Resistivity Tomography Surveys Research Articles

Geophysical and Geotechnical Investigation of Building’s Foundation around Crusher area, Lokoja, Kogi state, Nigeria

Integration of geophysical and geotechnical techniques had been utilised to investigate the foundation of a site characterised with building cracks. It was conducted at Muhammadu Buhari Estate, Crusher area in Lokoja, Nigeria, The study aims to delineate geological features responsible for the failure of the building through 2-D resistivity imaging and electromagnetic surveys in conjunction with geotechnical analysis. Seven 2-D electrical resistivity imaging traverse lines, seven electromagnetic profiles were run and seven soil samples were collected and analysed. Each of the sample point were located on each of the 2-D imaging traverse. Traverses 1-7 host samples 1-7 respectively. The findings revealed high conductivity in unstable areas (resistivity ranging from 11-42 Ohm-m, 1-38 Ohm-m, 4-12 Ohm-m, 1-15 Ohm-m, and 11-30 Ohm-m for traverse 1-5, respectively) and low conductivity in stable ones (resistivity ranging from 19-91 Ohm-m and 109-2056 Ohm-m for traverse 6 and 7, respectively) and they occur at near surface to a relatively shallow depths (ranging from 0-6m depth) in all the profile. Geotechnical analysis showed high clay content in the soil, affecting its engineering properties in most of the studied locations. Soil grain oversize data analysis indicated a range between 70.1% and 75.1% for unstable portions (samples 1-5), with an average of 72.22% fines while percentage coarse ranges between 24.9 and 29.9 with an average of 27.78%. Their optimum moisture content ranged from 10.4% to 11.50%, with a mean value of 11.18%. Also, their plastic index averaged at 10.044%, while linear shrinkage varies between 1.79% and 3.60%, with an average of 2.808%. All these are the responsible factors for the soil instability in this axis of the investigated site. But for the stable portions (samples 6 and 7), Soil grain oversize data analysis is between 18.1% and 18.2% with an average of 18.65% fines while percentage coarse range between 81.9 and 80.8 with an average of 81.35%. The optimum moisture content ranged from 10.4% to 11.50%, with a mean value of 11.18%, the optimum moisture content ranged from 11.3% to 12.0%, with a mean of 11.65%, the plastic index equals zero (0), linear shrinkage varies between 5.57% and 5.71%, with an average of 5.64% and all these parameters are responsible for the stability of these two portions. The California Bearing ratio (CBR) results graded the soil samples into different types with the unstable portion characterised with clay, silty clay and sandy clay while the stable portion are classified into well graded sands. The well graded sands are very suitable for the engineering construction and hence the buildings on this portion showed no cracks and failure because of lack of differential settlements unlike that of the unstable portion characterised with cracks due differential settlements that led to structural failure at those sides of the study site. The geophysical results corroborate well with geotechnical results and these highlight the value of combining geophysical and geotechnical techniques to evaluate the integrity of the subsurface, providing information that is essential for designing infrastructure foundations and determining stability.

Identification and Verification of the Movement of the Hidden Active Fault Using Electrical Resistivity Tomography and Excavation

Identifying the movement of the branches of the hidden Thakhek fault in Thailand is challenging due to the absence of evident landforms indicating an active fault. In this study, we analyzed a digital elevation model (DEM) to identify potential landforms. A 2D Electrical Resistivity Tomography (ERT) survey was conducted to locate the hidden Thakhek fault. The results reveal vivid images of resistivity contrast, interpreted as two reverse faults, with mudstone exhibiting low resistivity in the middle, flanked by thick sediment layers with higher resistivity. Three trenches were excavated perpendicular to the two interpreted reverse faults. The displacement of reverse faulting appears to have shifted mudstone over Quaternary sediments, with vertical offsets revealed in trenches NWY-1, NWY-2, and NWY-3. This movement could be identified as a positive flower structure. Additionally, lakes are identified as a negative flower structure along the traces. These features result from strike-slip strains under a locally appropriate compressional and extensional environment in a shearing strike-slip fault.

Aquifer vulnerability valorization via DRASTIC index-based assessment within litho-facies of a coastal environment

Determining the degree of an aquifer’s susceptibility to pollutants is an important tool for policy planning in subterranean water administration and conservation. In this work, subterranean water vulnerability was appraised with the use of DRASTIC methodology, which employs seven parameters: depth to groundwater (D); net groundwater recharge (R); aquifer media (A); soil media (S); topography (T); influence of vadose zone (I); and hydraulic conductivity (C). Different ratings were given to each parameter based on the hydro-geological and geo-electrical information obtained from the study area. Geo-electrostratigraphic data was acquired with vertical electrical soundings and electrical resistivity tomography surveys. The results demonstrated the existence of geoelectrical layers with variable resistivities ranging from high to low, with variable curve types. Longitudinal conductance measures indicated moderate subterranean water protective capacity. Analysis of DRASTIC vulnerability indices showed that the aquifer indicated moderate to high vulnerability to contamination. Vulnerability maps were generated to indicate locations of high vulnerability, the goal being to prevent indiscriminate haphazard and wildcat abstraction of groundwater in such locations. The quality category index (QCI) generated for the DRASTIC model indicated that underground water susceptivity is dependent on all 7 parameters employed in the model, with the most critical determinants being the depth to aquifer (D), net recharge (R), aquifer media (A), and impact of the vadose zone (I). The work recommends that anthropogenic activities that could contaminate groundwater resource be halted as the resource is indicated by DRASTIC models as being vulnerable.

Geo-electrical prognosis of aquifer protectivity, corrosivity, and vulnerability via index-based models within a major coastal milieu

This work was carried out to investigate the protective capacity, vulnerability, and corrosivity within a major coastal milieu in Southern Nigeria with the use of index-based geo-electrical modeling methods. Vertical electrical soundings were undertaken at twenty locations with the aid of Schlumberger array having a maximum electrode spacing of 400 m. The results indicated that the lithology comprised four subsurface layers having variable values of resistivity and thickness. The Dar-Zarrouk parameter, the Aquifer Vulnerability Index (AVI), and the GOD (Groundwater occurrence G, Overlying lithology O and Depth to aquifer D) models were employed to appraise measures of aquifer protectivity and vulnerability to contamination. The longitudinal conductance values ranged from 0.0071–1.95 mhos with a mean of 0.32 mhos, indicating moderate protectivity. AVI values ranged from 1.73–4.10 with a mean of 3.03, indicating moderate aquifer vulnerability. The GOD indices ranged from 0.35–0.63 with a mean of 0.49, indicating moderate aquifer vulnerability. Corrosivity was also computed based on topsoil resistivity values which ranged from 12.7 to 664.2 Ωm with a mean of 168.17 Ωm, indicating moderate corrosivity, and demonstrating the unsuitability of corrosive locations for laying underground pipes. All the index-based models gave similar interpretations, indicating moderate aquifer protectivity and susceptibility. These results were corroborated by 2D electrical resistivity tomography surveys conducted at four stations. This work has therefore delineated important aquifer geo-hydraulic properties with index-based geo-electrical modeling techniques. The results obtained are critical for effective aquifer management, conservation, and sustainability.

Preconditioning of mountain permafrost towards degradation detected by electrical resistivity

Warming permafrost has been detected worldwide and is projected to continue during the next century by many modelling studies. In mountain regions, this can lead to potentially hazardous impacts on short time-scales by an increased tendency for slope instabilities. However, time scales of permafrost thaw and the role of the ice content are less clear, especially in heterogeneous mountain terrain, where ice content can vary between zero and supersaturated conditions over small distances. Warming of permafrost near the freezing point shows therefore complex inter-annual behaviour due to latent heat effects during thawing and the influence of the snow-cover, which is governed by highly non-linear processes itself. Here, we demonstrate a preconditioning effect within near-surface layers in mountain permafrost that causes non-linear degradation and accelerates thaw. We hypothesise that a summer heat wave, as has occurred in the Central European summers 2003, 2015 and 2022, will enhance permafrost degradation if the active layer and the top of the permafrost layer are already preconditioned, i.e. have reduced latent heat content. This preconditioning can already be effectuated by a singular warm year, leading to exceptionally strong melting of the ground ice. On sloping terrain this ice-loss can be considered as irreversible, as large parts of the melted water will drain during the process, and an equivalent build-up of ice in cold years does not happen on similar time-scales as the melting. We propose a simple geophysical approach based on electrical resistivity tomography surveys that can assess the state of preconditioning in the absence of boreholes. In addition, we will show that resistivity data from a total of 124 permafrost sites in the Andes, Europe, and Antarctic adhere to a distinct power law behaviour between unfrozen and frozen states, which confirms the consistent electrical behaviour of permafrost and active layer materials over a wide range of landforms and material composition.

Salinity distribution in agricultural land by geophysical, hydrochemical and geostatistical approaches: a pilot area located in Qelabshowah–Belqas, East Nile Delta region, Egypt

The study area is situated in the Qelabshowah–Belqas region, known for its Quaternary deposits. This research aims to demonstrate the two-dimensional (2D) variation of subsurface layers and salinity distribution using geoelectrical data, hydrochemical analysis, and geostatistical analysis. DC resistivity measurements were taken at fifteen vertical electrical sounding (VES) survey points using a Schlumberger array (AB/2 = 100 m) along three profiles. In addition, an electrical resistivity tomography (ERT) survey was conducted with a dipole–dipole array across one profile. Seven surface water samples were collected in the area. From the 1D and 2D inversion of VES and ERT data sets, three-to-four geoelectric layers were identified, including unconsolidated surface deposits, saturated clayey sand, saturated sand, and a salt-rich layer. The 2D inversion of VES data revealed an ancient salt-rich layer deposited in swampy conditions over a conductive wet sand layer along profile one due to salt mineral infiltration and dissolution. The 2D inversion of ERT data showed accurate lateral geometric accuracy compared to the 2D inversion of VES data, highlighting geological features, such as caves in the second layer and a buried water canal on the ground surface. Surface water samples showed high salinity levels with sodium hazards, indicating an Na–Cl composition. Geoelectric and hydrochemical data sets were geostatistically analyzed using spherical variogram supported ordinary Kriging interpolation. The analysis indicated weak to moderate spatial dependency for true resistivity parameters, while sodium content (SC) and permeability index (PI) showed strong spatial correlation. The 2D spatial distribution resistivity maps based on the 1D inversion of VES data displayed a general decrease in resistivity with depth, likely due to clay minerals or moist soil in the second layer and saline irrigation water infiltration in the third layer. The 2D spatial distribution of SC and PI showed a high concentration zone, posing a potential risk to agricultural crops regardless of soil permeability. It is recommended to use these maps when cultivating plants that can tolerate high sodium levels during the reclamation process.

Structure and age of ancient walls from the city of Nan, northern Thailand

The structure and age of remains of two ancient walls located in the city of Nan, a former part of the Lanna Kingdom in modern northern Thailand, were investigated. The remains differ in appearance with a well-preserved brick wall present at Mahawong Road (MHW), while an earthwork with a brick reinforcement core is found at Phaya Wat Temple (PWT). We employed an electrical resistivity tomography (ERT) survey and optically stimulated luminescence dating (OSL) to determine the buried structure of the earthwork at the PWT site and the production age of bricks from both locations. The ERT results reveal lateral heterogeneous resistivity domains cross-cutting the wall, which presumably relates to the construction of a dam. According to OSL dating, the PWT wall was likely built during the time of the war with Burma that ultimately resulted in a foreign occupation of Nan in the sixteenth century. This structure played a role in both flood mitigation and military defence of the city. The structure at MHW represents a city wall that was built at the end of the nineteenth century after Nan was relocated to its present position and rather reflects representative and social needs.

Geophysical exploration to assess leachate percolation and aquifer protectivity within hydrogeological units at a major open dump in Eket, Nigeria

Leachate contamination was investigated with geo-electrical methodologies at a major open dump in Eket, Southern Nigeria. The open dump is heavily polluted, degrades the environment, and has released toxins into the atmosphere. To model leachate percolation, vertical electrical soundings (VES) and electrical resistivity tomography (ERT) surveys, constrained by well lithological information, were undertaken at the site. The field equipment used for the geo-electrical investigations was the ABEM Terrameter SAS 1000 and its accessories. WINRESIST software was employed to process the sounding data and RES2DINV was employed to generate 2D tomograms indicating subsurface heterogeneity. Results obtained from sounding data showed that the subsurface had three lithological units (motley topsoil, fine sand and coarse sand), with coarse sand being the main hydrogeological unit comprising water-bearing formations. The first layer’s resistivity ranged from 84.9 – 625.4 Ωm with a mean of 360.4 Ωm; the second layer’s resistivity ranged from 7.5 – 1008.1 Ωm with a mean of 142.9 Ωm, and the third layer’s resistivity values ranged from 11.6 – 745.6 with a mean of 205.5 Ωm. The 2D resistivity tomograms indicated that leachate had percolated into groundwater resources. This is concerning as groundwater is the major water resource within the area. Measures of the Dar-Zarrouk indices and electrical reflection co-efficient indicated that the highly heterogenous region had moderate aquifer protective capacity and moderate aquifer potentiality. To mitigate contamination at the site, sanitary landfills, phyto-remediation, impermeable liners, or impermeable earth materials should be employed to diminish the rate of leachate percolation.

Identification of sliding surface using electrical-resistivity tomography for landslide mitigation: A case study of the Cibitung Landslide

Landslide mitigation efforts require a knowledge of the geometry and depth of the sliding surface present in a landslide body. Electrical resistivity tomography is the most common geophysical method used in landslide investigation. This paper presents the results of a series of electrical resistivity tomography surveys performed using a dipole-dipole array configuration to identify the sliding surfaces within a landslide body located in the Cibitung landslide area. The ERT images parallel the landslide body suggest that the sliding surface located below the crown is characterized by a bedrock layer with very low resistivity values. This bedrock layer continues down-slope at a maximum depth of 9 m below the ground surface. In contrast, the landslide body is characterized by a higher resistivity value. Based on the interpretation of the ERT images, the landslide involved a non-circular deep sliding surface. The results of this study have been used, in combination with the geotechnical drilling data, to construct the landslide cross-section necessary to analyze landslide stability and subsequently to recommend a landslide stabilization measure.

An effective multiphysics toolkit for lithium prospecting: From geophysics to the static reservoir model in the Pozuelos salt flat, Argentina

Energy transition drives the energy sector to renewable energy and electrification, being the critical minerals key players in the industrial development map. They comprise rare-earth elements and 35 other elements, including lithium, which holds 60% of its world reserves in the so-called lithium triangle located in Argentina, Bolivia, and Chile. The low electrical resistivities, variations in salt concentrations, low acoustic impedances, and dynamics of the hydrogeologic system make brine monitoring a complex geophysical exploratory problem. So, the objective is to find a suitable combination of geophysical techniques that fit the lithium exploration objectives, which are the characterization of the salt flat in-depth, fluid detection, basement delineation, definition of the main structures and main faults, and detection of semifresh water aquifers that contribute to its recharge and that are key to the reservoir water balance of an endorheic basin, which has the resource in solution. For this purpose, the evaluation of several prospecting methods in different salt flats was executed, concluding that full tensor magnetotellurics (MT), electrical resistivity tomography, and gravity comprise a toolkit that fits the objectives set. The methodology was validated at the Pozuelos salt flat. The results show that the fresh water-brines contact and the recharge system were well-defined and imaged by ERT. The full tensor MT detected two ultraconductive units: a shallower multilayer system saturated with brines, has a 400 m thickness, whereas the deeper unit has a 500 m thickness. Both MT and gravity characterized the basement, and gravity successfully delineated the main structures. The geophysical interpretation is in concordance with shallow and deep exploration wells. Finally, the integration of geophysical and well data allowed the construction of a 3D static reservoir model that finds the deepest basement area at approximately 900 m depth and discriminates eight lithofacies.

Physicochemical Characteristics of Feldspar Deposits in Zango Daji, Kogi State Using Geological Methods

Six rock samples were randomly collected and photographs, locational coordinates and elevation points were taken with appropriate tools and devices. Portions of samples were prepared into thin sections and observed under hand lenses, microscope, tested for hardness and other diagnostic properties. Specific density was also determined. The remaining samples were subjected to geochemical and mineralogical analysis. An average percentage composition of 17.1958 mass % of K2O was obtained which is far greater than its Calcium and Sodium contents, indicating it is a K-feldspar. Geochemical result showed averages of Si2O, Al2O3, K2O, Na2O, CaO, and Fe2O3, contents of 59.54 mass %, 17.83 mass %, 17.19 mass %, 2.33 mass %, 0.08 mass % and 1.60 mass % respectively while mineralogical results revealed it is orthoclase in view of its high alumina content of 17.83 percent. The results showed that the sample specification meets the British International Standard specification. Furthermore, electrical resistivity tomography survey was conducted by setting up Schlumberger Electrode Array to determine the significant resistivity contrast between the feldspar and the compact bedrock in the profile. The resistivity data of eight points were processed into Resists curves after conversion and subjecting the data by inversion with WinResist software to obtain 1 - D and 2 – D maps. Together with results of chemical analyses and vertical electrical sounding, the location map of mineralogical composition of the Oxides were determined. Field and petrographic evidence showed a potential source of gemstones and niobium. The study also revealed the occurrence of feldspar deposit are hosted by granitic and pegmatitic intrusive. The quantum and reserve estimates of both overburden layer and target mineral thickness were determined to be 44.3 ± 2.18 million metric tons and 741,933 ± 132,205 m³ respectively.

A Multidisciplinary Approach for the Characterization of Artificial Cavities of Historical and Cultural Interest: The Case Study of the Cloister of Sant’Agostino—Caserta, Italy

In northern Campania (Southern Italy), the historic center of many towns is characterized by the widespread presence of cavities in the subsoil, excavated over the centuries for quarrying tuff blocks for buildings, along with cathedrals, churches and chapels. A singular feature of these places of worship is, in fact, the presence of a wide and frequently connected network of underground cavities and tunnels, which were used for hydraulic, religious or connecting purposes. The cavity network is often unknown, abandoned or even buried, thus representing a risk for their susceptibility to sinkholes. Such elements are important as cultural heritage of inestimable value and as attractors for tourism; for this reason, the multidisciplinary study conducted on a place of worship in the Caserta area is illustrated herein: the Cloister of Sant’Agostino, in Caserta (XVI century CE). A geological and geotechnical characterization of the subsoil was performed at first. A laser scanner survey of the accessible cavities and the external churchyard was carried out. The resulting 3D model of the underground sector allowed for a clear understanding of the room size, their location, the levels and the path of the corridors. To understand the extension and layout of the crypts, Electrical Resistivity Tomography (ERT) surveys were undertaken in the surrounding areas. The analysis of the ERT measurements revealed some anomalies that could be ascribed to unknown structures (crypts). Finally, numerical methods were applied to estimate the stress state of the soft rocks and the potential areas of crisis, with preliminary assessments of the influence of the presence of cavities on the stability of the subsoil. The results allowed us to improve the knowledge of the study site and provide useful data for the planning of future targeted investigations, underlining how integrated research between applied disciplines can provide indispensable support both in the management and mitigation of geological risks in urban areas and in the sustainable reuse of hypogea.

Engineering Geological and Geophysical Investigations to Characterise the Unstable Rock Slope of the Sopu Promontory (Gozo, Malta)

Different engineering geological and geophysical investigations were performed at the Sopu promontory in the island of Gozo (Malta), involved in an impressive lateral spreading process due to the superimposition of a stiff limestone (ULC) on a ductile clay (BC). The applied techniques include: traditional geological and engineering geological surveys, unmanned aerial vehicles (UAV) survey, electrical resistivity tomography (ERT) survey, ground-penetrating radar (GPR) investigations, single-station seismic ambient noise measurements, and array seismic ambient noise measurements. The integration of the obtained results allowed us to reconstruct a subsoil model of the promontory that includes features related to the local geology of the slope and to the landslide process, as well as to define a conceptual model that describes the main evolution phases of the expansion process. The presence of back-tilted rock blocks with no features of polarization of Rayleigh waves evidenced the different failure mechanism of the rigid UCL plateau at the Sopu promontory with respect to the Selmun promontory, located in the close island of Malta, where the lateral spreading due to the same geological setting tends to produce unstable rock blocks with a toppling mechanism. This result encourages further future observations and analyses of this topic.

Electrical Resistivity Tomography Investigation of Permafrost Conditions in a Thermokarst Site in Fairbanks, Alaska

<abstract> <p>The degradation of permafrost poses severe environmental threats to communities in cold regions. As near-surface permafrost warms, extensive topographic variability is prevalent in the Arctic and Sub-Arctic communities. Geologic hazards such as thermokarst are formed due to varying rates of permafrost degradation, resulting in ground subsidence. This gradual subsidence or abrupt collapse of the earth causes a danger to existing infrastructure and the economic activities of communities in cold regions. Understanding the causes of thermokarst development and its dynamics requires imaging its underground morpho-structures and characterizing the surface and subsurface controls. In this study, we conducted a two-dimensional (2D) electrical resistivity tomography (ERT) survey to characterize the permafrost conditions in a thermokarst prone site located in Fairbanks, Alaska. To increase the reliability in the interpretability of the ERT data, borehole data and the depth-of-investigation (DOI) methods were applied. By using the 2D and three-dimensional (3D) ERT methods, we gained valuable information on the spatial variability of transient processes, such as the movement of freezing and thawing fronts. Resistivity imaging across the site exhibited distinct variations in permafrost conditions, with both low and high resistive anomalies observed along the transects. These anomalies, representing taliks and ice wedges, were characterized by resistivity values ranging from 50 Ωm and above 700 Ωm, respectively. The results from this study showed the effectiveness of ERT to characterize permafrost conditions and thermokarst subsurface morpho-structures. The insights gained from this research contribute to a better understanding of the causes and dynamics of thermokarst, which can be instrumental for engineers in developing feasible remedial measures.</p> </abstract>

An Integrated Microgravity and Soil Analysis Methods to Assist the Resistivity Profiles Interpretation in an Engineering Site Southern Baghdad, Iraq

A 2D Electrical Resistivity Imaging survey was conducted at the Al-Jadiriya Campus of Baghdad University. The 2D resistivity survey was achieved on two profiles using the Wenner- Schlumberger configuration. The length of each profile is 80 meters, and the spacing between adjacent electrodes is 1 mr. The obtained resistivity data were interpreted using the RES2DINV program. Both profiles show high resistivity anomaly at depths more than 6 meters, extending from a distance of 32-44 m and 37-44 mrs for profiles 1 and 2, respectively. In addition to the deep anomalies, there are many shallow high-resistivity anomalies. A microgravity survey was carried out for a distance of 20- 60 m along the two profiles to check the nature of the deep high resistivity anomaly. The gravity profiles showed many positive gravity anomalies reflected the effect of shallow sources, which coincide with the shallow relatively high resistivity anomalies. Many soil samples were analyzed chemically and sedimentologically to investigate soil homogeneity. It is found that the soil is homogenous, composed mainly of silt and clay. According to the integrated interpretation of resistivity, gravity, and soil analyses, it is found that the high resistivity anomaly is related to dry compacted soil mass, which is indicated in the gravity study. This study confirms the importance of integrated research to access better geophysical interpretation. The water table level in the study sites ranges 5.6–6.2 ms.

Determination Water Level by Electrical Resistivity and Comparing Results with Drilled Well in Al-Naharwan, Baghdad

The 2D electrical resistivity imaging survey was done in Al-Naharawan area within Baghdad city for unknown subsurface to give a best location for digging a new well. Electrical resistivity imaging survey is implemented for along four lines that are 120 m long and all have 1m electrode separation. Measurements done with wenner-Schlumberger array and interpreted data by using RES2DINV program. The results for drilling the well at electrodes 62 and 63 in line 1 showed exactly corresponds to the lithological order of the well Fadhel Abass. Electrical resistivity imaging results showed high and low resistivity values, although they both contain clay, which corresponds to variances in water content. One Groundwater sample was taken from the new well and analyzed chemically to determine the concentration of the major ions. water analysis shows high TDS, electrical conductivity, and most parameters affecting human life out of permissible limits, so it is not suitable for human drinking. SAR =21.695, RSC=-474, which indicates the risk from sodium is improbable and lies within permissible ranges. PI% within Class II, meaning the water quality is good for irrigation. The findings indicated that the 2D imaging method is an effective and powerful enough tool for identifying new well with water table and detecting subsurface.

Characterization of seepage in an earthfill dam by the integration of geophysical surveys and geotechnical data

Seepage is a common phenomenon in earthfill dams, and one that typically warrants episodic assessment and monitoring by regulatory agencies. Geophysical surveys provide a convenient, cost-effective, and convenient means for assessing seepage, but only if they are applied in effective combinations and if the data are analyzed properly. In the present study, a self-potential survey of streaming potential and a 2D electrical resistivity tomography survey were notably successful in detecting seepage under the embankment of the nearly 90-year-old Standing Rock Dam No. 1 in South Dakota, USA. Self-potential (SP) measurements show a localized negative anomaly of up to 27 mV on the upstream side of the dam, which we interpret to be the inlet or recharge zone for seepage, and a positive anomaly of up to 20 mV on the downstream side, which we interpret to be the outlet or discharge zone. The spatial correlation between our resistivity models, SP data, and topography was excellent. Water-saturated, conductive deposits in the dam's embankment match a negative SP amplitude in the inlet or recharge area. Similarly, water-saturated electrically conductive sediments are characterized by a positive SP amplitude in the discharge area where surface seepage occurs in a topographic low. Six years of water-level measurements in the existing piezometers interpreted relative to the properties and spatial distribution of coarser-grained alluvium (sand and gravel) underneath the dam's embankment fill, suggest that most seepage occurs under the dam. Our approach to assessing seepage in this project should be widely applicable to earthfill dams worldwide, and our modeled results constitute a valuable case study.

Discovery of an Active Forearc Fault in an Urban Region: Holocene Rupture on the XEOLXELEK‐Elk Lake Fault, Victoria, British Columbia, Canada

AbstractSubduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments.

An attempt to use convolutional neural network to recover layered-earth structure from electrical resistivity tomography survey

The Electrical Resistivity Tomography (ERT) method is a geophysical prospecting technique for determining subsurface structure in terms of electrical resistivity distribution. Direct current is injected into the ground through a pair of electrodes, and the potential difference is measured using another pair of electrodes. The estimated resistivity model of the subsurface is then obtained using a mathematical optimization algorithm called the inversion algorithm, which incorporates multiple measured potential differences from different electrode configurations as the input data. The inverted resistivity model serves as a resource for the interpretation process of the survey. Specialists employ the model and the prior geological information in the area to extract the subsurface geological structure under the data measurement profile. The inverted model can be treated as a two-dimensional image in which each element represents resistivity. The image can be visualized as a blurred or distorted version of the actual resistivity image of the subsurface. We, therefore, developed a post-inversion image enhancement code using a neural network aiming to recover the true subsurface model with predetermined features. We employed the convolutional neural network (CNN) that uses the inverted models as input and returns an enhanced model as output. We tested our developed neural network with a case study that utilized the proposed method to recover the layered earth structure which is often found in real surveys. The electrode spacing was 5 meters in the Schlumberger array configuration. The true subsurface resistivity models of training data and validating data for evaluating the neural network were randomly generated based on the predetermined feature of the layered structure. We then generated their respective inverted models from those data through simulated inversion routine. As a result, 94.75% of the enhanced models in the validation dataset exhibited a lower loss metric compared to the original inverted models in the same validation dataset. We also applied the proposed method to practical field data to recover the subsurface layers and compared them with stratigraphic information from a borehole in each area. The result showed that the proposed method gives well-resolved structures in the enhancement process after the typical inversion.

Cavity detection using a pseudo-3D electric resistivity tomography at the Palaeolithic/Neolithic site of Scaloria Cave, Apulia, Italy: integrated assessment of synthetic and field data sets

A pseudo-3D electrical resistivity tomography (ERT) survey has been carried out to detect cavities at the Neolithic/Palaeolithic site of Grotta Scaloria, close to Manfredonia, Apulia, Italy. Scaloria Cave has a rich history of archaeological research of more than 80 years and is one of the most important Neolithic complexes in the Mediterranean. Synthetic data modelling allowed to check the adequacy of the geophysical method and to develop a proper experimental setup at the survey design stage. Indeed, the results of the field data inversion revealed high resistivity anomalies which can be related to cavities and provided a good definition of the main geological structures and boundaries. Moreover, the results suggest that unknown and speleological unexplored cavities are still present at the Scaloria Cave study site. These findings may provide further insights on pseudo-3D ERT applicability, particularly for cavity detection. Furthermore, the approach used in this study yields fruitful information for further archaeological survey design and for the interpretation of ERT investigations targeting similar geological features and structures.