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Enhancement and presentation of airborne geophysical data
Abstract
Airborne geophysical data can be enhanced by a range of linear and non-linear filtering algorithms which selectively enhance the anomalies due to one group of geological sources relative to anomalies due to other groups of geological sources. Mathematical enhancement techniques are complemented by a range of imaging routines which can be specified to visually enhance the effects of selected geological sources.
... This is a convenient means of defining a hue-invariant transformation by addition or subtraction of white light. Such a transformation has been compared with the sun-illumination of a natural scene, in which the saturation of the sunlit parts of the scene decreases and appears "washed out" (Milligan and Gunn, 1997). ...
... For example, variations in the data are perceived in more discrete intervals by changing the intensity with a sine function. This provides better definition of overall variations in the radioelement concentrations when the data are not gradient enhanced (Milligan and Gunn, 1997). Alternatively, user-defined intervals may be assigned a discrete colour. ...
... It is also an effective tool for quality control of gridded data, whether for line-to-line and flight-to-flight shifts, or general high-frequency noise. Milligan and Gunn (1997) outline a technique whereby a shaded colour image is prepared in IHS space, using the original colour image to modulate hue, the darker portion of the shaded relief to modulate intensity and the lighter portion of the shaded relief to modulate saturation. Shaded relief images may also be presented directly in grey-scale, to focus on the short wavelength information. ...
Guidelines to radioelement mapping using gamma-ray spectrometry data
... The data was collected along E-W flight line spacing with a spacing between 200 and 500 m, and a terrain clearance J o u r n a l P r e -p r o o f Journal Pre-proof between 80 and 100 m. The grids were micro-levelled, and merged using the method of Milligan andGunn, (1997) andMinty et al., (2009). The national grid cell size is ~85 m, which is sufficient resolution to image geological details appropriate for terrane-scale structural analysis. ...
... The data was collected along E-W flight line spacing with a spacing between 200 and 500 m, and a terrain clearance J o u r n a l P r e -p r o o f Journal Pre-proof between 80 and 100 m. The grids were micro-levelled, and merged using the method of Milligan andGunn, (1997) andMinty et al., (2009). The national grid cell size is ~85 m, which is sufficient resolution to image geological details appropriate for terrane-scale structural analysis. ...
... The RTP filter shifts the position of the magnetic signal to be directly above the causative body [Nabighian, 1972]. The filter shifts the magnetic field from the inclined field at the mid latitudes to the vertical field at the magnetic pole [Milligan and Gunn, 1997]. ...
Determining overprinting relationships in the geological record has underpinned our understanding of the evolution of the Earth and has provided the context in which to interpret geoscientific datasets. Geological overprinting relationships are generally determined from outcrop relationships. When outcrop is absent or limited, overprinting relationships using geological observations is often ineffective or ambiguous. We show how aeromagnetic data can be used to inform superposition of geological structures using a combination of folded stratigraphy, faults (both high angle and low angle), and plutons from the well-exposed Hill End Trough, in the Phanerozoic eastern Lachlan Orogen. Overprinting relationships reveal episodic inversion of the Late Silurian to Early Devonian back arc basin succession, preserved within the Hill End Trough, occurred during superimposed shortening events. The first event resulted in the development of regional folds associated with the Tabberabberan Orogeny and the second event resulted in the development of overprinting north-south and north-northwest trending faults during the Early Carboniferous Kanimblan Orogeny. Inversion structures are overprinted by a Carboniferous granite, which is overprinted by previously unrecognised northeast- and northwest conjugate faults, which we interpret to be related to the Permo-Triassic Hunter Bowen Orogeny.
... Generally, the high magnetic values arise from igneous and crystalline rocks. According to Gunn (1997), magnetic surveys have been mainly used in the past for mapping depth to magnetic basement of sedimentary basin and delineating igneous rock units. Recently, magnetic survey now have greater relevance to sedimentary basin studies due to some enhancements done to magnetic anomaly data (Mohammed and Mustapha, 2014). ...
... Enhancement techniques were applied to the data to enhance certain characteristics of the sources, thus facilitating the overall interpretation (Blakely, 1995). The circularly shaped magnetic anomalies according to Gunn (1997) suggest presence of magnetic aureole within the area (Fig. 2). ...
Proton precession magnetometer of G857 model with 0.1 gamma sensitivity was used to survey a field in Michael Okpara University of Agriculture in Umudike to ascertain the magnetic fabric of the area in order to determine its Geology. Edge detection techniques such as analytical signal, tilt derivative and its associated horizontal derivative, vertical and horizontal derivatives magnetic data have delineated structural features in the area significant for hydrocarbon prospectivity in the basin. Reduction-to-Equator (RTE) of the Total horizontal derivative (THDR) of tilt derivative (TDR) revealed more magnetic anomalies that was not seen on Total magnetic intensity (TMI) map. The analytical signal has shown the location of magnetic sources, contacts or edges. Applying threshold cutoff of 0.0, to the RTE tilt grid has delineated litho-structural boundaries/contacts at the study area. The vertical derivatives map enhanced high frequency anomalies relative to low frequencies.
... The gravity method is based on different earth materials with different bulk densities (mass) that produce variations in the measured gravitational field (Mariita, 2007). All sedimentary basins are underlain by the basement and presence of magnetic units at the basement surface will help in using depth to the basement to map the basin's morphology (Gunn, 1997). Depth to this crystalline basement is one of the most useful inferences derived from analyses of magnetic field data. ...
... Depth to this crystalline basement is one of the most useful inferences derived from analyses of magnetic field data. Depth to the basement has direct relevance to the structural and depositional history of an area (Gunn, 1997). Presence of intrusive (Intrasedimentary/Intrabasement) type magnetic sources cause difficulty in quantitative interpretation of magnetic data using conventional interpretational techniques (Mahbubar and Shaikh, 2013). ...
... It could provide a substitute to reduction to the pole (RTP) transformation especially at low geomagnetic latitudes (like the study area). It has a capability to outline causative positions irrespective of Koenigsberger ratio, strike direction of the magnetic sources, magnetic inclination and declination of an area (Milligan and Gunn 1997). Consequently, AS resolves asymmetric nature of magnetic anomalies to symmetrical signatures. ...
... Consequently, AS resolves asymmetric nature of magnetic anomalies to symmetrical signatures. Hence, this data Adeyemo et al. enhancement technique is capable of delineating the edges of magnetic sources like geologic contacts (Nabighian 1972;Roest et al. 1992;Macleod et al. 1993;Milligan and Gunn 1997;Olaniyan et al. 2012;Arisoy and Dikmen 2013). In this study, the analytical signal transformation was applied to the total magnetic intensity (TMI) map using the MAGMAP GX in the Geosoft Oasis Montaj program. ...
Many researchers have suspected saltwater occurrence in Agbabu in the eastern Dahomey Basin. However, there is a need to get an insight to the possible origin of saltwater in the area. The aim of this study is to determine the source of saltwater in the inland aquifers of Agbabu, southwestern Nigeria for proper management of scarcely available freshwater resource in the area. 23 vertical electrical sounding, 2D electrical resistivity tomography (ERT), aeromagnetic and hydrochemical data (11 subsurface water samples) analyses were utilized in this study. The results from the magnetic analysis revealed the inland extension of the Chain Fracture Zone (CFZ). This NE-SW trending fracture zone cuts across the entire eastern Dahomey Basin, including Agbabu and its environs. Four (4) geoelectric layers were delineated across Agbabu namely, the topsoil (4 - 2806 Ωm), clayey sand (3 - 1280 Ωm), clayey (3 - 161 Ωm) and bituminous sand (1 - 10390 Ωm) layers. Anomalously low resistivity (1 - 20 Ωm) zones suspected within the bituminous sand correspond to the saltwater intrusion zones. The bituminous sand unit harbouring freshwater exhibits characteristic resistivity values of 33 – 160 Ωm. The interface between the freshwater and saltwater intruded zones in the area was suspected at 10 m depth with resistivity values ranging between 21 Ωm and 33 Ωm. The results from the hydrochemical water analysis also confirmed the presence of saltwater intrusion in the area. The salinity extent chart which was generated from the VES results indicated the mobility of the saltwater into the groundwater system. The fracture connectivity between the complex aquifers of Agbabu and the Atlantic Ocean (CFZ), that was mapped from the aeromagnetic and electrical resistivity results, is suspected to be primarily controlling the localization of saltwater in the study area.
... These methods use the principle that anomalies related to deeper sources attenuate more slowly than those related to shallow sources (Grauch and Drenth 2009). The first vertical derivative has come to be a basic requirement in magnetic interpretation projects, but the second vertical derivative has a more resolving power (Milligan and Gunn 1997). The total horizontal derivative process involves a phase transformation in addition to an enhancement of high frequencies. ...
Groundwater potential of some parts of Cross River State, Nigeria, was assessed involving vertical electrical sounding (VES) and high-resolution aeromagnetic methods (HRAM). The VES and HRAM dataset were used to delineate the hydro-lithostratigraphic units and map the spatial distributions of geologic structures, respectively. The electro-stratigraphic sections revealed the various hydro-lithostratigraphic units composed of fractured/weathered crystalline basement, sandstones, siltstones, sandy shales, and highly baked and fractured shales occurring at different depths. Generally, the unsaturated top layer is composed of shales, marl, clay, silt, gravel and lateritic materials with resistivity ranging from ~ 3.0 to ~ 550 Ω m. This layer is underlain by various aquifer units like sandstones, siltstones, fractured shales and limestone/marl, and fractured/weathered basement occurring at different depths (~ 30 to ≤ 100 m) as revealed by the electro-stratigraphic sections obtained in the study area. In the Calabar Flank (CF), Oban Massif (OM) and Ikom-Mamfe Embayment (IME), the apparent resistivity for these hydro-lithostratigraphic units varied between ~ 8.50 and ~ 59.50 Ω m, ~ 120.2 and ~ 1562.6 Ω m and ~ 30.8 and ~ 277.2 Ω m, respectively. The enhanced total magnetic intensity maps and source parameter imaging result showed that the OM and IME are characterized by high concentration of short-wavelength anomalies, while the CF is dominated by relatively undisturbed thick sedimentation (~ 6217 m). HRAM results show that such areas are predominated by geologic structures like faults, fractures, fissure, dyke swamps, etc. caused by tectonics. Such geologic structures create secondary porosity and enhance permeability. From all the results obtained, OM and IME represent areas with moderate groundwater potential. The CF dominated by argillites with no significant tectonic activity is observed to have low groundwater potential.
... This enhancement filter calculates the vertical rate of change in the magnetic signal (Milligan and Gunn 1997). It emphasises short-wavelength anomalies associated with shallow geological structures. ...
Significant yet untapped resources still abound in the Nigerian sector of the Dahomey Basin. Although the presence of an extensive Cretaceous Petroleum System has been confirmed following recent discoveries in Aje and Ogo Fields offshore Lagos, exploration outputs in the Dahomey Basin has so far not been encouraging. Proper understanding of the basement architectural framework and controls on tectonic development remains key to unlock the unrealised potentials in the basin. Hence, a geophysical interpretation of the basement structure and architecture of the Dahomey Basin southwestern Nigeria has been carried out in this study. Various edge enhancement techniques were applied to the high-resolution residual magnetic intensity (HRRMI) grid of the area. This includes first vertical derivative (FVD), total horizontal derivative (THDR), tilt derivative (TDR) and total horizontal derivative of upward continuation (10 km). Determination of the depth to magnetic sources and sedimentary thicknesses in the study area were achieved using standard Euler deconvolution and source parameter imaging (SPI) techniques, with depth range of 4.5–6.3 km attained in the two identified sub-basins located offshore of the study area. Lineament analysis gave insights on the tectonic trends and stress-field orientation in the basin with major trends in the NNE-SSW, NE-SW, NW-SE, and WNW-ESE directions. 2D forward modelling of some selected profiles was employed to characterise the basement pattern and architecture, which depicted a horst-graben architecture. The basement structure and architecture have a major control on the distribution of sub-basins, petroleum systems elements and trap styles in the basin. The study demonstrates the robust application of high-resolution aeromagnetic data in basin-wide mapping of regional subsurface geological features, basement architecture and determination of sedimentary thickness in a frontier basin.
... Analytical signal maps capture the response of all magnetic bodies whether they are reversely magnetized or not. All the shape preserved on analytical signal maps is centered to one positive body and are not subject to the instability that occurs in transformations of magnetic fields from low magnetic latitude, thus able to define source positions regardless of any remnant magnetizations in sources [48]. In this paper, we analyzed and interpret residual and regional field AS at wavelength more and less than 20 km and 50 km. ...
Across the Luconia continental shelf, the nature and structures of the crust are lacking geological understanding and precise characterization. Newly acquired, aeromagnetic, and airborne gravity data were used to assess deep and shallow sub-surface signals within the Central Luconia Province, off the coast of Sarawak, offshore Malaysia. Regional aeromagnetic anomalies appear to primarily reflect deep crustal features while depth (Z) tensors of airborne gravity anomalies evidence shallow subsurface structures. Strike directions of the interpreted structural trend on aeromagnetic and airborne gravity anomalies maps are measured and plotted into rose diagrams to distinguish the structural orientations for all datasets. Signature patterns extracted from the depth profiles were correlated with parallel seismic lines and nearest exploration wells and coincide well with the top of carbonate for Cycle IV/V and structures seen within the Cycle I and II sediments. The orientation of faults/lineaments at shallower depth is dominated by a NW-SE orientation, similar with the faults extracted from two recently published structural maps. Deeper subsurface sections yielded E-W to NWW-SEE dominant directions which were never presented in the published literature. The E-W oriented anomalies are postulated to represent the remnants of the accretion between the Luconia crustal block and southern boundary of the Palawan block. The NW-SE trend follows the same direction as prominent faults in the region. The insight into shallow and deep subsurface structures in Central Luconia Province imaged through airborne gravity and aeromagnetic data should provide guidelines and complementary information for regional structural studies for this area, particularly in combination with detailed seismic interpretation. Further evaluation on the response of Air-FTG® gravity and aeromagnetic could lead to the zonation of potential basement highs and hydrocarbon prospects in this area.
... This filter minimizes or removes the high-frequency and noise signals and it does not produce edge effects. The UW is often used to compare magnetic and gravimetric data acquired at different altitudes (Jacobsen, 1987;Milligan and Gunn, 1997). ...
ABSTRACT Airborne and ground magnetic and gravimetric maps provide important information about the spatial distribution of causative sources in the geological substrate. These sources normally have different physical properties and geometric shapes and are located at different depths, making it difficult to identify the geological features that correspond to the sources. Filtering and enhancement techniques can be used to highlight features in potential field anomalies (such as center and edges of sources). Also, these techniques remove associated noise, isolate shallow from deep sources, and estimate depths. We applied enhancement and depth estimation techniques in airborne and ground potential field data to delineate a subsurface structural framework of two copper occurrences (Victor Teixeira and Capão Grande) in the northwestern border of Camaquã Basin with the Sul-Riograndense Shield. Our results confirmed previously recognized structures and revealed an interconnected structural framework, which has an estimated depth of 170 m by Euler solutions. This configuration suggests a possible connection between the two mineralization outcrops, which are separated by a distance of 2,300 m.
... Colored boxes refer to data: redgeophysics; green -DEM remote sensing; yellowgeologic; bluewater-wells; black -GIS analysis/products magnetic sources. Using low-pass, band-pass and high-pass filters (Spector and Grant 1970;Milligan and Gunn 1997;Geosoft 2009), the different depth ranges to the magnetic sources (semiquantitative analysis) were separated. ...
Productive regions in the Crystalline Basement Aquifer System (CBAS) in Paraná state, Brazil, were identified qualitatively and quantitatively through spatial correlation of wells and geological conditions such as lineaments, hydrography, aeromagnetometry and lithology. Different methods applied in Precambrian metamorphic and igneous aquifers around the world and some Brazilian states were integrated and applied in CBAS with the aim of understanding the best scale and approach to productivity. The median productivity of the 224 wells analyzed is 0.29 m3/h/m. Under a multi-scale regional assessment, the results showed that the best condition is associated with the distance of 350 m from the lineaments (1:100,000), especially those with directions N40W, N10E and N70E. Considering the hydrolithological units, gneisses are the most productive, especially where lineaments coincide with regional structures such as shear zones, foliations and Cenozoic tectonic reactivations. Quartzites, granitoids, schists, phyllites, and rhyolites were also favorable when close to important rivers and not necessarily coinciding with regional lineaments, the high density of lineaments, or with vertical fractures. The areas of intersection of the lineaments and the weathering mantle did not serve as a discriminatory parameter. As the median depths of the water wells reach 90 m from the surface, the extraction of oriented magnetic gradients referred to magnetic sources down to 800 m depth, thus corroborating surface mapped lineaments and new nonoutcroppable structures. Considering the complexity of the environment and the global use of groundwater from fractured aquifers, this work contributed by discriminating geospatial parameters to decrease the exploratory risk in CBAS.
... Filtering of the potential field data are procedures that consist of enhancing algorithms which carefully attenuate the anomalies from one set of geologic sources relative to anomalies due to other geologic sources (Milligan and Gunn, 1997). Typical potential field data filtering methods consisting of first vertical derivative (Blakely and Simpson, 1986), total horizontal and tilt angle derivatives (Verduzco et al., 2004;Miller and Singh, 1994;Roest et al., 1992;Hood and Teskey, 1989), analytical signal (Roest et al., 1992) and upward continuation (Nabighian, 1984;1972) were applied in this investigation. ...
Tectonic structures controlling mineralization in some parts of Southeast Nigeria were evaluated using airborne potential field data . High and low frequency filters and depth determination tools were adopted to evaluate short and long wavelength anomalies, resolve the spatial spreading of igneous intrusions, depths to geologic sources and basin topography. The high frequency results exhibited high concentration of short wavelength anomalies in the Obudu Plateau and Ikom-Mamfe Rift. The underlying main tectonics of the area elucidated by the low frequency results caused the widespread occurrences of short wavelength geologic structures that are revealed by the high frequency maps. The study area is characterized by comparatively thin (~13.0 to <3000 m) sedimentation. The observed thin thickness is as a result of the massive Precambrian basement outcrops in some locations in the Obudu Plateau and the proliferation of igneous intrusions within this part of the Lower Benue Trough. The 2-D models showed the undulating nature of the underlying basin topography, the location of intrusions, domal structures and related normal faults. The locations and neighbourhood of intrusions and/or short wavelength structures are viable sites for lead-zinc-barite, brine and metallogenic minerals.
... Some anomalies are too gentle to be noticed on an anomaly map, which requires their amplification by filtering. The Vertical Derivative Anomaly Filter (DZ) (Milligan and Gunn 1997) aims at amplifying, in the vertical direction, the boundaries of anomaly sources and the ones related to faults and other shallow features, being largely used in geological mapping to identify structural features. The Reduction to Pole filter (Baranov and Naudy 1964), in turn, approximately reallocates the anomalies over their sources, converting bipolar asymmetrical anomalies into monopolar symmetrical ones, which are usually simpler and easier to interpret. ...
Differences in tectonic style and kinematics in orogenic belts can result from either multiphase or progressive deformation. In eastern Borborema Province, a regional shallow dipping foliation is multiply folded and crosscut by transcurrent shear zones. Here, we investigate if these fabrics resulted from diachronous deformational events or from a single-phase progressive deformation. The study area, the Feira Nova region, mostly comprises metasedimentary rocks from the Surubim Complex, and is bounded by the NE-trending sinistral Gloria do Goitá (GGSZ) and the sinistral contractional Paudalho (PSZ) shear zones, which separate it from Paleoproterozoic basement rocks. Structures can be grouped into a contractional and a transpressional stage, both related to the Brasiliano Orogeny. The contraction-related structures are represented by a gently dipping foliation (S2) related to a top-to-the-NW tectonic transport. The contact between the metasedimentary and basement rocks is parallel to S2, indicating the fabrics in both lithotypes are of Brasiliano age, with strain localization having produced the PSZ. NW-verging macroscopic folds are consistent with this regime. The following transpression generated steep mylonitic foliation (S3) mainly along the GGSZ, and caused refolding. The data here presented are consistent with a progressive deformation history comprising a gradual transition from contraction to transpression.
... Continued1993 ;Milligan and Gunn, 1997 ;Olaniyan et al., 2012 ;Arisoy and Dikmen, 2013 ; Sanusi and Amigun, 2020a , b ) like hematite and goethite even in the sedimentary geologic setting. ...
This study focusses on the sedimentological and aeromagnetic characteristics within the Agbaja Ironstone Formation to establish the sedimentary depositional history and map oolitic ironstone zones in the Lokoja district of the Nupe Basin. Forty sandstone samples were collected for granulometric, sedimentary facies and petrographic analyses to reconstruct depositional processes of the Agbaja Ironstone Formation. Aeromagnetic data analysis was engaged to map the oolitic ironstone rich zones. The iron-rich sandstones in the Agbaja Ironstone Formation occur as oolitic ironstone, ironstone concretion and laterites. The bivariate grain size plots and multivariate discriminant analyses suggest that 92% of the sandstone in the formation were deposited in beach and coastal dunes under fluvial-river actions, while 8% are of the marine environment deposited under wave actions. The quartz-feldspar lithic (QFL) ternary diagram and palaeocurrent analyses of the samples revealed that the matured, sub-arkosic sandstones originated from the cratonic interior (Abakaliki uplifts). The bioturbation and reactivation by Ophiomorpha burrows and herringbone on the fine-medium grained sandstones in the formation contributed to the observed high magnetic intensity values. High magnetic signatures (-94.784 – 1.191 nT), analytic signal peaks and depth levels from the 2-D source parameter images are the basis for the produced oolitic ironstones prospectivity map in this study.
... 12 When the earth's field is inclined, magnetic anomalies due to induction are asymmetric with respect to the source, whereas magnetic anomalies occur directly above the source if the induced field is in the vertical direction. 13 The most prominent feature in gravity and magnetic methods is that although gravity anomalies tend to be above the mass density in gravity method, this situation differs in magnetic anomalies when both magnetization and ground magnetic field are not in vertical direction. Unless the unit vectors in the direction of magnetization (m) and the surrounding field (̂) are vertical, the phase angles of the magnetization ( m ) and the ground magnetic field ( f ) cause phase changes in magnetic anomaly, such as lateral shifts, distortions, and even sign changes 9 ( Figure 2). ...
In this study, which is the first part of quantitative analysis of total magnetic anomaly maps on archaeological sites, it is aimed to analyze the horizontal boundary values of potential buried structures that cause anomalies from the total magnetic anomaly maps of an archaeological excavation area. In the first step, the model was created in accordance with the archaeological structure. Noisy and noiseless anomaly maps of this model were calculated. These maps were enhanced using the reduction to pole and upward continuation methods. Tilt angle and automatic gain control methods were used. The responses of these methods on the total magnetic anomaly maps of the model were analyzed. In the second step, by applying the same procedures on the total magnetic anomaly map obtained from the magnetic measurements on the area that has not been excavated in the Amasya Oluz Mound archaeological excavation area, the horizontal boundaries of the potential structure elements of the region were analyzed.
... For each TMA the Radially Average Power Spectrum -RAPS was calculated (Equation 3), to identify the depth of the main magnetic sources. Using low-pass, band-pass and high-pass filters (Spector and Grant, 1970;Milligan and Gunn, 1997;Geosoft, 2009) of different depth ranges to the magnetic sources (semi-quantitative analysis) were separated. ...
https://acervodigital.ufpr.br/handle/1884/69151
... Airborne geophysical data can be enhanced by a range of linear and non-linear filtering algorithms. A range of imaging routines can be specified to visually enhance the effects of selected geologic sources using mathematical enhancement techniques [8]. The interpretation of aeromagnetic anomaly data for improved sources depth estimates and locations of structural features such as faults, folds and contacts requires the use of relevant standard techniques and good geological interpretation. ...
High-resolution aeromagnetic data covering an area of 24, 200 km2 in north central Nigeria has been acquired and analyzed with the aim of carrying out trend analysis, edge detection (structural delineation) and depth to magnetic source estimation using reduce to the pole (RTP), horizontal gradient magnitude (HGM), center for exploration targeting plug-in (CET), 3D Euler deconvolution and source parameter imaging (SPI) techniques. Trend analysis was applied to the RTP data to delineate structures that have dissected the area. The 3D Euler deconvolution and HGM were correlated by plotting the estimated Euler solutions for a structural index of one (SI=1) on HGM map and the resulting map produced have shown that both methods can contribute in the interpretation of the general structural framework of the study area. The structural delineation based on HGM and CET maps showed that two predominant trends (ENE-WSW) and (WNW-ENE) have affected the area. The trend/depth/contacts of these faults were classified into four groups: Faults <150 m, 150 m - 300 m, 300m - 450 m which are the most predominant fault system based on Euler solutions with a structural index of one (SI=1) and those deeper than 450 m while the result of source parameter imaging (SPI) revealed a depth to source varying from 58 m specifically for areas with shallow depth to the magnetic source to those from deeper source occurring at 588.153m depth especially the south-central portion and the south-eastern portion of the study area.
... Maps of digital elevation model, anomalous magnetic field (AMF), total count, eTh and eU were generated. This remote sensing of rock types and lineaments allowed the contouring of the geology and interpretation of structures (e.g., Milligan andGunn 1997, Nabighian et al. 2005). ...
Mineralogical evolution of ophiolites is significant to understand paleo-oceanic crust and mantle requiring multi-proxy techniques to identify steps in the processes. We studied the Bossoroca ophiolite from the southern Brasiliano Orogen, a prime example of Tonian accretion to an oceanic island arc. Integration of field geology, aeromagnetometry, aerogamaspectrometry, electron microprobe analyses, and compositional maps of minerals led to the decoding of oceanic and continental processes. The ophiolite is highly magnetic and low-K and is positioned at the base of the superstructure. We studied amphibolite, tourmalinite, and chromite-talc-magnesite granofels from the ophiolite, Capivaras diorite from the Cambaí Complex infrastructure and one metavolcanoclastic rock from the Vacacaí Group superstructure. Honblende is zoned in all rock types. Low-Ti hornblende is compatible with medium-pressure metamorphism at 7 kbar. This M1 to M2 amphibolite facies resulted in the widespread association of olivine + talc in metaserpentinite. Dravite is similar to tourmaline from the Ibaré ophiolite. Andesine and oligoclase are dominant and albite minor. Cr-spinel in granofels recrystallized in greenschist facies; host rock originated by carbonatization of serpentinite formed in the oceanic crust along with chloritite and tourmalinite. Serpentinite rare earth elements (REE) suggest origin in depleted mantle peridotite. The ophiolite evolved in the Adamastor Ocean until incorporation into the island arc.
... Reduction to the pole is the process of converting the magnetic field from magnetic latitude where the Earth's field is inclined, to the field at a magnetic pole, where the inducing field is vertical [20]. When the Earth's field is inclined, magnetic anomalies due to induction have forms that are asymmetrically related to their sources, but when the inducing field is vertical, the induced anomalies are directly over their sources [21]. Assuming that all the observed magnetic fields of a study area are due to induced magnetic effects, pole reduction can be calculated in the frequency domain as in equation (2) [22]. ...
Assessment of the structures and solid minerals was carryout to investigate subsurface structural characteristics and mineralization potential zones within part of north-central Nigeria. The residual magnetic intensity data of the area was reduced to magnetic pole after which several source edge detection/interpretation with depth determination techniques including, analytic signal; tilt derivative; first and second vertical derivatives and Euler deconvolution were applied to the aeromagnetic data. From the analytic signal map, three magnetic zones were delineated. These are: low to relatively low magnetic zone (LM) with amplitude range from 0.003 to 0.009, moderate magnetic zone (MM) with amplitude 0.009 to 0.106 and those with amplitudes above 0.106 were products of later magmatic intrusions into host with fractures, faults and joints. Tilt derivative helped in delineating location and extent of edges of causative sources while Euler deconvolution helps in determination of boundary, depth and geometry of the structures. From first vertical derivative map, structures were found to have high lineament density around the central portion of the area and span toward the western end of the map were delineated. The lineaments mapped trending in the ENE-WSW followed by WNW-ESE with some NE-SW, NNE-SSW and NNW-SSE trends. The second vertical derivative (SVD) map also helped in delineating structures and possible mineralization zones that are pronounced within the study area, around high analytic signal zones. Delineated possible and favorable mineralization zones from second vertical derivative map correlate with portion of the study area with rocks showing high analytic signal amplitude suggesting the rocks to be of later magmatic intrusions where mineralization fluids solidify within the host rocks.
... In this study, two selected ternary composite images, namely: radio-element composite (K, eU, and eTh) and Uranium Composite (eU, eU/eTh, and eU/K) are considered. Composite image map of the radioelements ( Figure 11) mixes the data for eU (in blue), eTh (in green) and K (in red), where blue colour tends to reduce the most reduced signal-to- noise ratio of uranium channel (Milligan and Gunn 1997). This map presents strong spatial correlations with the known geologic units. ...
Wadi Queih and Wadi Safaga lie in the central part of the Eastern Desert of Egypt. The airborne gamma-ray spectrometric data of the study area have been interpreted qualitatively and quantitatively and correlated with the surface lithologic units to define the anomalous uranium zones and to reveal if any association is present between the radiometric anomalies and the structural trends.The radiometric data have been treated statistically. The results revealed that the area has a wide range of radioactivity ranging from 0.1 to 23.6 Ur for the total-count (TC), 0.1 to 3.3 % for potassium (K), 0.01 to 19.1 ppm for equivalent uranium (eU), and 0.29 to 18.9 ppm for equivalent thorium (eTh). The calculated CV values for all the rock units in the study area are less than 100% for three radio-elements (K, eU, eTh), except for potassium in Thebes Formation. The younger granites and Duwi Formation have the highest radiometric values. The lowest values exist over ophiolitic metagabbro, basic metavolcanics, metasediments, serpentinites, and Umm Gheig Formation. The generated composite radio-elements and composite image maps define the locations of the high anomalous radiometric and eU zones as bright white areas. The most radioactive anomalies have NW-SE and NE-SW trends. ARTICLE HISTORY
... These methods use the principle that anomalies related to deeper sources attenuate more slowly than those related to shallow sources (Grauch and Drenth 2009). The first vertical derivative has come to be a basic requirement in magnetic interpretation projects, but the second vertical derivative has a more resolving power (Milligan and Gunn 1997). The total horizontal derivative process involves a phase transformation in addition to an enhancement of high frequencies. ...
Groundwater potential of some parts of Cross River State, Nigeria, was assessed involving vertical electrical sounding (VES) and high-resolution aeromagnetic methods (HRAM). The VES and HRAM dataset were used to delineate the hydro-lithostratigraphic units and map the spatial distributions of geologic structures, respectively. The electro-stratigraphic sections revealed the various hydro-lithostratigraphic units composed of fractured/weathered crystalline basement, sandstones, siltstones, sandy shales, and highly baked and fractured shales occurring at different depths. Generally, the unsaturated top layer is composed of shales, marl, clay, silt, gravel and lateritic materials with resistivity ranging from ~ 3.0 to ~ 550 Ω m. This layer is underlain by various aquifer units like sandstones, siltstones, fractured shales and limestone/marl, and fractured/ weathered basement occurring at different depths (~ 30 to ≤ 100 m) as revealed by the electro-stratigraphic sections obtained in the study area. In the Calabar Flank (CF), Oban Massif (OM) and Ikom-Mamfe Embayment (IME), the apparent resis-tivity for these hydro-lithostratigraphic units varied between ~ 8.50 and ~ 59.50 Ω m, ~ 120.2 and ~ 1562.6 Ω m and ~ 30.8 and ~ 277.2 Ω m, respectively. The enhanced total magnetic intensity maps and source parameter imaging result showed that the OM and IME are characterized by high concentration of short-wavelength anomalies, while the CF is dominated by relatively undisturbed thick sedimentation (~ 6217 m). HRAM results show that such areas are predominated by geologic structures like faults, fractures, fissure, dyke swamps, etc. caused by tectonics. Such geologic structures create secondary porosity and enhance permeability. From all the results obtained, OM and IME represent areas with moderate groundwater potential. The CF dominated by argillites with no significant tectonic activity is observed to have low groundwater potential.
... Typical magnetic data enhancement processes involving first vertical and second vertical derivatives, horizontal gradient, analytical signal and tilt angle derivative were used in this study. Enhancements of the magnetic dataset are treatment processes using filtering algorithms which prudently enhance the anomalies from one set of geological sources relative to anomalies due to other geological sources (Ekwok et al., 2019;Milligan and Gunn, 1997). Nevertheless, second vertical derivative process results in heightening of noise. ...
High resolution airborne magnetic and radiometric data were used to map probable sites for polymetallic-magmatic hydrothermal deposits in the study area. The first and second vertical derivatives, horizontal gradient, analytic signal and tilt angle derivative were used to enhance magnetic data in order to determine the location of short wavelength anomalies, magnetic sources and geologic boundaries. Also, to understand and detect radiometric anomalies controlling mineralization in host rocks, enhancement operations involving potassium (K), equivalent thorium (Th), equivalent uranium (U) concentrations and ternary imageries were carried out. All these operations enabled the delineation of responses associated with mineralization, lithology and geologic structures. Generally, lineaments trend in the NE-SW, NNE-SSW, NW-SE and E-W directions, and serve as potential pathway for hydrothermal fluid migration and mineralization. Radiometric anomalies from spectrometric imageries are caused by felsic minerals, hydrothermal rock alterations, widespread shales, residual clay, oxides and accessory minerals that constitute regolith of the area. The coincident high magnetic and potassium intensities sites in Anambra Bain (AB), Abakaliki Anticlinorium (AA), Ikom-Mamfe Rift (IMR) and Obudu Plateau (OP) are probable sites for polymetallic-magmatic hydrothermal deposits. Generally, the joint magnetic and radiometric results were able to establish link between lithology, geologic structures and hydrothermal alteration patterns.
... This filter minimizes or removes the high-frequency and noise signals. The UW is often used to compare magnetic and gravimetric data acquired at different altitudes (Milligan & Gunn, 1997). ...
This work describes the reconnaissance and detailed phases of a potential copper sulfide mineralized target situated between the Sul-Riograndense Shield and the Camaquã Basin. The study area comprises the Victor Teixeira and the Capão Grande copper occurrences. These were recognized and catalogued in the 1960s by the Brazilian Department of Mining Production (DNPM in Portuguese) in the countryside of Caçapava do Sul, Rio Grande do Sul State, Brazil. These occurrences are delimited in the surface by malachite and azurite disseminated mineralizations which fill fractures and sandstone and schist pores. The initial phase of reconnaissance involved a gravimetric survey that revealed an anomaly with high amplitude between the copper occurrences. The forward modeling of this anomalous peak showed a body with significant density, which was related to chalcocite, and motivated the use of resistivity and induced polarization in the detailed phase. Our data were associated with a conductive body and three resistive and three polarized bodies. The correlation between the conductive and the polarized bodies (situated below 60 m) was associated with sulfides in the subsurface. The resistive bodies situated above the polarized ones were attributed to silicified portions in the country rocks. The combination of procedures based on the geophysical analysis in an area delimited by structural and metallogenetic control showed efficiency in the prospect of the buried target, which is a common paradigm in current minerals research.
... Filtering algorithms deliberately enhance anomalies from a certain set of geologic sources relative to anomalies due to other geologic sources and they are used in potential field data enhancements [77]. The main igneous intrusions in the research area were delineated using typical potential field (PF) data enhancement methods such as the analytic signal (ASig) [78][79][80][81], low pass filter [82], and upward continuation (UPWC) method [53,54]. ...
Investigation into understanding the genesis of brines in southeast Nigeria was carried out utilizing high-resolution potential field (HRPF) data. This study reveals that igneous intrusions and associated hydrothermal fluids are responsible for brine generation. The obtained result of the analytic signal revealed the locations and spatial distribution of short- and long-wavelength geologic structures associated with igneous intrusions. The low pass filtering, upward continuation, and 2D modelling procedures showed key synclinal structures which coincided well with the location of brine fields. The results showed that salt ponds are common in the neighborhood of igneous intrusions. To validate this finding, a conceptual model describing igneous-related hydrothermal circulation systems that are driven by convective cells of the hydrothermal fluid and overburden loads was generated. This model fits reasonably well into the overall stratigraphic and geologic framework of the study area.
... The radioelement composite image provided in a single view an overall pattern of the radioelement distribution. This offered much in terms of lithologic discrimination based on colour variations (Milligan and Gunn, 1997). The uranium, thorium, and potassium images revealed areas where the particular radioelement has an absolute with relatively higher concentration (Duval, 1983). ...
... In this study, analytic signal (AS), being independent of the ambient magnetic field and source magnetisation directions, resolves asymmetrical nature of magnetic anomalies to symmetrical signatures. Hence, this AS filter can be used to delineate edges of magnetic sources like geologic structures (Nabighian, 1972;Roest et al., 1992;Macleod et al., 1993;Milligan and Gunn, 1997;Arisoy and Dikmen, 2013). In a similar vein, the total horizontal gradient (THG) technique was applied to locate the edges of magnetic sources in this study. ...
This study integrated aeromagnetics, aeroradiometric and electrical resistivity survey data to unravel the subsurface geological evidences which are related to rare earth element (REE) enrichment in the Ijero-Aramoko pegmatite field in southwestern Nigeria. Magnetic data enhancement filters were employed to delineate subsurface structures in the study area. The high Kd areas (eTh, eU) and high magnetic anomalies at the periphery of geologic structures were attributed to REE and gemstone mineralisation in the Ijero-Aramoko pegmatite field. The geophysical characteristics over the REE and gemstone mineralised zones were used to establish a relationship between REE mineralisation and geophysical signatures. The resultant mineral potential map showed a 74% agreement with the known mining pits in the study area. Thus, this agreement confirms the reliability of the produced mineral potential map for future resource management.
... the reduction of the long wavelength-regional effects and determine the properties of adjacent anomalies (Milligan and Gunn 1997). Using first vertical technique, it is easy to delineate the high wave anomalies number that reflects surface, near surface and local geological structures such as (ground water channel). ...
Aeromagnetic data of the western part of Koraimat-Alzafarana road, Eastern Desert, Egypt is interpreted to detect the subsurface structures that may resulted in presence of subsurface aquifer. To reach to the main target of this study, many procedures are done using some magnetic analysis techniques (e.g., technique of reduction to the magnetic pole, separation technique of regional-residual anomalies and edge detection methods). The results have been encouraging to merit further estimation of the magnetic depth and analyzing the trends of the study area. To increase the credibility, the depth is revised by the P-depth technique. The shallow and deep magnetic components are calculated to be 2046 and 5680 m. To ease the detection of the structure that encasing the study area and lack the rigorous analysis, reduced the magnetic pole map, residual map and 3D Euler deconvolution are integrated to depict the combined lineament map that prevailing tectonic pattern of the study area. Eventually, NE–SW trend is the predominant structural trend affecting on the study area as deducing from magnetic anomalies. Moreover, there are minor structural trends which were taken N–S, NW–SE, W–E, NNW–SSE and NNE–SSW directions. The presence of subsurface structures may assist in the occurrence and recharging of the groundwater aquifers.
... The RTP filter reconstructs the magnetic field of a data set as if it were at the pole. This means that the data can be viewed in map assuming a vertical magnetic inclination and a declination of zero (Milligan and Gunn, 1997, Hansen and Pawlowski, 1989and Mendonca and Silva 1993. As a result, RTP significantly simplifies the interpretation of magnetic data. ...
... The total magnetic field data were reduced to the pole prior to further processing. Filters used include (1) the First Vertical Derivative (1VD), (2) the Tilt Derivative (TD) and (3) the Horizontal Derivative (Blakely and Simpson, 1986;Miller and Singh, 1994;Milligan and Gunn, 1997;Pilkington and Keating, 2009;Verduzco et al., 2004). Regarding the radiometric data, the original bands K, eTh, and eU and the ternary RGB (Red, Green, Blue) color composite are used. ...
The mineral system concept, which combines integrated information from all kinds of geological and geophysical studies, is a very useful approach to predict and locate structures and lithologies potentially associated with gold mineralization at all scales (i.e., upscaling from craton-to district-to camp-to deposit-scale). This is demonstrated in this study where the Yaou deposit, located in French Guiana within Rhyacian terranes, formed and deformed during the Trans-Amazonian orogenic cycle (ca. 2600-1950 Ma), is used as a case study. The deposit comprises numerous intrusions hosting the bulk of the high-grade gold mineralization at Yaou, associated with a quartz-carbonate veining system. Both intrusive and gold events are dated in order to constrain Yaou in the regional litho-structural framework. Spatially associated with the Central Yaou Shear Zone (CYSZ), one of the quartz monzodiorite yields a U–Pb zircon age of 2131 ± 6 Ma, supra-chondritic εHft of ca. 2.5, and Hf model ages at ca. 2.6–2.4 Ga. These data suggest Paleoproterozoic rock formation due to reworking of juvenile crust, derived from the depleted mantle during the late Archean to early Paleoproterozoic. The main and economic gold event hosted by the quartz monzodiorite is dated at 2105 ± 25 Ma by Re–Os isotope analyses of Au-bearing pyrite, representing the first date for gold mineralization in French Guiana. A multi-scale approach using airborne magnetic and radiometric data allows to better understand the spatial distribution of the intrusions and possibly associated shear zones, both related to gold mineralization events. The intrusions and associated shear zones are interpreted as being parallel along a N60° trend and replicated laterally toward the northwest. A temporal association is defined between the shearing deformation and the magmatic event, both being synchronous with a possibility of pre-shearing intrusion and a protracted magmatic phase active while shearing. The Yaou-type intrusion-hosted mineralization can be targeted along these N60°-striking structures. This integrated approach opens some targeting options, previously unrecognized within the southern Paramaca Greenstone Belt.
... Filtering of the potential field data are procedures that consist of enhancing algorithms which carefully attenuate the anomalies from one set of geologic sources relative to anomalies due to other geologic sources (Milligan and Gunn 1997). Typical potential field data filtering methods consisting of first vertical derivative (Blakely and Simpson 1986), total horizontal and tilt angle derivatives (Verduzco et al. 2004;Miller and Singh 1994;Roest et al. 1992;Hood and Teskey 1989), analytical signal (Roest et al. 1992) and upward continuation (Nabighian 1984(Nabighian , 1972 were applied in this investigation. ...
Tectonic structures controlling mineralization in some parts of Southeast Nigeria were evaluated using airborne potential field data. High- and low-frequency filters and depth determination tools were adopted to evaluate short- and long-wavelength anomalies, resolve the spatial spreading of igneous intrusions, depths to geologic sources and basin topography. The high-frequency results exhibited high concentration of short-wavelength anomalies in the Obudu Plateau and Ikom–Mamfe Rift. The underlying main tectonics of the area elucidated by the low-frequency results caused the widespread occurrences of short-wavelength geologic structures that are revealed by the high-frequency maps. The study area is characterized by comparatively thin (~ 13.0 to < 3000 m) sedimentation. The observed thin thickness is as a result of the massive Precambrian basement outcrops in some locations in the Obudu Plateau and the proliferation of igneous intrusions within this part of the Lower Benue Trough. The 2-D models showed the undulating nature of the underlying basin topography, the location of intrusions, domal structures and related normal faults. The locations and neighbourhood of intrusions and/or short-wavelength structures are viable sites for lead–zinc–barite, brine and metallogenic minerals.
... First, the total magnetic intensity data ( Fig. 4) was reduced to the magnetic pole and overlaid with the area's geology. This was appropriate for the anomaly to be centered directly above its respective causative source [37][38][39][40][41] and was accomplished using -10.62 0 inclination and -2.76 0 declination of the centre point of the study area using (2005 IGRF). Since the RTP often introduces noise during aeromagnetic data interpretation, especially for data collected very close to the equator as well as the current study area, stable reduction to the pole method was used to avoid this effect by applying -90 0 amplitude correction inclination, producing a balanced RTP map (Fig . ...
The current study was designed to provide a consistent and detailed understanding of the Structural disposition and magnetic source depth of Sheet 266, southeast Nigeria in the Auchi region by applying three source edge mapping techniques: horizontal gradient magnitude; tilt derivative and Euler deconvolution to aeromagnetic data obtained from Nigeria Geological Survey Agency Abuja (NGSA). To achieve the above objective, there was a need to correct the latitudinal effect on data obtained at low latitudes just as the present case, the total magnetic intensity (TMI) data was reduced to the pole (RTP) and regional magnetic anomalies were extracted from the RTP data to obtain the residual anomaly data using the upward continuation technique. To establish the boundaries of the magnetic sources, various source edge mapping techniques such as Analytic signal (AS), Horizontal Gradient Magnitude (HGM) and Tilt Derivative (TDR) were applied. A strong correlation between these techniques has been found, suggesting that their incorporation may contribute to delineating the structural mechanism of the study area. A comprehensive structural map based on the findings was therefore built. The key tectonic patterns in the study region are typically interpreted to predominantly ENE-WSW trend followed by WNW-ESE trend. In the study Original Research Article Tawey et al.; AJOGER, 3(4): 1-15, 2020; Article no.AJOGER.61526 2 area, depth to magnetic source estimation using 3D Euler deconvolution and source parameter imaging (SPI) has also shown that the two techniques complement each other with depth estimation and general depth to magnetic source was 50 m in the basement portion of the study area and >2000 m in sedimentary portion as seen from both SPI and 3D Euler deconvolution techniques.
... One advantage of the Analytic Signal technique is that it defines source positions regardless of any remenant magnetization in the sources [15] hence it's independent of the direction of magnetization. Maxima (ridges and peaks) in the calculated analytic signal of a potential field anomaly map locate the anomalous source body edges and corners. ...
Lineament are extended mappable linear or curvilinear feature of a surface which can be utilized in mineral, oil, gas, and underground water studies. They are obvious in satellite images, aerial images or aerial photographs. The aim of this study is to analyse an automatically generated lineament from part of the Maru schist belt using a combination of Digital Elevation Model (DEM) data and Aeromagnetic data. The area of study is within the Maru schist belt in Zamfara State. A shaded relief map was produced from the two data sources and the lineament was extracted automatically from them. The extracted lineament was combined to obtain a final lineament map of the area. Trends such as N-S, NW–SE, ENE–WSW and NW–SE, with very little E-W were all present. The areas of very high, high, moderate and low population of lineaments was delineated. Regions with very high lineament intersections and lineament concentration, covers an area of 153.6 km2 and accounts for 16.1% of the study area, these regions are known to harbour gold and iron ores from literature reviews. These target regions are located at the Northern and South part of the study area. Also, the gold artisan miners have dominated these same regions and they have only started from exposed vein of gold mineralization. With this rather fast and easier approach of obtaining such a map, it could be used as a start point for planning a more detailed geophysical or geological survey
... Modeling uses a multi-technique approach including upward and downward continuations, vertical derivatives and total horizontal gradient (Blakely, 1995;Milligan & Gunn, 1997;Hinze et al., 2013). Euler deconvolution is used to estimate depths to the top of source bodies on the reduced to the pole and derivatives (Marson & Klingele, 1993;Reid & Thurston, 2014). ...
Modeling gravity and magnetic anomalies over the Chicxulub crater are used to constrain the structure, stratigraphy, and asymmetries. Chicxulub is a multiring ~ 200 km rim diameter structure with a central uplift and well-preserved peak ring. The low relief terrain and physical property contrasts have facilitated geophysical modeling of the structure and impactite deposits. Nevertheless, contrasting models have been obtained due to data resolution limitations, uneven coverage, non-uniqueness solutions, boundary conditions, and heterogeneous/anisotropic media. We employ a multi-technique approach based on regional–residual separation, spectral analysis, first and second derivatives, upward and downward analytical continuations, horizontal gradients, analytical signal, Euler deconvolution, reduction to the pole, and forward modeling to constraint the anomaly sources, geometry and depths. Forward modeling of gravity anomaly favors central uplift flat-top models, whereas magnetic models show irregular shapes with a peak towards the NE, at 4–5 km depth. Analysis shows the effects of intersecting regional anomalies in the semicircular pattern that limit the definition of asymmetries, which constrains impact angle and trajectory, crater structure and pre-existing target features. Models link lateral–vertical density and magnetic property contrasts, distinguishing non-magnetic pre-and post-impact carbonates and carbonate-rich breccias from melt and basement rich breccias, and displaced, fractured impactites and basement uplift.
... following the application of derivatives, such as the first-order vertical derivative 337 (Milligan and Gunn, 1997) In relation to the gamma-ray spectrometry, the individual channels were 343 qualitatively analyzed and a ternary map, based on the RGB color system linked to the 344 K, Th and U channels, respectively, was produced to characterize the Tavares pluton 345 and nearby lithologies and their compositional variation (e.g., Dickson and Scott, 1997 strongest anomalies occur in the western portion of the Tavares pluton, coinciding with 350 the area of occurrence of syenitic dikes (Fig. 1b). 351 ...
... Analytical signal maps capture the response of all magnetic bodies whether they are reversely magnetised or not. All the shape preserved on analytical signal maps is centred to one positive body and are not subject to the instability that occurs in transformations of magnetic fields from low magnetic latitude, thus able to define source positions regardless of any remnant magnetizations in sources(Milligan & Gunn, 1997). Regional AS data represents deep seated magnetic bodies, with high contrast in amplitude and frequency while residual AS data shows the sudden changes in magnetic relief that accompanies shallow seated geological bodies (Al-Ibiari et al. 2018). ...
The Luconia and Balingian basins in Sarawak formed during the Eocene. The area
underwent various tectonic deformations including extension and compression. It was
infilled by thick sedimentary sequences, including widespread limestone. The basins
also exerted a strong control of the pre-existing basement inhomogeneities reflected by
structural variation in the basins. However, the degree of structural preservations within
the thick sedimentary sequences is uncertain. Thus. this study aims to produce a
comprehensive geological model that incorporates tectonic deformation and limestone
growth as a regional marker. This was done by conducting seismic interpretation over
3025.7 km seismic lines, assisted with lithological interpretation from deep-seated
wells. The depth tensor from airborne gravity and aeromagnetic data were evaluated to
assess deep and shallow sub-surface structures. Results from seismic and well
interpretation were combined to calculate the amount of total and tectonic subsidence.
A chronological-based geological model was developed to demonstrate the effects of
tectonic deformations at three different time ranges. The model is supported by the
amount of tectonic subsidence since 37 Ma. The basins experienced an increase in
tectonic subsidence from 37 to 18 Ma. Then, delay in tectonic subsidence from 15.5 to
11.8 Ma and increase in tectonic subsidence from 11.8 Ma. This area was formed partly
by crustal extension at the beginning before it formed as a foreland basin. The insight
into subsurface structures revealed NW-SE orientations at shallow depth and E-W to
NWW-SEE directions in deeper depth. The E-W anomalies are postulated to represent
the remnants of the crystalline basement. The findings of this study proved the existence
of the “Basement Rule” in controlling the basin development and revealed the orientation of potential crystalline basement of the Luconia continental block. The model had elevated the regional understanding of prolong tectonic and sedimentation history in the area. It could be an advantage in determining the potential hydrocarbon reserves, particularly in Eocene-Oligocene successions.
The study area is located in Iran central zone and Urumieh-Dokhtar
volcanoplutonic belt. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data was used to identify alteration zones associated with Fe-Skarn mineralization in the Ravanj village, Markazi Province, Iran. Argillic, phyllic and propylitic alteration zones are typically associated with Fe-Skarn mineralization in the study area. In this research, the Selective Principal Component Analysis (SPCA) method was applied to VNIR + SWIR bands of ASTER remote sensing data. Bands 1, 4, 6 and 8 were designated for identification clay minerals. Bands 4, 5, and 6 were selected for argillic
alteration mapping. Bands 1, 2, and 4 were used to identification iron oxides/hydroxide minerals. Bands 5, 6, and 7 were chosen to map phyllic alteration zones. Bands 7, 8, and 9 were nominated to specify propylitic alteration mapping. According to the eigenvector statistics calculated using SPCA for ASTER, inverse SPC4 image identified clay minerals and SPC2 images detected argillic alteration, oxides/hydroxide minerals, phyllic alteration and propylitic
alteration. In this paper, SPCA technique is an appropriate method because of the distinction between alteration minerals and vegetation for Fe-Skarn mineralization exploration.
The aeromagnetic and electrical resistivity tomography technique was used to evaluate the foundation conditions and stability of the Apodu earth dam located around Malete, Ilorin Sheet 201, Southwestern Nigeria. The Aeromagnetic data set was interpreted to identify dykes, lineaments, and magnetic sources controlling subsurface geology. The extracted lineament structures were employed to examine features controlling the distribution of surface and subsurface hydraulic substances cum stability of dam slope. Magnetic lineaments' length and parallelism in some areas suggested installation under a tensional stress field along pre-existing zones of weakness. Magnetic susceptibilities are not uniform in the area and were categorized viz: high, moderate, and low. The magnetic intensity range at Apodu dam is -644.19 to 285.40 nT. The northeastern part of the study area where the Apodu dam is located shows little or no sign of faulting and appears to be more stable structurally. Characterization of the Apodu dam subsurface for seepage appraisal using the electrical resistivity tomography technique revealed that the dam is well compacted at the surface but with observed low resistivity values indicative of possible occurrences of weakness zones that are water-bearing in some areas of the dam embarkment body at depths of around 8m and 25m. These observed low resistivity zones indicating a water-bearing weakness zone were connected in RTM1 and RTM2, and a possible water pathway was established. This water pathway is indicative of a possible seepage pathway and adequate measures are expected to be made to arrest the situation. In conclusion, Apodu dam is situated in a geologically stable
environment as deduced from the aeromagnetic study but its earth embarkment is suspected to possess areas with possible seepages zones as obtained from the electrical resistivity techniques of investigations. There is therefore a need to quickly remedy the dam earth embankment to ensure the longevity of the dam and avert possible failure
The structural and tectonic influence on the relief of a region can be studied based on subsurface magnetic structures, especially those correlated to the basement (deep structures) and shallow structures linked to geomorphological context, faults/fractures, and drainage network. Aiming to understand the structural and tectonic influence on the relief of the Araucárias Plateau, airborne magnetic geophysical data was processed using Oasis Montaj software (Geosoft TM, 2004) and used to generate magnetic anomaly maps. Interpretations of the vertical derivative applied to the anomalous magnetic field enabled the identification of 49 magnetic lineaments. The characteristics of these lineaments allowed to divide the study area into three distinct geotectonic compartments that are in agreement with the structural blocks of the Paraná basin. The resultsshow that these magnetic lineaments mapped show directional agreement with shallow lineaments and, in many cases, constitute structures that extend from the basement to the surface. It was also possible to observe that the drainage network is adjusted to important mapped magnetic lineaments. This reinforces the importance of these structures in the genesis andevolution of the Araucárias Plateau. The analysis of magnetic lineaments, faults/fractures mapped, and geomorphological lineaments showed NW-SE orientations as the most important trend, suggesting that structures with this orientation exert the greatest influence on the relief in the study area.
Aeromagnetic methods are primarily used for regional geological mapping and in the exploration of natural resources such as oil and gas, minerals, and groundwater. They are also extremely useful in research studies on the internal structure and evolution of the earth, plate tectonics, and earthquakes. Such studies require the potential field aeromagnetic data to be digitally accessible. Aeromagnetic data coverage has already been completed and published for many countries and continents; digitally compiled anomaly maps were prepared and are being used in the exploration and exploitation of natural resources, disaster studies, and research activities. In contrast, India is not entirely covered by aeromagnetic surveying. But certain selected regions covering approximately 70% of the geographic land area of the country were covered with diverse technical specifications and the data is available in a number of disparate individual surveys and flying seasons, tied-up individually to their respective tie-line set up, largely non-digital and poorly archived. The gap areas include the Himalayas, Gangetic plains, Bengal basin, Deccan traps, and Aravalli mountains. The aeromagnetic coverage of these areas is also extremely important with regard to natural resources and hazards. Hence, the gap areas are also required to be covered and the whole data of the country need to be digitally linked together, on top priority, for generating a digitally compiled aeromagnetic anomaly map and database for India. The aeromagnetic data of approximately 48,400 km2 of area flown in three different blocks and time periods with varied survey specifications comprising parts of Andhra Pradesh, Tamil Nadu, and Karnataka states of southern India was digitally compiled and is presented.
The aeromagnetic and electrical resistivity tomography technique was used to evaluate the foundation conditions and stability of the Apodu earth dam located around Malete, Ilorin Sheet 201, Southwestern Nigeria. The Aeromagnetic data set was interpreted to identify dykes, lineaments, and magnetic sources controlling subsurface geology. The extracted lineament structures were employed to examine features controlling the distribution of surface and subsurface hydraulic substances cum stability of dam slope. Magnetic
lineaments' length and parallelism in some areas suggested installation under a tensional
stress field along pre-existing zones of weakness.Magnetic susceptibilities are not uniform in the area and were categorized viz: high, moderate, and low. The magnetic intensity range at Apodu dam is -644.19 to 285.40 nT. The northeastern part of the study area where the Apodu dam is located shows little or no sign of faulting and appears to be more stable structurally. Characterization of the Apodu dam subsurface for seepage appraisal using electrical resistivity tomography technique revealed that the dam is well compacted at the surface but with observed low resistivity values indicative of possible occurrences of weakness zones that are water-bearing in some areas of the dam embarkment body at depths of around 8m and 25m. These observed low resistivity zones indicating a water-bearing weakness zone were connected in RTM1 and RTM2, and a possible water pathway was established. This water pathway is indicative of a possible seepage pathway and adequate measures are expected to be made to arrest the situation. In conclusion, Apodu dam is situated in a geologically stable environment as deduced from the aeromagnetic study but its earth embarkment is suspected to possess areas with possible seepages zones as obtained from the electrical resistivity techniques of investigations. There is therefore a need to quickly remedy the dam earth embankment to ensure the longevity of the dam and avert possible failure.
The responses of the lead-zinc (Pb–Zn) ore-bearing veins in the Asu River Group shales in Abakaliki area to ground magnetic, resistivity, and induced polarization (IP) methods of geophysics were studied in Nkpuma-Ekwoku. Results of the study indicate that low magnetic susceptibility values are associated with shallow tectonic structures within the sedimentary basin which could host sulfide ores, while high susceptibility values suggest shallow intrusive/volcanic rocks. The ore-bearing veins within the sedimentary rocks have susceptibilities of ≤ 1.2 nT, while their resistivity and IP values are ≤ 80 Ωm and ≥122 ms respectively. The ores display mineral zoning in their electrical data profile. The primary occurrence of the ores is fracture controlled. However, they usually overflow into the unfractured sedimentary zones and become interstratified with the sediments. The ores maintain NW-SE trend even within structures of different orientations. The earlier view that structures with NE-SW trend are barren in the study area is not supported by the result of the present work. Shallow ore bodies occur before 30-m depth, while deep-seated ones are as far as 50 m below the surface.
O uso e interpretação de dados e imagens obtidos através de sensores remotos permitem importantes aplicações nas geociências. Neste artigo, foram utilizados dados aeromagnéticos e SRTM (Shuttle Radar Topography Mission) com resolução de 30 metros para, respectivamente, capturar indicadores geológicos e geomorfológicos na porção oeste do Batólito Ipojuca-Atalaia. Técnicas de geoprocessamento foram utilizadas para correlacionar os fatores geológicos e geomorfológicos e como produto final obter a compartimentação geomorfológica da área. Para tanto, aplicou-se as metodologias de extração da hipsometria, grau de declividade, imagem sombreada e drenagem no MDT (modelo digital de terreno) do SRTM, além dos filtros da amplitude do sinal analítico (ASA) e da primeira derivada vertical (DZ) nos dados aeromagnéticos. Como resultado foi possível distinguir nove compartimentos geomorfológicos distintos, enquadrados em cinco categorias de acordo com os processos geológico-geomorfológicos dominantes na região, sendo elas a cimeira estrutural, os planaltos, o pediplano, o pedimento e as planícies. A compartimentação da área auxiliou o entendimento dos processos ocorridos e atuantes na porção oeste do Batólito Ipojuca-Atalaia e como o regime tectônico atua como agente primário na evolução do relevo na região.
The interaction of structure development and magmatism in rift-settings provide systemic controls on the emplacement of ore deposits both during rifting and subsequently, and a knowledge of these may help to predict better the likely locations of major deposits. The Paleoproterozoic Bryah Rift Basin includes substantial mafic magmatism and deep-crust penetrating structures and possesses syn-rift volcanogenic massive sulphide (VMS) and later gold mineralisation, providing a good opportunity to study interactions between magmatism, structures, and mineralisation. The volcano-sedimentary Bryah Group was deposited in a continental rift developed within the broader Yerrida Basin on the northern margin of the Yilgarn Craton at ca. 2030 Ma. Multi-scale interpretation and forward modelling of gravity and magnetic data was applied to characterise the magmatic and structural patterns of the Bryah Group, enabling a better understanding of its tectono-magmatic development, and controls on VMS and gold mineralisation. The interpretation highlights the bounding extensional faults and the inward-deepening structural pattern of the Bryah Rift Basin that reflects the initial rift geometry. Deep-rooted gravity sources show magmatism focused in the southern-central part of the basin, where mafic rocks' thickness can reach more than 10 km. At shallower levels, the rift magmatism extends in three east to east-northeast trending magmatic corridors with an en-echelon arrangement along the rift, oblique to major structures. These magmatic corridors have an intrinsic relationship with the internal structure of the rift. VMS mineralisation associated with mafic magmatism shows a spatial connection with magmatic corridors and with major syn-rift faults, whereas VMS mineralisation associated with felsic volcanic rocks are associated with off-axis volcanism and regional pre-rift faults. Later orogenic gold mineralisation is also spatially associated with the borders of the magmatic centres and rift faults, despite occurring ∼200 Ma later. These associations suggest that primary rift architecture has substantially focused syn-rift VMS mineralisation, and also later gold mineralisation, as a consequence of rheological and compositional contrasts.
The Archaean Madibe-Kraaipan granite-greenstone terrain, largely located in northwest South Africa but extending into Botswana, is characterized by mafic metavolcanic rocks interlayered with ferruginous and siliceous metasedimentary rocks, mainly banded iron formations (BIF) and ferruginous chert, flanked by granite gneiss. In South Africa where the terrain has been reasonably well-mapped and sampled, the greenstones consist of three narrow NNW-striking belts (Stella, Kraaipan and Madibe, from west to east respectively) while the surrounding granitoid rocks include tonalitic and trondhjemitic gneisses (TTG), granodiorites and adamellites. In Botswana in the north, the geological outcrop is largely obscured by cover of Tertiary–Recent sediments and Kalahari calcretes and the area is intruded by the NeoArchaean Gaborone Igneous Complex that includes A-type granitoids, anorthosites, rhyolites and subsidiary mafic rocks, represented by the Mmathethe granite. The aim of this study between longitude 24°E to 26°E and latitude 25°S to 26.5°S was to carry out geophysical surveys to gain a better geological and structural understanding of the extension of the Madibe-Kraaipan granite-greenstone terrain in southeast Botswana through the use of potential field data. Specifically, the new collected gravity measurements at 2 to 4 km intervals are reduced and later merged with existing gravity data in Botswana and South Africa for an integrated interpretation with regional aeromagnetic data.
The analysis of gravity and aeromagnetic data revealed the existence of relatively narrow N-S trending rocks of dense and high magnetic intensity extending to the village of Mmathethe, corresponding to the northern extent of the greenstone belt. Gravity and aeromagnetic anomalies of the region appear to present coincident medium to high amplitude regions. Magnetically quiet areas are widely distributed in the north-central part of the study area corresponding to areas covered by a blanket of the Kalahari sediments that range from 0 to 55 m thick as indicated by borehole data. The area lies within an ENE-trending Pre-Transvaal dyke swarm clearly visible on the high resolution aeromagnetic data. The derivative and analytic signal techniques applied for both gravity and magnetic data spatially map the greenstone belt and granite plutons very well. The depth estimation results obtained by the 3D Euler method in combination with two-dimensional power spectrum technique locate the high magnetic intensity horizon at around 4.0 km. The depths were further confirmed by gravity model results along two profiles across the mentioned granite-greenstone terrain in Botswana and South Africa in a W-E direction. The models show a maximum depth extent of 4.7 km for the greenstones and 4.4 km for the plutons. It is recommended that more detailed gravity traverses across the greenstone belt and plutons be conducted at 1.0 km spacing with regional coverage elsewhere at 1-3 km spacing.
This research aims to model mineral favorability for intrusion-related gold in the central portion of the Tapajós Mineral Province (TMP) in the southwestern Pará state. The scope of the study includes the experimentation and evaluation of knowledge and data-driven methods applied to multisource data aiming at the prediction of potential targets for the occurrence of gold mineralization, considering the critical factors of the mineral system sought and the characteristics of the known mineralization. The aerogammaspectrometric data processing allowed identifying a hydrothermal alteration signature common to gold mining sites, providing information in regions with little or no field data. The aeromagnetometric data analysis enhanced by an upward continuation filter allowed the identification of high magnetic response zones associated with these deposits, probably related to hydrothermal fluids' transport. Linear features extracted from radar images revealed the predominance of features of general direction NNW-SSE, which are consistent with the main structural control of deposits in this region. The analyzed gamma-spectrometric, magnetometric and topographic data were integrated by the application of three favorability modeling techniques – Fuzzy Logic (knowledge-driven), Bayesian Logic (or Weights of Evidence; data-driven), and another based on machine learning algorithm (data-driven) – resulting in the conception of three prospective models for the studied area. The fuzzy model indicated targets in a more effective and sectorized manner, mainly in the eastern portion of the area, proportionally reflecting the panorama of data availability and mapping areas of known mineralization occurrences reasonably well. However, some deposits were not mapped by the model or scored low prospective values. Nevertheless, its validation indicated confidence in the degree of randomness explored to map the mineralizing event. The Bayesian model indicated areas of greater potential distributed regularly in the area, with elongated geometry in directions compatible with the prospective structural trends, mapping most known deposits in the most likely areas. The machine learning algorithm was more accurate in indicating areas of known deposits as prospective. In all models, the regions indicated as prospective coincide, in their majority, with the most expressive deposits, such as São Jorge and Tocantinzinho. The results obtained in the models were combined to generate a concordance map, which mapped the overlapping of their highest prospective scores, indicating new areas of prospective interest in the central portion of the TMP.
Over the past few decades, geological and geophysical studies in the Borborema Province (NE Brazil) have evolved considerably, especially in the North and Central subprovinces. However, the Pernambuco-Alagoas Domain, inserted in the South subprovince, does not have, up to now, detailed geophysical and geological studies, mainly due to a thick soil profile, which make difficulty the geological survey in the region. The aerogeophysical study of an area located in the Ipojuca-Atalaia Batholith, Eastern part of the Pernambuco-Alagoas Domain allowed the delimitation of the lithogeophysical and magnetic geophysical domains, main directions of the lineaments, main shear zones, and their behavior in depth through the upward continuation and the Euler Deconvolution. The set of integrated magnetometric data revealed that the region was intensely deformed by a set of transcurrent shear zones of NE-SW direction. Aerogammaspectrometric data enabled the improvement of the geological mapping (1) of the Western part of the Ipojuca-Atalaia Batholith granitic plutons and gneisses and (2) of the limits of the batholith with the adjoining crustal sub-domains. The integration of the whole set of aero magnetometric and aero gammaspectrometric data also provides a basis for a better understanding of the tectonic evolution of the studied area and the Ipojuca-Atalaia Batholith.
Sheet 201 and 223 part of Ilorin, southwestern Nigeria was studied. The aim was to evaluate the foundation conditions to assess the stability of the selected earth dams. The data set was interpreted to identify dykes, lineaments and magnetic sources controlling subsurface geology. The extracted lineament structures were employed to examine features controlling the distribution of surface and subsurface hydraulic substances cum stability of dam slope. The length and parallelism of magnetic lineaments in some parts of the area suggested emplacement under tensional stress field along pre-existing zones of weaknesses. Magnetic susceptibilities are not uniform in the area and were categorised viz: high, moderate, and low. Magnetic intensity range at Asa, Agba and Apodu dam are 126.56 to-20.90 nT,-215.09 to 111.42 nT, and-644.19 to 285.40 nT. The north western part of the Asa and Agba dam area shows little or no sign of faulting and appears to be more stable structurally than the other part of the study area. Thus, the northwestern part where the two dams are located, is fairly stable structurally. Also, the north eastern part of the study area where the Apodu dam is located shows little or no sign of faulting and appears to be more stable structurally. In conclusion, all the three dams are in less structurally active zones and could only likely to have a functional failure rather than structural because the areas near the dams are structurally stable.
This study, performed in the Doropo region (northeastern Côte d'Ivoire) in the Paleoproterozoic domain (southern part of the WAC), aims to produce a detailed lithostructural map of this area. The geology of this region is less known due to the thick lateritic overburden and the scarcity of outcrops. Recent airborne geophysical data (aeromagnetic) integrated with field structural and lithological observations allowed to distinguish several lithologies including biotite granite, gneissic and migmatitic granite, granodiorite, tonalite, gabbro, amphibolite, rhyolite and dolerite. Filtering techniques of derivatives, upward continuation, tilt, analytical signal, spectral analysis and 3D Euler deconvolution showed the presence of faults, shear zones and intrusions. The structural analysis revealed four deformation events (D1-D4): (i) D1 is a N–S compression marked by E-W foliation (S1), conjugate NE-SW and NW-SE sinistral and dextral shears, respectively, as well as F1 folds with E-W axial planes; (ii) D2 is a major transpressive phase characterized by an intense NE-SW foliation (S2), E-W dextral shear zones, a weakly dipping stretching L2 lineation plunging N-E, and NE-SW faults; (iii) D3 is a NE-SW compressional phase generated NW-SE foliation (S3) with ∼E-W shears, and NW-SE axial plane F3 folds; (iv) D4 is a late, rather brittle phase characterized by brittle structures (faults, fractures, etc.) and the emplacement of NE-SW and NW-SE dolerite dykes. This tectonic evolution demonstrates that the Doropo region was subject to ductile and then brittle deformations. Furthermore, we suggest that the geodynamic model which could have prevailed in the Doropo region occurred in an oceanic arc collisional tectonic context.
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