Electrical Resistivity Tomography Data Research Articles

Geoelectric Characterization of Hyporheic Exchange Flow in the Bedrock‐Lined Streambed of East Fork Poplar Creek, Oak Ridge, Tennessee

AbstractA multimethod geoelectric survey was implemented between January and March 2022 along a 220‐m long reach of the bedrock‐lined streambed of East Fork Poplar Creek in Oak Ridge, Tennessee to identify locations of surface‐water and groundwater exchange and characterize the subsurface flow paths that convey water between the stream and flood plain. A waterborne self‐potential (WaSP) survey was completed in January 2022 to measure the electric streaming‐potential field in the stream. Electric resistivity tomography (ERT) was performed in March 2022 on the flood plain adjacent to the WaSP survey reach to map the electric resistivity distribution and characterize the hydrogeology and subsurface flow paths that facilitate surface‐water and groundwater exchange in the bedrock‐lined stream. The combination of WaSP and ERT data support the qualitative interpretation that surface‐water and groundwater exchange likely occurs along fractures in outcropping bedrock and along two fault lines that intersect the limestone creek bed.

Coastal sedimentation and topographic changes in the Mataram Plain, Lombok (Indonesia) following the 1257 CE eruption of Samalas volcano

AbstractThe 1257 CE eruption of Samalas volcano, Indonesia, buried the entire island of Lombok beneath various thicknesses of pyroclastic material during this Volcanic Explosivity Index (VEI) 7 event. This study aims to reconstruct the paleo‐topography of western Lombok before the Samalas eruption in 1257 CE and analyse the sedimentation processes that led to its landscape evolution over the last 700 years. Stratigraphic data were collected from various surveys, such as outcrops, coring, drilling, hand auger and wells observation. Electrical resistivity tomography (ERT) measurements were performed to complement the stratigraphic data. A combination of stratigraphy, ERT and topographic data from various sources (topographic map, Digital Elevation Model Nasional [DEMNAS] and Real Time Kinematic [RTK] measurement) is employed to determine the depth of the paleo‐surface. Topographic modelling was performed by subtracting the delta‐DEM (DDEM) from the current‐DEM (CDEM). The result of this operation is a paleo‐DEM, which is used to reconstruct the paleo‐hydrographic features such as shoreline positions and river channels. Modelling results demonstrate that the relief has not been significantly modified, except in lowland areas, which is the sediment accumulation zone. River channels have experienced minor changes, except for the location of the river mouth and the degree of meandering. Significant changes occurred at the shoreline that has prograded by approximately 1.6 km during the last 700 years. A schematic model is built to illustrate the evolution processes of the study area, consisting of the pre‐eruption condition, the immediate post‐eruption condition and development of the current condition. This model further develops the previously proposed model with a higher resolution and simultaneously revises the estimated boundary of the pyroclastic density current (PDC) deposit from previous work.

Seasonal groundwater salinity dynamics in the mangrove supratidal zones based on shallow groundwater salinity and electrical resistivity imaging data

Soil salinity plays an essential role in the growth of mangroves. Mangroves usually grow in intertidal zones. However, in Karimunjawa National Park (KNP), Indonesia, mangroves are also found in supratidal zones. Thus, this study aims to determine why mangroves can grow in this supratidal zone, even during the dry season. We analyze seasonal changes in groundwater flow and salinity dynamics using the hydraulic head, shallow groundwater salinity, and electrical resistivity imaging (ERI) data. The result shows that variation in groundwater salinity is caused by seawater intrusion, which is generated by a hydraulic gradient due to the sea level being higher than the water table in KNP. Rainfall and evapotranspiration, which change seasonally, likely affect the water table fluctuation and salt concentration. ERI images indicate this seawater intrusion in the top sediment up to the bedrock boundary. However, the resistivity difference in the wet and dry seasons shows that remarkable resistivity change occurs at the deeper layer (50–60 m below ground level (BGL)), likely due to freshwater recharge from rainwater on the land side. Groundwater in the KNP is shallow and saline; thus, mangroves in this zone, e.g., Ceriops tagal and Lumnitzera racemosa, can grow because their roots can reach this groundwater. These mangrove species can still grow in this zone even though the shallow groundwater is very saline (46–50 ppt). However, this condition might cause these mangroves to grow stunted. Thus, freshwater availability is crucial for mangrove growth in this supratidal zone to dilute this high groundwater salinity.

Application of the electrical resistivity tomography in groundwater detection on loess plateau

Agricultural irrigation of the South Jingyang tableland in Shaanxi Province, China has led to a continuous rise of the groundwater level and has triggered a series of loess landslides, thereby seriously affecting the life and property safety of local residents. Research shows that the major cause of the landslide in the loess layer of the South Jingyang tableland is the rising groundwater level. Therefore, the research on the formation mechanism of landslide in this area should include the investigation of the stratigraphic structure and groundwater level distribution characteristics. On this basis, a series of approaches, such as electrical resistivity tomography (ERT), borehole, and laboratory tests, was carried out on the South Jingyang tableland, and the groundwater level distribution and stratigraphic structure in the study area were determined. The qualitative relationship between resistivity value and water content at different depths was detected using the inversion results of ERT and borehole data. Through laboratory tests, the quantitative relationship between resistivity values under different water contents was established. The precise depth of the groundwater level was inferred by connecting the qualitative relationship with the quantitative relationship, and then a detailed 3D geological model was established by linking the inversion results of ERT with the field borehole lithology data and geological survey data. The detection results show that when the qualitative and quantitative analyses of the ERT inversion results were combined, the distribution of the groundwater level was accurately judged. The ERT is effective in reflecting the stratigraphic structure and hydrological characteristics of the Loess Plateau, and its potential as a supplementary technology for detecting the groundwater level is reasonable. This study addresses the limitation and inaccuracy in determining the stratum structure and groundwater level by solely relying on borehole information or ERT. The established 3D geological model not only provides a basis for the study of groundwater table fluctuation, but also a technical guidance for the stability evaluation of loess slope, landslide prediction, and early warning in the study area.

Application of 2-D and 3-D Geo-electrical Resistivity Tomography and Geotechnical soil Evaluation for Engineering site Investigation: A Case Study of Okerenkoko Primary School, Warri-Southwest, Delta State, Nigeria

In the design of building structures, joint efforts must be decided to resolve the depth to competent layers across the intended site, and periodic subsidence monitoring and deformation assessment of all buildings, specifically high-rise buildings, should be a regular practice, to safeguard the durability of civil engineering structures, to avert the disastrous consequences of structural failure and collapse prevalent of late. It was this extremity that necessitated the adoption of an integrated methodology which employed DC resistivity tomography involving 2-D and 3-D techniques and geotechnical-soil analysis to evaluate subsoil properties for engineering site investigation at Okerenkoko primary school, in Warri-southwest area of Delta State, to adduce the phenomena responsible for the visible cracks/structural failure observed in the school buildings. Rectilinear set of 2-D resistivity data consisting of five (5) parallel and five (5) perpendicular lines were obtained in a 100 x 80 m2 rectangular grid using the Wenner array. Thirteen (13) Schlumberger soundings were also obtained on the site with half-current electrode separation of 200 m. The results brought to light the geological structure beneath the subsurface, which consists of four geoelectric layers identified as top soil, dry/lithified upper sandy layer, wet sand (water-saturated) and peat/clay/sandy clayey soil (highly water-saturated). The deeply-seated peat/clay materials (ρ ≤ 20 Ωm) were delineated in the study area to depths of 17.1 m and 19.8 m from 2-D and 3-D imaging respectively. The dominance of mechanically unstable peat/clay/sandy clay layers beneath the subsurface, which are highly mobile in response to volumetric changes, is responsible for the noticeable cracks/failure/subsidence detected on structures within the study site. The DC resistivity result was validated using geotechnical test of soil samples collected from boreholes covering the first 8.0 m on three of the profiles. Atterberg’s limits of the soil samples revealed plasticity indices of zero for all samples. Thus, the soil samples within the depth analyzed were representatives of sandy soil which does not possess any plasticity and their plasticity index is taken as zero. These findings apparently justify the subsoil conditions defined in the interpretation of 2-D and 3-D resistivity imaging data. 3-D images presented as horizontal depth slices revealed the dominance of very low resistivity materials i.e. peat/clay/sandy clay within the third, fourth and fifth layers at depths ranging from 5.38-8.68 m, 8.68-12.5 m and 12.5-16.9 m respectively. Hence, 3-D tomography amplified the degree of accuracy of the geoelectrical resistivity imaging. Resistivity contour maps of second, third and fourth layers for VES 1 to 13, displayed low resistivity direction predominantly towards the northeastern part of the site, and signifies that rocks within the northeastern part have low resistivity values, which connotes high porosity and establishes the groundwater flow trend in the study area. The methods employed in this study justifiably gave relevant information on the subsurface geology beneath the study site and its suitability for engineering practice. Thus, it is suggested that these methods should be appropriated as major tools for engineering site assessment projects and groundwater future studies.

Monitoring spatiotemporal soil moisture variability in the unsaturated zone of a mixed forest using electrical resistivity tomography

AbstractEuropean forests are suffering considerably from the consequences of the droughts of recent years, and the exact reasons and influencing factors for this are still not fully understood. This study was conducted to characterize the changes and dynamics of soil moisture in a mixed forest in northern Bavaria within 1 year. Since electrical resistivity correlates well with soil water content, we used two‐dimensional electrical resistivity tomography (ERT) monitoring and time‐lapse analyses to supplement punctual measurements by sensors and soil analyses to show soil moisture changes throughout a whole year (2020–2021). While the topsoil dries out significantly from summer to autumn down to a depth of about 3 m, a clear increase in soil water content and a decrease in resistivity below 3 m can be observed during winter period. Anomalies in the topsoil (0–1 m) showing lower resistivities than the surrounding substrate could be related to tree positions by additional terrestrial laser scans. A significant relationship could be found between tree crown projection area and resistivity in 1–2 m depth. We found a trend that mean resistivity below pine is lower as below beech. ERT data were also used to estimate the soil water content via Archie's law and the results correlate strongly with the measured values, but the degree of correlation varies depending on the depth level. ERT as a noninvasive method, in combination with additional data, for example, on the vitality status of individual trees, could help to better understand root water uptake and water supply to trees, especially during periods of drought.

Analysis of soil electrical resistivity and hydraulic conductivity relationship for characterisation of lithology inducing slope instability in residual soil

Slope instability occurrences as damaging shallow-landslides in the residual soil around mountains has been widely studied with geophysical, geotechnical and hydrogeological techniques but relating soil electrical resistivity to hydraulic conductivity for characterisation of lithology inducing of these landslides is not common. In this study, we used Electrical Resistivity Tomography (ERT) data and Hydraulic Conductivity (HC) data obtained from soil samples collected within 1–4.5 m depth in the borehole to assess the characteristics of soil that can induce landslide in the study location. The HC data were derived empirically from Beyer, Kozeny-Carman and Slitcher formula which were validated with HC obtained from laboratory experiment. The Empirical Derived Hydraulic Conductivities (EDHC) were correlated with the soil resistivity. The result shows a strong correlation between soil resistivity and HC with regression values of R2 = 0.9702, R2 = 0.9153 and R2 = 0.9232 for Beyer, Kozeny-Carman and Slitcher formula, respectively. The ERT model revealed a possible sliding surface between two contrasting resistive top material and underneath conductive materials at about 4 m depth. The HC assessment result corroborated the ERT model result because high and low-HC values were obtained in the borehole soil samples within 0–4 m and > 4 m depths from EDHC, respectively. The low-HC zone below 4 m depth was responsible for the occurrences of the shallow-landslides in the study.

English

The Osogbo central solid waste dumpsite in Osun State, Nigeria falls within the Ilesha Schist Belt, southwestern basement complex of Nigeria. Thirty-three (33) Vertical Electric Sounding (VES), and 2-D Electric Resistivity Tomography (ERT) data were acquired across various locations in the study area using ABEM Terrameter SAS 300, respectively. Schlumberger configuration with a maximum spread length of 100 m, for the VES was used, while four traverses of ERT were conducted, using Dipole-Dipole Array with average traverse length of 112.50 m. A smoothness-constrained least-squares inversion was then applied to the ERT data using a 2-D inversion algorithm [that is, DIPRO for Windows, version 4.01. Generally, the results defined the contaminated leachate plumes as electrically conductive, with anomalous resistivity values. Both the VES and the 2-D tomography identified zones of low resistivity values, between 24 and 67 ?m, from 10 to 20 m deep, as leachate contaminant plumes. Some of the weathered and fractured basement rocks indicated that the subsurface soil and groundwater have been contaminated by the leachates, which appears to have migrated outside landfill site. Zones within the sub-surface, with resistivity values relatively high (> 300 Ωm) and depth above 30 m might be pollutant free. Solid wastes management calls for the adoption of the best practicable environmental management approach through the governments’ enforcement of circular economic policies, using extended producer responsibility (EPR) model and construction of engineered landfills to replace unsanitary landfills. The conduct of environmental impact assessment (EIA), prior to commencement of operations and phytoremediation of closed sites is also crucial. Key words: Tomography, leachate, pollution, contaminant plume.

Integrated Geophysical and Geotechnical Approaches for Evaluating Dam Seepage in Lesser Himalayan Region of Pakistan

Dam leakage is a major problem in earth-filled reservoir dams. The present research is aimed at detecting the seepage zones in an earth-filled reservoir dam in the vicinity of Sarobi village, North Waziristan, Pakistan. The objective was achieved by integrating geological, geotechnical, and geophysical datasets. Geological survey was carried out in the area to identify the surface exposures. Geotechnical data involved was used to estimation of permeability and Lugeon values to identify network of joints/fractures whereas electrical resistivity tomography (ERT) technique (utilizing Wenner-Schlumberger configuration) was applied for the identification of the leakage zones. Geological data revealed highly deformed alluvium towards the right abutment underlain by compressed shale and highly fractured limestone beds. Geotechnical data indicated high permeability, Lugeon values, and concentration of sand and gravel towards the right abutment whereas low permeability, Lugeon values, and higher concentration of silt and clay were observed towards the left abutment. ERT data identified a conductive zone embedded between impermeable lithologies having resistivity between 10 and 60 Ω·m. This conductive zone was observed in parallel profiles acquired in upstream, dam crest, and downstream parts of the dam. The integration of all the datasets revealed a network of joints that are interconnected and provide a path for water at the upstream side to pass through right abutment of dam and leak towards the downstream.

Three-dimensional subsurface architecture and its influence on the spatiotemporal development of a retrogressive thaw slump in the Richardson Mountains, Northwest Territories, Canada

ABSTRACT The development of retrogressive thaw slumps (RTS) is known to be strongly influenced by relief-related parameters, permafrost characteristics, and climatic triggers. To deepen the understanding of RTS, this study examines the subsurface characteristics in the vicinity of an active thaw slump, located in the Richardson Mountains (Western Canadian Arctic). The investigations aim to identify relationships between the spatiotemporal slump development and the influence of subsurface structures. Information on these were gained by means of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR). The spatiotemporal development of the slump was revealed by high-resolution satellite imagery and unmanned aerial vehicle–based digital elevation models (DEMs). The analysis indicated an acceleration of slump expansion, especially since 2018. The comparison of the DEMs enabled the detailed balancing of erosion and accumulation within the slump area between August 2018 and August 2019. In addition, manual frost probing and GPR revealed a strong relationship between the active layer thickness, surface morphology, and hydrology. Detected furrows in permafrost table topography seem to affect the active layer hydrology and cause a canalization of runoff toward the slump. The three-dimensional ERT data revealed a partly unfrozen layer underlying a heterogeneous permafrost body. This may influence the local hydrology and affect the development of the RTS. The results highlight the complex relationships between slump development, subsurface structure, and hydrology and indicate a distinct research need for other RTSs.

Žune Ba-F epithermal deposit Part 2: Geophysical characterization and exploration perspective

The Žune barite-fluorite ore body in northwestern Bosnia and Herzegovina has been explored in order to determine spatial distribution of the ore body. At the Žune site, barite mineralization occurs in the form of veins and is hosted by dolostone. A geophysical survey was carried out with the aim to detect anomalies associated with the barite-fluorite deposit and to characterize the abandoned old deposit. Therefore, 2D electrical resistivity tomography (ERT) was measured along four profiles. Prior to surface geophysical measurements, detailed geological field mapping of the ore body and host rocks was undertaken. The geometry of the abandoned mining cut with steep walls and complex structural setting with barite that incorporates coarse fragments of host rock and fluorite makes interpretation of the results challenging. The collected geological data were used to calibrate the geophysical models. Forward modelling in addition to inversion of the ERT data helped to reduce the ambiguity of geophysical data interpretation. The results revealed that the barite vein in the area of mining cut is limited to a depth of about 10 m and length is about 40 m. There is no indication for barite mineralization in the area south of the mining cut, while anomalies that may be associated with ore bodies are present in the northern and northeastern part.

Evaluation of Engineering Site and Subsurface Structures Using Seismic Refraction Tomography: A Case Study of Abydos Site, Sohag Governorate, Egypt

Because of the strategic importance of the Abydos archaeological site in Egypt as a source of wealth for Egyptian tourism, this study was concerned with carrying out geophysical measurements to detect subsurface succession and measure variations in the geotechnical engineering features of the soils/rocks in order to protect this significant area. The findings will assist geologists and seismologists in collaborating with archaeologists for future site development, revitalization, and investment. The primary objectives of this work were to determine the subsurface lithology, evaluate the engineering geotechnical properties of soils/rocks, identify the layer thicknesses, and identify the site class by calculating Vs30. To achieve these goals, seventeen (17) seismic refraction tomography (SRT) P- and S-wave measurements were executed in front of the Osirion location. SeisImager Software was used for the processing and interpretation of the outcomes. The results were the travel time–distance curves, which were used for building the 2D seismic models that exhibited the velocity and the depth of the layered models. These models were validated by our previous works using electric resistivity tomography and borehole data. The results indicated that this site consisted of three geoseismic subsurface layers. The first layer was the surface that was made up of wadi deposits, which were a mixture of gravel, sand, and silt and were characterized by incompetent to slightly competent materials. The second layer corresponded to the sand and muddy sand deposits of competent rock that was of fair to moderate quality. The third layer (clay deposits) had a higher velocity and was more compact and may be employed as a bedrock layer. The elastic moduli, Vs30, petrophysical, and geotechnical properties of the three geoseismic layers were appraised as essential parameters. Integration of petrophysical and geotechnical parameters and elastic moduli revealed that the third layer was composed of competent clays, which were characterized by low values of porosity, void ratio, Poisson ratio, and stress ratio. It also had a high rigidity, Young’s and bulk moduli, concentration and material indexes, N-value, ultimate bearing capacities, and high density values, and vice versa for the first layer. The standard NEHRP site class was B (rocks). These parameters are ordinarily used as key indications and serve as significant inputs for any future work.

Brief communication: Combining borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography measurements to investigate seasonal changes in ice-rich mountain permafrost

Abstract. A novel combination of borehole temperature, borehole piezometer and cross-borehole electrical resistivity tomography (ERT) data is used to investigate changing ice and/or water contents in the creeping ice-rich Schafberg rock glacier in the Eastern Swiss Alps. Instrumentation techniques and first results are presented. The rock glacier ice is close to its melting point, and the landform has locally heterogeneous stratigraphies, ice and/or water contents and temperature regimes. The measurement techniques presented continuously monitor temporal and spatial phase changes to a depth of 12 m and should provide the basis for a better understanding of accelerating rock glacier kinematics and future water availability.

Massively parallel modeling and inversion of electrical resistivity tomography data using PFLOTRAN

Abstract. Electrical resistivity tomography (ERT) is a broadly accepted geophysical method for subsurface investigations. Interpretation of field ERT data usually requires the application of computationally intensive forward modeling and inversion algorithms. For large-scale ERT data, the efficiency of these algorithms depends on the robustness, accuracy, and scalability on high-performance computing resources. In this regard, we present a robust and highly scalable implementation of forward modeling and inversion algorithms for ERT data. The implementation is publicly available and developed within the framework of PFLOTRAN, an open-source, state-of-the-art massively parallel subsurface flow and transport simulation code. The forward modeling is based on a finite-volume discretization of the governing differential equations, and the inversion uses a Gauss–Newton optimization scheme. To evaluate the accuracy of the forward modeling, two examples are first presented by considering layered (1D) and 3D earth conductivity models. The computed numerical results show good agreement with the analytical solutions for the layered earth model and results from a well-established code for the 3D model. Inversion of ERT data, simulated for a 3D model, is then performed to demonstrate the inversion capability by recovering the conductivity of the model. To demonstrate the parallel performance of PFLOTRAN's ERT process model and inversion capabilities, large-scale scalability tests are performed by using up to 131 072 processes on a leadership class supercomputer. These tests are performed for the two most computationally intensive steps of the ERT inversion: forward modeling and Jacobian computation. For the forward modeling, we consider models with up to 122 ×106 degrees of freedom (DOFs) in the resulting system of linear equations and demonstrate that the code exhibits almost linear scalability on up to 10 000 DOFs per process. On the other hand, the code shows superlinear scalability for the Jacobian computation, mainly because all computations are fairly evenly distributed over each process with no parallel communication.

Field evaluation of semi-automated moisture estimation from geophysics using machine learning.

Geophysical methods can provide three-dimensional (3D), spatially continuous estimates of soil moisture. However, point-to-point comparisons of geophysical properties to measure soil moisture data are frequently unsatisfactory, resulting in geophysics being used for qualitative purposes only. This is because (1) geophysics requires models that relate geophysical signals to soil moisture, (2) geophysical methods have potential uncertainties resulting from smoothing and artifacts introduced from processing and inversion, and (3) results from multiple geophysical methods are not easily combined within a single soil moisture estimation framework. To investigate these potential limitations, an irrigation experiment was performed wherein soil moisture was monitored through time, and several surface geophysical datasets indirectly sensitive to soil moisture were collected before and after irrigation: ground penetrating radar, electrical resistivity tomography (ERT), and frequency domain electromagnetics (FDEM). Data were exported in both raw and processed form, and then snapped to a common 3D grid to facilitate moisture prediction by standard calibration techniques, multivariate regression, and machine learning. A combination of inverted ERT data, raw FDEM, and inverted FDEM data was most informative for predicting soil moisture using a random regression forest model (one-thousand 60/40 training/test cross-validation folds produced root mean squared errors ranging from 0.025-0.046 cm3/cm3). This cross-validated model was further supported by a separate evaluation using a test set from a physically separate portion of the study area. Machine learning was conducive to a semi-automated model-selection process that could be used for other sites and datasets to locally improve accuracy.

A Comparison of Hydrological and Geophysical Calibration Data in Layered Hydrologic Models of Mountain Hillslopes

AbstractBoth hydrological and geophysical data can be used to calibrate hillslope hydrologic models. However, these data often reflect hydrological dynamics occurring at disparate spatial scales. Their use as sole objectives in model calibrations may thus result in different optimum hydraulic parameters and hydrologic model behavior. This is especially true for mountain hillslopes where the subsurface is often heterogeneous and the representative elementary volume can be on the scale of several m3. This study explores differences in hydraulic parameters and hillslope‐scale storage and flux dynamics of models calibrated with different hydrological and geophysical data. Soil water content, groundwater level, and two time‐lapse electrical resistivity tomography (ERT) data sets (transfer resistance and inverted resistivity) from two mountain hillslopes in Wyoming, USA, are used to calibrate physics‐based surface–subsurface hydrologic models of the hillslopes. Calibrations are performed using each data set independently and all data together resulting in five calibrated parameter sets at each site. Model predicted hillslope runoff and internal hydrological dynamics vary significantly depending on the calibration data set. Results indicate that water content calibration data yield models that overestimate near‐surface water storage in mountain hillslopes. Groundwater level calibration data yield models that more reasonably represent hillslope‐scale storage and flux dynamics. Additionally, ERT calibration data yield models with reasonable hillslope runoff predictions but relatively poor predictions of internal hillslope dynamics. These observations highlight the importance of carefully selecting data for hydrologic model calibration in mountain environments. Poor selection of calibration data may yield models with limited predictive capability depending on modeling goals and model complexity.

Geomorphometric and Geophysical Constraints on Outlining Drained Shallow Mountain Mires

Long-term draining of peatlands results in transformation of vegetation and obliteration of their morphological features. In many areas, efforts are made to restore the original ecosystems and increase their water retention potential. Using combined analyses of a LiDAR-based digital terrain model (DTM), colour-infrared (CIR) imagery data, ground-penetrating radar (GPR) data and electrical resistivity tomography (ERT) data, we tested the applicability of these methods in outlining the extent and subsurface structure of drained mires located in the Stolowe Mountains National Park area, Poland. The LiDAR-DTMs enabled parameterisation of physiographic features of the mires and determination of their extent, runoff directions and potential waterlogging areas. CIR analysis enabled classification of vegetation types. GPR prospecting revealed the bedrock morphology, thickness and internal structure of the peat deposits, showing that this technique can also provide data on variability in the decomposition of phytogenic deposits. The obtained ERT sections indicate both the thickness of peat deposits and variability in the bedrock internal structure. The results show that integrated analyses of data obtained with different methods can be an effective tool in outlining the original extent of peatlands, with potential application in the planning of peatland ecosystem restitution.

Use of electrical resistivity tomography to reveal the shallow freshwater–saline interface in The Fens coastal groundwater, eastern England (UK)

The Fens is a region that contributes 11% of the agri-food economy from just 4% of the agricultural land covering England (UK). This region is vulnerable to soil salinisation from sea-level rise with estimated 100-year flood events projected to be observed up to every 2 years by 2100. Seawater intrusion and upwelling of saline groundwater can provide an additional pathway; however, the area’s groundwater has not been assessed and the risk is unknown. This study used data from the British Geological Survey’s stratigraphic core archive to produce the first stratigraphic map of the loosely consolidated Holocene deposits in the South Holland–Holbeach Marsh region. There is a sandy unconfined aquifer towards the coast, a semiconfined central region with a silty cap and a clay confining cap in the north region. Electrical resistivity tomography data indicate water level depths of 0.58 ± 0.37 m above mean sea level (msl) in February 2021 and 0.01 ± 0.72 m msl in August 2021. The saline–freshwater boundary was at 1.70 ± 0.82 m msl in February 2021, deepening to 2.00 ± 1.02 m msl in August 2021, but the only evidence of seasonal fluctuation was within 10 km of the coast. A potential, but unverified, freshwater lens up to 3.25 m thick may exist beneath the surface. These results suggest that freshwater–saline interface fluctuations may primarily be driven by surface hydrology and would be vulnerable to climate-change-induced future variations in factors that affect surface water.

Modelling Cover-Collapse Sinkholes That Appeared after the M6.2 Petrinja Earthquake in Croatia Using Electrical Resistivity Tomography Data

After the destructive earthquake in Petrinja measuring M6.2 occurred on 29 December 2020, which was followed by a series of foreshocks and aftershocks in the area of the rural settlements in Mečenčani and Borojevići, cover-collapse sinkholes suddenly appeared. The investigated area is located 20 km southeast of the epicentre. Some months later, more than 90 cover-collapse sinkholes appeared, jeopardising local infrastructure and residential buildings. The sinkholes appear in the area of covered karst, where there are clastic deposits 2–10 m thick on the fractured and weathered limestone bedrock. There are two geological models located in the investigated area: GM-1, where the base consists of clastic strata covering comprising Lithothamnium limestone, which in turn leads to the formation of underground cavities and cover-collapse sinkholes, and the GM-2, where the base comprises clay deposits without any cover-collapse sinkholes. These models can be effectively distinguished due to tomographic resistivity models; hence, numerous measurements were undertaken using two-dimensional electrical tomography in several phases. An estimate of the threat to infrastructural facilities was conducted, and the boundaries of the geological models were precisely determined according to which underground cavities and cover-collapse sinkholes did not develop. Tomographic measurements were also conducted over the largest cover-collapse sinkhole measuring 25 m in diameter and helped to more precisely define the entire hydrogeological model and the mechanisms involved in the formation of cover-collapse sinkholes.

Study of the anisotropy of frozen rocks at the Khanovei test site (Komi Republic) based on the data of controlled source radio-magnetotelluric soundings and electrical resistivity tomography

The results of a joint inversion of the data of controlled source radio-magnetotelluric soundings (CSRMT) and electrical resistivity tomography (ERT), obtained at the Khanovei permafrost test site near Vorkuta, are presented. The geological section in the work area is composed of Quaternary fluvio-glacial, lacustrine-glacial and alluvial deposits up to 60 m thick, overlying a sequence of sandstones, siltstones and mudstones of Permian age. The CSRMT survey was carried out using a grounded electrical line 480 m long as a source in the frequency range 1-1000 kHz. The results of various inversion options were compared: 1) isotropic one-dimensional (1D) inversion of CSRMT data; 2) isotropic separate two-dimensional (2D) inversion of CSRMT and ERT data; 3) joint anisotropic 1D inversion of CSRMT and ERT data. Based on the results of a joint anisotropic inversion of CSRMT data in the far and intermediate zones of the source and ERT data, a vertical anisotropy associated with horizontal layering of frozen Quaternary clay deposits in the depth range from 2 to 15-20 m was revealed and its parameters (horizontal and vertical resistivity and anisotropy coefficient) were determined. On the geoelectric section obtained from the ERT data, the thickness of the anisotropic layer of frozen clayey deposits is significantly overestimated. The middle part of the section, in the depth range from 15-20 to 60-70 m, is characterized by relatively low resistivity values (tens of ohmmeters). This is consistent with the results of previous studies, which established that the upper part of the Permian sediments is in a thawed state and is fractured and water-saturated. A high-resistivity layer (hundreds of ohmmeters) was revealed at the base of the geoelecric section according to CSRMT data. A significant increase in the resistivity of Permian rocks with depth is confirmed by the data of vertical electrical soundings previously performed in the Khanovey area.