Electrical Resistivity Tomography Models Research Articles

A new insight for geophysical and geospatial mapping quaternary alluvium of Indo-Gangetic plains for exploration and assessment of groundwater in Haryana, India

A comprehensive study on groundwater exploration and assessment was conducted, integrating geophysical, geological, lithological, and resistivity logging techniques for a holistic evaluation of groundwater resources in Haryana, India. The primary research objective was to gain insight into the geological composition of alluvial sand and its implications for groundwater occurrence in Haryana's semi-arid region. The study showcases high-resolution electrical resistivity tomography results alongside conceptual cross-sectional models and 3D lithological models, revealing buried channels that serve as groundwater sources and saturated sand as well as recharge conduits within the alluvial region of Haryana. Moreover, borehole drilling at two sites and resistivity logging data demonstrate a strong correlation between various lithologies and resistivity logs. Overall, groundwater prospect zones are delineated at depths ranging from 18 m to over 100 m based on resistivity values of aquifer zones inferred from electrical resistivity tomography models, as well as mapped thick sand layers with significant hydrogeological importance in the study area. This emphasizes the stress on the aquifer system, where groundwater is being extracted from deeper depths. The conceptual litho-sectional model reveals a thick sand signature at intermediate depths, indicating the presence of buried paleochannels extending from Galedwa (Kurukshetra) to Jubbal (Yamunanagar). Additionally, the 3D lithological models, along with the electrical resistivity tomography results, facilitate strategic planning for shallow and deep boreholes for groundwater exploration and prospecting, aiming at sustainable groundwater development and management of resources in both the present and future.

Evaluation of the hillslope fine-scale morphology under forest cover with pit-mound topography - Integration of geomorphometry, geophysical methods, and soil features

Forested hillslopes are zones of specific surface hydrology and geomorphic activity regimes. Their distinct properties, namely terrain microrelief (<10 m in diameter and height), are often a result of past disturbances that control forest stand conditions, soil formation processes, and superficial processes. A clear bioindicator of the past forest disturbance is pit-mound topography, which, however, is challenging to study because of its complexity and relatively small sizes of individual forms (usually <5 m in diameter). The present study analyzed the spatial representation, geomorphometric, and geophysical evaluation of the rare pit-mound topography in the Karkonosze Mountains National Park, SW Poland, Central Europe. For this task, two digital terrain models (DTMs) have been considered and were based on different quality point clouds collected during airborne and terrestrial laser scanning (ALS and TLS, respectively). The first data allowed the production of the DTM in 1 × 1 m spatial resolution, while the second data offered the DTM in 0.025 × 0.025 m resolution. Various geomorphometric derivatives (Terrain Ruggedness Index, Geomorphons, Topographic Wetness Index, Valley Depth, and Negative Openness) were applied and compared based on these models. In the further part of the study, we applied electrical resistivity tomography (ERT) and electromagnetic induction (EMI), assisted by shallow soil sampling and analyses to support the interpretation of geophysical models.Our TLS-based DTM offered higher-quality models and a better representation of pit-mound topography. The high-quality TLS-based DTM elevation model can support close-to-reality hydrological and geomorphic modeling. The geophysical investigation allowed us to isolate a critical difference between treethrow pits and mounds better represented by ERT models than shallow EMI models. The differences were partly supported by soil properties, namely lower electrical resistivity in treethrow pits were related to higher moisture conditions, organic matter, organic carbon, and silt content in pits. As a general property, pit-mound topography resulting from tree uprooting adds to the complexity of forested hillslope hydrology and geomorphic activity. The surficial heterogeneity in hillslope topography was also evident in soil properties with sharp changes in short distances between treethrow mounds and pits.

High-resolution geophysical investigations in the central Apennines seismic belt (Italy): Results from the Campo Felice tectonic basin

The Campo Felice basin, in the central Apennines seismic belt (Italy), developed in the hangingwall of a 30 km-long system of NW-trending normal faults with Holocene paleoseismic activity and potential sources of M 6–7 earthquakes. We provide the first subsurface images of a key portion of the basin bounded by the Mt. Cefalone fault along two intersecting profiles trending NNE-SSW (CF-Dip, 1195 m-long) and WNW-ESE (CF-Strike, 1315-m long). We combined high-resolution depth-migrated reflection sections with P-wave velocity and electrical resistivity tomography models. CF-Dip profile displays a wedge-like syn-tectonic sedimentary sequence of alluvial and glacial deposits with Vp ∼ 2500–3000 m/s and resistivity > 500 Ωm in the hangingwall of Mt. Cefalone fault, overlying a high-Vp (>4000 m/s) limestone bedrock ∼ 300 m deep. The whole sequence displays reflectors truncated by the Mt. Cefalone fault zone and subsidiary antithetic faults. CF-Strike profile, tied to three 80–110 m-deep boreholes, shows a thick fluvio-lacustrine sequence with low-Vp (<2000 m/s) and low resistivity (<100 Ωm), and a bedrock that deepens to the southeast (>450 m). Single-station ambient noise measurements display Horizontal to Vertical Spectral Ratios with peaks at ∼1 Hz, decreasing to ∼0.8 Hz to the southeast in agreement with the bedrock deepening indicated by seismic profiling. According to our results, the Campo Felice basin is a deep asymmetric half-graben controlled by faulting whose activity likely started before the Middle Pleistocene. Our minimum displacement estimate accrued in the past 0.5 Ma by the Mt. Cefalone fault is in the range of ∼100–250 m.

PyMERRY: A Python solution for an improved interpretation of electrical resistivity tomography images

Electrical resistivity tomography (ERT) is a widely used geophysical method for studying geologic hazards, civil engineering, and environmental remediation. It provides information about the subsurface’s resistivity distribution by analyzing electrical data collected at the surface or in boreholes. However, interpreting ERT images can be complex due to ambiguities in their resolution. To address this issue, we develop a postprocessing method called Python iMprovement of Electrical Resistivity tomography ReliabilitY (PyMERRY) to improve the reliability of ERT images. The PyMERRY code can be applied to any 2D resistivity model obtained from ERT inversion software. It computes a coverage mask that defines the domain well constrained by the data and the inversion process. It also evaluates the resistivity uncertainties in the ERT models. In addition to the sensitivity approaches, PyMERRY provides low- and high-resistivity values for all covered cells. Synthetic tests indicate that the approach is efficient and highlight the importance of resistivity contrasts, mesh selection, electrode spacing, and profile length in the reliability of ERT images. Compared with previous studies, using PyMERRY in south-central Bhutan allows a more accurate interpretation of ERT images. It confirms a high-resistivity contrast across the topographic frontal thrust and reveals the existence of small-scale variations.

New Insights from Geophysical, Hydrogeological and Borehole Data into the Deep Structure of the Louta Phosphatic Deposit (Gantour Basin, Morocco): Mining Implications

The Gantour Phosphatic Basin (GPB) is formed by a sedimentary series of Maastrichtian to the Eocene age, which consists of alternating phosphate layers and sterile levels. This series outcrops in the northern part of the basin, where it is exploited in open-pit mines. The exploration methodology employed by the Office Chérifien des Phosphates (OCP) group to investigate the GPB is based on direct recognition with boreholes drilled on a 500 × 500 m grid. This research is concerned with the compilation and analysis of data collected during several drilling campaigns conducted on the central segment of the GPB, namely the Louta deposit. This research also includes acquiring, processing, and interpreting new geophysical and hydrogeological data. Its main objective is to provide a better understanding of the deep structure of the phosphatic series. Therefore, the present study was carried out according to a multidisciplinary approach that comprises three parts. (i) The first one involves geological modeling of the exploration borehole data using Datamine Studio RM software (version 1.4), developed by Datamine Corporate Ltd., (Bristol, United Kingdom). It results in establishing a series of geological cross-sections that display a detailed view of the deep structure of the phosphatic series and its lateral variations. (ii) The second part is related to the hydrogeological study, whose purpose was to elaborate on an accurate and updated piezometric map of the study area. The new map helps understand the groundwater flow in the Louta deposit. Furthermore, the superimposition of the piezometric level with the geological sections throws light on the flooded volume of phosphate in this deposit. (iii) The third part of the study focuses on implementing the Electrical Resistivity Tomography (ERT) method. The interpretation of the recorded geoelectrical data not only highlights the main features controlling the mode and the proportion of the phosphate series deepening under the Plio-Quaternary cover but also confirms the evolution of the overall structure of the studied area. The obtained 2D ERT models generally corroborate the cross-sections produced by geological modeling. They also correlate with the information provided by the hydrogeological study. Such information will help guide future hydrogeological and mining extraction planning in the studied area.

Tracing past extreme floods on an alluvial fan using geophysical surveying

SummarySedimentary units in alluvial fans may record gradual transport and deposition during multiple floods or sediment‐laden flows or, conversely, during few catastrophic events. While outcrops are a valuable source of direct information to constrain past geomorphic and hydrologic processes, such exposures are scarce, especially along aggrading rivers or those that have been subject to recent catastrophic sedimentation. In this context, near‐surface geophysical techniques can constrain the dimensions, internal architecture, composition, and petrophysical properties of different sedimentary units. We consider the Grimmbach alluvial fan in the cuesta landscape of southwestern Germany, which was heavily impacted by sediment and wood loads during a flash flood in 2016; published radiocarbon dates indicate that at least three floods similar to the one in 2016 may have occurred since the 17th century. To test whether and to which detail near‐surface geophysics might reveal the sedimentary legacy of these floods, we survey the Grimmbach alluvial fan using detailed topographic data and geophysical imaging based on electromagnetic induction, electrical resistivity tomography (ERT), and ground‐penetrating radar. Our geophysical results indicate former channel courses and two coarse bar deposits up to 3 m below the surface, which are comparable with the more extensive bar deposits of the 2016 flood. From the ERT models, we interpret coarse, up to 5 m thick, gravel lag overlying bedrock at a maximum depth of 10 m. Our geophysical results also highlight patches of finer materials derived from gradual sedimentation and soil development. Overall, our results show that the Grimmbach alluvial fan may have formed and reshaped during catastrophic flows, which likely caused channel avulsions. Our findings point to the need to reconsider flash flood and debris‐flow hazards in similar headwaters and fans of this seemingly quiescent cuesta landscape in southern Germany.

Groundwater Investigation through Electrical Resistivity Tomography in the Galhareri District, Galgaduud Region, Somalia: Insights into Hydrogeological Properties

Electrical resistivity tomography (ERT) was conducted to delineate groundwater potential zones in villages located in the Galhareri district of the Galgaduud region, central Somalia. A total of four ERT profiles were examined using the gradient configuration, chosen for its practical advantages over other configurations. The study revealed that all profiles were situated within similar geological environments, characterized by comparable rock types. However, notable disparities were observed in lithological variations, particularly in the texture of rocks encountered at different locations and in the thicknesses of the encountered geo-electric layers. The two-dimensional inversion results derived from the electrical resistivity data unveiled the presence of four geo-electrical layers. The first layer was interpreted as sand dunes. The second layer exhibited relatively higher resistivity values, indicating the presence of compact limestone and sandstone. The resistivity of the third layer suggested the existence of a lower resistivity layer, interpreted as weathered limestone, while the fourth layer demonstrated very low inverted resistivity, interpreted as sandy clay with sandstone. The ERT models constructed for the survey area effectively delineated the aquifer zone, represented by layer 3, which likely consists of weathered limestone, sandy clay, and sandstone. The resistivity values obtained for the aquifer zone, specifically at depths ranging from 200 to 300 m, were relatively low, suggesting that the groundwater quality is brackish in nature.

Analysis of soil electrical resistivity and hydraulic conductivity relationship for characterisation of lithology inducing slope instability in residual soil

Slope instability occurrences as damaging shallow-landslides in the residual soil around mountains has been widely studied with geophysical, geotechnical and hydrogeological techniques but relating soil electrical resistivity to hydraulic conductivity for characterisation of lithology inducing of these landslides is not common. In this study, we used Electrical Resistivity Tomography (ERT) data and Hydraulic Conductivity (HC) data obtained from soil samples collected within 1–4.5 m depth in the borehole to assess the characteristics of soil that can induce landslide in the study location. The HC data were derived empirically from Beyer, Kozeny-Carman and Slitcher formula which were validated with HC obtained from laboratory experiment. The Empirical Derived Hydraulic Conductivities (EDHC) were correlated with the soil resistivity. The result shows a strong correlation between soil resistivity and HC with regression values of R2 = 0.9702, R2 = 0.9153 and R2 = 0.9232 for Beyer, Kozeny-Carman and Slitcher formula, respectively. The ERT model revealed a possible sliding surface between two contrasting resistive top material and underneath conductive materials at about 4 m depth. The HC assessment result corroborated the ERT model result because high and low-HC values were obtained in the borehole soil samples within 0–4 m and > 4 m depths from EDHC, respectively. The low-HC zone below 4 m depth was responsible for the occurrences of the shallow-landslides in the study.

Noninvasive Monitoring of Subsurface Soil Conditions to Evaluate the Efficacy of Mole Drain in Heavy Clay Soils

Soil degradation and low productivity are among the major agricultural problems facing farmers of the newly reclaimed agricultural area in the Nile Delta region, Egypt. High content of clay and silt characterizes the soil texture of all farms in the area, while farmers still rely on the traditional mole drainage (MD) system to reduce the salinity of the farm soil. We present a comparison of innovative geo-resistivity methods to evaluate mole drains and the salinity affected clay soils. Geoelectrical surveys were conducted on three newly reclaimed farms to image the subsurface soil drainage conditions and to evaluate the efficiency of using the traditional MD systems in these heavy clay environments. The surveys included measuring the natural spontaneous potential (SP), apparent resistivity gradient (RG), and electrical resistivity tomography (ERT). Integrating the results of the three methods reduced the ambiguous interpretation of the inverted ERT models and allowed us to determine the subsurface soil structure. The inverted ERT models were suitable for locating the buried MDs and delineating the upper surface of the undisturbed clay beds. The proximity of these layers to the topsoil reduces the role played by MDs in draining the soil in the first farm and prevents the growth of deep-rooted plants in the second farm. Time-lapse ERT measurements on the third farm revealed a defect in its drainage network where the slope of the clay beds opposes the main direction of the MDs. That has completely obstructed the drainage system of the farm and caused waterlogging. The presented geo-resistivity methods show that integrated models can be used to improve the assessment of in situ sub-surface drainage in clay-rich soils.

Mass-balance threshold matching of geoelectric and seismic data: a case study from Ketzin

Abstract Seismic and geoelectric methods are used as complementary tools for the identification of fluid/gas effects in underground storage and production scenarios. Both methods present a different resolution. Seismic data tend to be acquired by much more dense geometrical layouts, and geoelectric acquisition as a potential field method shows information integrated over spatial distances. We present results matching seismic and electrical resistivity tomography (ERT) results from two repeat surveys performed during CO 2 storage operations at the Ketzin pilot site in Germany. The approach is based on data acquired in 2009 and 2012 at different stages of total injected CO 2 volume. Masses are obtained from the averaged change in acoustic impedance and are compared to masses inferred from inverted electrical resistivities. Archie's equation is used to obtain saturations from the ERT models. The sensitivity of the background resistivity, R 0 , as a relevant parameter in determining the mass of injected CO 2 is analysed. A specific threshold analysis based on seismic thickness by bin has been applied to investigate the matching of seismically and ERT obtained masses. Both mass estimates match comparatively well, but show inherent sensitive parameters and their corresponding impact as temporal and spatial uncertainties that may cause mass differences for the areas of investigation.

Identification of potential artificial groundwater recharge sites in an alluvial setting: A coupled electrical resistivity tomography and sediment characterization study

Alluvial aquifers are one of the most exploited and easily accessible aquifer networks across the world, as they coincide with regions of intensive human settlements. Over the years, improper management and overexploitation of groundwater has severely stressed the natural replenishing capacity of these aquifer systems. Identification of potential artificial recharge networks and their monitoring are urgently needed for long-term sustainable groundwater management. In this study, an efficient and cost-effective strategy to locate prospective groundwater recharge zones is devised by integrating electrical resistivity tomography (ERT), sediment characterization, and temporal monitoring techniques. The efficacy of the above strategy is evaluated in the central part of Indo-Gangetic basin (IGB). Correlation of 2-D ERT models with borehole lithology and depth-wise variation of hydraulic parameters deciphered the existence of prospective regions composed of highly permeable strata within ∼6–48 m depth. In addition, the behavior of the delineated prospective regions was monitored by applying time-lapse ERT study. The comparison of time-lapse 2-D ERT models highlighted a significant reduction in post-monsoon resistivity profiles, suggesting an increase in saturation levels due to infiltration of rainwater. This study provides an integrated cost-effective approach for identification of prospective artificial recharge sites and monitoring their efficacy in replenishing shallow aquifers, thereby enabling sustainable groundwater management.

Towards understanding the extent of saltwater incursion into the coastal aquifers of Akwa Ibom State, Southern Nigeria using 2D ERT

The hydro-lithostratigraphic units of the coastal area of Akwa Ibom State, Southern Nigeria were investigated involving electrical resistivity tomography (ERT) and Wenner array. This procedure and electrode configuration were adopted because of their wide range of sensitivity and inherent characteristics to geological materials. This study is aimed at defining the extent of saltwater invasions into the coastal and inland aquifers. The investigated area is divided into three zones A (≤0 < 20 km), B (<20 < 40 km), and C (>40 km)) based on the distance from the survey locations to the shoreline. The saline aquifer of the control tomogram obtained at the shoreline (Ibeno Beach) is characterized by the electrical resistivity of 0.159–3.55 Ωm. Within Zone A, (Ukpenekang, Eastern Obolo, and Okoro Ette), the saline aquifer is defined by electrical resistivity values of 0.33–44.0 Ωm, 2.07–48.5 Ωm, and 8.0–60.0 Ωm respectively. In general, the Zone B saline aquifers are defined by electrical resistivity values of 0.602–64.5 Ωm. Furthermore, the third zone which is situated a distance >40 km from the shoreline, is described by relatively higher electrical resistivity values (38.9–3260 Ωm) and indicates a region free from saltwater incursions. Generally, the 2D ERT models showed that electrically resistivity values increase progressively inland.

Subsurface imaging of fluvial deposits of the Vistula River valley in Kraków (southern Poland) by 2D ERT survey

We describe the application of 2D ERT (electrical resistivity tomography) surveys to investigate the spatial complexity of fluvial deposits of the Wisła River valley in the eastern part of Kraków (southern Poland). All ERT survey lines were completed within the industrially influenced floodplain of the Wisła River at two research sites. Due to the transformation of the natural state of the environment through many years of industrial activity of the ArcelorMittal Kraków plant, some of the geomorphological elements analysed have been irretrievably transformed and hidden by anthropogenic accumulations such as waste landfills and engineering structures. Hence, many years of soil contamination have changed the primary resistivity characteristics of the subsurface. For this purpose, the measurement array applied combines standard arrays, i.e., Wenner-Schlumberger and Dipole-Dipole, which gave improved results (higher resolution) in comparison to the standard single array. The data interpretation method was supported by the calculation and visualization of the vertical and horizontal gradients of the interpreted resistivity within the resistivity sections. This approach allowed accurate determination of resistivity boundaries on the ERT resistivity sections and thus helped lithological interpretation of the fluvial deposits in the research area. The resistivity of water in a channel located within one of the analysed areas has impacted some of the research results. Furthermore, 2D ERT forward modeling was implemented to generate synthetic datasets. The synthetic data allowed investigation of the influence of groundwater contamination on the resistivity distribution within superficial layers, and also tested the ability of the 2D ERT model to recognize the detailed spatial distribution of palaeomeander (meander scar) infills. All methods have provided new information on the industrially influenced floodplain of the Wisła River in Kraków.

Electrical resistivity tomography for characterization of carbon-rich rocks (shungites)

Electrical resistivity tomography (ERT) is rightly considered an effective tool for studying subsurface, but some particular geological conditions can lead to results that are difficult to explain. Carbon-rich rocks undoubtedly belong to such cases because of their very high resistivity contrast compared to the host rock. Shungite-bearing rocks have total organic carbon (TOC) in the range of 5 to 95 weight percentage (wt%), and such a range accounts for a wide variety of electrical rock properties. We used ERT at three shungite deposits to study their internal structure and to evaluate the reliability of the resulting geoelectrical models. The study of carbon-rich rocks (shungites) of the Zaonega Formation in the Onega basin is interesting in the Precambrian carbon accumulation and the exploration of shungite deposits. As a result, we clarified the main features of geological sections, such as topsoil base, primary shungite-rich successions, individual zones of metasomatic rocks, and gabbro intrusions. Drilling data and geological reference sections show the reliability of the ERT cross-sections and accuracy of main geological boundaries with an average discrepancy of 1–2 m. Laboratory studies of drill cores samples revealed the nonlinear relationship between resistivity and TOC, which, in the future, may facilitate identifying rocks with relatively low carbon. However, it was impossible to quantitatively assess TOC by ERT models due to the resolution-dependent correlation loss. Comparison of synthetic model with natural field one shows that the significant considerable contrast of resistivity for geological layers about 1:1000 ratio and more produces lower sensitivity and higher uncertainty parts of the inverse models. At the same time, despite high RMS errors, all obtained models retain their geological validity. Our research results demonstrate the efficiency of ERT for mineral exploration of shungites and reveal its internal limitations when the survey these specific highly conductive carbon-rich rocks.

Application of petrophysical relationships to electrical resistivity models for assessing the stability of a landslide in British Columbia, Canada

Landslides in the Thompson River Valley, British Columbia, Canada, threaten the serviceability of two railway lines that connect Vancouver to the rest of Canada and the US. To minimise the impact of slope instability on vital transport infrastructure, as well as on terrestrial and aquatic ecosystems, public safety, communities, local heritage, and the economy, and to better inform decision making, there is a need for monitoring. Since 2013, the Ripley Landslide – a small, slow-moving, translational landslide – has been the focus of monitoring efforts in the Thompson River Valley transportation corridor. In November 2017, a novel Electrical Resistivity Tomography (ERT) monitoring system was installed on the site, providing near-real-time data collection via a telemetric link. 4-Dimensional resistivity models are presented in the context of moisture content and soil suction, two parameters known to influence slope stability in the Thompson River Valley. Here, we discuss the development of laboratory-based petrophysical relationships that relate electrical resistivity to moisture content and soil suction directly, building on relationships developed in the field. The 4-D ERT models were calibrated using these petrophysical relationships to provide insights into the complex spatial and temporal variations in moisture content and soil suction. This study highlights the utility of geoelectrical monitoring for assessing slope stability in the context of moisture-driven landslides.

МЕТОДИКА ГЕОФІЗИЧНОГО КАРТУВАННЯ ТА МОНІТОРИНГУ МІСЦЬ ЗАХОРОНЕННЯ РАДІОАКТИВНИХ ВІДХОДІВ

Sites of radioactive waste disposal in Chernobyl exclusion zone still contain significant amounts of fissile materials. At the same time, the information on the amount of radioactive material buried during the accident cannot be called satisfactory. Within the strengthening international safety and environmental requirements, special activities were initiated to survey the radioactive waste disposals created during the liquidation of the accident at the Chernobyl nuclear power plant. The work involved the following combination of non-invasive geophysical methods: 1) high-resolution magnetometry; 2) ground penetrating radar (GPR) 3) direct current electrical resistivity tomography (ERT). Magnetometry provided the detailed map of facilities, and highlighted the sites where containers with fissile substances had been buried. ERT models revealed variations in the resistivity across the disposal site, which is associated with different water content, and may indicate flooding. Ground water level was determined from GPR mesurements. The combined interpretation of geophysical results served a basis for designing the geometric model of the "Chernobyl 3rd stage" radioactive waste disposal facility. It was used to determine the places for subsequent applying direct radioactive methods for investigation and monitoring of this disposal site.

Performance Evaluation of Copper and Stainless-steel Electrodes in Electrical Tomographic Imaging

Geophysicists use electrical methods to investigate and characterise the earth’s subsurface geology. This study aims to evaluate the performance of copper and conventional stainless-steel electrodes in subsurface tomographic investigations using electrical resistivity tomography (ERT) and induced polarisation (IP) at two sites in Penang, Malaysia. Site 1 and Site 2 employed profile lengths of 200 m and 100 m, with electrodes spacing of 5.0 m and 2.5 m, respectively. In the results of the final data inversion, it was observed that the ERT and IP tomographic models of Site 1 have the best convergence limits with percentage relative differences (copper as reference model) ranging from –70% to 70%, while Site 2 recorded –8% to 8%. The electrodes performance evaluation showed that population root mean square (RMS) error and population mean absolute percentage error (MAPE) of data points between copper and stainless-steel electrodes yielded large values for Site 1 with values above 28% and that of Site 2 was less than 4%. Hence, copper (good electrical conductivity and non-polarisable) electrodes have improved the quality and quantity of infield data which give low values of population RMS error and population MAPE compared to conventional stainless-steel electrodes, especially for large unit electrode spacing surveys. Most notably, this work has contributed to the understanding of the capability of copper electrodes in providing precise and reliable inversion models for subsurface tomographic investigations in pre- and post-land uses (engineering work), hydrogeology/groundwater, environmental studies, etc.

Investigating the effective resistivity of reinforced concrete waste storage tanks at the Hanford Site

Industrialized sites pose challenges for conducting electrical resistivity geophysical surveys because the sites typically contain metallic infrastructure that can mask electrolytic-based soil and groundwater contamination. The Hanford Nuclear Site in eastern Washington State, USA, is an industrialized site with underground storage tanks, piping networks, steel fencing, and other potentially interfering infrastructure that could inhibit the effectiveness of electrical resistivity tomography (ERT) to map historical and monitor current waste releases. The underground storage tanks are the largest contributor by volume to subsurface infrastructure and can be classified as reinforced concrete structures with an internal steel liner. Directly measuring the effective value for the electrical resistivity of the tanks, that is, the combination of individual components that comprise the tank’s shell, is not reasonably possible because they are buried and are dangerously radioactive. Therefore, we indirectly assess the general resistivity of the tanks and the surrounding infrastructure by developing synthetic ERT models with a parametric forward-modeling study using a wide range of resistivity values from [Formula: see text] to [Formula: see text], which are equivalent to steel and dry rock, respectively. The synthetic models use the long-electrode ERT (LE-ERT) method, whereby steel-cased metallic wells surrounding the tanks are used as electrodes. The patterns and values of the synthetic tomographic models are then compared with LE-ERT field data from the AX tank farm at the Hanford Site. This indirect method of assessing the effective resistivity reveals that the reinforced concrete tanks are electrically resistive and the accompanying piping infrastructure has little influence on the overall resistivity distribution when using electrically based geophysical methods for characterizing or monitoring waste releases. Our findings are consistent with nondestructive testing literature that also indicates reinforced concrete to be generally resistive.

Joint geophysical prospecting for groundwater exploration in weathered terrains of South Guangdong, China.

Groundwater occurrence in a hard rock terrain is strongly controlled by the weathered/fractured zones. However, delineation of such zones is a challenging task given their structural heterogeneity. Traditionally, large numbers of well tests are conducted to assess the subsurface formation. But, such tests suffer from efficiency in terms of cost, time, and data coverage. Non-invasive geophysical methods can be the best alternative of expensive drilling methods. However, a geophysical method alone is ambiguous to interpret the highly heterogeneous subsurface formation. In this study, joint application of electrical resistivity tomography (ERT), magnetic method, and joint profile method (JPM) was conducted for groundwater exploitation in a weathered terrain of South Guangdong, China. ERT, magnetic, and JPM data were acquired along different geophysical profiles via a variety of survey parameters. The interpreted 2D models of electrical resistivity and magnetic data coupled with the local accessible boreholes and hydrogeological information constrain the subsurface geologic formation into four discrete layers with specific electrical resistivity range, i.e., topsoil cover, highly weathered saturated layer, semi-weathered saturated layer, and un-weathered substratum. Incorporation of JPM (ER, SP, and IP methods) with ERT and magnetic models reveal three faults (F1, F2, and F3) and several saturated intense fractures/discontinuities. The groundwater reserves associated with the weathered/fractured rock were estimated via hydraulic parameters, namely hydraulic conductivity and transmissivity. The results suggest that high-yield groundwater resources are found within the weathered/fractured zones. Geophysical results of this joint application fit pretty well to the local hydrogeological data of the study area. Our novel approach reduces any ambiguity caused in the geophysical interpretation and provides clearer insight of the subsurface formation with more confident solutions to the most challenging problems of the hard rock sites. This hydrogeophysical study provides important contributions to groundwater exploration in areas where weathering has significant effects on the hard rock aquifer system. Compared with traditional methods, this approach is more advantageous for assessment of groundwater resources in hard rock terrains.

2D and 3D ground model development for mountainous landslide investigation

The 2D and 3D ground models were developed on mountainous landslide investigation. The ground model in the landslide area provided information on the relationship between the landslide structure, characteristics of soil, rock, and groundwater. This study presents an integrative approach in 2D and 3D ground modeling on landslide events in mountainous areas. The methods used geophysical investigations (Electrical Resistivity Tomography (ERT)) to develop 2D and 3D resistivity distributions, and geotechnical investigations (soil sampling, laboratory testing, and borehole testing) to develop the relationship between resistivity and soil properties, as well as to validate 2D and 3D ERT models. The results showed that low resistivity (<350 ohm.m) refers to well-graded soils and water-saturated materials; medium resistivity (350–722 ohm.m) refers to granular soils, weathered rock, and shale; and high resistivity (>722 ohm.m) refers to bedrock. The 2D and 3D resistivity distribution is able to explain the relationship between resistivity and the characteristics of soil, rock, and groundwater in the mountainous landslide area.