Three‐Dimensional Electrical Resistivity Tomography Schemes Optimized for Imaging Wall Foundations

Electrical resistivity imaging (ERI) method is very useful in mapping subsurface contamination zones. In this study, 3D ERI is used to identify the distribution and depth of subsurface soil contamination zone near Al_Furat state company for chemical and pesticides industries in Hilla city, Iraq. A 3D pure image of electrical resistivity distribution is obtained from a perpendicular square grid (80 × 80 m) which consists of three parallel and three perpendicular lines. The electrical resistivity values range from < 1 to 21 O.m, and the total root mean square (RMS) after four iterations is about 7%. The image gives eight different depth slices for the estimated model with a depth interval of about 1 m. The maximum depth of investigation is 13.7 m. The results mapped the contamination zone, where high electrical resistivity values (about 21 O.m) are observed possibly due to accumulation of alkaline waste disposed from the company. The ERI results show that the subsurface layers up to a depth of 4.99 m are of moderate-to-high electrical resistivity values. The study reveals that 3D ERI is an effective tool for subsurface contamination zone mapping in various depths, which extends in this study from the near-surface to a depth of about 5 m below the ground level.

Two-Dimensional (2D) Electrical Resistivity Tomography (ERT) survey was carried out at 11 sites within an area of 10 km2 to delineate deeper potential groundwater zones in a complex geological terrain underlain by quartzite, shale and limestone formations with varied resistivity characteristics. The area is in medium rainfall zone in Tadipatri mandal of Anantapur district, Andhra Pradesh state, India. The investigation was carried out to meet the growing demands of water supply. Interpretation of the high-density 2D resistivity dataset results revealed potential zones at only three sites in Tummalapenta, Ayyavaripalle and Guruvanipalle villages within the depth zone of 24–124 m. A major fault zone oriented in EW direction is mapped at Tummalapenta site. Based on high resolution geophysical data interpretation and significant anomalies, four boreholes were drilled in complex, viz., limestone, shale and quartzite formations up to a maximum depth of 192 m in the area with the yield ranging from 300 to ~5000 liter per hour (lph). These four anomalous drilled borehole sites corroborates with the aquifer zone delineated through ERT technique. The aquifer parameters estimated from pumping tests show that the transmissivity varies between ~0.3 and 179.5 m2/day while the storage coefficient ranges from 0.137 to 0.5 indicating large variation in aquifer characteristics of the system in a smaller area. Suitable water conservation measures were suggested for improving the groundwater condition and yield of the pumping wells.

Electrical resistivity tomography (ERT) has been shown to be effective for surveying and monitoring dams, due to the method's sensitivity to moisture content and relevant physical properties (e.g., porosity). Automated ERT systems, capable of time‐lapse monitoring, can be used to detect variations in ground conditions. However, dam environments are often structurally heterogenous due to, for example, zoned embankments, supporting bedrock or concrete structures and adjoining headponds. If such factors are not accounted for, off‐grid effects may obscure and distort features of interest (e.g., leakage zones) in resulting ERT images. Synthetic modelling simulating conditions at Mactaquac Dam, Canada, was carried out to evaluate whether the abutting concrete structure and properties of the adjacent headpond (water resistivity and level variations) need to be accounted for in an inversion of ERT data. This was achieved through a synthetic numerical model of the dam, including headpond, concrete abutment, core and dry and wet rockfill components. The results show that internal features and dynamic changes through time (e.g., headpond level and resistivity variation) can induce 3D effects in the inversions, which have the potential to be misinterpreted. The modelling revealed that leakage zones could be resolved, showing that features of interest in dam monitoring can still be identified despite potential 3D effects. Overall, these results show that 3D effects from internal structure and a water body are likely to distort modelled resistivity distributions in dam settings. This research sheds light on how ERT can be impacted by structural complexity in dams, using synthetic modelling to understand and quantify the nature of expected artefacts resulting from heterogeneities outside the footprint of the survey area.

Application of 2D-Electrical resistivity tomography in delineating groundwater potential zones: Case study from the voltaian super group of Ghana

A joint research project between Statoil and Schlumberger is focusing on permanent cross-well geophysical methods for reservoir monitoring during steam assisted gravity drainage (SAGD). A feasibility study indicated detectable differences in seismic and electrical reservoir properties based on expected changes in temperature and fluid saturation during oil production. Based on these results, a permanent cross-well system was installed at Statoil’s Leismer Demonstration Area (LDA) in Alberta, Canada. Baseline datasets, including cross-well seismic, 3D vertical seismic profiling (VSP) and cross-well electrical resistivity tomography (ERT), have been acquired. Comparisons between conventional surface seismic and downhole seismic data show an increase in resolution and frequency content as expected. The ERT baseline shows clear separation between zones of high and low electrical resistivity. Several time-lapse studies have been carried out over the last year, and production-induced time-lapse effects are observed in the reservoir section, both for the time-lapse 3D VSP and ERT datasets. Continuous remote ERT monitoring and annual 3D VSP acquisitions will give the potential of each data type as monitoring tools for oil sand reservoir, as well as possibilities for integrating acoustic and electric data.

Yazoo clay is highly expansive in nature, which experiences shrink-swell behavior with seasonal moisture variations. Currently, concerning Yazoo clay characteristics, several natural and artificial slopes in Mississippi are vulnerable to failure due to a high volume of precipitation. Among available nondestructive methods to investigate the subsurface conditions, the Electrical Resistivity Imaging (ERI) method is becoming popular in recent years. Using the 2D ERI technique, the moisture variation at different soil layer depths can be easily detected. The current study investigates the failure of artificial highway slopes on Yazoo clay in Jackson, Mississippi. Qualitative evaluation of temporal moisture variation of the slope was conducted using ERI. The 2D ERI tests were conducted at the crest and toe of the 3H:1V slope. Based on the ERI test results, the overall degree of saturation was between 80% and 90%. The high-saturation condition indicated the formation of perched water zones.

Renewable energy resources are intermittent and need buffer storage to the bridge time-gap between production and demand peaks. North German Basin has a very large capacity for CAES in saline aquifers and cavities. Replacement of brine by gas in aquifer cause strong changes in electrical resistivity. Previously we have shown the applicability of 2D electric resistivity tomography (ERT) for monitoring sequestered CO2 in these reservoirs. Towards more real world problems, this paper aims at studying resolution of 3D ERT in boreholes for CAES mapping in deep saline aquifers. We used for this 3D surveys 3-9 vertical arrays of borehole electrodes installed surrounding the targets (caprock, aquifer, CAES plume, aquitard). We applied non classical electrode configurations of pseudo (optimized) and real 3D measurements in different boreholes. The results show that the applied 3D technique can generally map the dome structure targets with varying resolution, smearing and artefacts. However, resistivity amplitudes are less correctly recovered. The resolution increases with increasing number of borehole electrode arrays, increasing of layer thicknesses and the a priori fixing of boundaries in the inversion. Optimized 2D arrays have better resolution than the other arrays and thus needs to be developed to real 3D arrays.

Electrical resistivity tomography (ERT) is one of the most widely used geophysical methods in geological, hydrogeological, and geo-environmental investigations. Although 3D ERT is now available, 2D ERT remains state-of-the-practice due to its simplicity in fieldwork and lower space requirements. 2D ERT assumes that the ground condition is perpendicular to the survey line and outside the survey line is homogeneous. This assumption can often be violated in conditions such as geologic strikes not perpendicular to the survey line and topographic changes or buried objects near the survey line. Possible errors or artifacts in the 2D resistivity tomogram arising from violating the 2D assumption are often overlooked. This study aimed to numerically investigate the boundary effects on 2D ERT under various simplified conditions. Potential factors including resistivity contrast, depth and size of buried objects, and electrode spacing were considered for the parametric studies. The results revealed that offline geologic features may project onto the 2D tomogram to some extent, depending on the aforementioned factors. The mechanism and implications of boundary effects can be drawn from these parametric studies.

Landslides occur worldwide. Global damages range in billions of dollars and cost hundreds of lives each year; Sweden is not an exception. In this study, we have acquired radiomagnetotelluric (RMT), electrical resistivity tomography (ERT) and high-resolution reflection seismic along 4 profiles with the main aim of imaging the geometry of quick clays and associated unconsolidated sediments that have caused landslides in a pilot study area in southwest Sweden. Comparison between the electrical resistivity models from the 2D inversions of RMT and ERT data with the reflections observed in the seismic data suggests a good correlation between the small-scale structures and those observed in the RMT model. However, the ERT model better delineates the resistive bedrock than the RMT model. The resistivity models are also compared with the CPT-R data collected in two geotechnical boreholes and further confirm some of the layer boundaries imaged with the acquired surface geophysical data.

A new stochastic inversion algorithm to invert three-dimensional (3D) electrical resistivity tomography (ERT) data has been implemented. The statistical information about the correlation among the model parameters and the correlation among observed data was employed to stabilize the ill-posed 3D ERT inverse problem. The data noise covariance matrix was diagonal based on the assumption of uncorrelated data noises. The model parameter covariance matrix, however, could be a full matrix depending on the correlation length between the model parameters. The resulting unsymmetric linear system was solved by bi-conjugate gradient (BICG) methods, such as BICGSTAB, a stabilized variant of BICG algorithm, and TFQMR, a transpose-free quasiminimal residual algorithm. An advantage of this algorithm is that any prior knowledge about the model parameters can be easily incorporated in the inversion process in the form of covariance model type and correlation length. This is the basis for our future implementation of a joint hydrologicalgeophysical inversion. This stochastic inverse technique will be used for characterizing, monitoring, and predicting fluid movement in heterogeneous vadose zone.

ABSTRACTAn Automated time‐Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the long‐term monitoring of coastal aquifers. This ALERT system has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land‐use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry riverbed with electrode take‐outs at regular intervals of 10 m. The maximum depth of investigation is about 160 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Volumetric geoelectric images of the subsurface can be obtained ‘on demand’ or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is fully automated and seamless. The ALERT technology can provide early warning of potential threats to vulnerable water systems such as over‐exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater‐freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.

Electrical resistivity tomography (ERT) is an important method for exploring oil and gas formations, mineral deposits, and groundwater resources. However, most ERT instruments have limitations such as complicated human–computer interaction, few measurement parameters, difficulties in remote monitoring, and lack of four-dimensional (4D) exploration functions. To overcome these limitations, in this study we proposed and developed software for a remote control 4D ERT system based on a cloud platform and Internet of Things (IoT) technology. Specifically, acquisition device control, channel and control, acquisition control, and remote monitoring systems were designed, with various modules being debugged and optimized multiple times. The system is equipped with a 4G module, through which the collected data are transmitted to the cloud platform to realize real-time, stable, and long-distance data acquisition and transmission. Furthermore, the system realizes remote monitoring, real-time measurement, and other functions via the cloud and IoT technology, thereby providing strong technical support for achieving 4D exploration. The developed system was empirically evaluated by applying it to explore the Gequ Landslide area in Jainca County, Qinghai Province, China. The exploration results indicate that the system software has stable functions and high intelligence; thus it satisfies the requirements for field exploration.

This article presents a tutorial of the bases for, and problems encountered with, the resistivity and induced polarization methods in geotechnical and environmental applications. It commences with a discussion of aqueous electrolytic conduction in rocks including consideration of the effects of temperature, rock texture, rock type, geological processes, and of the presence of clay minerals. Both electrode and membrane polarization in soils and rocks are described. The elementary theory for the resistivity and induced polarization methods is introduced via formulas for electrodes on homogeneous and layered half-spaces. The notion of apparent resistivity is presented. Vertical electric sounding is described in relation to curve types, inversion, equivalence, anisotropy, and correlation. Profiling with resistivity is briefly treated while combined sounding-profiling is treated in more detail for both the resistivity and induced polarization methods. The parameters used to describe the induced polarization phenomena are introduced. A brief discourse follows on data acquisition and processing, including design considerations for transmitters and receivers, electrodes, and wire logistics. A section on arrays and the factors involved in selecting an array provokes the disclosure that while their application in geotechnical and environmental studies usually is straightforward much remains to be learned about the resistivity and induced polarization methods in general. Specific topics requiring study are comparative studies of various arrays in (a) depth of exploration in sounding and in profiling, (b) resolution of horizontal structures, (c) sensitivity to depth, (d) sensitivity to lateral effects, (e) sensitivity to buried and surface topography, and (f) the effectiveness of focused arrays. References to recent significant publications on one-, two-, and three-dimensional forward and inverse interpretation procedures follow. Problems encountered with these methods are initially treated under the section on arrays and are expanded with specific reference to geologic noise and negative induced polarization effects. The advantages of focused arrays are illustrated. The paper concludes with nine representative examples of applications.

Electrical resistivity tomography (ERT) is widely used in environmental investigation such as contaminated sites surveys since it is fast and economic. Analysis and processing of the ERT data using computer have become one of the key issues to promote the application of ERT. A method is proposed to recognize contaminated area in ERT profiles using K-means algorithm. The effectiveness of the proposed method is evaluated using a synthetic model of typical contaminated sites. The study results show that when the resistivity difference between the contaminated area and the background area is large (more than 30%), the contaminated area identified by the K-means algorithm is similar to the contaminated area set in the synthetic model; when the difference between the contaminated area and the background area is small (less than 30%), the recognition accuracy of the method is reduced. The main reason is that when the resistivity of the contaminated area is close to the background, the difference between the two categories obtained by K-means algorithm is small, so some background areas may be misjudged as contaminated areas because of the data errors in the acquisition and inversion process.

&amp;lt;p&amp;gt;The growing demand for renewable energy leads to an increase in the development of alternative energy applications. In this way, shallow geothermics assumes an important role in the global energy transition of building air conditioning. The design of Ground Source Heat Pumps (GSHP) requires a multidisciplinary approach including a good understanding of the underground geological setting, such as hydrogeological aspects and heat flow conditions. Classic monitoring strategies often rely on local and point-based measurements to monitor changes of underground temperature in time, with the limit of not succeeding in a whole delimitation of the Thermal Active Zone (TAZ). In this context, Electrical Resistivity Tomography (ERT) can bring relevant information on the temperature distribution for monitoring the induced thermal plume within BHEs (Borehole Heat Exchangers) systems. Geophysics helps the understanding of the thermal processes, in order to front the difficulties arising from Ground Source Heat Pumps (GSHP) implementation. Thermal conductivity and electrical resistivity depend equally in a complex way on different common subsurface and environmental attributes such as, among the main, mineralogical composition, grain size, density, porosity and saturation. Besides, thermal conductivity increases significantly with temperature in wet ground, by making it clear a relationship between both parameters.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;ERT is particularly sensitive to the porous medium temperature and, when applied in time-lapse (TL), could provide spatially distributed information on the changes over time of water content, salinity or temperature. For this reason, in this work we monitored the complex TAZ temporal evolution during a heat injection experiment using a 3D time-lapse ERT survey, arranged in a reduced scale physical model. For a better understanding of measured electrical resistivity values, focused on mapping the extent of a geothermal plume around a borehole, a specific laboratory device was utilized. Grain size distribution, bulk density and saturation of the porous medium are known and established, as well as reliable temperature values acquired through sensors with which calibrate the ERT results. Thus, changes in resistivity can be interpreted to track the evolution of the plume of heated water and used to estimate the temperature change. The propagation of the heat plumes into the ground is also highly sensitive to interstitial water flow rate, thus also this condition was recreated and monitored varying the hydraulic gradient in the experimental device.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The present work aims to demonstrate the ability of ERT to provide complementary insights about the sub-surface spatio-temporal dynamic for monitoring the extension of TAZ caused by BHEs probes. In addition, the detailed scale adopted and the variable control within a laboratory setup ease the study of the interaction between thermal and electrical properties.&amp;lt;/p&amp;gt;