Applying FDEM, ERT and GPR at a site with soil contamination: A case study

A geophysical investigation that included seismic refraction tomography, electrical resistivity tomography (ERT) and ground penetrating radar (GPR) was carried out on the cliff of Roca in the south-eastern part of Puglia, Italy. The objective was to investigate the physical– mechanical characteristics of the rock mass behind a cliff face. Seismic refraction tomography has been conducted along the free surface of the cliff on an area of 836 m2. The results show strong velocity gradients, from 600 to 1200 m s−1. Low velocity zones can be correlated with the low quality of the rock mass. An ERT survey has also been acquired on the same area along 23 parallel profiles spaced 1 m apart. The results show a larger area with high resistivity variations in the first 0.6 m of the subsoil. In the deep subsoil the results shows a fracture system with NNW–SSE alignment. GPR surveys have also been acquired in the same area of ERT and seismic refraction tomography. A 100 MHz centre frequency antenna was used. The penetration depth is limited to about 2 m, due to the conductive materials present in the first metre of the subsoil. The main reflectors are near-vertical and most of them can be correlated with fractures. Geophysical data were compared with geological and geomorphological data. The integrated interpretation of all data indicates that a cliff-parallel fracture set develops in response to the presence of a free surface at the cliff face. This fracture system divides the cliff in two parts.

The structure of karst hydrosystems is complex and the related hydrodynamic functioning mechanism can be highly heterogeneous in both unsaturated and saturated zones. The Low-Noise Underground Laboratory of Rustrel (LSBB) is located in unsaturated zone (UZ) of Fontaine de Vaucluse (FdV) karst hydrosystem. In this way, the LSBB is a unique place to apply surface-based geophysical surveys due to numerous and various geological and hydrogeological knowledge under the investigated zone. We exhibit in this paper complementarity and efficiency of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR). Both GPR and ERT are well suited for the first phase of our study, in order to characterize the general structure of the investigated chosen zone. GPR results highlight the tectonic features which are also, but less accurately, seen by ERT apparent resistivity maps. ERT 2D inverted results have shown the presence of moderate resistivity zones within the limestone. The provided information allowed carrying out following step of our study to enhance understanding of local karst UZ e.i. apply Magnetic Resonance Sounding (MRS), Gravimetry and Low Frequency Electromagnetics (LF-EM).

An integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to detect the existence of archaeological remains of the Roman’s temple devoted to August. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most famous temple of Spain (12th century) evolving from Romanesque to Gothic styles. A project including electrical resistivity tomography (ERT), ground probing radar (GPR) and frequency domain electromagnetics (FDEM) has been planned over one year and conducted during a week of intensive field survey. From all the methods applied, both ERT and GPR have provided some detailed information on subsoil structures. The ERT method has been applied with different techniques and arrays, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data have been recorded extensively and therefore, 70.000 apparent resistivity data were available to obtain a full 3D image after an integrated inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists.

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.

Legacy landfill sites often contain buried trenches that are not well delineated, not documented to modern standards, with subsurface voids that can act as preferential pathways for water, sediment and contaminants. To evaluate whether geophysical methods can help identify these features in a landfill, we conducted integrated electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) survey along three collocated transects; at a closed 1950’s landfill site in Ontario, Canada. The survey used a multi-electrode ERT array and a 250 MHz GPR to image shallow structures associated with historical waste trenches and potential soil-pipe development.The survey results reveal a consistent pattern: high-resistivity anomalies in ERT between 1-2 m depth align with wide, crossing hyperbolas in the respective GPR profiles. The high-resistivity anomalies are interpreted as waste trenches. Located between the identified trenches, elongated resistive zones corresponding to GPR troughs and dipping reflectors, are tentatively interpreted as sedimentary layers with possible soil-pipe-like connections. These results, overlaid with site monitoring data, will improve the overall clarity and allow a better understanding of these subsurface structures. Three trench-like structures with two connecting anomalies are imaged along each transect, demonstrating a repeating subsurface pattern. Deeper ERT anomalies (4-9 m) lack GPR counterparts due to the limited penetration of the 250 MHz antenna Further surveys will use different antenna types to achieve deeper resolutions.The study results show that combining ERT resistivity contrasts with GPR hyperbola geometry, provides a reliable means of mapping buried trenches and potential erosion pathways at legacy landfill sites. Next steps include expanding the survey grid, forward modeling, integrating other geophysical methods that complements ERT and GPR, and developing 3D interpretations to support long-term environmental monitoring and risk assessment.

During the extreme precipitation event of 15th–17th June 2013 in Garhwal Himalaya, glacial lake outburst flooding accompanied by numerous landslides and flash flood events caused widespread death and destruction. Many hill slopes and steep river banks had developed fractures and fissures indicative of landslides and one such scarp was observed at Kunjethi (Kalimath) village on satellite images during routine analysis. Rainfall threshold analysis for years 2012, 2013, 2014 and 2015 has confirmed that it was the main triggering factor which happened due to extreme precipitation in June 2013 leading to scarp development and initiation of landslide. In order to characterize the landslide and get subsurface information, two highly cost-effective and fast non-invasive geophysical techniques, electrical resistivity tomography (ERT) and ground penetrating radar (GPR), were used. Four ERT profiles and one GPR radargram profile were used to determine the approximate depth to slip surface, which is inferred around 15–19 m. Integration of satellite remote sensing, geophysical studies and field observations have been used to demarcate the maximum possible slide zone. This study reiterates that earth observation tools in integration with faster, non-invasive and cost-effective geophysical techniques can establish the slip surface, which is an essential information required for landslide hazard mitigation measures.

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

As part of the NAWA-EMMAT project, geophysical surveys were carried out on selected sites in Poland, i.e. on the flood dike and in the surrounding of the concrete water dam. The goal of the surveys was the non- invasive detection of loose zones in the flood dike and fractured zones located in the subsoil of the water dam. Terrain geophysical measurements were conducted with the use of electrical and electromagnetic methods as well as applying seismic methods. Due to the limitations of the length of this paper, only selected results recorded on the flood dike using selected geophysical methods, i.e. electrical resistivity tomography (ERT) and ground penetrating radar (GPR) are presented. The presence of several higher hydraulic permeability zones in the dike was suggested by the results of the ERT method. The GPR method allowed, due to its high resolution, to identify small leakage zones in the body of the dike. An attempt at a quantitative interpretation of the geophysical data was made; for this purpose, the synthetic results obtained from numerical modelling of electrical and electromagnetic fields were applied. For proper construction of numerical models and further interpretation, the results of geotechnical sounding, geological drilling and the results of petrophysical measurements were also used.

The study investigated the importance and image behavior of integrated geophysical methods in mapping contaminant spread beneath the surface of a pollution site in Ogoniland, Southern Nigeria. Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques constrained by Vertical Electrical Sounding (VES) data were employed to investigate the electrical properties of hydrocarbon contaminated soils that resulted from recent oil spills/leakage into the environment. Five (5) ERT and GPR lines and twenty-nine (29) VES data were acquired at the spill site. Basically, the electrical signatures from the resistivity measurements were able to image the subsurface layers and the associated contamination zone. GPR equally imaged the subsurface stratigraphy to a depth of 10.0 m beneath the surface. The interpretation of the five (5) ERT data showed consistency in the resistivity structure indicative of contaminant plumes with anomalously high electrical resistivity in the range of 1000-10,000 Ωm, a possible indication of hydrocarbon contamination. On the GPR radargram, regions of high electrical resistivities were in agreement with reduced GPR reflection behavior (shadow zones) and were limited to the near surface of the surveyed areas. Vertical electrical sounding delineated layers with high resistivity predominantly within the second and fourth geoelectric layers within pollution depths of 2.4 m and 11.9 m, respectively. As a result, the underground aquifer, relatively between 7.5 and 10.5 m, has been infiltrated by hydrocarbons. It can be seen from the study that geoelectric measurements on the surface can describe the distribution of hydrocarbon resistive zones as well as their conductive behavior that may be linked with the biodegradation of oil spills in the subsurface. Thus, the employment of these integrated methods for contaminant monitoring, hydrogeologic studies and remediation planning reduced the uncertainties, and they are of extensive relevance in mapping the geological behavior of polluted soils in contamination sites.

Summary This work presents novel evidence concerning the well-known contaminated site in Trecate, NW Italy, which was affected in February 1994 by crude oil contamination as a consequence of an oil-well blowout. In the Trecate area, between 2009 and 2012, an extensive geophysical survey, combining electrical resistivity tomography (ERT), frequency-domain electromagnetic (FDEM) mapping, and ground-penetrating radar (GPR), was conducted to assess the contamination distribution of hydrocarbon in the shallow subsurface. In March 2025, a follow-up survey was performed to verify changes in the contamination and extend the site characterization. New ERT and GPR measurements, alongside deeper FDEM surveys, were conducted to evaluate the effect of bioattenuation and persistence of contamination. The results showed minimal variation compared to the previous surveys, with the contaminated zone continuing to exhibit significantly higher electrical conductivity and lower range of oscillation in soil moisture in the smear zone layer. The study underscores the effectiveness of geophysical methods, especially FDEM for large-scale contamination mapping and ERT for detailed detection, in monitoring long-term contamination processes.

Ground penetrating Radar (GPR) surveys conducted at Fort IX in Kaunas, Lithuania have uncovered substantial evidence of the existence of mass execution trenches used by the Nazi regime between 1941 and 1944. Between 1941 and 1944 as many as 45,000 people were systematically murdered by the Nazi Einsatzgruppen during the implementation of the “Final Solution”. In the search for these mass execution trenches a GPR survey of the Fort was conducted using both 225 MHz and 450 MHz antennae to complete two grids. Results from the GPR survey show dipping in the stratigraphy of the GPR profiles at 0.5 to 1.3 meters in depth. These were found in multiple locations throughout the grids in intervals between 5 – 8 meters. Along with dipping reflections in the stratigraphy of the profiles, areas of possible different soil composition were also located in the same areas using electrical resistivity tomography (ERT). These areas that differed in soil composition formed unnatural rectangular patterns in the subsurface indicating the presence of the execution trenches. These discoveries combined with past Soviet archaeological investigations of the fort from the 1960's have resulted in further evidence of the existence and location of Nazi mass execution trenches at Fort IX.

Ground penetrating radar (GPR) surveys have been carried out at Schirmacher Oasis and Dakshin Gangotri located at Queen Maud Land, East Antarctica, during the 22nd Indian Antarctic Summer Expedition, 2002-2003. The present study confirmed the ability of the high-resolution GPR for monitoring the glaciers. It gives information about the health of the glaciers before it collapses. GPR survey over three frozen lakes near the Maitri Station provided the lakes' top ice thickness and bedrock depth. Similarly, the internal layers of the glaciers have been mapped in-situ using GPR. The results can be correlated with the results obtained by ice-core drill wells to understand the different field parameters (i.e., thickness of each layer, dielectric constant, etc.).

Geophysical investigations were conducted utilizing Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to assess a Non-Aqueous Phase Liquid (NAPL) contaminated site in the southeast of China. Traditional drilling and sampling methods combined with geochemical analysis are limited in deriving reliable spatial interpolations due to low sampling density and high heterogeneity of shallow groundwater. Variations in soil physical properties, such as conductivity and dielectric properties, resulting from NAPL infiltration, provide the physical basis for geophysical detection. While the combined use of ERT and GPR is established in geophysics, its effective application to NAPL sites remains challenging due to complex site conditions and ambiguous signatures. Our ERT results reveal high resistivity anomalies potentially indicative of NAPL contamination, and overlaying GPR attribute analysis (including amplitude, phase, coherence, and texture) onto these results enhances subsurface characterization and anomaly discrimination. The integrated approach demonstrates its capability to clarify subsurface contamination patterns under heterogeneous conditions, providing a spatially continuous interpretation framework that complements sparse direct sampling.

A geophysical investigation that involves the integration of Vertical Electrical Sounding (VES), Electrical Resistivity Imaging (ERI), and Ground Penetrating Radar (GPR) methods was conducted on Papalanto, Ogun River floodplain, a location underlain by sedimentary terrain of Southwestern Nigeria. This research aimed to image the underground lithological units and delineate the shallow geologic structures in order to characterize the area for agricultural suitability throughout the dry season. VES results typically outlined three geologic layers which are topsoil, saturated loamy clay, and alluvium. From 2D inverted resistivity results, three major geologic layers, namely topsoil, saturated loamy clay, and alluvium, were outlined and are in very good agreement with the results attained through VES. The first three layers of 3D inverted resistivity sections display a great amount of variation in the distribution of resistivity at superficial depth, made up of low resistive content. From the GPR survey, three geologic layers were also outlined from the results, namely the topsoil, saturated loamy clay, and alluvium. Consequently, the study location can be said to be semi-competent to competent luxuriant farming land in consideration of the resistivity distribution of the floodplain subsurface. Therefore, VES, ERI, and GPR are very effective geophysical methods for describing and classifying the shallow subsurface in reference to the measured physical properties. Hence, they should be applied in related geophysical investigations for better insight into the geology of the subsurface. Keywords: Agricultural suitability; Dry season; Floodplain; Geophysical investigation; Underground lithological units.

The characterization of the deep vadose zone is traditionally a difficult task, since\naccess without disturbance of samples or modification of in-situ conditions is\npractically impossible. On the other hand, the presence and flow of water in the\nvadose zone controls a number of phenomena of great environmental interests, such as\ncontamination of water resources, catchment hydrology, floods and slope stability. As\na consequence, non invasive – geophysical - techniques, such as electrical\nresistivity tomography (ERT) and ground-penetrating radar (GPR), are being\nincreasingly used to investigate the vadose zone. The use of cross-hole methods is\nparticularly effective and increasingly applied. The dependence of the geophysical\nresponse on changes in soil moisture content, e.g. via changes in electrical\nresistivity or dielectric properties, is the key mechanism that permits the use of\nnon-invasive techniques to monitor the vadose zone in time-lapse mode, i.e. via\nrepeated measurements over time. The data from non-invasive techniques can be\nsubsequently used to calibrate physical-mathematical models of water flow in the\nunsaturated zone. In this paper, we present the results of a forced infiltration\nexperiment conducted at the research site in Gorgonzola, close to Milan, Italy. The\nsite has been equipped to monitor the unsaturated flow dynamics in the local alluvial\nquaternary sediments of the Po river plain, characterized by a fairly coarse\nsandy-gravel grain size distribution. Four 20 m deep boreholes were drilled at the\nsite; three of them are permanently equipped with stainless steel borehole electrodes\nfor ERT imaging. Each borehole has a set of 24 electrodes, spaced 0.8 m vertically.\nThe same boreholes are also being used for cross-hole GPR tomography. Both MOG\n(multiple offset gathers) and ZOP (zero offset profiles) have been adopted as\nconfigurations for GPR surveys. Conversion from GPR traveltimes to moisture content\nvalues has been provisionally performed using the semi-empirical relationships. ERT\nhas been used so far only to provide qualitative images of changes in resistivity due\nto water saturation changes. Inversion of ERT data has been performed using the\nCRTomo 2D code (Kemna, 2000), while the code Migratom (Jackson and Tweeton, 1994) was\nused for the inversion of GRP MOG data. ZOP profiles have been analyzed under the\nassumption of straight ray horizontal propagation. Monitoring of natural infiltration\nprocesses started in January 2005. In July 2005 an artificial water injection\nexperiment was performed using a 2 m deep, 2.60 m long trench. A total of 3 cubic\nmeters of tap water infiltrated in about 2 hour time. A number of ERT and GPR MOG\ndatasets were acquired from July 5 (date of water injection) to July 11, 2005. The\ndata were used to calibrate a 3D unsaturated flow model, thus helping the\nidentification of field-saturated hydraulic conductivity and other unsaturated soil\nparameters. Strengths and limitations of the methodology are analysed and discussed.