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Accelerated climate warming is causing significant reductions in the volume of Arctic glaciers, such that previously ice‐capped bare ground is uncovered, harboring soil development. Monitoring the thermal and hydrologic characteristics of soils, which strongly affect microbial activity, is important to understand the evolution of emerging terrestrial landscapes. We instrumented two sites on the forefield of a retreating Svalbard glacier, representing sediment ages of approximately 5 and 60 years since exposure. Our instrumentation included an ERT array complemented by adjacent point sensor measurements of subsurface temperature and water content. Sediments were sampled at each location and at two more additional sites (120 and 2000 years old) along a chronosequence aligned with the direction of glacial retreat. Analysis suggests older sediments have a lower bulk density and contain fewer large minerals, which we interpret to be indicative of sediment reworking over time. Two months of monitoring data recorded during summer 2021 indicate that the 60‐year‐old sediments are stratified showing more spatially consistent changes in electrical resistivity, whereas the younger sediments show a more irregular structure, with consequences on heat and moisture conductibility. Furthermore, our sensors reveal that young sediments have a higher moisture content, but a lower moisture content variability. [ABSTRACT FROM AUTHOR]

Subsurface formations with low compaction, often, due to the presence of underlying cavities, are potential sources of hazards. Thus, understanding the occurrence, properties, and extension of these weak zones poses a major concern in engineering geophysics. In this study, we examine the ability of geophysical methods to map weak areas over carbonates in Northern Oman. The weak zones are known to cause surface depression in many areas. The geophysical methods examined involve Ground-Penetrating Radar (GPR), Seismic Refraction Tomography (SRT), and Electrical Resistivity Tomography (ERT). This integrated geophysical survey was conducted near the Bimmah sinkhole, in the Quriya area, Northern Oman. The survey covers both an area with ground truth (low compaction sediments overlaying a cave) and a part with unknown subsurface properties. GPR velocity analysis using selected diffraction's fitting helped to identify high-velocity anomalies that were attributed to the cavity. The GPR interpretation was calibrated with SRT and ERT. The former showed a clear drop in P-wave velocity and low ray coverage at the cavity zone, while the latter demonstrated high resistivity anomalies caused by the air filling the cavity. The scope was to examine the geophysical methods response, especially the GPR, and utilize the results of this preliminary approach for a wider exploration investigation in the area. The results from the study indicated that the GPR is capable to serve as a pioneer method in detecting the cavities. Hence, the GPR will cover large area in the site and the other two methods will be used as complementary for the final subsurface conditions' evaluation. [ABSTRACT FROM AUTHOR]

Ground improvement by permeation grouting is widely utilized for improving engineering properties of soil. The accurate evaluation of a region improved by grouting is a critical issue to ensure the performance of grouting; nevertheless, current methods to evaluate the area of grouted bulbs under the ground are not well-documented. This study aims to use cross-hole electrical resistivity tomography (CHERT) to evaluate a grouted bulb region by considering the effect of electrode configuration at laboratory scale using a cylindrical container filled with saturated sand. Curing time monitoring was also conducted on grouted bulbs with various recovered Carbon Black (rCB) concentrations. Based on the inverted result, the predicted area of the grouted bulb was addressed, and then the error value between the actual and predicted areas was examined. The results of this study show that CHERT can be employed to assess the location and area of a grouted bulb. Also, it was observed that electrode spacing, damping factor, and curing time had significant influences on the image resolution and error value. Unlike the above-mentioned factors, change of rCB concentration ratio had only a slight effect on the image resolution. Thus, the use of a small amount of rCB as an additive material to enhance the grout strength can also provide a distinct contrast of measured resistivity between the grouted bulb and surrounding soil. In this study, a grouted bulb containing 3% of rCB is recommended based on the test results on strength characteristics. In addition, CHERT measuring can be conducted even in early stages of curing, during which it exhibits a better electrical resistivity contrast between the objective area and the surrounding soil. [ABSTRACT FROM AUTHOR]

A geoelectrical survey was carried out outside the walls of the ancient Egnazia (Puglia, Italy) with the aim of enriching the knowledge about its defense system. Nine Electrical Resistivity Tomographies (ERTs) were realized using the dipole-dipole (DD) electrode array, approximately transversal to the walls and equally spaced. The new Extended data-adaptive Probability-based Electrical Resistivity Tomography Inversion Method (E-PERTI) was applied, for the first time, to model the resistivity distribution of a large dataset. Considering some peculiar aspects of the general theory of probability, an optimization of results was reached giving major emphasis to one dataset portion rather than another and inspecting selectively vertical or lateral resistivity variations. In this way, sets of aligned resistivity lows attributable to the trace of an ancient ditch were found. [ABSTRACT FROM AUTHOR]

This study explores the hydrogeological conditions of a landslide-prone hillslope in the Upper Mosel valley, Luxembourg. The investigation program included the monitoring of piezometer wells, hydrogeological field tests, analysis of drillcore records, and geophysical surveys. Monitoring and field testing in some of the observation wells indicated very pronounced preferential flow. Electrical resistivity tomography (ERT) and self-potential geophysical methods were employed in the study area for exploration of the morphology of preferential flowpaths. Possible signals associated with flowing groundwater in the subsurface were detected; however, they were diffusively spread over a relatively large zone, which did not allow for the determination of an exact morphology of the conduit. Analysis of drillcore records indicated that flowpaths are caused by the dissolution of thin gypsum interlayers in marls. For better understanding of the site's hydrogeological settings, a 3D hydrogeological model was compiled. By applying different subsurface flow mechanisms, a hydrogeological model with thin, laterally extending flowpaths embedded in a porous media matrix showed the best correspondence with field observations. Simulated groundwater heads in a preferential flow conduit exactly corresponded with the observed heads in the piezometer wells. This study illustrates how hydrogeological monitoring and geophysical surveys in conjunction with the newest hydrogeological models allow for better conceptualization and parametrization of preferential flow. [ABSTRACT FROM AUTHOR]

Subsurface formations with low compaction, often, due to the presence of underlying cavities, are potential sources of hazards. Thus, understanding the occurrence, properties, and extension of these weak zones poses a major concern in engineering geophysics. In this study, we examine the ability of geophysical methods to map weak areas over carbonates in Northern Oman. The weak zones are known to cause surface depression in many areas. The geophysical methods examined involve Ground-Penetrating Radar (GPR), Seismic Refraction Tomography (SRT), and Electrical Resistivity Tomography (ERT). This integrated geophysical survey was conducted near the Bimmah sinkhole, in the Quriya area, Northern Oman. The survey covers both an area with ground truth (low compaction sediments overlaying a cave) and a part with unknown subsurface properties. GPR velocity analysis using selected diffraction’s fitting helped to identify high-velocity anomalies that were attributed to the cavity. The GPR interpretation was calibrated with SRT and ERT. The former showed a clear drop in P-wave velocity and low ray coverage at the cavity zone, while the latter demonstrated high resistivity anomalies caused by the air filling the cavity. The scope was to examine the geophysical methods response, especially the GPR, and utilize the results of this preliminary approach for a wider exploration investigation in the area. The results from the study indicated that the GPR is capable to serve as a pioneer method in detecting the cavities. Hence, the GPR will cover large area in the site and the other two methods will be used as complementary for the final subsurface conditions’ evaluation.

Two-dimensional electrical resistivity tomography (2-D ERT) is one of the most common geophysical tools employed to satisfy the ever-growing need for obtaining subsurface information. Most of the conventional electrode arrays used for 2-D ERT survey are built with the theoretical assumption that the survey lines are straight to guarantee four collinear electrodes at every point of measurement. However, due to surface constraint associated with most survey areas, it is rarely possible to conduct a two-dimensional resistivity survey on a straight line. Therefore, 2-D ERT survey conducted on a surface constraint field requires shifting one or more electrodes off the survey line, which contrasts with the underlying assumption. Consequently, the result might be prone to false anomalies. Thus, this study aimed to device a new approach that could mitigate the false anomalies posed by noncollinearity of electrodes in 2-D ERT result. In view of this, ABEM Terrameter SAS4000 using Wenner array configuration was adopted for the survey. The data was acquired with all electrodes inline and one or more electrodes offline at stepwise distances, respectively. Based on the result obtained, the new approach mitigates the offline electrodes effect, as the inverse resistivity tomograms resolves the geometries of the true model reasonably well. More so, it has high R-value >90% which is an indication of proximity to the true model. Hence, it is concluded that the new approach is effective in mitigating offline electrode effect on a 2-D ERT result. [ABSTRACT FROM AUTHOR]

A geoelectrical survey was carried out outside the walls of the ancient Egnazia (Puglia, Italy) with the aim of enriching the knowledge about its defense system. Nine Electrical Resistivity Tomographies (ERTs) were realized using the dipole-dipole (DD) electrode array, approximately transversal to the walls and equally spaced. The new Extended data-adaptive Probability-based Electrical Resistivity Tomography Inversion Method (E-PERTI) was applied, for the first time, to model the resistivity distribution of a large dataset. Considering some peculiar aspects of the general theory of probability, an optimization of results was reached giving major emphasis to one dataset portion rather than another and inspecting selectively vertical or lateral resistivity variations. In this way, sets of aligned resistivity lows attributable to the trace of an ancient ditch were found.

Sinkholes are natural phenomena caused by the dissolution of limestone rocks due to the infiltration of acidic surface water. Initially, this infiltration creates voids, and then a sudden collapse occurs when the roof structure fails to support the weight load of the overlying deposits. In Algeria, a sinkhole occurred in February 2015 after a severe snowstorm, on an agricultural field near the city of El Ouldja (South-Eastern Setif). The sinkhole occurred only after the dissolution of a quaternary lacustrine limestone. This dissolution created a karstic cavity, covered by an alluvial deposit. These alluvium deposits progressively pulled down from the base to the top through a sediment removal phenomenon at the deep karstic cavity opening. The sinkhole is circular with 16-m diameter and 16-m deep. This phenomenon represents a risk for local populations living with the fear of a worsening and expansion of the existing cavity or the formation of new sinkholes under the same circumstances. This work aims at identifying high-risk zones in the entire site surrounding the karstic collapse phenomenon through geological analysis of the sinkhole walls as well as two geophysical methods, namely, electrical resistivity tomography (ERT) and electromagnetic induction (EMI). Results revealed the existence of developing superficial karst cavities, in the NNW and SSE part, beneath the Quaternary alluvium of the study area, bordered by a fracture network. Such structures are a natural risk that needs to be considered not only for agricultural land management, but also for rural construction. [ABSTRACT FROM AUTHOR]

An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed open-pit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (≤ 2 m thick) and dip between 28° and 32° south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from ~10 to ~150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20–50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400–1200 m/s) and resistivity (150–800 Ωm). The deeper layer reveals an increase in resistivity and velocity, and it's characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area. [ABSTRACT FROM AUTHOR]

The application of resistivity tomography to detection for hidden dangers of embankment can grasp the safety status of embankment and ensure their safe operation. Electrical detection is usually based on the theory of semi-infinite space, but the section of embankment does not satisfy the condition of semi-infinite space. In order to reduce the impact of embankment terrain to the electrical method and improve the detecting precision of resistivity tomography, the paper established a trapezoidal embankment. Through changing the arrays and the distance of electrodes, the paper established a numerical simulation and model test. It is better to choose Schlumberger array in the detection of the hidden dangers. Electrode distance should not be more than 1.0 m to ensure the the detection resolution. [ABSTRACT FROM AUTHOR]

This study explores the hydrogeological conditions of a landslide-prone hillslope in the Upper Mosel valley, Luxembourg. The investigation program included the monitoring of piezometer wells, hydrogeological field tests, analysis of drillcore records, and geophysical surveys. Monitoring and field testing in some of the observation wells indicated very pronounced preferential flow. Electrical resistivity tomography (ERT) and self-potential geophysical methods were employed in the study area for exploration of the morphology of preferential flowpaths. Possible signals associated with flowing groundwater in the subsurface were detected; however, they were diffusively spread over a relatively large zone, which did not allow for the determination of an exact morphology of the conduit. Analysis of drillcore records indicated that flowpaths are caused by the dissolution of thin gypsum interlayers in marls. For better understanding of the site’s hydrogeological settings, a 3D hydrogeological model was compiled. By applying different subsurface flow mechanisms, a hydrogeological model with thin, laterally extending flowpaths embedded in a porous media matrix showed the best correspondence with field observations. Simulated groundwater heads in a preferential flow conduit exactly corresponded with the observed heads in the piezometer wells. This study illustrates how hydrogeological monitoring and geophysical surveys in conjunction with the newest hydrogeological models allow for better conceptualization and parametrization of preferential flow.

We discuss a strategy capable of a quantitative long‐term monitoring of water saturation and volume variation of light non‐aqueous phase liquids in the soil. The goal was reached monitoring a controlled sand cell contaminated with classical gasoline over 124 days, using geophysical methods such as electrical resistivity tomography, induced polarization and ground penetrating radar. We show that empirical relations, linking the water saturation to the physical parameters measured as resistivity from electrical resistivity tomography and travel time from georadar with advanced processing, are good tools for this purpose. The consistence of the proposed process is validated by both good overlap of results carried out from electrical resistivity tomography and georadar and theoretical models simulating the actual experiment. [ABSTRACT FROM AUTHOR]

Highlights • Integrated geophysical case study to delineate geologic features beneath hot spring. • Electrical Resistivity Tomography effectively delineated the lateral extent of fracture zones. • Deciphered shallow alteration/fracture zones that act as conduit for the hot springs. Abstract An integrated geophysical study consisting of magnetic, very low frequency (VLF) electromagnetic and electrical resistivity tomography (ERT) surveys has been carried out in the vicinity of hot springs at Atri and Tarabalo in central Odisha, India. The study aims to delineate near-surface structural settings of these regions (e.g., faults, fracture network etc.) that controls the flow of geothermal fluids. Processed magnetic data shows distinct low anomaly as well as high-low transition zone near the hot springs and high anomaly away from the hot springs of both the regions. VLF current density sections along the selected profiles reveal existence of conductive fractures near to the hot springs of both the areas. Resistivity study in both the areas advocates three layered subsurface configurations: low resistive (<80 Ohm.m) top layer, moderate resistive middle layer, and high resistive basement (>700 Ohm.m). Coincidence of low magnetic anomalies, and low resistivity (or high current density) values at same location indicate the existence of alteration/fracture zones. Resistivity models at Tarabalo depict a low resistive (∼75-100 Ohm.m) dipping faulted/fractured zone extending beyond a depth of 80 m. VLF depth sections and reduced to pole magnetic anomaly maps also reveal high current density and low magnetic signatures, respectively in this zone. This fracture zone is extending about 600 m in EW direction with the marshy area at the center and may have acted as a conduit for the warm water movement from depth to surface. Indication of faulted/fractured zones around the Atri hot spring are also there but the inferences are not as conclusive as that of Tarabalo area due to the nearby power line hindrances. Thus, the study provides a cost effective approach to delineate near surface fracture zones as well as helps to understand the thermal fluid flow mechanism of the hot springs. [ABSTRACT FROM AUTHOR]

The main objective of this study was to assess the impact of the NamSon landfill on subsurface geological structure and hydrological environment by geophysical techniques and hydrochemical analysis of surface and groundwater. The electrical resistivity tomography (ERT), self-potential (SP) and very low frequency (VLF) methods were used for the investigation of geological structure near the landfill. Three profiles (900 m long in total) of the two-dimensional ERT, VLF density sections and 180 SP data points scattered throughout the study area near the disposal site constituted the basis of the data used in analysis. Additionally, surface water and groundwater samples were collected from six sites in the area for the chemical analysis. Interpretations of geophysical data show a low resistivity zone (< 15 Ωm), which appears to be a fully saturated zone with leachate from landfill. The results of the geophysical investigations are not always fully confirmed by the results of hydrochemical analysis. The quality of water in the vicinity of the landfill dramatically decreased over the year (2015-2016) and actions should be taken to inverse this negative trend. [ABSTRACT FROM AUTHOR]

Self potential (SP) and electrical resistivity tomography (ERT) methods are used together with the results of groundwater samples hydrogeochemical analysis to assess the impact of the water leak from the landfill garbage site at NamSon located in Northern Hanoi on causing pollution to the surrounding environment and affecting geological structure. Selected survey area covers an area of 180 × 300 m lying in the low land of the NamSon site with a slope ranging about 8 m in direction NW-SE. There are three geophysical measurements lines denoted as T1, T2 and T3. Processing 180 SP data points has allowed to draw maps of equipotential epoch in the two periods in 2015 and 2016. The maps show four zones of SP positive anomalies with maximum amplitudes of about +20 mV where the groundwater flow direction is downward and five zones of SP negative anomalies with minimum values in a range from −180 to −260 mV where the groundwater flow direction is upward. Resistivity values of the subsurface layers of soils and rocks have been aquired from 2D inverse model for measuring ERT in March 2015 and March 2016. The results of the ERT allowed to define the low resistivity in the range 15-20 Ωm related to leachate plume from NamSon landfill site. Results of the physico-chemical analysis of groundwater samples from the existing six boreholes show increases in concentration of the measured pollutant parameters indicating contamination of the groundwater as a result of solid waste leachate accumulation. This result is affirmative evidence for the survey results by geophysical technique. The rapid decrease in quality of groundwater over the last year is probably due to the influence of the leachate from the NamSon landfill site. [ABSTRACT FROM AUTHOR]

ABSTRACT In this study, a new two-dimensional inversion algorithm was developed for the inversion of cross-hole direct current resistivity measurements. In the last decades, various array optimisation methods were suggested for resistivity tomography. However, researchers have still collected data by using classical electrode arrays in most cross-hole applications. Therefore, we investigated the accuracy of both the individual and the joint inversion of the classical cross-hole arrays by using both synthetic and field data with the developed algorithm. We showed that the joint inversion of bipole-bipole, pole-bipole, bipole-pole, and pole-tripole electrode arrays gives inverse solutions that are closer to the real model than the individual inversions of the electrode array datasets for the synthetic data inversion. The model resolution matrix of the suggested arrays was used to analyse the inversion results. This model resolution analysis also showed the advantage of the joint inversion of bipole-bipole, pole-bipole, bipole-pole, and pole-tripole arrays. We also used sensitivity sections from each of the arrays and their superpositions to explain why joint inversion gives better resolution than the any individual inversion result. [ABSTRACT FROM AUTHOR]

Deterioration of freshwater aquifers adjacent to saline lakes is a serious environmental and water resources management problem. Generally, different phenomena including saltwater intrusion, upconing, free and forced convection flow can lead to the salinization of fresh aquifers in the vicinity of saline lakes. The severity of this problem depends on hydrogeological and hydrogeochemical conditions of coastal saline lake aquifers. Accordingly, Maharlu Lake in the southeast of Shiraz plain is one of these saline lakes and has an important impact on its adjacent fresh aquifers so that the salinity of these aquifers has increased in recent years. Since the lake has special conditions such as very flat and low permeable floor and variable water level fluctuation during a year, with the onset of the rainy season after a long-term well pumping in the adjacent aquifer, the lake water covers a wide extent of the lake area in a short time. Then, because of the higher permeability of the surface of the lake in its southeast margin, conditions for convection flow are provided there. In this study, salinity distribution in one of the aquifers (Bokat aquifer) in the southeast of the lake is investigated using resistivity tomography along several profiles. Interpretation of inverse models of these profiles showed that convection flow is an important process in increasing the salinity of fresh aquifer. Comparison of the results of geo-tomography sections with convection index map of the Bokat aquifer showed that getting closer to the lake, free convection flow becomes the dominant process in the salinity distribution. In addition, in farther distances relative to the lake, due to increasing the pumping rate of the extraction wells, forced convection flow is an important process. [ABSTRACT FROM AUTHOR]

Tailings dams are traditionally monitored by routine visual inspections combined with the use of instruments such as piezometers, topographical landmarks, and water level indicators. As they are structures that have high potential damage associated, the use of robust investigation methods is essential to ensure the operational safety of these dams. Geoelectrical methods have been successful in mapping humid zones inside the tailings dam structures, using the proper acquisition parameters and data sampling that these methods allow. This is a promising approach to investigating a tailings dam with a heterogeneous embankment made of limestone tailings, that has been in operation for 50 years in São Paulo state, Brazil. Considering its structural complexity, electrical resistivity sections were acquired with a tight profile spacing – of 10 to 15 m, processed, and modeled by applying a specific workflow. The Electrical Resistivity Tomography (ERT) operational workflow applied to the dataset obtained from two field campaigns, conducted in the embankment and the right abutment of the dam, performed well in mapping possible high water content zones inside the structure, defining a relevant element for the risk assessment. For this purpose, compartmentalization of the resistivity ranges, and three-dimensional modeling techniques were used, both constrained by the geotechnical instruments of the structure. The embankment campaign mapped zones of attention in the right abutment driving the decision to perform a follow-up survey dedicated to this region. The right abutment campaign mapped zones of high resistivity, associated with the natural terrain, composed of sandy soils, saprolite, and rock, as well as zones of low resistivity related to high moisture content regions. The geometry of these conductive zones made it possible to infer the depth of the water table in the region between the direct measurements from the instrumentation and to model zones with low resistivity values. The 3D modeling revealed that part of the conductive zones, present in the right abutment and extending to the embankment indicate a possible contribution of moisture from the natural terrain towards the embankment of the dam. Thus, the achieved results demonstrated that 3D modeling applied to the combination of the geotechnical and geophysical datasets contributes to expanding the knowledge of the internal structure of the dam and consequently the safety of the operations, especially where there is no instrumentation, improving the tailings dam risk management. • Three-dimensional model, correlating geoelectric and geotechnical information • Numerical model for modeling high moisture zones • Volumetric investigation in correlation with located geotechnical data [ABSTRACT FROM AUTHOR]

It is well known that borehole fluid effects in crosshole electrical resistivity tomography (ERT) can produce serious image artifacts using conventional inversion algorithms that do not account for borehole effects. Using the analytical solution for an infinitely long borehole model in a half-space, we demonstrated that the borehole effects depend on the borehole radius and on the resistivity contrast between the borehole fluid and the surrounding rock formation. The results also show that the borehole effects arise mainly for the inline data where the current and potential electrodes are placed at the same borehole, and they are not severe for the crosshole data. To overcome the borehole effects in the crosshole ERT surveys, we developed a new 2.5D inversion algorithm incorporating the boreholes. The new inversion algorithm yielded an electrical resistivity image that was devoid of artifacts around the boreholes. [ABSTRACT FROM AUTHOR]

Geophysical prospecting is commonly used in solving environmental problems because of time saving, low cost, and nondestructive. The main objective of this study was to investigate the environmental impact assessment of groundwater contamination near El-Dair waste disposal site, west of Sohag city, Egypt to delineate a contaminant plume. The resistivity and self-potential (SP) methods can be helpful complement to geochemical measurements. The correlation between results of 2D electric resistivity tomography and SP method was applied and confirmed by hydro-chemical analysis of groundwater. The probable leachate plume is prolonged to out of disposal sites boundaries, and in general, those plumes have the same directions of the groundwater flow. Three profiles (3 km long) of two-dimensional resistivity tomography and 725 SP data points scattered within the area near and far the disposal site were implemented. Groundwater samples were collected from 13 tubewells at various locations for hydro-chemical analysis. Interpretations of resistivity tomography show a low resistivity zone (<7 Ω m), which appears to be a fully saturated zone with leachate from an open dumpsite. The results of SP show a large negative SP anomaly (−120 mV) with respect to a reference station taken outside the contaminant plume. SP method supported the results of resistivity method relating contaminated zones in the survey area. There is a good correlation between the geophysical investigations and the results of hydro-chemical analysis. The results of hydro-chemical analysis proved that there are concentrations of heavy metals within the groundwater which were percolated from wastewater disposal. [ABSTRACT FROM AUTHOR]

An indigenous apparatus was devised for measuring the true resistivity of core samples in laboratory or at the drilling site and tested for its efficacy. The true resistivities of selected core samples from a shallow borehole of Brahmaputra alluvium were measured using an in-situ DC resistivity meter and compared with corresponding High-resolution Electrical Resistivity Tomography (HERT) cross section acquired across the borehole site. We find a very good match between the direct measurements and inverse model data of HERT within the limits of resolution. Thus, the present work proves the validity and utility of the resistivity survey in identifying the nature of the sub-surface material and its stratification by measuring the core sample resistivity with the help of in-situ DC resistivity meter. [ABSTRACT FROM AUTHOR]

Indiscriminate use of agro-chemicals for intensive agriculture is mainly responsible for deterioration of groundwater quality in west Delhi of India. Geophysical and hydrochemical investigations were carried out to decipher subsurface geologic formation and assess groundwater salinity. Electrical resistivity tomography surveys revealed the presence of potential groundwater zones at 20-30 m below ground level. The groundwater quality was moderate to poor as evidenced from the low resistivity value of the potential aquifer. Only 20.2 % of the study area showed good quality groundwater potentiality, situated at few pockets in eastern, western, and northern parts of the study area. Low resistivity formation at southern part of the study area was due to infiltration of highly polluted drain water along with fertilizer rich surface water to the aquifer. Hydrochemical investigation also confirmed the result. The average electrical conductivity of groundwater was around 5,950 μS cm. Longitudinal unit conductance and transverse unit resistance values were calculated and used for delineation of saline groundwater zones. A direct relationship between longitudinal unit conductance and groundwater salinity made it possible to develop empirical equations by which salinity can be predicted from computed longitudinal unit conductance value. It helps for periodic monitoring of the pollution level by spending less labour and money and benefits immensely the policy makers. The article suggests further study and research work that can lead to sustainable exploitation/use and management of groundwater resources in west Delhi. [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) soundings performed in the past in the volcanic area of Mt. Vesuvius by two independent research groups showed in the same places MT apparent resistivity curves with very similar shape, but statically shifted by one order of magnitude, at least. To try to resolve this ambiguity new controlled source audio-magnetotelluric (CSAMT) measurements have been carried out in the same MT sites. The interpretation of the CSAMT dataset, combined with that of two shallow dipole-dipole geoelectrical resistivity tomographies previously carried out in the area have allowed a reliable electrical structure to be recovered down to a few km of depth, which will next be used for a best constrained re-interpretation of the deep MT soundings.

This paper presents the results of combined geophysical and morphostructural research of a significant tectonic lineament forming the boundary between the core Bohemian Forest (Šumava) Mts. and its foothills of Pošumaví. The exact course, length and character of the fault have not yet been studied in detail despite its possible role in the uplift of the mountain range. To assess the fault course, length and continuity, we have employed a combination of geophysical, morphological and morphostructural methods. These indirect methods had to be applied as the fault only rarely outcrops along its course, and the morphological border is not straightforward. In the beginning, GIS morphometric methods have been applied to assess the influence of the fault on the present relief. Thereafter, structural measurements of joint systems were undertaken together with the analyses of linear structures within the relief. Finally, resistivity profiling at multiple sites across its estimated course has helped to localise the exact position of the fault. Altogether, fifteen profiles were measured using pole-dipole and dipole-dipole electrode configurations. To obtain more detailed results, the resistivity profiling was supplemented by electrical resistivity tomography on three profiles. The paper brings two main results. Firstly, the combination of morphostructural and geophysical methods brings information that each separately cannot, particularly when the faults have no outcrops. Secondly, it was found that the studied fault stretches along the whole study area. Moreover, indicators point to its possible continuation towards the south-east. [ABSTRACT FROM AUTHOR]

Sinkhole collapse is one of the main limitations in the development of karst areas, especially where bedrock is covered by unconsolidated material. Studies of sinkhole formation have shown that sinkholes are likely to develop in cutter (enlarged joint) zones as a result of subterranean erosion by flowing groundwater. Ground-penetrating radar (GPR) and electrical resistivity imaging or tomography (RESTOM) are well suited to mapping sinkholes because of the ability of these two techniques for detecting voids and discriminating subtle resistivity variations. Nine GPR profiles and two-dimensional electrical resistivity tomography have been applied, with relative success, to locate paleo-collapses and cavities, and to detect and characterize karst at two sinkhole sites near Cheria City where limestone is covered by about 10 m of clayey soils. The survey results suggest that GPR and RESTOM are ideal geophysical tools to aid in the detection and monitoring of sinkholes and other subsurface cavities. [ABSTRACT FROM AUTHOR]

Abstract: The assessment of hydraulic conductivity of heterogeneous aquifers is a difficult task using traditional hydrogeological methods (e.g., steady state or transient pumping tests) due to their low spatial resolution. Geophysical measurements performed at the ground surface and in boreholes provide additional information for increasing the resolution and accuracy of the inverted hydraulic conductivity field. We used a stochastic joint inversion of Direct Current (DC) resistivity and self-potential (SP) data plus in situ measurement of the salinity in a downstream well during a synthetic salt tracer experiment to reconstruct the hydraulic conductivity field between two wells. The pilot point parameterization was used to avoid over-parameterization of the inverse problem. Bounds on the model parameters were used to promote a consistent Markov chain Monte Carlo sampling of the model parameters. To evaluate the effectiveness of the joint inversion process, we compared eight cases in which the geophysical data are coupled or not to the in situ sampling of the salinity to map the hydraulic conductivity. We first tested the effectiveness of the inversion of each type of data alone (concentration sampling, self-potential, and DC resistivity), and then we combined the data two by two. We finally combined all the data together to show the value of each type of geophysical data in the joint inversion process because of their different sensitivity map. We also investigated a case in which the data were contaminated with noise and the variogram unknown and inverted stochastically. The results of the inversion revealed that incorporating the self-potential data improves the estimate of hydraulic conductivity field especially when the self-potential data were combined to the salt concentration measurement in the second well or to the time-lapse cross-well electrical resistivity data. Various tests were also performed to quantify the uncertainty in the inverted hydraulic conductivity field. [Copyright &y& Elsevier]

Geodynamic activity in the area of West Bohemia is typified by the occurrence of earthquake swarms, Quaternary volcanism and high flux of mantle-derived CO. The highest swarm activity occurs beneath the eastern edge of the Cheb basin, which is delineated by the NW-SE trending morphologically pronounced Mariánské Lázně Fault (MLF) controlling the formation of the basin. The previous trenching survey across the MLF zone has identified several fault strands with possible Quaternary activity. In this paper we present the results of the geophysical survey focused to trace the faults signatures in geophysical sections and to build an image of near surface tectonics. The method of electric resistivity tomography (ERT) along two profiles parallel to the trench identified a strong resistivity contrast between the bodies of sandy gravels in the middle and conductive clayey sands to the west and weathered crystalline basement to the east. The 2-D ground penetration radar (GPR) sections show direct correlation of reflections with lithological boundaries identified in the trench. As expected, the GPR signal amplitudes increase with the resistivities found in the ERT sections. Two of the four faults identified in the trench are indicated in the resistivity and GPR sections. A 3-D GPR measurement has identified a spot of high amplitudes elongated parallel to the MLF trend, which coincides with the high resistivity body. To improve the signal-to-noise ratio of the time slices we stacked the GPR time slices within vertically homogeneous blocks. This provided a contrast image of the sand-gravel body including its boundaries in three dimensions. The detailed analysis of the 3-D GPR cube revealed additional fault that limits the highly reflective sands and appears to be offset by another younger fault. Our results suggest a complex fault pattern in the studied area, which deserves a further study. [ABSTRACT FROM AUTHOR]

Abstract: Since the beginning of the 20th century, more than 150 sinkhole occurrences, mainly dropout (or cover-collapse) sinkholes, have been reported in the Tournaisis area (south-eastern Belgium). Land-use planning in such a context has to take into account hazards linked with sinkhole subsidence and collapse. Management maps, drawn at a regional scale, point out zones where karstic risks have to be taken into account when dealing with infrastructure or building projects. However, karst hazard is highly variable in three dimensions at the local scale. Therefore, for such purposes, an accurate methodology is needed to detect and delineate covered karst features, especially when located in urbanized areas. As geophysical investigations are sensitive to contrasts in physical properties of soils, these methods can be useful to detect such targets. The specific karstic context encountered in the Tournaisis area strongly guides the choice of investigation techniques. Electrical resistivity imaging (ERI) methods were tested on a well-known site where dropout sinkholes occurred formerly. This site was also studied using static cone penetration tests (CPT) and boreholes. A 3D inverted resistivity model was computed based on the 2D ERI models obtained after inversion. Resistivity profiles were extracted at each CPT location and compared to geotechnical results to determine an empirical and site-specific resistivity law that allows discrimination between weathered zones and sound limestone. Performance tests were conducted to evaluate the potential of the proposed methodology for two typical engineering problems based on two current hypotheses. Borehole data were used as ground truth. Similar performance tests were also computed using the CPT depth to bedrock model. The results of these performance tests are compared and discussed. Finally, an ERI-based investigation strategy is proposed to assess karst hazard in palaeokarstic context, such as encountered in the Tournaisis area, at the scale needed for building and infrastructure purposes. [Copyright &y& Elsevier]

Abstract: The forward and inversion methods of resistivity tomography with a vertical line current source (RTVLCS) were introduced in this paper. The technique was applied to the evaluation of residual oil saturation of the reservoir studied in an old oil region. The observed water saturation during the course of the measurement of RTVLCS and subsequent drilling records proved the reliability of our result. However, our model has not included the influence of all factors such as electrical contact resistance between the metal casing and surrounding rocks and of the size of the metal cased well taken as a vertical line current source (VLCS). In addition, RTVLCS was carried out after the well had been in use for many years, while the initial resistivity values used in the inversion were obtained from the resistivity curve logged upon the completion of the well. Therefore, the residual oil saturation obtained by RTVLCS is relative and has not complete consistency with real situations, and can only be used as reference for adjusting a plan for oil production in the mid- or post-phase of an oilfield. [Copyright &y& Elsevier]

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This article presents possibilities of application of selected geophysical methods in railway engineering. Geophysical methods are used to observe a physical field. The measured data are interpreted in order to provide findings on geotechnical conditions of railway substructure and its surroundings. Geophysical methods have been unsystematically applied on European railways in the past. This paper's text is supported mainly by the measurements performed under the INNOTRACK projects at the sites in the Czech Republic, France, Spain, and Sweden. The geophysical testing has proved that the geophysical methods can reliably, quickly, in detail, and at a relatively low cost, inform about the problematic zones of the track. The effect of the geophysical application can be enhanced by long-term monitoring of track segments. [ABSTRACT FROM AUTHOR]

Abstract: In the study of rock avalanche phenomena, numerical modelling makes use of back analyses of the rock avalanche propagation for calibration of the modelling assumptions and parameters. The back analyses require knowledge of the run-out area boundaries and the thickness distribution of the deposit. Geophysical methods can be applied to retrieve the thickness distribution, but, due to strong heterogeneities and logistic problems they are seldom applied. The aim of this work is to assess the potential of integrated geophysical methods to recognise and characterise a deposit created by two rock avalanches which occurred in the Sandalp valley (Switzerland) in 1996. The topography of the site before and after the rock avalanche is known and can be used as a benchmark. Resistivity tomography, seismic P-wave tomography, and active and passive surface wave analysis have been applied on several profiles deployed both on the rock avalanche deposit and in the surrounding area. Innovative approaches for surface wave analysis based on laterally constrained inversion and multimodal inversion have been applied to the data. A comparison of the results of the geophysical investigations with the topographic benchmark has shown the capability of the geophysical methods to locate the bottom of the deposit in the areas where the contrast with the host sediments properties is significant. In these areas, the deposit has higher resistivities and lower seismic velocities than the underlying materials. In the areas where the deposit is thicker and richer in fine-grained materials the geophysical parameters are not able to discriminate between the rock avalanche deposit and the underlying sediments. As a secondary task, the geophysical methods also allowed the bedrock pattern to be outlined. [Copyright &y& Elsevier]

Located in the northern part of the Ardenne massif, the Ninglinspo catchment shows morphologic features (valley floor benches, slope dissymmetry, river superimposition, etc) which constitute the likely heritage of periglacial processes. Here, we consider chiefly the geomorphic meaning of the valley floor benches. Assuming that bench deposits resulted at least partially from periglacial processes during the last glacial means that, during the Holocene warming, the river was unable to evacuate them totally. Reasoning in the same way for previous glacial and interglacial episodes (e.g. with a river unable to transport during warm periods the entirety of the materials provided by periglacial slope processes), implies that a polygenic genesis of these benches should be considered. In this study, we try to bring to the fore this polygenic origin using different various techniques, including geophysical surveying and tephrochronology. Finally, catchment denudation rates by periglacial slope processes ranging from 0.45 to 2 mm/ka have been inferred since the last glacial. [ABSTRACT FROM AUTHOR]

Abstract: Geophysical monitoring is used principally to interpret the locations and amounts of ground condition changes. To achieve these objectives, differences are computed and examined using time-lapse images calculated under the time-invariant static assumption, that any material property changes during the data measurement can be practically ignored. These monitored data, however, can be contaminated with noise and frequently generate false anomalies of ground condition changes. Furthermore, the assumption of the static model can be invalid if the material property changes significantly during data acquisition. To alleviate these problems, we developed a new least-squares inversion algorithm that allows for the subsurface properties to continuously change in time. We define the subsurface structure and the entire monitoring data in the space–time domain, allowing us to obtain a four-dimensional space–time model using just one inversion process. We introduce the regularizations not only in the space domain but also in time, resulting in reduced inversion artifacts and improved stability of the inverse problem. We demonstrated the performance of the proposed algorithm through numerical experiments that assumed several scenarios of ground condition changes and data acquisition sequences. Finally, the applicability to field data was proven by applying the developed algorithm to the monitoring data of crosshole resistivity tomography jointly performed with a dye tracer flooding experiment. This experiment had a small enough scale that we could not ignore the change of material properties during the data measurement. [Copyright &y& Elsevier]

In the analysis of resistivity tomography data, boreholes are assumed to be drilled vertically and have no borehole deviation with depth. However, such a condition is seldom encountered in real field situations. Moreover, in two-dimensional (2-D) tomographic imaging, it is assumed that the boreholes belong to the section to be imaged, while this is often far from the case in real situations. Given these discrepancies between assumptions and reality, unrealistic subsurface images may be obtained due to the improper handling of borehole deviation. In this study, the borehole deviation effect in 2-D resistivity tomography is analyzed. The effect is analyzed using a 3-D finite element forward modeling program to handle a general case of 3-D borehole deviations. As a further step, the borehole deviation has been included in the inversion process, which is implemented by assuming that the subsurface structure is 2-D within a 3-D resistivity tomography inversion code. By inverting the resistivity data obtained from a site where boreholes are significantly deviated, it is shown that the borehole deviation should be taken into account to obtain accurate subsurface images. The enhancement was also demonstrated with the 3-D ERT imaging of field data set. [ABSTRACT FROM AUTHOR]

Identification of undercover karst areas can be essential for engineering or hydrogeological studies. Geological reconnaissance of karst areas can be challenging due to their complex and highly heterogeneous internal structure. Herein, we present a case study carried out at the carbonate coastal Montgrí Massif in Costa Brava (Spain), which, in the study area, is characterized by a well-developed karst system covered by Quaternary sediments. The objectives of the work are to delineate and characterize karst structures using geophysical techniques and to produce a subsurface geological model that goes beyond the individual physical properties determined independently by each geophysical method. We acquired electrical resistivity and seismic datasets for five collocated profiles. We analyzed the seismic datasets to retrieve P-wave (V p) and S-wave (V s) velocity models. Clay-filled karst formations may produce seismic and electrical signatures that are difficult to interpret due to the wide range of values that can be present depending on the properties of the karst's infill. In this type of environment, the Fuzzy c-means (FCM) data clustering approach is a way of producing an integrated model that can help to constrain the lithological interpretation. In this study, FCM is first applied to an electrical resistivity-V s dataspace in order to characterize near surface sediments. Four different groups are recognized corresponding to different lithologies, fluid conductivities, and saturations in the unconsolidated sediments. Secondly, FCM analysis of the resistivity-V p datasets provides integrated models that help to delineate the karst areas. The interpretive constraints used derive from the results of the first FCM application and the lithological description of a single borehole that reaches the karst below the Quaternary sediments. In this way, karst is identified in all five profiles. This study shows that the integration of seismic and electrical resistivity information using the FCM approach can extend borehole information throughout the study area and thus provide comprehensive geological models in karst environments. • Fuzzy-c means data clustering provides comprehensive subsurface models in karst environment. • Vs and electrical resistivity integration delineates fine/coarse sediments distribution. • Vp and electrical resistivity zonal models image karst zones. • Membership degree higher than 70% has been used as reliability criterion of the models. • Zones with lower resistivity in shallow sediments and in karst delineate saline intrusion paths. [ABSTRACT FROM AUTHOR]

We present a novel, inexpensive, and fast microimpedance tomography system for two-dimensional imaging of cell and tissue cultures. The system is based on four-electrode measurements using 16 planar microelectrodes (5 μm×4 mm) integrated into a culture chamber. An Agilent 4294A impedance analyzer combined with a front-end amplifier is used for the impedance measurements. Two-dimensional images are obtained using a reconstruction algorithm. This system is capable of accurately resolving the shape and position of a human hair, yielding vertical cross sections of the object. Human epithelial stem cells (YF 29) are also grown directly on the device surface. Tissue growth can be followed over several days. A rapid resistivity decrease caused by permeabilized cell membranes is also monitored, suggesting that this technique can be used in electroporation studies. [ABSTRACT FROM AUTHOR]

The first high-resolution images of the fault-controlled High Agri Valley Basin obtained with the electrical resistivity tomography are presented. The Agri Valley is one of the more complex components of the Quaternary fault network of the Apennine chain. The geometry of the pre-Quaternary bedrock, the location and dip of the master fault and the tectonic evolution of the basin are still a subject of debate. Therefore, deep electrical resistivity tomography integrated with stratigraphic analyses has been carried out to improve knowledge of the geological setting of the basin. The high-resolution electrical images allow us to reconstruct the complex geometry of the basin. The electrical imaging highlights the irregular shape of the basin, which is bordered by shallow faults and filled with Pleistocene alluvial deposits. In longitudinal cross-section, the basin appears as a mosaic of fault-bounded blocks forming three different depocenters separated by intrabasinal highs. In transverse cross-section, the basin is an irregular graben, locally asymmetric to the northeast, and with secondary grabens due to antithetic faults. [Copyright &y& Elsevier]

This paper shows how it is possible to use wells drilled during geotechnical pre-investigation of a tunneling site to obtain a 2-D image of the resistivity close to a tunnel boring machine. An experimental apparatus is presented which makes it possible to perform single and borehole-to-borehole electrical measurements independent of the geological and hydrogeological context, which can be activated at any moment during the building of the tunnel. This apparatus is first demonstrated through its use on a test site. Numerical simulations and data inversion are used to analyse the experimental results. Finally, electrical resistivity tomography and single-borehole measurements on a tunneling site are presented. Experimental results show the viability of the apparatus and the efficiency of the inverse algorithm, and also highlight the limitations of the electrical resistivity tomography as a tool for geotechnical investigation in urban areas. [Copyright &y& Elsevier]

The apparent resistivity of the samples with water-bearing configuration was measured by an electrode-array and 2-D resistivity images of these samples were reconstructed then. The obtained series of tomograms reveal the distribution and its variation of true resistivity within the samples caused by the changes of crack and liquid distribution. Applying this method to the simulation experiment on the electrical properties of rocks, the fracturing and water filling process, which produces the electrical changes, can be brought to light clearly. [ABSTRACT FROM AUTHOR]