Electrical Resistivity Tomography Images Research Articles

Soil electrical resistivity monitoring as a practical tool for evaluating irrigation systems efficiency at the orchard scale: a case study in a vineyard in Central Chile

In many orchards, irrigation scheduling is designed based on data from meteorological networks and considering homogeneous soil properties. Such assumptions may result in inefficient irrigation, which is difficult to constrain without expensive or invasive techniques. Here we have evaluated the ability of the electrical resistivity tomography (ERT) for detecting meter-scale irrigation uniformity and deep percolation during irrigation. The spatiotemporal variability of soil volumetric water content (VWC) in a vineyard located near Santiago (Chile) was inferred using ERT monitoring of two irrigation cycles. The electrical resistivity structure up to 4 m depth was estimated using two-dimensional inversion of ERT data. ERT results were verified by comparing resistivity models with VWC measured with soil moisture sensors, soil properties mapped in a 2 m-depth soil pit, and the spatiotemporal evolution of VWC obtained by solving numerically Richards equation. Largest temporal variations of resistivity were observed within the root depth (1 m) and are consistent with expected relative changes in VWC during irrigation. ERT images exhibit lateral changes in resistivity at these depths, likely indicating non-uniform infiltration of water controlled by observed soil texture variations. Resistivity changes were also observed below the root zone, suggesting that a fraction of the irrigation water percolates downward. These findings can be explained by an excess of irrigation water applied during the monitoring, which was planned considering regional evapotranspiration (ET) data that overestimated the actual ET measured at the vineyard. Altogether, our results suggest that ERT monitoring during irrigation is a cost-effective tool to constrain the performance of irrigation systems.

Development of an Electrical Resistivity Tomography Monitoring Concept for the Svelvik CO<sub>2</sub> Field Lab, Norway

Abstract. Within the ERA-NET co-funded ACT project Pre-ACT (Pressure control and conformance management for safe and efficient CO2 storage – Accelerating CCS Technologies), a monitoring concept was established to distinguish between CO2 induced saturation and pore pressure effects. As part of this monitoring concept, geoelectrical cross-hole surveys have been designed and conducted at the Svelvik CO2 Field Lab, located on the Svelvik ridge at the outlet of the Drammensfjord in Norway. The Svelvik CO2 Field Lab has been established in summer 2019, and comprises four newly drilled, 100 m deep monitoring wells, surrounding an existing well used for water and CO2 injection. Each monitoring well was equipped with modern sensing systems including five types of fiber-optic cables, conventional- and capillary pressure monitoring systems, as well as electrode arrays for Electrical Resistivity Tomography (ERT) surveys. With a total of 64 electrodes (16 each per monitoring well), a large number of measurement configurations for the ERT imaging is possible, requiring the performance of the tomography to be investigated beforehand by numerical studies. We combine the free and open-source geophysical modeling library pyGIMLi with Eclipse reservoir modeling to simulate the expected behavior of all cross-well electrode configurations during the CO2 injection experiment. Simulated CO2 saturations are converted to changes in electrical resistivity using Archie's Law. Using a finely meshed resistivity model, we simulate the response of all possible measurement configurations, where always two electrodes are located in two corresponding wells. We select suitable sets of configurations based on different criteria, i.e. the ratio between the measured change in apparent resistivity in relation to the geometric factor and the maximum sensitivity in the target area. The individually selected measurement configurations are tested by inverting the synthetic ERT data on a second coarser mesh. The pre-experimental, numerical results show adequate resolution of the CO2 plume. Since less CO2 was injected during the field experiment than originally modeled, we perform post-experimental tests of the selected configurations for their potential to image the CO2 plume using revised reservoir models and injection volumes. These tests show that detecting the small amount of injected CO2 will likely not be feasible.

Integrated MASW and ERT Imaging for Geological Definition of an Unconfined Alluvial Aquifer Sustaining a Coastal Groundwater-Dependent Ecosystem in Southwest Portugal

This paper integrates multichannel analysis of surface waves (MASW) and time-lapse electrical resistivity tomography (ERT) to define aquifer geometry and identify transient groundwater features of the Cascalheira Stream Basin Holocene alluvial aquifer (aquifer H), which contributes to the Santo André Lagoon, part of a coastal groundwater-dependent ecosystem (GDE), located in southwest Portugal. MASW measures shear-wave velocity (VS), allowing one to obtain steady geological models of the subsurface, and ERT measures subsurface electrical resistivity (ER), being subjected to ambient changes. MASW enables disambiguation of geological structures in low ER environments, such as coastal areas. This research covered one natural year and involved one MASW campaign, four ERT campaigns, and additional geological field surveys and groundwater monitoring to assist interpretation of results. In the area, the conjugate NW–SE and NE–SW strike-slip fault systems determine compartmentalization of geological structures and subsequent accommodation space for Holocene sedimentation. MASW and ERT surveys show how the NW–SE system deepens these structures toward the coast, whereas the NE–SW system generates small horsts and grabens, being one of these occupied by aquifer H. From upstream to downstream, aquifer H thickness and width increase from 10 m to 12 m and from 140 m to 240 m, respectively. Performance of VS and ER models was satisfactory, with a normalized error of the VR and ER models in the 0.01–0.09 range, meaning that a quantitative quota of uncertainty can be segregated from the overall uncertainty of groundwater models without substantially affecting its simulations accuracy. This methodology seeks to improve the design of shallow groundwater research in GDE preservation policies.

Electrical resistivity tomography monitoring of two managed aquifer recharge ponds in the alluvial aquifer of the Llobregat River (Barcelona, Spain)

ABSTRACTOver the past 20 years, there has been growing interest in the use of the subsurface for water storage using shallow ponds, where water is infiltrated to the subsurface and subsequently groundwater is recovered from pumping wells. This scheme is designed as a surface‐managed aquifer recharge. Llobregat artificial recharge ponds are managed aquifer recharge systems located in alluvial aquifers near Barcelona, with strong significance for water supply to the city. The recharge ponds have shown low infiltration rates since the beginning (e.g., Ca n'Albareda) and a significant decrease after some months (e.g., Sant Vicenç). Consequently, different methodologies were designed for monitoring the systems and evaluating the effectiveness of the selected areas and maintenance procedures. For this purpose, we combined the use of electrical resistivity tomography with standard hydrogeological methodologies, including water table monitoring from piezometers and infiltration tests. The combination of direct and indirect methods have allowed us to improve the diagnosis of the subsurface involved in the managed recharge system. The electrical resistivity tomography technique has shown to be a cost‐effective and high‐resolution tool, flexible and well adaptable for surveying at different scales without disturbing the recharge process. As a consequence, we demonstrate the usefulness of electrical resistivity tomography imaging to unveil hydrogeological heterogeneities and monitoring infiltration, the effect of clogging and clean‐up processes in surface‐managed aquifer recharge projects.

Integrated hydrogeophysical modelling and data assimilation for geoelectrical leak detection

Time-lapse electrical resistivity tomography (ERT) measurements provide indirectobservations of hydrological processes in the Earth's shallow subsurface at high spatial and temporal resolution. ERT has been used in the past decades to detect leaks and monitor the evolution of associated contaminant plumes. Specifically, inverted resistivity images allow visualization of the dynamic changes in the structure of the plume. However, existing methods do not allow the direct estimation of leak parameters (e.g. leak rate, location, etc.) and their uncertainties. We propose an ensemble-based data assimilation framework that evaluates proposed hydrological models against observed time-lapse ERT measurements without directly inverting for the resistivities. Each proposed hydrological model is run through the parallel coupled hydro-geophysical simulation code PFLOTRAN-E4D to obtain simulated ERT measurements. The ensemble of model proposals is then updated using an iterative ensemble smoother. We demonstrate the proposed framework on synthetic and field ERT data from controlled tracer injection experiments. Our results show that the approach allows joint identification of contaminant source location, initial release time, and solute loading from the cross-borehole time-lapse ERT data, alongside with an assessment of uncertainties in these estimates. We demonstrate a reduction in site-wide uncertainty by comparing the prior and posterior plume mass discharges at a selected image plane. This framework is particularly attractive to sites that have previously undergone extensive geological investigation (e.g., nuclear sites). It is well suited to complement ERT imaging and we discuss practical issues in its application to field problems.

Characterization of displacement and internal structure of landslides from multitemporal UAV and ERT imaging

Preliminary knowledge of the surface movement and internal structure of a landslide is essential to improve the understanding of landslide behavior as an initial step to develop suitable mitigation measures. This paper characterizes displacement and internal structure of landslides using multitemporal unmanned aerial vehicles (UAV) and electrical resistivity tomography (ERT) imaging. Multitemporal orthophotos, digital elevation model (DEM) profile lines, DEMs of difference (DoD), and ERT images allowed us to measure the landslide morphometry and areal changes, detect the trend of surface displacement, calculate the surface movement rate and direction, characterize the spatial variability in surface movement rates, and relate spatial variation in displacement to the internal structure of the landslide. The Kalisari Landslide experienced retrogressive movement where the scarp of landslide moved backward 38 m from 2015 to 2019. The surface movement ranged from 0.7 to 8.1 m with the direction mostly to the northeast. The depletion and accumulation ranged from 0.1 to 5 m, with the most active change located northwest of the landslide. The variability in movement was influenced by the spatial variability in the subsurface material inferred from the ERT images, i.e., surface material, clay, weathered breccias (possibly altered), and breccias. The interpretation from multitemporal UAV and ERT images indicates that the Kalisari Landslide has a nonhomogeneous multiple retrogressive behavior with a curved slip surface located at the clay layer.

Snapshot of a magmatic/hydrothermal system from electrical resistivity tomography and fumarolic composition, Whakaari/White Island, New Zealand

Combined interpretation of Electrical Resistivity Tomography (ERT) data, with plume and fumarolic gas emissions provides a snapshot of the Whakaari/White Island hydrothermal system 11 months prior to an eruption on December 9, 2019. Two and three dimensional inversion of the ERT data images the F0 fumarole as a low resistivity (2–5 Ωm) feature reflecting the two-phase zone surrounding the single phase vapour conduit. Crucially, interpretation of the inversion images is well constrained by a large existing dataset of rock electrical properties, including surface conductivity, porosity and intrinsic formation factor, alongside new measurements of liquid phase electrical conductivity taken from a range of hot-springs on the island. Pore-filling liquids are an order of magnitude more conductive than sea-water with their low to very low pH (3.4 to −0.3) contributing to their extremely conductive nature. An extensive low-resistivity feature (0.2 Ωm) at ~125 m depth is therefore interpreted as a liquid saturated layer exposed to hydrothermal alteration by acid fluids. The intersection of the F0 fumarole trace with this layer is coincident with a previously determined source of deformation, suggesting that heat transport inside the fumarole conduit pressurises surrounding liquid and vapour filled pore space, generating the deformation signal. The timeseries of fumarolic and plume emissions in the year prior to the ERT survey show that the gases transported by the fumarole are a mixture of high temperature magmatic vapour and lower temperature gas equilibrated within the hydrothermal environment. Interpretation of the gas data suggests that the snapshot captured by the ERT image shows the single-phase vapour and two-phase vapour-liquid regions adjacent to the conduit were likely at close to their smallest extent, consisting of mostly hydrothermal derived gases.

Application of principal component analysis to geo-electrical recordings

Nowadays, electrical resistivity tomography (ERT) is able to furnish a global reconstruction of the main features of buried structures at a shallow scale, in a faster and cost-effective manner. The use of a controlled source makes ERT less sensitive to noise with respect to other geophysical methodologies based on natural sources, such as magnetotelluric. During field practice, the depth of exploration is limited by the relative distances between the source and receiver dipoles. The latest generation resistivimeters favor the simultaneous recording of signals in correspondence with multiple and physically separated receivers. These characteristics concur towards a more profitable application of ERT imaging in the case of deeper targets distributed within harsh or densely inhabited large areas. At the same time, the ease of placing a good number of receivers far from the electric sources enhances the need of tools apt for extracting weaker signals from background noise of both natural and anthropic nature. For this purpose, the use of the well-known method, i.e., principal component analysis (PCA) as a filtering tool is tested on the geoelectrical time series acquired in the Mt. Vesuvius area (Naples, Italy). A field and a test dataset have been derived from such recordings and subsequently processed through an original procedure based on PCA. Subsequently, the results have been compared with the ones obtained using more typical estimators, such as stacking or frequency filtering, thus evaluating the usefulness of PCA in the processing of the geoelectrical time series. A good estimate of the said parameters can also be obtained in the case of source-to-receiver distance of more than 5 km.

Identification of underground water-bearing caves in noisy urban environments (Wuhan, China) using 3D electrical resistivity tomography techniques

Caves formed in hard limestone due to groundwater erosion and rainwater infiltration may cause damages of buildings. The electrical resistivity tomography (ERT) with high lateral resolution and low economic cost is a good way to detect the caves, as the electrical resistivity of water-bearing caves is much lower than that of surrounding rocks. In this study, we arranged three sets of intersected ERT profiles to detect the unknown water-bearing caves in an area of Wuhan city of China. Both two-dimensional and three-dimensional inversions were performed to obtain the underground electrical resistivity tomography images. The reliability of imaged water-bearing caves was analyzed in terms of the reliability analysis tool. The inversion results not only agree well with the drilling results, but also detect five possible locations of water-bearing caves in the limestone bedrock. The findings of these underground water-bearing caves in advance can effectively avoid potential safety hazards as this area will become the commercial center of Wuhan city.

The Effect of Hurricane Irma Storm Surge on the Freshwater Lens in Big Pine Key, Florida using Electrical Resistivity Tomography

Animals and plants on low elevation oceanic islands often rely on a thin lens of fresh groundwater and this lens is vulnerable to seawater contamination from storm surge. Documentation of the impact of the storm surge on the freshwater lens and its subsequent recovery is limited. In September 2017, Hurricane Irma made landfall in the Florida Keys as a category 4 storm with storm surge heights in excess of 2 m. This study used Electrical Resistivity Tomography (ERT) to investigate the effect of the storm surge on the freshwater lens of Big Pine Key, FL. The study compared ERT images along three profiles ranging between 220 and 280 m length collected in 2011 with post storm data collected about 3 to 4 months (November 2017/January 2018) and eight (May 2018) months after Irma. The post storm data documented that the storm surge impacted the freshwater lens on all three profiles with low resistivity (i.e., high salinity) zones in the upper 2 m of the groundwater. The increase in salinity was most pronounced in the lower elevations of the profiles. The May 2018 data were collected immediately after 2 weeks of intense precipitation. These data showed 40% recovery of the freshwater lens, most pronounced in the lower elevation of the profiles. This suggests that both the impact of storm surge and the freshwater recovery due to precipitation are most pronounced in low elevation regions where both saline and freshwater can collect at the surface.

Geoelectrical characterization and monitoring of slopes on a rainfall-triggered landslide simulator

In this paper, we present the results of time-lapse electrical resistivity tomography (ERT) monitoring of rainfall-triggered shallow landslides reproduced on a laboratory-scale physical model. The main objective of our experiments was to monitor rainwater infiltration through landslide body in order to improve our understanding of the precursors of failure. Time-domain reflectometry (TDR) data were also acquired to obtain the volumetric water content. Knowing the porosity, water saturation was calculated from the volumetric water content and we could calibrate Archie's equation to calculate water saturation maps from inverted resistivity values. Time-lapse ERT images proved to be effective in monitoring the hydrogeological conditions of the slope as well as in detecting the development of fracture zones before collapse. We performed eight laboratory tests and the results show that the landslide body becomes unstable at zones where the water saturation exceeds 45%. It was also observed that instability could occur at the boundaries between areas with different water saturations. Our study shows that time-lapse ERT technique can be employed to monitor the hydrogeological conditions of landslide bodies and the monitoring strategy could be extended to field-scale applications in areas prone to the development of shallow landslides.

Mapping the contaminant plume of an abandoned hydrocarbon disposal site with geophysical and geochemical methods, Jiangsu, China.

The management and remediation of abandoned hydrocarbon-contaminated sites require detailed information on the distribution of contaminant plumes. In areas where groundwater is active, the formation of contaminant plumes is associated with hydrodynamics, the nature of the sedimentary layers, and the nature of the pollutants and the degradation process. A comprehensive survey is needed to determine this information. An abandoned hydrocarbon disposal site is located in an area where groundwater is very active. In the investigation of contaminant plumes, we combined the geophysical method with accurate geochemical analysis of subsoil and groundwater samples. Ground-penetrating radar and electrical resistivity tomography images of the electrical anomalies potentially originating from hydrocarbon pollution were used to select sites for subsurface sampling. Total petroleum hydrocarbons, total dissolved solids, and groundwater pH were measured. The results showed that the source zone had undergone long-term natural attenuation, and it was unable to continuously output organic matter to support the expansion of contaminant plumes. Low-resistivity anomalies and enhanced attenuation in the study area were caused by hydrocarbon degradation products and enhanced mineral weathering. Delineating the distribution of contaminant plumes in areas where the resistivity was below 15Ωm. The distribution of the plume in the vertical direction was related to the hydrocarbon release history (release rate and volume) and was affected by fluctuations in the groundwater level. The contaminant plume moved very slowly along the direction of the hydraulic gradient and was in a basically stable state. The results showed that the combined application of the geoelectrical method and the geochemical method can effectively describe the distribution of underground contaminant plumes in an aged pollution site.

Urban landslide monitoring by combined use of multiple methodologies - a case study on Sv. Anton town, Slovakia

ABSTRACTWe employed integrated methods to assess the landslide movement in Sv. Anton town in the Western Carpathians Neogene Volcanic Field (Central Slovakia). The integrated diagnostics required study of the landslide kinematic activity by a combination of Global Navigation Satellite Systems (GNSS) and Electrical Resistivity Tomography (ERT) imaging from November 2013 to March 2015. A topographic model with 2-cm accuracy was constructed from Unmanned Aerial Vehicles (UAV) photogrammetry. Continuous spatial datasets of movement and displacement field vectors were interpolated from the measured movements over the entire study period. Although deformation studies in Slovakia have a long-term tradition, complex interdisciplinary studies in urbanized areas are still lacking. This inspired our main objectives: to identify landslide kinematics and to reconstruct and define the rates of annual landslide movement obtained from geodetic measurement at the monitoring points. Our results demonstrate how landslide integrated diagnostics contribute to the detection of slope instability, with a maximum velocity of 60.82 mm/yr during the summer period. The precipitation effects are consistent with the Sv. Anton landslide displacement acceleration, and the following increases in total monthly precipitations are staggering compared to long-term monthly averages: July precipitation increased by 175.3%, August by 203.3%, and September by 198.1%.

A Probability Electrical Resistivity Tomography Imaging of complex tectonic features in the Kissamos and Paleohora urban areas, Western Crete (Greece).

A number of 2D Electrical Resistivity Tomographies (ERT) were conducted in northwest and southwest Crete with the aim to model geotectonic features at selected urban sites. The study area is located within the central forearc of the Hellenic subduction zone. Thirteen ERT profiles were realized using the dipole–dipole electrode and/or the Wenner-Schlumberger arrays . In order to model the resistivity distribution, the probability-based electrical resistivity tomography inversion (PERTI) method has been applied . The main features of the PERTI method are: (i) unnecessity of a priori information; (ii) full, unconstrained adaptability to any kind of dataset, including the case of nonflat topography; (iii) independence from data acquisition techniques and spatial regularity; (iv) capability to resolve complex continuous resistivity variation. Since the present case-study is the first application of the PERTI algorithm to locate fault zones and near subsurface irregularities, its routine has been tested using the well-known RES2DINV commercial software as reference. The comparison has shown a common coherence between the results of the two inversion methods and confirmed the better filtering capacity and greater versatility of the PERTI algorithm, already outlined in a number of previous papers dealing with archaeological applications.

Geophysical Mapping of Shallow Rock Salt at Borabue, Northeast Thailand

ABSTRACTIn the northeast of Thailand, many swamps and lakes show evidence of rock salt dissolution, with attendant environmental problems due to saline groundwater and soil salinization. The knowledge of the depth and structure of the rock salt would be useful to achieve a better understanding of the impact of its dissolution on the environment and to facilitate effective land management strategies. Geophysical methods can provide key subsurface information. Four seismic refraction profiles and three electrical resistivity tomography profiles were carried out between Nong Bo and Nong Pong reservoirs in the salt‐affected Borabue district of Maha Sarakham province. The seismic results indicate the top of shallow rock salt beneath Nong Bo reservoir as a diagnostic zone of high P‐wave velocity >3400 m/s at depths >20 m. The electrical resistivity tomography investigation shows electrical resistivity values ≳5 Ωm at similar depths, indicative also of shallow rock salt. Zones interpreted to be saline groundwater are electrically conductive, identified by electrical resistivity values ≲5 Ωm in the electrical resistivity tomography images. The combined seismic–electrical resistivity tomography interpretation suggests that shallow rock salt lies beneath the reservoirs and ponds in the Borabue district but does not appear to form a continuous body between the two largest reservoirs.

Application of electrical resistivity tomography to monitor the dewatering of vertical and horizontal wells in municipal solid waste landfills

The performance of vertical and horizontal wells is usually evaluated using water level wells or piezometers in landfills. In this study, pumping tests in vertical and horizontal wells were performed at the Chang'an and Tianziling landfills, respectively. During the pumping tests, the leachate level drawdown around the wells and the resistivity of the surrounding waste were both measured to investigate the feasibility of applying electrical resistivity tomography (ERT) to monitor the dewatering in landfills. It was found that the resistivity ρ of the waste samples drilled from the Tianziling landfill decreased as volumetric moisture content θ increased, and the relationship could be fitted with Archie's law (ρ = ρwaθ-m). For waste samples with the same θ, ρ tended to increase as the depth of the samples increased. It was because the waste at deeper layers experienced longer degradation and tended to have greater proportion of inert particles like cinder and dust. During a vertical well pumping test in the Chang'an landfill, the resistivity of waste on the right side of the well increased by 10% to 30%. The distribution of areas where resistivity increased was heterogeneous. During two horizontal well pumping tests in the Tianziling landfill, the areas where resistivity increased were distributed in an ellipse around one well and were horizontally distributed around another well. The resistivity of waste near the two wells increased by 4% to 18%. According to the ERT testing results, the influence radii of the two wells were estimated as 38.7 m and 33.2 m, respectively. These were close to the values (i.e., 42.1 m and 34.8 m, respectively) obtained through leachate level testing results. Artifacts produced by the inversion process were both found in the ERT images of vertical and horizontal well pumping tests. The combined use of ERT with other hydrogeological monitoring methods can improve the understanding of draining effects by vertical and horizontal wells in landfills.

Delineation of subsurface variability in clay-rich landslides through spectral induced polarization imaging and electromagnetic methods

Although commonly triggered by heavy or long-lasting precipitation events, landslides in clay-rich formations are hardly predictable as their triggering mechanisms are still not fully understood. Hence, detailed information about the internal structure of landslides with high spatial resolution is fundamental for an improved understanding of triggering mechanisms and management. In this context, electrical resistivity tomography (ERT) is a well-established method used for the delineation of lithological interfaces in landslides and changes in water content. However, the quantitative interpretation of the ERT images can be challenging, particularly for clay-rich landslides, considering that the electrical signatures are dominated by the variations in the clay fraction rather than saturation. Moreover, large scale investigations also demand the development of faster surveying techniques. Hence, in this study we discuss the combined investigation of a landslide with low induction number electromagnetic (EMI) mapping and induced polarization (IP) imaging. We investigate the application of such methods to assess subsurface variability across different scales: near surface EMI mapping and their correlation with geomorphological data, whereas IP images are used to investigate the extension at depth from the shallow structures. Measurements are performed at a shallow clay-rich landslide in Lower Austria (Austria), characterized by lithological structures of the Flysch and Gresten Klippen Zone, with both formations known to be highly susceptible to landsliding. The interpretation of the imaging results for data collected along 15 profiles is performed using separately acquired hydrogeological and geotechnical data. We observe a relationship between the electrical properties and geotechnical parameters which permits to delineate areas associated to different weathering stages controlling the groundwater flow. Moreover, spectral induced polarization (SIP) data collected along one transect shows sensitivity to changes in textural composition, such as the fraction of clay minerals. The study shows the suitability of an integrated investigation with EMI mapping and IP imaging in combination with extensive geotechnical data for an improved characterization of subsurface variability and thus, understanding of clay-rich landslides.

Estimating anisotropic heterogeneous hydraulic conductivity and dispersivity in a layered coastal aquifer of Dakshina Kannada District, Karnataka

The solution for the inverse problem of seawater intrusion at an aquifer scale has not been studied as extensively as forward modeling, because of the conceptual and computational difficulties involved. A three-dimensional variable-density conceptual phreatic model is developed by constraining with real-field data such as layering, aquifer bottom topography and appropriate initial conditions. The initial aquifer parameters are layered heterogeneous and spatially homogeneous that are based on discrete field measurements. The developed conceptual model shows poor correlation with observed state variables (hydraulic head and solute concentration), signifying the importance of spatial heterogeneity in hydraulic conductivity and dispersivity of all the layers. The conceptual model is inverted to estimate the anisotropic spatially varying hydraulic conductivity and the longitudinal dispersivity at the pilot points by minimizing the least square error of state variables across the observation wells. The inverse calibrated model is validated for the hydraulic head at validation wells and the solute concentration is validated with equivalent solute concentration derived from the electrical resistivity, which shows good results against the field measurements. The verification of estimated anisotropic hydraulic conductivity with the electrical resistivity tomography image shows good agreement. This investigation gives an insight about constraining the highly parameterized inverse model with real-field data to estimate spatially varying aquifer parameters for an effective simulation of the seawater intrusion in a layered coastal aquifer.

Detection of Cavity Using Electrical Resistivity Tomography (ERT) at Patherdih, Jharia Coal Field, Dhanbad, India

Geophysical survey for cavity detection is one of the most common near surface application. The usage of resistivity methods are also very straightforward for the air filled underground voids, which should have theoretically very high resistivity in the ERT image, whereas the water filled cavities have low resistivity in Electrical Resistivity Tomography (ERT) image. Three different geoelectric arrays Wenner, Schlumberger and Dipole-dipole are applied for detecting cavities. The survey results suggest that ERT is a viable geophysical tool for the detection and monitoring of mining voids and other subsurface cavities. The resolution capacity of joint Wenner-Schlumberger, Dipole-dipole array for the detection of cavity is greater than that of the individual one. Using the ERT survey in Patherdih Thana Basti village (Jharia coal field), a number of air filled cavity or water filled mining voids are detected at around 25-30m of below the ground level at different profile distance of 630m profile length along the road that extending S-E to N-W direction.

ERT imaging of the interior of the huge tumulus of Kastas in Amphipolis (northern Greece)

AbstractThe geophysical investigations aiming to image the interior of the huge tumulus of Kastas in Amphipolis in northern Greece commenced on November of 2014 and continued sporadically until the first months of 2017. The electrical resistivity tomography (ERT) method was exclusively employed in both conventional and innovative schemes. Due to the immense size of the tumulus, an irregular grid was established on the tumulus whereas regular grids of ERTs were established on a small terrace created by the excavations and at the south extremity. Also, typical two‐dimensional (2D) ERTs carried out to help clarifying the geological and tectonic setting. Kastas tumulus was created by covering a hill with large amounts of transported earth material. In this respect it constitutes a huge human construction. Further, the excavations revealed a unique building containing ancient art masterpieces. The irregular grid on top constitutes the first effort of such an approach to investigate the interior of a tumulus since all known efforts have been implemented employing regular grids. Consequently, a three‐dimensional (3D) smoothness constrained inversion algorithm was modified. At first, the irregularly placed electrode locations had to fit in a finite element forward solver having particular specifications. Further, the hexahedral elements of the created mesh were manipulated to cope with neighbouring electrode positions in order to make a less dense grid. Finally, a so called ‘nudging’ process was followed to merge closely positioned electrodes belonging to different, branching, 2D data acquisition lines. The geophysical imaging helped clarify the geological and tectonic structure of Kastas. Further, it yielded several findings some of them being interpreted as natural features and some others as probable manmade. The latter comprise the targets of future excavations.