Two-dimensional Electrical Resistivity Tomography Research Articles

High-resolution electrical resistivity tomography applied to patterned ground, Wedel Jarlsberg Land, south-west Spitsbergen

This article presents results of two-dimensional electrical resistivity tomography (ERT) applied to three types of patterned ground in Wedel-Jarlsberg Land (Svalbard), carried out in late July 2012. The structures investigated include sorted circles, non-sorted polygons and a net with sorted coarser material. ERT was used to recognize the internal ground structure, the shape of permafrost table below the active layer and the geometric relationships between permafrost, ground layering and surface patterns. Results of inversion modelling indicate that the permafrost table occurs at a depth of 0.5–1 m in a mountain valley and 1–2.5 m on raised marine terraces. The permafrost table was nearly planar beneath non-sorted deposits and wavy beneath sorted materials. The mutual relationships between the permafrost table and the shape of a stone circle are different from those typically presented in literature. Ground structure beneath the net with sorted coarser materials is complex as implied in convective models. In non-sorted polygons, the imaging failed to reveal vertical structures between them.Keywords: Patterned ground; permafrost; active layer; electrical imaging; Svalbard.(Published: 18 June 2015)Citation: Polar Research 2015, 34, 25678,http://dx.doi.org/10.3402/polar.v34.25678

Construction of Indistinguishable Conductivity Perturbations for the Point Electrode Model in Electrical Impedance Tomography

We explain how to build indistinguishable isotropic conductivity perturbations of the unit conductivity in the framework of the point electrode model for two-dimensional electrical impedance tomography. The theoretical approach, based on solving a fixed point problem, is constructive and allows the implementation of an algorithm for approximating the invisible perturbations. The functionality of the method is demonstrated via numerical examples.

Combining Multiple Electrode Arrays for Two-Dimensional Electrical Resistivity Imaging Using the Unsupervised Classification Technique

This article describes the use of k-means clustering, an unsupervised image classification technique, to help interpret subsurface targets. The k-means algorithm is employed to combine and classify the two-dimensional (2D) inverse resistivity models obtained from three different electrode arrays. The algorithm is initialized through the selection of the number of clusters, number of iterations and other parameters such as stopping criteria. Automatically, it seeks to find groups of closely related resistivity values that belong to the same cluster and are more similar to each other than resistivity values belonging to other clusters. The approach is applied to both synthetic and field data. The 2D postinversions of the resistivity data were preprocessed by resampling and interpolating to the same coordinate. Following the preprocessing, the three images are combined into a single classified image. All the image preprocessing, manipulation and analysis are performed using the PCI Geomatics software package. The results of the clustering and classification are presented as classified images. An assessment of the performance of the individual and combined images for the synthetic models is carried out using an error matrix, mean absolute error and mean absolute percent error. The estimated errors show that images obtained from maximum values of the reconstructed resistivity for the different models give the best representation of the true models. Additionally, the overall accuracy and kappa values show good agreement between the combined classified images and true models. Depending on the model, the overall accuracy ranges from 86 to 99 %, while the kappa coefficient is in the range of 54–98 %. Classified images with kappa coefficients greater than 0.8 show strong agreement, while images with kappa coefficients greater than 0.5 but less than 0.8 give moderate agreement. For the field data, the k-mean classifier produces images that incorporate structural features of the three electrode array configurations. Consequently, some clusters that overwhelmingly correspond to the lithologic units of the investigated areas are better identified than the tomographic images of each data set considered separately, underscoring the relevance of the unsupervised classification technique in this study.

Numerical method for solving a two-dimensional electrical impedance tomography problem in the case of measurements on part of the outer boundary

The two-dimensional electrical impedance tomography problem is considered in the case of a piecewise constant electrical conductivity. The task is to determine the unknown boundary separating the regions with different conductivity values, which are known. Input information is the electric field measured on a portion of the outer boundary of the domain. A numerical method for solving the problem is proposed, and numerical results are presented.

Electrical resistivity tomography applied to groundwater aquifer at downstream of Chih-Ben Creek basin, Taiwan

Groundwater resource investigations primarily involve inferring groundwater levels and recharge sources from factors such as the distributed location and range, subsurface geological structure, and flow paths of groundwater sources. This study conducted an investigation on the underground aquifer at the downstream of the Chih-Ben Creek basin, Taiwan. In addition, convenient, rapid, and accurate two-dimensional electrical resistivity tomography (ERT) was employed for evaluations. The cross-sectional ERT image indicates that the low-resistivity region in the stratum at survey line CB01 may be a groundwater deposit or fracture zone. This region can be divided into three major segments. The segment that is 0–50 m from the survey line exhibits a decreasing trend and is thus inferred to be a recharge source. Survey line CB02 can be generally divided into four layers, with the second layer containing a greater amount of groundwater. This layer is an aquifer with a depth of approximately 25–100 m below the ground surface and 75–100 m in thickness. The base of the aquifer slopes toward the east and primarily possesses a lithological composition of slate and sandstone with explicit and fractured joints.

Prospecting for clay minerals within volcanic successions: Application of electrical resistivity tomography to characterise bentonite deposits in northern Sardinia (Italy)

Electrical resistivity tomography (ERT) is applied to prospect for and characterise a bentonitic clay deposit in northern Sardinia. Sardinian bentonites derived from the hydrothermal alteration of thick successions of pyroclastic flows and epiclastites are associated with the Oligo-Miocene calc-alkaline volcanic cycle. The alteration of these rocks is generally controlled by faults that control the local circulation of hydrothermal fluids. Two-dimensional ERT investigations were performed close to a faulted area to define the location, thickness and lateral continuity of the clayey body, and determine how it relates to faulting and stratigraphy. A line-based three-dimensional ERT data acquisition was carried out in a selected area to estimate the available clay reserves. The reliability of these resistivity models was assessed by comparison with local borehole data. Finally, the interpretation of the ERT results was optimised through synthetic modelling of the electrical resistivity imaging technique. The results define the extent and geometry of the bentonitic deposit with good accuracy and outline the scenarios where the ERT method may provide optimal results when prospecting for clay deposits.

Electrical resistivity tomography investigations on a paleoseismological trenching study

Two-dimensional electrical resistivity tomography (ERT) investigation was performed in a paleoseismological trenching study. Data acquisition strategies such as the selection of electrode configuration and electrode intervals of ERT application were investigated in this paper. The ERT results showed that the Wenner and Wenner–Schlumberger arrays yielded similar results for subsurface characteristics whereas the DD array provided slightly different results. The combined usage of these arrays produced satisfactory images of the subsurface resistivity distribution. In addition, the electrode spacing tests revealed that a suitable interpretation of subsurface geology can be obtained from a 5 m electrode interval. However, a suitable trenching location defined by successful 2D resistivity models was obtained for 1 m electrode spacing. Therefore, the comparison of the trench and ERT results was also possible. The results of trenching and ERT studies substantially support each other.

Application of ERT and Aerial Photographs Techniques to Identify the Consequences of Sinkholes Hazards in Constructing Housing Complexes Sites over Karstic Carbonate Bedrock in Perak, Peninsular Malaysia

The position and condition of carbonate bedrocks such as limestone or dolomite and its appearance are important for engineering construction sites such as buildings, housing complexes and road projects. In spite of the subsidence damage resulting from carbonate dissolution causing massive losses all over the world, the causes are well addressed in a few areas only. The application of a geophysical field survey and aerial photograph will be represented through identification techniques. In this case study, a two-dimensional (2D) electrical resistivity tomography (ERT) survey was performed across two housing complex construction sites, north of Ipoh city, in the Perak state, Peninsular Malaysia, to image the subsurface and locate evidence for the near surface karstic features such as voids or cavities including sinkholes. Furthermore this survey was carried out to estimate the depth of the bedrock and to assess the reliability of the (ERT) electrical resistivity method whether it can identify such features or not. Six resistivity traverses or profiles were conducted along the survey area at each of the two construction sites. The orientation, extension and the degree of inclination of those profiles are shown in the location map. The correct resistivity data was interpreted using res2dinv software. The interpretation of E R Tomography/ image sections revealed that many anomalies with very low resistivity and high conductivity extend along the project areas in these construction sites. Massive sinkhole has affected many sections of the project area in construction site #1. Thus it contains non-stiff clay and is saturated with water, rendering it less resistant to electrical currents (high conductivity). Enormous longitudinal channel pipe containing both stiff and sandy clay has affected many sections of project area in construction site #2. An assessment of the situation was surmised from the subsurface images. Subsequently, an estimation of the possibility of a collapse occurring in the near future due to the sinkhole was prepared. This study also demonstrated that the high-resolution Electrical Resistivity Tomography (ERT) can be effectively applied to reflect and differentiate surficial soil, clay, weathered rocks, compact or intact rocks, and air-filled karstic voids or cavities. The appearance of many sinkholes in the area is mostly attributed to karstic activity. In accordance to the classification of the characteristics of morphological features of karstic ground conditions by Waltham and Fookes (2003), the karst level in construction site#1 found between profile 1 and profile 6 is an older or complex karst type KIV, while the karst level in construction site#2 found between profile 1 and profile 3 is a mature karst type KIII. Afterwards, the karst type changed over profile#4 to profile #6 to an older karst or complex karst type KIV. The hazards of the sinkholes and other karst features such as, cavities and dissolution channel pipe can cause problems to the construction projects in the near future resulting to mismanagement during the initial phase of the projects, as the developers did not carry out prior geophysical technique and geological studies. Moreover, the borings within these karsts regions is incapable of providing sufficient subsurface data for analysis, and then might misrepresent the subsurface geological model, which might in turn lead to an additional cost for corrective design or an ad-hoc analysis. Consequently, early engineering subsurface remediation techniques are needed to minimize the potential of geohazard of sinkholes and other karst features in these construction sites over karstic carbonate bedrock. Initially, it was considered to utilize the reverse graded filter technique to fill the huge sinkhole in construction site#1. Skin friction piles driven into the layers contained non-stiff materials (soil, clay, silt, sand). New chemical grouting techniques such as deep injection chemical grouting technique are utilized in construction site#2. Controlling of the surface and ground water drainages will be in force when work begins at these construction sites.

Hydrogeological and geophysical study for deeper groundwater resource in quartzitic hard rock ridge region from 2D resistivity data

Electrical resistivity method is a versatile and economical technique for groundwater prospecting in different geological settings due to wide spectrum of resistivity compared to other geophysical parameters. Exploration and exploitation of groundwater, a vital and precious resource, is a challenging task in hard rock, which exhibits inherent heterogeneity. In the present study, two-dimensional Electrical Resistivity Tomography (2D-ERT) technique using two different arrays, viz., pole–dipole and pole–pole, were deployed to look into high signal strength data in a tectonically disturbed hard rock ridge region for groundwater. Four selected sites were investigated. 2D subsurface resistivity tomography data were collected using Syscal Pro Switch-10 channel system and covered a 2 km long profile in a tough terrain. The hydrogeological interpretation based on resistivity models reveal the water horizons trap within the clayey sand and weathered/fractured quartzite formations. Aquifer resistivity lies between ∼3–35 and 100–200 Ωm. The results of the resistivity models decipher potential aquifer lying between 40 and 88 m depth, nevertheless, it corroborates with the static water level measurements in the area of study. The advantage of using pole–pole in conjunction with the pole–dipole array is well appreciated and proved worth which gives clear insight of the aquifer extent, variability and their dimension from shallow to deeper strata from the hydrogeological perspective in the present geological context.

Application of 3D electrical resistivity imaging for mapping frozen ground conditions exemplified by three case studies

Periglacial landscapes comprise landforms that are inherently 3D structures, often exhibiting small-scale spatial heterogeneity of surface and subsurface conditions. The objectives of the present paper are to illustrate the potential of the novel application of 3D electrical resistivity imaging for mapping frozen ground conditions exemplified by three case studies with different geomorphological problems to be addressed and to consider the efficacy of the 3D approach to geomorphological investigations in mid-latitude high alpine and high latitude lowland permafrost environments.The approach described in the three case studies includes reconnaissance surveys using two-dimensional electrical resistivity tomography (2D ERT) followed by a detailed mapping using three-dimensional electrical resistivity imaging (3D ERI). The latter approach enables a spatial imaging of the subsurface resistivity distribution and clearly improves the delineation and characterization of subsurface structures compared to state-of-the-art 2D ERT that is limited to findings gained along single profiles or extrapolation between several profiles. Although it can be challenging and time-consuming to apply this technique in periglacial environments, the promising results demonstrate its value for the 3D delineation of frozen ground conditions. In the case of the described case study sites, characterizing the subsurface heterogeneity is close to impossible using drilling or 2D geophysical surveying alone because of the complex 3D nature of the frozen ground characteristics comprising permafrost and permafrost-free areas (alpine permafrost test site) as well as permafrost with variable characteristics (subarctic lowland permafrost test site) at close distance. Even in environments that seem homogeneous at first sight, this method allows us to detect substantial subsurface property variations that can be attributed to different frozen ground conditions. Furthermore, 3D ERI allows the linking of different data sources (e.g., site-specific geomorphology and hydrology) to enhance the spatial understanding of surface and subsurface characteristics and dynamics in permafrost environments. The improved knowledge of the geophysical anatomy and subsurface architecture of the permafrost occurrences revealed by this study suggests a more widespread use for glacial and periglacial landform studies in the future.

Shallow Seismic Refraction, Two-Dimensional Electrical Resistivity Imaging, and Ground Penetrating Radar for Imaging the Ancient Monuments at the Western Shore of Old Luxor City, Egypt

A combined near surface geophysical survey conducted in archaeological site at the western bank of Luxor area shows that, the geophysical methods offer the possibility to characterize and reconstruct the geometry of subsurface structures without destroying the deposits, providing a way to find solutions to the questions of archaeological or engineering significance. The two dimensional electrical imaging (R2D) and the shallow seismic refraction (SSR) techniques were carried out through twelve profiles measured across the study area in NW-SE directions with 5 m profile spacing. The interpretation of the 2-D resistivity imaging shows that there are two layers which are arranged from the top to the bottom as soil layer consists of weathered clay and its thickness varies between 3.82 m and 4 m; wetted clay and mud with depth varies from 25.3 m to 26.23 m and a massive body may composed of Alabaster, limestone or granite is present within the second layer. Following, the inverted 2-D models were merged to form a quasi-3D resistivity model which was visualized as depth slices. A high resistive anomalous feature was detected in successive depth slices. Moreover, analysis of shallow seismic data shows that, P-velocity ranges from 400 - 1350 m/sec whereas S-wave ranges from 280 - 460 m/sec. The massive body with high resistivity is detected and shows also a high velocity. Following a detailed Ground Penetrating Radar (GPR) a survey was carried out to image the subsurface. 200 MHz antenna frequency was used on the survey. Inspection of the 2-D GPR profile shows the presence of hyperbolic anomalous features is present at a depth nearly 4 m. Moreover, three dimensional depth slices shows also the presence of an anomalous feature which appears at depth slices ranging from 3.5 m to 5 m. Finally, the interpreted geophysical results have been verified through archaeological direct excavation started by the Egyptian supreme council of Antiquities in cooperation with the European Commission working in the restoration project of Memnon’s and the Temple of Amenhotep III. They have found a body of Pharaonic statue, the statue has been known by the archaeologists as a statue of Queen Tie, wife of Amenhotep III and mother of Pharaoh Akhenaton unification, carved in Alabaster stone, and this result confirmed that the integrated geophysical method can be effectively contributed in the archaeological prospection in Egypt.

Mapping of quick clay by electrical resistivity tomography under structural constraint

abstract Article history:Received 11 June 2013Accepted 7 September 2013Available online 19 September 2013Keywords:ERTRCPTConstrained inversionQuick clayCase histories Geotechnicalprojectsusuallyrelyontraditionalsoundinganddrillinginvestigations.Drillingonlyprovidespointinformation and the geology needs to be interpolated between these points. Near surface geophysical methodscan provide information to fill those gaps. Norwegian case studies are presented to illustrate how two-dimensional electrical resistivity tomography (ERT) can be used to accurately map the extent of quick clay de-posits. Quick clay may be described as highly sensitive marine clay that changes from a relatively stiff conditiontoaliquidmasswhendisturbed.Quickclayslidespresentageo-hazardandthereforelayersofsensitiveclayneedto be mapped in detail. They are usually characterized by higher resistivity than non-sensitive clay and ERT istherefore a suitable approach to identify their occurrence. However, our experience shows that ERT cannot re-solvethissmallresistivitycontrastnearlargeanomaliessuchasabedrockinterface.Forthisreason,aconstrainedinversion ofERTdata wasappliedtodelineatequickclayextent both vertically andlaterally.As comparedtotheconventional unconstrained inversions, the constrained inversion models exhibit sharper resistivity contrastsand their resistivity values agree better with in situ measurements.© 2013 Elsevier B.V. All rights reserved.

Groundwater quality assessment of a freshwater wetland in the Selangor (Malaysia) using electrical resistivity and chemical analysis

Groundwater quality of the Paya Indah Wetland (PIW) was targeted for the present study using integrated two-dimensional electrical resistivity imaging (ERI) and hydrochemical surveys. Electrical resistivity and the influence of variations in the lake level were investigated using daily water balance measurements, water sampling and geochemical analysis, and well-logging. The geoelectrical resistivity surveys comprised 14 resistivity traverses in the PIW. The resistivity surveys predicted the high-permeability areas separated in order to provide pathways for lake drainage. The resistivity inverse model showed that the freshwater zone is extremely clear. Chemical analyses of the groundwater samples obtained from seven boreholes from 2010 to 2013 were studied. The total dissolved solids and electrical conductivity were represented as being less than 500 mg/L and 600 μmhos/cm, respectively, and freshwater was therefore indicated. The lake water level was proportionate to rainfall fluctuation with the level measured in 2010. The soil and groundwater confirmed there to be fresh and brackish water zones in the study area, this result being obtained from ERI and the hydrochemistry of the groundwater samples.

LiDAR and 2D Electrical Resistivity Tomography as a Supplement of Geomorphological Investigations in Urban Areas: a Case Study from the City of Wrocław (SW Poland)

In urbanized areas, particularly in lowland terrains and floors of large river valleys, the natural land configuration is often hard to recognize due to a long history of human activity. Accordingly, archaeological works in cities, which supply knowledge on settlement conditions, are usually accompanied by geological and geomophological research. Lately, data from light detection and ranging (LiDAR) have become a valuable source of information on urban land configuration. Geophysical methods are also becoming increasingly popular in background studies. The paper presents a method of using and linking these sources of spatial information about landforms in such areas. The main aim is to identify to what extent these complementary sources of data and the proposed method can be used in such a specific environment to reconstruct natural, buried terrain morphology. The city of Wroclaw in Central Europe serves as an example. To this end geomorphometric studies were conducted with the use of digital elevation models (DEMs) based on LiDAR scanning and derivated land-surface parameters—SAGA Wetness Index, Channel Network Base Level and Altitude above Channel Network. The study also involved determining morphological edges and measurements of the meanders of the Odra, as well as expanding information on the spatial distribution of alluvia and the structure of slope breaks. To this end, geophysical measurements were conducted using the Two-Dimensional Electrical Resistivity Tomography method. Additionally, five typical sequences of man-made ground present within the perimeter of the city were distinguished. As a result, a map of the main landforms of Wroclaw is presented. Finally, we argue that although high resolution DEM and derivate land-surface parameters are very useful in terrain analysis, places with thick man-made ground or strongly levelled areas must be recognized by geoarchaeological excavations or geological bore holes. The geophysical survey is useful to identify buried morphological edges and older relief elements in open areas.

Heat transport dynamics at a sandy intertidal zone

[1] Intertidal zones are spatially complex and temporally dynamic environments. Coastal groundwater discharge, including submarine groundwater discharge, may provide stabilizing conditions for intertidal zone permeable sediments. In this study, we integrated detailed time series temperature observations, porewater pressure measurements, and two-dimensional electrical resistivity tomography profiles to understand the coupled hydraulic-thermal regime of a tropical sandy intertidal zone in a fringing coral reef lagoon (Rarotonga, Cook Islands). We found three heating patterns across the 15 m study transect over tidal and diel periods: (1) a highly variable thermal regime dominated by swash infiltration and changes in saturation state in the upper foreshore with net heat import into the sediment, (2) a groundwater-supported underground stable, cool region just seaward of the intertidal slope break also importing heat into the subsurface, and (3) a zone of seawater recirculation that sustained consistently warm subsurface temperatures that exported heat across the sediment-water interface. Simple calculations suggested thermal conduction as the main heat transport mechanism for the shallow intertidal sediment, but deeper and/or multidimensional groundwater flow was required to explain temperature patterns beyond 20 cm depth. Temperature differences between the distinct hydrodynamic zones of the foreshore site resulted in significant thermal gradients that persisted beyond tidal and diel periods. The thermal buffering of intertidal zones by coastal groundwater systems, both at surface seeps and in the shallow subsurface, can be responsible for thermal refugia for some coastal organisms and hotspots for biogeochemical reactions.

The impact of subsurface geological structure on the groundwater occurrence using geophysical techniques in wadi El Kallabiyyah and wadi As Sabil, East Esna, Eastern Desert, Egypt

The present study aims to explain some hydrogeological problems that are related to geological setting in two wadis: El Kallabiyyah and As Sabil East Esna. For this purpose, land magnetic, vertical electrical soundings, and two-dimensional electrical resistivity imaging profiles were measured. The lithological succession comprises Upper Cretaceous to Quaternary deposits that rest on a Precambrian basement rock. The results of the interpretation of the land magnetic measurements in wadi El Kallabiyyah represent the depth to the basement rock which ranges from 260 to 660 m and increases towards the northeast trend, but in wadi As Sabil, there is a large basin at the center of the wadi with the recorded depth to the basement reaching 1,300 m due to the effect of two normal faults (F1 and F2). The results of the interpretation of the vertical electrical sounding and two-dimensional imaging confirmed the presence of these faults in wadi As Sabil. The results of the geoelectrical measurements in wadi El Kallabiyyah represented the water-bearing layer at the third unit that consists of sands, gravels, boulders, and clays. The presence of clay will affect the potentiality of a drilled well. For wadi As Sabil, the results of geoelectrical measurements led to the lower part of the second unit which acts as a water-bearing layer that consists of sands, boulders, gravels, and clays. It can be concluded that the best site for drilling wells in wadi El Kallabiyyah is at vertical electrical sounding no.5 with a total depth of not less than 230 m. In wadi As Sabil, the drilled wells must be concentrated at the center and the downstream.

Sinkhole detection using electrical resistivity tomography in Saudi Arabia

Karst phenomena exist in different areas in the Kingdom of Saudi Arabia, causing serious environmental problems that affect urban development and infrastructure (buildings, roads and highways). One of the most important problems are sinkholes, which most of the time consist of unfilled voids. These sinkholes are formed as a result of the chemical leaching of carbonate and evaporite formations by percolating water. Field investigations show that there are many surface expressions of sinkholes in the area; some appear on the ground surface and others are hidden in the subsurface. Geophysical data were collected at the study area using two-dimensional electrical resistivity tomography (ERT) with different electrode spacings to delineate buried sinkholes and associated subsurface cavities. Our findings indicated that the dipole–dipole method using an electrode spacing of 1 m was successful in detecting a known subsurface sinkhole. According to the ERT method the detected sinkhole depth ranges from 2 to 4 m, its height ranges from 2 to 4 m, and its width ranges from 5 to 7 m. Field observation has verified the geophysical data, especially along the profile A-A\. Finally, closely spaced ERT profiles were successful in determining the three-dimensional volume of the subsurface sinkhole.

Relationship between electrical resistivity and basic geotechnical parameters for marine clays

Recently, considerable efforts have been made in the attempt to map quick clay areas using electrical resistivity measurements. However there is a lack of understanding regarding which soil parameters control the measured resistivity values. To address this issue, inverted resistivity values from 15 marine clay sites in Norway have been compared with basic geotechnical index properties. It was found that the resistivity value is strongly controlled by the salt content of the pore fluid. Resistivity decreases rapidly with increasing salt content. There is also a relatively clear trend of decreasing resistivity with increasing clay content and plasticity index. Resistivity values become very low (≈5 Ω·m) for high clay content (>50%), medium- to high-plasticity (Ip ≈ 20%) materials with salt content values greater than about 8 g/L (or corresponding remoulded shear strength values greater than 4 kPa). For the range of values studied, there is poor correlation between resistivity and bulk density and between resistivity and water content. The data studied suggest that the range of resistivity values corresponding to quick clay is 10 to 100 Ω·m, which is consistent with other published limits. A comparison is made between two-dimensional electrical resistivity tomography (ERT) and resistivity cone penetration test (RCPTU) data for two of the sites and the two sets of data show similar trends and values irrespective of scale effect.

Efficacy of electrical resistivity tomography technique in mapping shallow subsurface anomaly

Electrical Resistivity Tomography is a versatile, fast and cost effective technique for mapping the shallow subsurface anomaly. It covers a wide spectrum of resistivity ranging from <1 Ohm.m to several thousands of Ohm.m. In this paper applications and utility of two-dimensional Electrical Resistivity Tomography (ERT) technique are discussed to look into huge data density coverage, different signal strengths of data from subsurface and their implications in resolving the aquifer zones, related geological structures etc. of the substratum ranging from alluvium to tectonically disturbed hard rock ridge region of the country. The major advantages and flexibility of ERT over conventional resistivity methods are also discussed.

2-D and 3-D Resistivity Imaging of Karst Sites in Missouri, USA

Two-dimensional (2-D) electrical resistivity tomography data were acquired across and in proximity to active sinkholes at two highway construction sites in Missouri. Site 1 is located in Greene County in southwestern Missouri; site 2 is located in Jefferson County in east-central Missouri. Eleven 2-D electrical resistivity tomography profiles were acquired along a suite of parallel traverses (spaced at 7.5 m intervals) at site 1 as part of the geotechnical investigation of a proposed interchange. The primary objectives were to identify and map air-filled karstic cavities (if present) and the top of bedrock. The resistivity data from site 1 were processed as both conventional 2-D data and as three-dimensional (3-D) data using two different inversion schemes. The top of bedrock was imaged, and five prominent north-trending, clay-filled, solution-widened fractures were identified. Interpreted air-filled voids were not imaged. At site 2, a pattern of 17 parallel electrical resistivity tomography profiles was acquired across and in proximity to a preexisting sinkhole (approximately 15 m × 10 m × 5 m L/W/D). During construction of the highway, the soil was emptied from the sinkhole overnight following a heavy rainstorm. Electrical resistivity tomography data were acquired to determine if the sinkhole was underlain by a large air-filled cavity. Interpretation of the 2-D and 3-D resistivity images from site 2 showed that the sinkhole was not underlain by a substantive cavity. Rather, the soil in the sinkhole appears to have “flowed” into the subsurface through solution-widened fractures that do not pose a significant risk to the overlying roadway.