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Application of shallow seismic exploration in detection of buried fault in coastal areas
Abstract
The depth of Quaternary in China coastal zone varies greatly. In most areas, silt, clay and sand are always multiply interbedded. This kind of formation can attenuate greatly the high frequency signal, and is unfavorable to propagation of seismic wave. At the same time, the density of population in the area is much higher and the industrial noise is strong, so the trigger and receiver conditions are not ideal. The paper presents some application examples of detecting buried faults with near-surface seismic methods in east Guangdong, Tianjin and Tangshan where the thickness of overburden varies greatly, and systemically introduces the methods of fieldworks, data processing and explanation approaches. It also illustrates the effective methods of how to improve SNR and resolution in environment with strong noise. In the application, the signal of refection is strong and the buried faults can be reflected evidently. Proofs of drilling at both sides of the faults approve that the depth of interface, location of fault point and throw of basement determined by seismic exploration are all reliable. The techniques and methods mentioned in this paper are of important reference value in urban active fault detection and engineering geophysical prospecting in similar areas.
... research has demonstrated that shallow seismic profiles can effectively identify the geometry, upper breakpoints, burial depths, and dips of buried faults, thus providing information for subsequent joint geological multi-drill profile deployment (He et al., 2007;Liu et al., 2008;Xu et al., 2016;Liang et al., 2018;Zhang et al., 2021). ...
... (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) depth and dip of buried faults, which provides information for composite drilling profile deployment (He et al., 2007;Liu et al., 2008;Xu et al., 2016;Liang et al., 2018;Zhang et al., 2021). ...
... The underground longitudinal wave velocity can be obtained and analyzed to obtain a continuous geological section [4] . This not only facilitates the use of multiple coverage techniques to suppress interference, improving the signal-to-noise ratio of seismic data, but also utilizes the rich reflection wave group features on the reflection profile to determine subsurface anomalies and their basic parameter characteristics [5] . This paper takes the pre-selection of a tunnel project as an example, which is located in the pre-desert area of the northern foot of the Dunhuang Mogao Grottoes in Gansu Province. ...
Shallow seismic exploration has significant application effects in terms of bedrock surface and underground structure exploration, detection of underground cavities and unfavorable geological bodies. It is a scientific and economical technical method to provide basic information of feasibility study and engineering planning for site selection of highways, airports and high-rise buildings. Taking the pre-selection of a tunnel project in Gansu as an example, this paper elaborates on the interpretation methods and techniques of the shallow seismic exploration data processing in the aeolian sand stratum based on the geophysical characteristics of the work area. Generally speaking, the interpretation results are highly consistent with the drilling results and can be applied to engineering surveys in similar strata.
... Therefore, selecting suitable geophysical exploration methods and technical parameters and correctly applying them to achieve relatively good detection results is critical for active identification of the target fault because of its complicated seismo-tectonic environment. In comparison with other methods, shallow seismic exploration techniques can play a crucial role in fault location in the covered terrains, as well as preliminary evaluation of fault activity (Dolan 1997, China Earthquake Administration 2005a, 2005b, Chai et al 2006, He et al 2007, Liu et al 2008, Gu et al 2013, 2015. Over the past few decades, the shallow seismic reflection method has been widely applied to the detection of deep geological and active faults, and has effectively solved various geological problems (Robert et al 2000, Haberland et al 2007. ...
In order to redefine the activity of the Benchahe fault, according to target layers with different burial depths, we followed the work rationale of 'from known to unknown, from deep to shallow and step by step'. We adopted not only the multi-level seismic detection method but also the joint exploration of P- and SH-waves. We obtained high-resolution images of the stratum structure in the depth range from 1350 m to near the surface and clear image results of the Benchahe fault. This exploration research accurately locates the Benchahe fault, and further shows that it is not a single fault but rather a fault-terrace belt consisting of two N-dipping faults within the effective detection depth. We clarify that the up-breakpoint of the southern section (F1-1) of the Benchahe fault is shallower and more active. The Benchahe fault is a near EW-trending fault, dipping north with an apparent dip angle of around 60°-75°. Subsequently, through geologic borehole section verification, we show that the results of the multi-level and multi-wave shallow seismic reflection method, determining the relevant parameters and crossing the location of the fractures, are reliable and accurate. We can observe that the reflective layers and fault characteristics revealed by the seismic profiles correspond well with the geologic borehole section. The depth of the upper breakpoint discernible from the borehole section is 93.8 m, which belongs to the top of the middle Pleistocene strata. Therefore, we may safely conclude that the newest active time of the Benchahe fault is late mid-Pleistocene to early late Pleistocene.
... The geophone is usually used as a string of 12 channels, and using 24 channels in single acquisition, migrating 2 channels, to realize 6 times fold and stack, it can improve the SNR of seismic signal. The field application parameters are set with 4 ksps sampling rate, 5 m trace interval, 50 m shot offset and 2048 sampling point [5,6]. So, the data quantity of single acquisition is 144 k (2k 24 3) bytes when using A/D with 24 bits resolution, plus the header information of message, it has nearly 200 k bytes to single transmission by the wireless network. ...
It has some issues such as cable layout trouble and low efficiency to wired shallow seismograph when working in complex environment. In this paper, it takes advantage of the current non-standard wireless communication technology, and according to the requirements of distributed seismic data acquisition with multi-node and multi-parameter, a star-shaped wireless network was built which is suitable for seismic data transmission using the networking mode of multi-channel and multi-address switching, and the distributed seismic data acquisition system was accomplished which had the function of synchronous control and asynchronous transmission adopting the method of address matching identification and custom packet combination, thus the additional accessories of traditional wired seismograph was greatly reduced to improve its capacity and efficiency of construction in a harsh environment. The key technical problems and solutions of wireless seismic data transmission is analyzed in detail in this paper, and provided a reliable wireless networking method which is easy to implement and its highly efficient and stable data transport control protocol.
This study focuses on the key structural locations to the south of the 1679 M8.0 Sanhe–Pinggu earthquake. In conjunction with prior deep seismic reflection exploration in the area, we conducted four shallow seismic investigations to the south of Sanhe–Pinggu seismic area to delineate the exact structure of identified faults and to ascertain the precise location, characteristics, and activity levels of active faults within the region. By analyzing the burial depth of the fault’s breakpoint as revealed by high-precision shallow seismic profiles, we postulate that the fault has been active since the middle and late Pleistocene epochs. In addition, we conducted a high-density borehole investigation in tandem with composite drilling profile at the corresponding sites of shallow breakpoints. Using chronological data from neighboring boreholes and accounting for the ages of samples acquired from these boreholes and staggered strata, the fault manifests as a Holocene active fault within the composite borehole–geological section. This study contradicted the previous conception that to the south of 1679 Sanhe–Pinggu seismic area contained no active faults. This new discovery not only has significant application value for evaluating the risk of large earthquakes in the southern part of the capital circle and understanding the earthquake disaster risk in Beijing but also has scientific significance for studying the development and evolution of faults and their deep–shallow coupling characteristics in North China since the late Cenozoic.
For city planning and reducing potential earthquake risk, it’s necessary to detect the information of the buried faults in an urban area especially, including the location and activities. An integrated technique with geophysical and geological methods, including the shallow seismic reflection profile, electrical resistivity measurement, geologic borehole section, and exploration trench, was used to detect the Chengnanhe fault, which is one of the two main faults passing through the Weihai urban area in Shandong province, China. The results show that it is a normal fault striking with E-W direction, and it is relatively inactive and stable. By using the thermoluminescence (TL) dating, we found that the Chengnanhe fault initiated in mid-Pleistocene and there was no offset after late Pleistocene. Such an integrated technique with multiple geological and geophysical methods provides a significant assessment of earthquake risk for city planning in urban areas.
The investigation and study of fault activities are a basic work for urban earthquake prevention and disasters reduction. In order to find out the location, characteristics and activities of the Laoyachen Fault in Zhengzhou, the high-resolution shallow seismic P and S wave survey profiling across the Laoyachen Fault was carried out at the end of 2006, and different seismic sources along with combinations of diverse observation geometries with different parameters were used. The fine structures in different depths beneath the profile were obtained and the patterns as well as characteristics of the Laoyachen Fault were determined. The results show that the Laoyachen Fault, running in NW and dipping in NE, is a normal fault and its dip angle is about 60°-70°, which incises strata of Eocene, Permian, Carboniferous or Ordovician epoch and goes up to the top boundary of Eocene stratum at the 800-850m depth. There is no any reflector of offset stratum found in Q + N strata. The borehole geological sections across steep slopes of earth surface present that the layers inferred from reflected seismic wave groups of shallow seismic profile are well correspondent with boring geological layers. The borehole results reveal that the three reference laminas, i. e. the boundary between Malan loess and silt with clay soil at about 21 m in depth, the calcareous gravel clay layer of 53.9m deep, and the calcareous silt layer of 61.9m deep, all have not depth variations at the two sides of surface steep slopes and are situated almost at the same ground surface elevations, which suggests that the steep slopes at the earth's surface should not result from the activities of Laoyachen Fault. In this study, through shallow seismic P wave and S wave exploration as well as combination of joint borehole geological sections, not only the location and characteristics of Laoyachen Fault was determined, but the geological and seismological evidences for the fault activity estimations were provided.
The work experience of shallow seismic exploration in land and water areas for earthquake safety evaluation of nuclear power sites is reviewed in this paper. By analyzing different typical engineering cases, some key technical issues, to which more attention should be paid in the buried fault detection, are discussed for different geological and geophysical conditions. For different geological conditions some critical schemes to implement the seismic exploration are presented, such as equipment selection, layout design, quantitative parameters optimization and the method to identify the fault. This study provides a practical and technical guide for detection of capable fault in nuclear power sites with similar geological conditions.
Many Quaternary folds and faults are identified on the seismic reflection profiles acquired in the adjacent areas of Qianliyan Island of southern Yellow Sea. The faults are mainly of reverse fault. The folds are mainly gentle and tilted, caused by fault activities. Most folds and faults occurred below a plane of unconformity between middle Pleistocene and its overlying upper Pleistocene. Some faults, such as Qianliyan fault, are late Pleistocene active fault. Many faults and folds were synsedimentarily developed in the lower Pleistocene and underlying strata, revealling rather strong tectonic activity, especially at the basin boundary where the faults even cut into the late Pleistocene stratum. The formation of folds and faults was attributed to the last tectogenesis of the Yellow Sea in middle Pleistocene, which maybe correspond to the Kunlun-Huanghe movement of Qinghai-Tibet plateau. At that time, the N-S striking horizontal stress drove the early-developed basin boundary faults reactive and further induced the NEE or E-W striking tectonic deformation in the overlying sedimentary layer.
Urban areas are mostly covered by thick Quaternary loose sediments in China. Many sizable hidden faults are developed in this loose layer. These hidden faults can be detected by using seismic method. Whether or not the upper offset points of the hidden faults detected by seismic method represent the real upper offset points of the faults? On the basis of the discussion on resolution of seismic records, the influence of S/N ratio of seismic records, the seismo-geological conditions and the data processing and interpretation methods on the seismic exploration results is analyzed in this paper. Study results show that appropriate technical measures are necessary for improving the S/N ratio and resolution of seismic records. For example, seismic profiles shall be laid vertical to fault strike during the data acquisition. Under conditions of guaranteed seismic records with high signal-to-noise ratio, the use of the methods with appropriate fold number and small group interval will help to get reflections from shallower wave impedance interfaces. In the course of seismic data processing and interpretation, it would be helpful to improve the effect of detecting the upper offset point of buried fault by obtaining accurate reflected wave velocity and adopting the processing technology that can enhance resolution and S/N ratio of seismic records as well as abandoning some mixed wave processing methods. It is difficult to detect with seismic method the interfaces in stratum without wave impedance difference, including the interface broken by buried fault. Even though the upper offset point of buried fault detected by seismic method is not necessarily the upper offset point in a real sense, the seismic prospecting result still provides scientific basis for the layout of drill hole profile, design of drilling depth, and the determination of fault activity.
The Yarlung Zangbo River suture zone is characterized by the presence of active faults in southern Tibet. This site is prone to moderate-large earthquakes due to the particular tectonic setting. The Xigazê region is located in this suture zone; the unique geological character of this region increases the likelihood of earthquake occurrence. The three key active faults zones within this region include the Yarlung Zangbo River, Xaitongmoin-Namling, and Jaggang-Xaitongmoin fault zones. Scientists have conducted large number of research projects in relation with the geophysical and regional geological structures in the Tibetan plateau. The amount of geological data remains insufficient and geophysical prospect data is relatively less in the Xigazê area than at other study sites. Furthermore, shallow seismic prospecting work has not been implemented yet in this specific region. This paper describes the application of a method that utilizes an artificial source of shallow seismic reflections. This method aims to detect active faults and survey low-velocity layers, in addition to shallow seismic refraction. The key technical problems are addressed, including details about the selection of seismic instruments, deployment design, parameters, data processing, and interpretation methods used to identify faults. The Geometrics Strata Visor NZXP seismograph was used to collect data and a 70 kg hammer was used as a seismic source, in addition to 60 and 100 Hz geophones. Seismic wave energy was maintained at constant high and low frequencies. The wave reflection range was between 10 and 120 Hz, and the high frequency energy attenuation gradually diminished. The seismic reflection observation system is comprised of a receiver arrangement in the front, the shotpoint in the rear, shooting in the downdip direction, reception in the updip direction, and continuous forward tracing. The spread geometry is determined by the receiver interval of 1 to 3 m, with a 2 to 9 m offset, 72 channels, and 6 to 9 folds. The acquisition parameters were set as 0.25 ms sampling intervals and 600 to 800 ms record lengths. This region is located in the Yarlung Zangbo River drainage catchment, which is influenced by a low velocity layer. The study area was chosen to determine the degree of influence of the low velocity layer and to isolate its effects during data processing to improve the quality of the profile data. Data processing was completed using the Vista seismic reflection wave software system. For high-frequency random noise, one-dimensional filtering of 10 to 90 Hz frequency band widths was adopted, which applied a two-dimensional filtering to address the apparent velocity difference of interference wave (e.g. F-K). The main parameters such as the trend, orientation, and upper offset point were evaluated to ascertain the effects on the Ladoi-Nêdong, Biding-Jiashala, Zhuogeluo and Jaggang-Xaitongmoin fractures. By analyzing the seismic reflection profile, additional groups of apparent interface reflection characteristics were identified. Based on the characteristics of seismic reflection groups in each fault line, we determined eight breakpoints of fault interpretation. The results show that the top of the bedrock reflective interface (i.e., the quaternary bottom), TN, is common in the surveyed area. Furthermore, TN has strong reflective energy in most of the profiles, significantly apparent seismic phase characteristics, and reflective interface that records changes in fluctuation. The two-way time of TN is 0 to 220 ms, and the buried depth is 0 to 170 m. There are two main internal reflection interfaces in the Quaternary bottom layer. Unique identifiers were assigned, namely TQ1 and TQ2, from the top to bottom, respectively.
Seismic exploration is an effective geophysical means for detecting buried faults. The spread geometries of seismic exploration are the main factors to affect the exploration results as well as the accuracy of fault location. Based on the shallow seismic exploration data from an active fault survey in Yinchuan city and in combination with the petroleum seismic exploration profiles and drilling data, this paper proposes an upward fault-tracing method for determining the geometry of the target fault and its upper breakpoint in thick Quaternary sediments. The result shows that the combination of high resolution shallow seismic exploration and borehole geological section is an effective way for locating the buried fault and determining its activeness.
Shallow reflection profiles over crystalline rocks of the Brevard zone show events that correlate with projections of mapped lithologic contacts, in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering (saprolite). The profiles, which were collected in western South Carolina about 5 km from Appalachian ultra Deep Core Hole (ADCOH) regional lines 1 and 3, straddle the contact between the Brevard fault zone and the wider Brevard mylonitic shear zone to the southeast. The principal goal of these experiments was to test the feasibility of using a hammer source and 24-channel seismograph to image near-surface brittle and ductile structures to a depth of several hundred meters. We recorded two dip lines and one strike line with a total common midpoint (CMP) coverage of 1 km. Processing focused on eliminating static shifts and separating reflections from overlapping refracted S-waves. A statics approach based on the alignment of first arrivals on common-offset g