Electrical Resistivity Profiles Research Articles

Hydrogeology of a landslide: a case study in the Montagne de Reims (Paris basin, France)

Investigations of hydric and hydrogeological mechanisms that contribute to the current reactivation of mass-movements feed predictive models that can be useful for risk management. Our model is based on an approach combining (1) a large-scale analysis of the landforms, using LiDAR imagery and field observations, (2) a characterization of the groundwater (piezometry, conductivity and temperature), both in the landslide and in the stable slope, and (3) electrical resistivity profiles. These combined data allow the reconstruction of the structure of the landslide to determine its influence on groundwater flow. The Saint Maur landslide bars the main aquifer of the Upper Ypresian sands whose slow flow results in lower piezometric level fluctuations and stronger mineralization than found in the unconfined aquifer on either side of the slip. The confined aquifer emptying occurs through the sliding plane so that the landslide aquifer is fed essentially by capillary rise from the shear plane and to a lesser extent by surface infiltration. Therefore, the amount of water in the landslide and the resulting probability of mass movement are weakly correlated with rainfall events. In addition, the wide dispersion of the conductivity values of the landslide aquifer reflects a weak mixing of the water. Groundwater circulation modelling allows us to correct in situ (i.e. underground) the flows causing reactivations, and confirms that stabilization by hydrogeological action is more efficient than a surface drainage. Thematic collection: This article is part of the Role of water in destabilizing slopes collection available at: https://www.lyellcollection.org/cc/role-of-water-in-destabilizing-slopes

Application of Analytical Hierarchy Process and Geophysical Method for Groundwater Potential Mapping in the Tata Basin, Morocco

Ensuring water availability for agriculture and drinking water supply in semi-arid mountainous regions requires control of factors influencing groundwater availability. In most cases, the population draws its water needs from the alluvial aquifers close to villages that are already limited and influenced by current climatic change. In addition, the establishment of deep wells in the hard rock aquifers depletes the aquifer. Hence, understanding the factors influencing water availability is an urgent requirement. The use of geographic information system (GIS), and remote sensing (RS), together with decision-making methods like analytical hierarchy process (AHP) will be of good aid in this regard. In the Tata basin, located in SE Morocco, ten factors were used to explain the groundwater potentiality map (GWPM). Five categories of potential zones were determined: very low (8.67%), low (17.74%), moderate (46.77%), high (19.95%), and very high (6.87%). The efficiency of the AHP model is validated using the ROC curve (receiver operating characteristics) which revealed a good correlation between the high potential groundwater zones and the spatial distribution of high flow wells. Geophysical prospecting, using electrical resistivity profiles, has made it possible to propose new well sites. It corresponds to conductive resistivity zones that coincide with the intersection of hydrogeological lineaments.

Groundwater Flow Rate Prediction From Geo‐Electrical Features Using Support Vector Machines

AbstractUnsuccessful drillings are issues in groundwater exploration using electrical resistivity profiling (ERP) and vertical electrical sounding (VES). Many geophysical companies spend a lot of money without obtaining the flow rate (FR) required during the campaigns for drinking water supply (CDWS). To solve this problem, we applied the support vector machines (SVMs) to real‐world data to predict the FRs before any drilling operations. First, from the ERP and VES, the features such as shape, type, power, magnitude, pseudo‐fracturing index, and ohmic‐area were defined including the geology of the survey area. Second, the FRs were categorized into four classes (dry: FR0 (FR = 0), unsustainable: FR1 (0 < FR ≤ 1), and productive boreholes: FR2 (1 < FR ≤ 3) and FR3 (FR > 3 m3/hr)) and associated with the features to compose two separated data sets: a multiclass data set () for common prediction during the CDWS and a binary data set (FR < FR2, FR ≥ FR2) addressed to the population living in a rural area. Features were vectorized and data were transformed before feeding to the SVM algorithms. As a result, the SVM models performed 77% of good predictions on and 83% on . Better performances with the optimal hyper‐parameters in (81.61%) and (87.36%) were achieved using the polynomial and radial basis function kernels respectively. Furthermore, the learning curves have shown that the performance scores on can be improved if larger training data becomes available (275 test samples at least) while it is not necessarily so for . As a benefit, the proposed approach could minimize the rate of unsuccessful drillings during future CDWS.

Effect of Groundwater Extraction and Artificial Recharge on the Geophysical Footprints of Fresh Submarine Groundwater Discharge in the Western Belgian Coastal Area

Human activities, such as managed aquifer recharge (MAR) and groundwater pumping, are influencing the natural groundwater flow in coastal areas. This might induce saltwater intrusion and impact fresh submarine groundwater discharge (FSGD). The use of resistivity methods (electrical resistivity tomography and continuous resistivity profiling) in coastal studies is very effective to investigate the salt–freshwater distribution, but it can be difficult to interpret quantitatively. In this study, the Western Belgian coast is investigated with resistivity methods, and image appraisal tools are systematically used to quantitatively interpret inversion models. Synthetic resistivity models, which reflect the existing situation at the Western Belgian coast, are first created and assessed quantitatively by means of the model resolution matrix, cumulative sensitivity matrix, and depth of investigation index. They reveal that no quantitative interpretation is possible of the FSGD, although lateral qualitative changes can be deduced from the inversion models. The field data show freshwater outflow from the lower beach to below the low water line, and they indicate that MAR has a positive impact on FSGD, while groundwater extraction reduces the outflow of freshwater to the North Sea. A comparison with existing groundwater models indicates their inability to reproduce the actual FSGD footprints.

Characterization of the Dynamics of Leachate Contamination Plumes in the Surroundings of the Hulene-B Waste Dump in Maputo, Mozambique

The contamination of areas around solid urban waste dumps is a global challenge for the maintenance of environmental quality in large urban centres in developing countries. This study applied a geophysical method (electrical resistivity) to identify leachate contamination plumes in the subsoil and groundwater, as well as to describe their temporal dynamics (2020 and 2021) in the surroundings of the Hulene-B waste dump, Maputo, Mozambique. Eight 400 m electrical resistivity profiles were performed, four profiles in January 2020 and four profiles in May 2021 overlapped, and the data were inverted with RES2D software. The electrical resistivity models predominantly indicate an E-W movement of large contamination plumes that are successively diluted with saturated media and groundwater, creating zones of less resistive anomalies (<4.2–8.5 Ω·m) possibly contaminated at the two analysed seasons, between 2020–2021. The thickness of the contamination plumes was higher in summer (2020) for profiles 1 and 2, and we associate it with the production and migration mechanisms of leachate that are intense in the hot and rainy season. Southwest of the dump, profile 4b showed the propagation of anomalous areas on the surface and at depth, which are associated with the production of leachate resulting from the continuous decomposition of waste that is continuously deposited in a new area southwest of the dump, thus generating a slow and continuous migration of leachate at depth, mainly in winter (2021). The spatial distribution of contamination plumes during both seasons was reduced significantly farther away from the waste deposit, revealing the attenuating effect of groundwater and lithological substrate (Profile 3).

Electrical Resistivity Contrasts and High Flow Rates Discontinuous Aquifers Identification in a Sheared Crystallophyllian Basement Zone at Boniérédougou (North-Central Côte d’Ivoire)

This study proposes an alternative approach to the investigation of high flow hydrogeological fractures within the basement in the Dabakala region of north-central Côte d’Ivoire. The used approach consists of exploring the subsurface by measuring electrical resistivity contrasts along the main shear direction within crystallophyllian rocks. Electrical resistivity profiling and vertical electrical sounding techniques, coupled with boreholes monitoring, have identified fractured aquifers whose best flow rates are around 96 and 116 m3/h. These aquifers mostly hosted in granodiorite have an average strength of 10 meters and are located at depth of around 100 meters. They are associated with open fractures created by tangential shear stresses that have affected the Dabakala volcano-sedimentary trench formations. The search for fractured aquifers along the main shear direction offers great perspective for obtaining high flow rates.

High-resolution teleseismic tomographic crustal imaging for potential seismogenic segment of the central Tan-Lu Fault Zone, East China

The Tan-Lu Fault Zone (TLFZ) is the largest strike-slip system in East Asia. The central TLFZ has four main branch faults across a width of 20– 40 km, and both the great 1668 Tancheng Earthquake (M8.5) and the Anqiu Earthquake (M7.0) occurred on its two eastern branch faults. Due to the lack of recent earthquakes and stations around a seismic-quiescent segment of the TLFZ centered near the 70 BCE Anqiu Earthquake, previous tomographic images have resolutions coarser than 20 km in the middle and lower crust, unable to show how the crust is faulted along the TLFZ. To improve the resolution, we deployed a 70-km-long W-E-trending seismic array with 38 portable seismographs across the TLFZ at 36.1°N. Using one month of teleseismic records and a newly developed multiscale teleseismic tomography method, our P-wave velocity profile has achieved a resolution of 5 km × 5 km in shallow crust, and 10 km × 10 km in deep crust, as shown by the resolution tests. The profile's velocity variation correlates well with surface geology and crustal electrical resistivity profiles. The new profile provides clues for the depth distribution of major TLFZ branches, and indicates the two eastern TLFZ branch faults cut through the crust along a column-shaped low-velocity anomaly. This low-velocity column is associated with low-resistivity anomalies in the upper and lower crust, indicating a likely presence of fluids in the faulted rocks. This crustal faulting interpretation is consistent with the Moho geometry of previous receiver function studies, with an alignment between a receiver function Moho dome and a large low-velocity anomaly. Though seismicity is sparse near the profile, our interpreted crust-cutting location is in line with a linear cluster of recent TLFZ earthquakes about 20 km south of the profile, indicating that this quiescent segment could be a 150 km seismic gap along the central TLFZ.

Groundwater prospecting using remote sensing and geoelectrical methods in the North Cameroon (Central Africa) metamorphic formations

Remote sensing imagery and geoelectrical surveys used in this study help to evaluate the groundwater potentiality in the metamorphic formations of the North Cameroon. The methodology of the work is then mainly based on the extraction and identification of lineaments or fracture zones which acts as the passageway for groundwater. Manual and automatic extraction technics applied on the Landsat-8 images made it possible to find sixty one (61) lineaments. The main directions of these lineaments are N-S and N22°E (NNE-SSW) and the secondary directions are N10°E, NE-SW, then W-E to N100°E. According to the lineament density map, high density values are located at the centre and at the western part of the study area. The lineament density contrasts show the level of fracturing in the area. This fracturing is also evidenced by the geoelectrical survey. Indeed, results of the electrical resistivity profiling and 2D electrical imaging around or near lineaments, reveal the existence of fractures and shear zones characterized by the conductive discontinuities in the electrical current responses. Usually, the discontinuities sus-evocated above form potential groundwater targets. So, based on the combined results from remote sensing and geoelectrical method, the present study thus demonstrates the effectiveness of using this approach for assessing the availability of potential groundwater zones in the metamorphic formations. However, methods used in this study should be supported by the drill hole methods for the validity of results.

Monitoring of horizontal subsurface flow constructed wetlands for tertiary treatment of municipal wastewater

This paper explores the feasibility of the electrical resistivity (ER) method as non-invasive technique to detect dysfunctions of full-scale tertiary wastewater treatment wetlands, such as clogging, that ultimately limit the system lifetime and performance. ER measurements were carried out according to the Werner’s method on two (overloaded) horizontal subsurface wetlands (HSF-CW1 and HSF-CW2) operated in parallel since the end of 2017, while still achieving satisfactory treatment efficiencies. Layered clog-induced preferential flow pathways through the HSF-CW beds and premature deterioration of the wetland’s liner with a possible development of dead zones associated with a low/heterogeneous density of vegetation could be confirmed by analysis of ER profiles. These results suggest that the ER method is a promising and feasible technique, as simplified then adapted/adopted to local context, for better monitoring and assessment of treatment wetlands with early process failure.

An Experiment of Determining Buried Channel Using ERP and IP

Geophysical methods became an important technique for engineer since several decades. They have been used to study the engineering problem which has to be solved urgently. Detecting underground water channels is one of the most important issues in prevention and mitigation of water surge hazards in underground engineering. In this research, we use electrical resistivity profiling (ERP) and induced potential (IP) as the guide is proposed. By applying different configurations and varied electrode spacing, the water channel can be analysed by its resistivity and chargeability variation. The ERP method is sensitive to resistive layers which indicates the empty channel while IP can detect the presence of water. This is seen from both synthetic and measured data, where the resistivity of 60 Ω.m approximately implies that the channel is not filled with water, while in the area with moderate chargeability, ranging from 15 to 25 ms, indicates that water exists shrouding the channel. By understanding the known underground channel well, we can expand the study for other engineering problems and as references to apply the method in similar cases, such as the underground cavity which can be very important for infrastructure.

Subsurface Geological Characterization of the Late Neogene–Quaternary Argive Basin, Peloponnese, Greece Using Transient Electromagnetic Data and Vintage Stratigraphic Logs

The onshore and offshore clastic deposits of the Argive Basin and the Argolic Gulf, respectively, in Peloponnese, Greece, form a Late Neogene–Quaternary half-graben that connects with the Aegean Sea. The onshore Late Neogene–Quaternary sequence, comprised of chaotically intercalated cohesive and granular clastic deposits, is in angular unconformity with bedrock comprised of Triassic–Upper Cretaceous strongly-weathered, highly-fractured karstic limestones thrusted against Paleogene flysch deposits. While the surface geology of the Argive Basin is well-known, the subsurface geology remains both poorly mapped and understood. We utilized transient electromagnetic (TEM) soundings coupled with 185 vintage stratigraphic logs, current surface geology knowledge, and insights from available geophysical surveys to characterize the subsurface conditions of this sedimentary basin. We estimated the thickness of the young deposits (the depth to bedrock) and detected potential subsurface tectonic structures. The TEM-FAST 48HPC data acquisition system with integrated inversion and visualization software package was used with a single-loop dimension of 50 m × 50 m to collect a total of 329 TEM soundings at 151 stations scattered throughout the basin. The TEM station spacing varied from 200 to 750 m allowing the mapping of 80 km2. The total depth of investigation with the inverted TEM data and the lithology logs was 130 m and 183 m, respectively. The joint interpretation produced several quasi-two-dimensional electrical resistivity profiles that traverse the sedimentary basin in various azimuths and depth slices of average electrical resistivity covering the basin. The depth slices and the vintage stratigraphic logs revealed an uneven bedrock topography overlain by an irregularly thick (over 180 m) Late Neogene–Quaternary heterolithic sediment cover.

Geoelectrical Investigation of a Proposed Landfill Site in Ogbomoso, Southwestern Nigeria

An integrated geophysical investigation was carried out at a proposed landfill site along the new Ibadan-Ilorin Expressway in Ogbomoso, Southwestern Nigeria in order to characterize the subsurface and determine the suitability or otherwise of the site for landfill. The study location is underlain by rocks of the basement complex. 29 Schlumberger Vertical Electrical Soundings were conducted along seven traverses each about 100m long, about 20m apart, with station spacing of about 30m and half current electrode spacing varying between 1 and 100m. The VES data were interpreted quantitatively by partial curve matching and 1-D computer iteration and the layer parameters were used to generate the geoelectric sections. Wenner electrical resistivity profiling was also conducted along the traverses with station spacing of 25m and electrode spacing varied between 5 and 15m, and the data were interpreted using 2D inversion procedures. The VES results revealed three layers defined by the topsoil, clay layer and fractured/fresh bedrock. Depth to the bedrock is generally shallow with a mean of 9.0 3.7m. The mean thickness of the saprolite (mainly comprising clay) is 8.6 4.0m. 2D inversion of the resistivity profiling data revealed intense fracturing of the bedrock. The water table is shallow, ranging from 2 to 6m and may be rather close to the landfill base or fall within the landfill wastes. The study area is underlain by clayey overburden capable of protecting groundwater in the underlying aquifer unit from contamination by the leachate dischargeable from the landfill wastes. However, the presence of shallow water table and bedrock fractures may promote contamination of groundwater in the underlying aquifer unit and pose substantial health risk to the host community. It can therefore be concluded that the study area is not a suitable location for siting landfill. Keywords: Contamination, Leachate, Clay, Water table, Bedrock fractures DOI: 10.7176/JEES/11-16-04 Publication date: June 30 th 2021

Grounding Electrodes with Internal Resistance: Application to Feasibility Study of the Driven-Rod Method for Modeling the Soil Electrical Resistivity Profile

The paper presents a model to include the internal resistance of the grounding electrodes in the calculation of its electrical features. The semi-analytical expressions for the calculation of the grounding resistance arising from the model are used to study the feasibility of the driven-rod method for the estimation of the soil resistivity profile since, unlike other methods, the internal resistance of the conductors can be of great influence for a correct estimate. From the grounding resistance profile an inverse problem based on the minimization of the quadratic differences between the resistance measured and that calculated from the model is posed. Several synthetic examples are used to assess the limitations of the method in conditions close to real situations. Finally, some real cases involving data measured in the field are analyzed. Whether in synthetic examples or in real soils it is found that the spatial frequency of the driven-rod resistance sampling is a determinant factor in order to study the feasibility of the driven–rod method.

Frequency and voltage-dependent electrical parameters, interface traps, and series resistance profile of Au/(NiS:PVP)/n-Si structures

Frequency and voltage-dependent electrical parameters, interface traps, and series resistance profile of Au/(NiS:PVP)/n-Si structures

Water resources exploration and management based on remote sensing and geophysical data analysis: Al-Naqab Watershed, East Central Sinai, Egypt

This study integrates remote sensing and geophysical data to identify the hydrological conditions of the Naqab reservoir watershed. The main objectives of this study are identification and the interpretation of subsurface structures and their impact on groundwater flow, the relationships between structures and groundwater and the optimum way for watershed management in this sub-basin. Geophysical data including, ground magnetic survey and 1D electrical resistivity sounding helped in identifying the lithology and delineating zones of groundwater occurrence. The magnetic data delineated the basement rock, aided in characterizing the geometry of the subsurface structures that control the land surface features and constrain groundwater flow system. The interpreted subsurface structure elements include six sets of faults trending NE-SW, NW-SE, NWW-SEE, NEE-SWW, N-S and E-W. The basement depth was estimated at zero at the southern part and about 5187 m at the northern part. Two aquifer systems were characterized, the shallow aquifer of the Wata Formation (Upper Cretaceous) and the deep aquifer of the Malha Formation (Lower Cretaceous). The probability of groundwater occurrence increases towards the central part of the study area to north direction, where the thickness of the sedimentary basin reaches its maximum at the center of the study area. Different trends of faults were interpreted from are the geo-electrical cross-sections along two transects. Three patterns of faults were characterized including step faults, graben faults and horst faults. These faults could be indicated on the geo-electrical section by a marked difference in the layers` thicknesses. The magnetic data confirmed the locations of the faults delineated by the electrical resistivity profiles. The outlined faults are trending mainly in NE-SW, NW-SE, NWW-SEE, NEE-SWW and E-W directions. Lineament structures delineation and drainage pattern analysis were evaluated and interpreted from the analysis of remote sensing (RS) data and geographic information system (GIS) technique. The SRTM-DEM (Shuttle Radar Topographic Mission-Digital Elevation Model) was also utilized to automatically identify and extract drainage network. Interpretation and analysis of the inferred lineament structures indicate the presence of a number of main lineament populations that trends: NE-SW, NW-SE, NNW-SSE, NWW-SEE, E-W and N-S. Meanwhile, the interpretation and analysis of drainage pattern network indicate the presence of three main lineament structures that trends: NW-SE, NE-SW and NWW-SEE. Azimuth distribution analysis of both the measured structures and drainage channels shows similar trends, except for very few differences in the prevailing trends. Similarity in orientation of lineament structures, drainage system, and subsurface structural trends were recognized in the area under study. In conclusion, the integration between remote sensing and geophysical data revealed a close matching between the surface structural lineaments and subsurface structural trends could be verified. The preferred conduits of surface run-off and groundwater flow, as well as, the proper locations for groundwater exploitation could be determined.

Shallow geophysical and hydrological investigations to identify groundwater contamination in Wadi Bani Malik dam area Jeddah, Saudi Arabia

Abstract The integration of geophysical techniques with hydrological investigation is frequently used for solving different geological and environmental problems including groundwater quality and exploration and seismic vulnerability assessment. In this research, the shallow geophysical techniques comprising of electrical resistivity profiling, vertical electrical sounding, and ground magnetic were used to identify the contaminated areas lying in the upstream and downstream of Almisk Lake in Jeddah. The chemical analyses of water samples collected from the wells located in the downstream and upstream areas were used to support these results as an increase in the total dissolved solids (TDS) shows a decrease in the resistivity value. The results of geophysical techniques and hydrochemical analyses show that the TDS values are significantly higher in the upstream area of dam than those of the downstream, which suggests that the contaminating source is lying in the upstream of the reservoir. Moreover, the dam was not completely successful to block the contamination because of improper base. The hydrochemical analysis and geophysical results clearly indicate that the groundwater is not suitable for drinking and irrigation purposes.

Investigation of groundwater salinity using geophysical and geochemical approaches: heuningnes catchment coastal aquifer. Western Cape Province, South Africa

Seawater intrusion is recognised as a major factor affecting the quality of groundwater in coastal aquifers around the world. To determine the occurrence and extent of saline groundwater intrusion prevailing in the Heuningnes Catchment, a 2D resistivity imaging survey was carried out in three sites within the catchment. Eleven resistivity profile lines were conducted using the multi-electrode ABEM SAS resistivity meter system, employing the Wenner Array to map the extent of saline groundwater. A 2D resistivity image was acquired for these resistivity profile lines and the data were processed using Res2DINV software to produce the 2D inverse resistivity models. The interpretation of the resistivity models revealed three resistivity zones. The first zone is characterised by low resistivity (1–3 Ω.m); this represents an aquifer filled with saline water. The second resistivity zone has values ranging from (3–30 Ω.m); this represents a brackish saturated formation. The third resistivity zone denotes a sandy layer/sandstone saturated with freshwater having high resistivity values above 30 Ω.m. Geochemical analysis results of water samples at 11 locations along the electrical resistivity profile lines also suggest high salinity, indicated by high levels of Cl−, Na+, Total Dissolved Solids, and Electrical Conductivity. It is concluded that some parts of the study area are impacted by seawater intrusion, based on the lower resistivity of < 3 Ω.m and ionic ratios which were close to the standard seawater ratios for Na+/Cl− and SO42−/Cl−. In addition, the high ionic ratios of Na+/Cl− and SO42−/Cl− suggest that salinity in the area is a result of dissolution of evaporates and dilution of marine clays rich in sodium. Therefore, salinization in the study area is the result of combined processes. Integration approach proved to be significant in mapping saline groundwater in coastal aquifers. Integrated assessment of resistivity data, ionic analysis and chemical interpretation indicate that the study area is a transition zone from freshwater to saltwater and only few locations are actually impacted by seawater intrusion. The overall groundwater quality is not apt for human ingestion.

Late Holocene Deformation near the Southern Limits of the Wabash Valley Seismic Zone of Kentucky and Indiana, Central United States, with Seismic Implications

ABSTRACTThe Wabash Valley seismic zone (WVSZ) is a region of diffuse, modern intraplate seismicity in the central United States with a history of strong, late Quaternary and Holocene seismicity as determined through paleoliquefaction studies. Yet, there are no specific faults linked to these strong WVSZ paleoearthquakes, some of which were as large as Mw 7.2–7.5. A multidisciplinary investigation of a linear, 5-kilometer-long and ∼3-meter-high scarp on the Ohio River floodplain in the southernmost WVSZ in western Kentucky evaluated whether the scarp is a fluvial landform or a tectonic feature. Geomorphic mapping and optically stimulated luminescence geochronology show that the age and orientation of the scarp are inconsistent with surrounding fluvial landforms. Trenching, core drilling, seismic reflection, electrical resistivity profiling, and cross sections of petroleum well logs all indicate a blind fault directly underlies the scarp. The scarp is interpreted to be the fold axis of a down-to-the-west monocline formed in alluvium by slip on the underlying blind fault, herein named the Uniontown fault. The Uniontown fault connects the Hovey Lake fault, striking N20°E and having ∼0.5 km of documented strike-slip offset, with an unnamed fault complex to the south that strikes N40°E, suggesting the Uniontown fault is part of a larger, Paleozoic structure that has been reactivated with strike-slip deformation. Geomorphic mapping utilizing luminescence and radiocarbon geochronology indicates that folding and faulting occurred ∼3.5 ka. Paleoliquefaction was suppressed by a thick clay cap in the main Ohio Valley, but paleoliquefaction features are widespread on Ohio River tributaries. Gravel dikes at one site had a maximum age of 3.4±0.4 ka, confirming the region has experienced strong, late Holocene shaking. Estimates using vertical displacement and rupture length indicate that slip on the Uniontown scarp could produce an Mw 6.2–7.7 earthquake, which is comparable to other large paleoearthquakes in the WVSZ paleoseismic record.

High resolution tomography of the Tanlu fault zone near Hefei with passive seismic and magnetotelluric linear array data

As the largest fault trending NNE-SSW to NE-SW in the eastern Eurasia Continent, the Tanlu fault zone (TLFZ) extends over 2,400 km within China, roughly from Wuxue, Hubei Province, to Russia. Since the Quaternary period, the TLFZ has been an earthquake-prone area in eastern China where several major earthquakes resulted by tectonic compression occurred, causing tremendous casualties and significant economic losses. Many studies on different segments of the TLFZ have been carried out in the past few decades. However, numerous key questions regarding the fault zone remain unanswered due to a lack of clear subsurface characterization and fault delineation. In this study, we present high-resolution tomographic results across the TLFZ to the east of Hefei, where one 22-km-long passive seismic array with densely spaced short-period nodes, and a 24-km-long magnetotelluric array were deployed adjacent to each other. We find the velocity and resistivity variations are highly consistent with known surface geology. Sharp property contrasts in both the seismic shear wave velocity and electrical resistivity profiles clearly delineate the Tanlu F1 fault (TLF-1) near Hefei. More interestingly, an upwelling with distinct high velocity is imaged within the Hefei Basin to the west of the TLF-1, whereas a slanted block with low-velocity and low-resistivity seems to cut into or thrust upon the high-grade to low-grade middle-pressure rocks in the Zhangbaling uplift right below the Tanlu F2 ductile shear fault (TLF-2). The presented results show a new approach to characterize deep subsurface structure of the TLFZ beyond 2-km depths using passive data, which it is often difficult for active seismic surveys with refracted and reflected waves to image.

INTEGRATION OF ELECTRICAL RESISTIVITY IMAGING WITH BOREHOLE AND HAND AUGER DATA FOR PEAT CHARACTERISATION: A CASE STUDY AT KUALA LANGAT, SELANGOR, MALAYSIA

Peat is one of the Quaternary sediments found in the Kuala Langat area. Peat contains organic matter where its high natural water content can cause the subsurface to becomes unstable and unsuitable for supporting any building structures. It can either react with a steady settlement, which is generally less harmful or suddenly experience shear failure of the peat, causing sudden subsidence. This study aims to characterise a subsurface peat layer by integrating borehole data and hand auger samples with electrical resistivity imaging (ERI) results. The collected soil samples physical and chemical properties were analysed to determine its moisture content, total organic content (TOC) and von Post’s degree of humification. Three electrical resistivity survey lines were conducted using Wenner-Schlumberger protocol at certain key locations in the study area to map the vertical subsurface electrical resistivity profile. The studies show that the thickness of the peat layer from the borehole data and hand auger data is perfectly matched with the electrical resistivity pseudo section. The study area is covered up to 5 meters thick of peat with a resistivity value ranging from 16 Ωm to 24 Ωm, followed by a thick layer of marine clay with a resistivity value ranging from 24 Ωm to 46 Ωm. This study can support the engineer’s selection of site suitability and equipment for engineering construction.Keywords: electrical resistivity imaging (ERI), peat characterisation, peat resistivity, soil sampling