiometers in areas such as environmental and archaeological mapping.
This paper presents a methodology in geophysics applied to the study of porous aquifers. This methodology is based on an improved “very low frequency-resistivity” equipment, on greater importance being attached to the local geological heterogeneity of the environment, and also on a method of data processing which is simple but well adapted to a large number of studies concerning prospecting, protection and management of underground water.The radio magnetotelluric-resistivity equipment (RMT-R) is derived from the classical VLF-resistivity equipment. Its increased reception band (12–240 kHz) allows us to realize rapid frequency soundings.In the example to be discussed, a geostatistical analysis of semivariance, made on a series of more than 600 measurements, shows a spatially structured geophysical heterogeneity, both for the values of resistivity and for those of phase. The correlation between this geophysical heterogeneity and the lithological heterogeneity of the environment is confirmed by mechanical soundings (geological test holes). The indirect relations between this geophysical heterogeneity and the hydraulic behaviour of the aquifer system can be estimated and are confirmed, on this specific site, by the observation of the piezometric levels.The results of our study illustrate the positive contribution RMT-R can make to hydrogeology. Above all they show that those porous environments, frequently considered as homogeneous, cannot be considered as such either hydrogeologically or geophysically.
In the past decade, both inductive electromagnetic survey instrumentation and associated interpretive techniques have become refined to the point that electromagnetic techniques are widely used for geological mapping as well for the direct detection of conductive ore bodies. Electromagnetic survey techniques have been particularly successful in exploration for potable groundwater, for measuring salinity levels in aquifers and monitoring coastal saline intrusion, and for mapping soil salinity in connection with crop growth.Regardless of the techniques employed, it is the terrain conductivity that is measured, and it is a particular advantage of electromagnetic techniques that small variations in the bulk conductivity of the terrain can often be detected. A further advantage is that most electromagnetic techniques allow measurements to be made rapidly, and survey costs are generally less than those associated with conventional DC resistivity surveys or, conversely, larger areas can be surveyed in greater detaul for comparable cost. A disadvantage of electromagnetic instrumentation is that although the shallower units cost about the same as resistivity equipment, the deeper penetration systems are relatively expensive. In general, electromagnetic systems are most effective in looking for the better conductors and are ineffective in searching for resistive material. In all cases some knowledge of electromagnetic theory is desirable for a successful interpretation.In this paper we present several case history selected from the literature in which a variety of electromagnetic systems (horizontal loop EM, ground conductivity meters and VLF) are used either alone or in conjunction with conventional resistivity to explore for groundwater.
Cover title. Thesis (doctoral)--Université de Dijon, 1975. Articles originally published in Ann. spéléol. 1974 and 1975. Abstracts in English. Includes bibliographical references (p. 116-124).
Observations tectoniques dans une carrière de Chaumont (Jura Neuchatelois, Suisse).
- Kiraly L.
Kiraly L. and Meia J. 1967. Observations tectoniques dans une carrière
de Chaumont (Jura Neuchâtelois, Suisse). Société Neuchâteloise des
Sciences Naturelles 90, 276-283.
Structure géologique et orientation des cavités karstiques: la Grotte de
- L Kiraly
- G Simeoni
Kiraly L. and Simeoni G. 1971. Structure géologique et orientation des
cavités karstiques: la Grotte de "Chez le Brandt" (Jura neuch.).
Bulletin de la Société Neuchâteloise des Sciences Naturelles 94,
91-97.
Erläuterungen zu Geologischer Atlas der Schweiz - Blatt: 1065
- H Liniger
Liniger H., 1970. Erläuterungen zu Geologischer Atlas der Schweiz -
Blatt: 1065 Bonfol (Attlasblatt 55), Schweizerische Geologische
Kommission, Bern.
Society of Exploration Geophysicists
- H Ward
H. Ward), Society of Exploration Geophysicists.
Geological mapping using VLF radio fields Application, Parts A and B, Investigations in Geophysics No
- J D Mcneill
- V F Labson
McNeill J.D. and Labson V.F. 1991. Geological mapping using VLF
radio fields. In: Electromagnetic Methods in Applied Geophysics,Vol.
2, Application, Parts A and B, Investigations in Geophysics No. 3 (ed.
M.N. Nabighian), Society of Exploration Geophysicists.
La grotte de Chez le Brandt " (Jura Neuchâtelois, Coord Essai de synthèse des donnees géologiques et hydroéologiques
- I Müller
Müller I. 1981. La grotte de " Chez le Brandt " (Jura Neuchâtelois,
Coord; 526 425/ 199 000). Essai de synthèse des donnees géologiques
et hydroéologiques. Cavernes (Neuchâtel) 25(1), 8–14.
Investigating the near-surface conductance with an integrated EM experiment in Bad Königshofen
- K Spitzer
- B Fluche
- W Storz
Spitzer K., Fluche B. and Storz W. 2000. Investigating the near-surface
conductance with an integrated EM experiment in Bad Königshofen.
Proceedings of the Elektromagnetische Tiefenforschung, Altenberg,
Germany, 18, Deutsche Geophysikalische Gesellschaft, 70-80. ISSN
0946-7467.
Comparative studies of VLF-R and VLF-EM geophysical methods; 1-D and 2-D numerical modelling at the tracer test-field Wilerwald (BE, Switzerland)
- T Steiner
- A De Carvalho Dill
- L Szarka
- I Müller
Steiner T., de Carvalho Dill A., Szarka L. and Müller I. 1992.
Comparative studies of VLF-R and VLF-EM geophysical methods; 1-D and 2-D numerical modelling at the tracer test-field Wilerwald (BE,
Switzerland). Bulletin du Centre d'Hydrogéologie de l'Université de
Neuchatel 11.
Radio Frequency Electromagnetics (RF-EM): Kontinuierlich messendes Breitband-VLF, erweitert auf hydrogeologische Problemstellungen
- W Stiefelhagen
Stiefelhagen W. 1998. Radio Frequency Electromagnetics (RF-EM):
Kontinuierlich messendes Breitband-VLF, erweitert auf hydrogeologische Problemstellungen. PhD thesis, Centre of Hydrogeology,
University of Neuchâtel, Neuchâtel, Switzerland.
La méthode inductive VLF‐EM pour la prospection hydrogéologique en continu du milieu fissuré.
- Turberg P.
Turberg P. and Müller I. 1992. La méthode inductive VLF-EM pour la
prospection hydrogéologique en continu du milieu fissuré. Annales
Scientifique de l' Université de Besancon, Mémoire hors série 11,
207-214.
Comparison of RF-EM, RMT and SP measurements on a karstic terrain in the Jura Mountains (Switzerland)
- F P Bosch
- M Gurk
Bosch F.P. and Gurk M. 2000. Comparison of RF-EM, RMT and SP
measurements on a karstic terrain in the Jura Mountains (Switzerland).
Proceedings of the Elektromagnetische Tiefenforschung, Altenberg,
Germany, 18, Deutsche Geophysikalische Gesellschaft, 51-59. ISSN
0946-7467.
Structure géologique et orientation des cavités karstiques: la Grotte de “Chez le Brandt” (Jura neuch.).
- Kiraly L.
La grotte de “Chez le Brandt” (Jura Neuchâtelois, Coord; 526 425/ 199 000).
- Müller I.
On determining the electric properties of deep layers of the earth’s crust.
- Tikhonov A.N.
Comparative studies of VLF‐R and VLF‐EM geophysical methods; 1‐D and 2‐D numerical modelling at the tracer test‐field Wilerwald (BE, Switzerland).
- Steiner T.
- Militzer H.