Innovative subsurface stratigraphy interpretation by integrating electrical resistivity tomography and borehole data

A common practice used by the Germans and collaborators in World War II, as part of the Holocaust, was to use existing Jewish cemeteries as places for mass burial. Research was completed at the Old Jewish Cemetery in Riga, Latvia, the Livas Jewish Cemetery in Liepaja, Latvia, and the Zaliakalnis Jewish Cemetery in Kaunas, Lithuania. The Old Jewish Cemetery in Riga was adjacent to the Riga Ghetto and was used to bury individuals murdered in the ghetto. In Kaunas, an area of the Zaliakalnis Jewish Cemetery is devoid of grave stones, and literature sources and testimony indicate that this area was used for the mass burial of Jews from the Kaunas Ghetto and other mass killings. In Liepaja, the local Jewish Heritage Foundation believes that there are mass graves within the Livas Cemetery. Methodologies for this research include the use of a pulseEkko Pro 500-megahertz ground-penetrating radar (GPR) system. Electrical resistivity tomography (ERT) data were collected through a linear array of electrodes coupled to a direct current (DC) resistivity transmitter and receiver. Analysis of aerial photography and satellite images was also employed at each location. ERT and GPR data indicate three separate trench anomalies in the Old Jewish Cemetery in Riga. The presence of these anomalies corroborates Holocaust survivor testimony that bodies were buried in mass graves in that area. In the Zaliakalnis Jewish Cemetery in Kaunas, ERT and GPR data indicate an anomaly in the western part of the cemetery, and ERT data further indicate two other possible mass graves. In Liepaja, preliminary GPR analysis indicates an anomaly in a cleared section of the cemetery. Based on the presence of geophysical anomalies in all three cemeteries, which correlate with literature sources and Holocaust survivor testimony, there is a high probability that mass graves are present at each site. Future research directions include expanding the search areas in each cemetery, additional literature and testimony-based research, and the addition of other geophysical methodologies.

Durusu Lake is the biggest and most important freshwater source supplying drinking water to the European side of Istanbul. In this study, electrical resistivity tomography (ERT) and transient electromagnetic (TEM) measurements were applied to detect a possible salt water intrusion into the lake and to delineate the subsurface structure in the north of Durusu Lake. The ERT and TEM measurements were carried out along six parallel profiles extending from the sea coast to the lake shore on the dune barrier. TEM data were interpreted using different 1-D inversion methods such as Occam, Marquardt, and laterally constrained inversion (LCI). ERT data were interpreted using 2-D inversion techniques. The inversion results of ERT and TEM data were shown as resistivity depth sections including topography. The sand layer spreading over the basin has a resistivity of 150–400 Ωm with a thickness of 5–10 m. The sandy layer with clay, silt, and gravel has a resistivity of 15–100 Ωm and a thickness of 10–40 m followed by a clay layer of a resistivity below 10 Ωm. When the inversion of these data is interpreted along with the hydrogeology of the area, it is concluded that the salt water intrusion along the dune barrier is not common and occurs at a particular area where the distance between lake and sea is very close. Using information from boreholes around the lake, it was verified that the common conductive region at depths of 30 m or more consists of clay layers and clay lenses.

Combined application of GPR and ERT for the assessment of a wall structure at the Heptapyrgion fortress (Thessaloniki, Greece)

We conducted a field experiment at the agricultural field site Voulund within the Danish hydrological observatory, HOBE, with the purpose of estimating recharge using geophysical methods. In September 2011, a saline tracer was added across a 142‐m2 area at the surface at an application rate mimicking natural infiltration. The movement of the saline tracer front was monitored using cross‐borehole electrical resistivity tomography (ERT); data were collected on a daily to weekly basis and continued for 1 yr after tracer application. The ERT data were inverted and corrected for temperature changes in the subsurface, and spatial moment analysis was used to calculate the tracer mass, position of the center of mass, and thereby the downwardly recharging flux. The recovered mass was underestimated by the ERT data by up to 50%. Mass balance errors are widely recognized and are a result of variable resolution of the tomographic models and smoothing applied in the inversion routine. The results were nonetheless in very good agreement with pore water samples collected and analyzed from five cores extracted within the tracer application area during the same period. Recharge during the 7.5 mo from September 2011 to the end of April 2012 was estimated to be about 500 mm using the ERT data. This value is in good accord with recharge estimates made based on drainage data from buried lysimeters located only meters away from the cross‐borehole ERT array. This suggests that long‐term automated ERT monitoring of a surface‐applied tracer is a promising technique for estimating groundwater recharge.

ABSTRACTA large‐scale electrical resistivity tomography (ERT) survey was undertaken around the Neolithic tell of Szeghalom‐Kovácshalom in southeast Hungary, covering an area of almost 6 ha. High‐resolution ERT data were collected along 28 uniformly distributed transects of variable length using the roll‐along technique. A recently presented two‐dimensional fast non‐linear resistivity inversion algorithm was used to invert the ERT data and recover the true subsurface resistivity distribution along the specific cross‐sections. The algorithm calculates and stores in an efficient manner the part of the Jacobian matrix that is actually important within the inversion procedure, thus rendering it almost 4.8 times faster than the algorithm that calculates the complete Jacobian matrix, without losing quality. The algorithm was further modified to account for any non‐standard electrode configuration. A recently established iterative algorithm for sparse least squares problems (LSMR) was incorporated for the first time into the algorithm to solve the inverse resistivity problem. The effectiveness and robustness of the LSMR solver was highlighted through the processing of all the ERT lines. The processing and evaluation of the ERT data made it possible to map the thickness of the anthropogenic layer below the surface of the tell, to outline the horizontal and vertical dimensions of the palaeochannel adjacent to the tell, and to determine the general stratigraphy of geological layers up to 10 m below the ground surface. The ERT results also were used to update an older topographic map of the site showing the course of the palaeochannel around the tell. A synthetic model verified and enhanced the conclusions based on the field data. This study illustrates the added value that a systematic ERT survey can provide in reconstructing the ancient fluvial geomorphology of a microregion as well as the depth and horizontal extent of deposits associated with human habitation at archaeological sites. Copyright © 2014 John Wiley & Sons, Ltd.

<p>A full understanding of the moisture regimes in agricultural soils is critical when considering a climatic future that is potentially both drier and hotter. The study of near-surface hydrodynamics necessitates a high temporal frequency of measurements to allow the capture of fast-moving processes. In addition to this, heterogeneities in soils, coupled with spatially variable moisture contents, mean that point sensors alone can miss valuable information when studying soil hydrodynamics.</p><p>The sensitivity of Electrical Resistivity Tomography (ERT) to soil water content means that it is ideally suited to studying soil hydrodynamics. As a non-invasive technique, it is able to effectively capture spatial heterogeneities, while repeating ERT measurements at short time intervals enables the monitoring of rapid soil moisture changes.</p><p>In order to study Conservation Agriculture (CA) - an agricultural technique shown to have increased water and heat stress resilience - we collect frequent ERT measurements (≤ daily) over a long period of time (> 2 years) using PRIME - a low-cost, low-power, ERT monitoring instrument developed by the British Geological Survey. Our equipment is permanently installed at three agricultural observatories in southern Africa (Malawi, Zimbabwe, and Zambia), and is complemented by many co-located point sensors monitoring soil temperature, water content, and matric potential.</p><p>We compare the hydrodynamics - derived from ERT data - found under CA, with those under conventional agricultural tillage methods, to better understand the root of CA’s resilience to hot and dry weather conditions. We use laboratory measurements together with the high number of co-located point sensor measurements to build pedological / geophysical relationships for each site, comparing those derived from laboratory scale samples to those derived from in-field measurements.</p>

Monitoring the perturbation zone near a foundation excavation with electrical resistivity tomography: Comparison between time-lapse 3D and 2D inversions in single-profile study

Sea level rise and coastal erosion have inundated large areas of Arctic permafrost. Submergence by warmer and saline waters increases the rate of inundated permafrost thaw compared to sub-aerial thawing on land. Studying the contact between the unfrozen and frozen sediments below the seabed, also known as the ice-bearing permafrost table (IBPT), provides valuable information to understand the evolution of sub-aquatic permafrost, which is key to improving and understanding coastal erosion prediction models and potential greenhouse gas emissions. In this study, we use data from 2D electrical resistivity tomography (ERT) collected in the nearshore coastal zone of two Arctic regions that differ in their environmental conditions (e.g., seawater depth and resistivity) to image and study the subsea permafrost. The inversion of 2D ERT data sets is commonly performed using deterministic approaches that favor smoothed solutions, which are typically interpreted using a user-specified resistivity threshold to identify the IBPT position. In contrast, to target the IBPT position directly during inversion, we use a layer-based model parameterization and a global optimization approach to invert our ERT data. This approach results in ensembles of layered 2D model solutions, which we use to identify the IBPT and estimate the resistivity of the unfrozen and frozen sediments, including estimates of uncertainties. Additionally, we globally invert 1D synthetic resistivity data and perform sensitivity analyses to study, in a simpler way, the correlations and influences of our model parameters. The set of methods provided in this study may help to further exploit ERT data collected in such permafrost environments as well as for the design of future field experiments.

To investigate the pathways for saline water intrusion into fresh water aquifers the electrical resistivity tomography (ERT) and ground penetrating radar (GPR) have been pooled for Nagapattinam area of Southern India. The study area has two river sub-basins, Uppanar River Basin (URB) and Cauvery River Basin (CRB). ERT carried out in URB shows very low resistivity ranging from 0.003 to 1.5 Ω m at shallow depth indicating saline water, whereas the resistivity ranges away from the river stream are 1.7–5.0 Ω m. Cauvery river basin shows resistivity range varying from 3.6 to 150 Ω m indicating a salt-fresh water mix. Similarly, few GPR profiles were carried out along ERT profiles to confirm the saline water intrusion and its pathway. Besides, the hydrochemical study reveals the same signature of saline and fresh water zones in both river basins. Water samples collected during field investigation show high total dissolved solids (TDS) content near URB and CRB river course. ERT data was correlated with GPR data to distinguish fresh and saline water zones more accurately. URB has more saline water zones at shallow depth nearer to river course and intermediate depth of saline water zones away from the river course, while CRB is having fresh water towards the inland.

ERT and salinity – A method to determine whether ERT-detected preferential pathways in brackish water-irrigated soils are water-induced or an artifact of salinity

An integrated electrical resistivity imaging (ERI), seismic refraction, and borehole geophysical investigation was conducted at a former retail petroleum site to assist in the detection and mapping of subsurface bedrock fractures and non-aqueous phase liquid (NAPL) within the shallow subsurface. These methods were used to characterize the subsurface geologic conditions and to assist in the detection and mapping of bedrock fractures that may be present within the investigation area and may act as groundwater preferential pathways. ERI data, collected with induced polarization (IP) information, was used in an effort to further characterize anomalies and prioritize potential drilling locations. Predetermined profile locations were selected based on available survey area, suspected utility locations, adequate line length, and location of existing groundwater monitoring wells and soil borings. Results were based on correlations between geophysical data processing of both geo-electric and seismic refraction cross sections and existing subsurface information. Cross sections were constructed to correlate site-wide geology and aid in the geophysical interpretation based on previously collected borehole geophysical data from on-Site wells and newly collected data from on-Site wells. The geophysical investigation, in cooperation with existing geologic and analytical information, was integral in identifying the locations for further investigation of subsurface bedrock fractures and NAPL. The project objectives were successful in locating anomalous features that were selected for additional investigation and satisfied state regulatory agencies on providing a path forwards to reaching site closure.

Monitoring of salt tracer experiments by electrical resistivity tomography (ERT) has been shown to be a valuable tool for characterizing the hydraulics of an aquifer, but efficient approaches of determining the spatial hydraulic conductivity distribution from ERT data are still missing. Standard inversion of ERT data obtained during salt tracer tests may even lead to estimates of the concentration distribution that are in contradiction to flow and transport of conservative compounds in porous media. In order to avoid nonphysical behavior, we consider the governing equations of groundwater flow, solute transport, and geoelectrics as a coupled system. While the tracer passes through the part of the domain that is sensitive for a particular electrode configuration, the measured electrical potential differences are perturbed. We characterize these perturbations by their temporal moments and relate them to the temporal moments of concentration, which themselves depend on hydraulic conductivity. We present temporal moment‐generating equations leading from the hydraulic conductivity field via heads and velocities to the temporal moments of concentration and electrical potential perturbations. The approach makes use of a linearized version of the Poisson equation. On the basis of this system of coupled steady state equations, we compute the sensitivity of electrical potential perturbations with respect to the log hydraulic conductivity distribution by the continuous adjoint state method for coupled systems. For demonstration, we simulate salt tracer experiments in a virtual quasi‐two‐dimensional sandbox, monitored by ERT. We show that the ratio of the first over the zeroth temporal moment of potential perturbation is less affected by the linearization of the Poisson equation than the zeroth and first moments themselves. Thus, it appears recommendable to use the ratio of first to zeroth moments also as data in inversion. We compare sensitivity patterns resulting from different electrode configurations. The methods of forward simulations and sensitivity calculations presented in this paper can be combined with any inverse kernel to develop a complete inverse model. Altogether, using temporal moments of potential perturbation appears promising for fully coupled hydrogeophysical inversion of ERT surveys during salt tracer tests.

Electrical resistivity tomography (ERT) has been extensively used in the investigation of contaminated sites. We report a case study at a DNAPL (dense non-aqueous phase liquid) remediation site in a factory, where the ERT data are complicated by a reinforced concrete layer with an embedded wire mesh on top of the soil. The wire mesh, as part of the infrastructure, imposes significant difficulties in the imaging of DNAPL. First, the inversion, as used in the conventional ERT data processing, cannot properly image the thin and highly conductive wire mesh. Second, the wire mesh creates a short-circuit for the current on the surface, so little current can reach the depth of DNAPL, and the data become insensitive to the sought targets. In this study, we improve the ERT imaging resolution for DNAPL by two ways. First, we recover a high conductivity for the top layer to sufficiently represent the effect of the wire mesh; then, this layer is treated as known and excluded from the inversion. Second, additional downhole electrodes are placed into the pumping wells used for remediation, so more information from the depth can be obtained. Our simulation and field data inversion both demonstrate the effectiveness of our advanced inversion techniques and downhole electrodes in improving the resolution of ERT imaging in complex exploration environments.

Two important mechanisms affect our ability to estimate solute concentrations quantitatively from the inversion of field-scale electrical resistivity tomography (ERT) data: (1) the spatially variable physical processes that govern the flow of current as well as the variation of physical properties in space and (2) the overparameterization of inverse models, which requires the imposition of a smoothing constraint (regularization) to facilitate convergence of the inverse solution. Based on analyses of field and synthetic data, we find that the ability of ERT to recover the 3D shape and magnitudes of a migrating conductive target is spatially variable. Additionally, the application of Archie's law to tomograms from field ERT data produced solute concentrations that are consistently less than 10% of point measurements collected in the field and estimated from transport modeling. Estimates of concentration from ERT using Archie's law only fit measured solute concentrations if the apparent formation factor is varied with space and time and allowed to take on unreasonably high values. Our analysis suggests that the inability to find a single petrophysical relation in space and time between concentration and electrical resistivity is largely an effect of two properties of ERT surveys: (1) decreased sensitivity of ERT to detect the target plume with increasing distance from the electrodes and (2) the smoothing imprint of regularization used in inversion.

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.