Electrical Resistivity Tomography Profiles Research Articles

Geophysical Characterization and Seepage Detection of the Chimney Rock Dam Embankment Near Salina, Oklahoma

The operator of Chimney Rock Dam observed the emergence of increasing seepage at the toe of the dam when the water level in the reservoir exceeded a particular elevation. However, the source and the pathways of the seepage were not identified. To address this issue, integrated geophysical methods were employed to delineate the different units of the dam embankment and identify potential seepage zones and pathways. The methods utilized in this study included electrical resistivity tomography (ERT), self-potential (SP), and multichannel analysis of surface waves (MASW). The ERT profiles revealed variations in the dam’s fill properties, including areas with anomalously low resistivity, interpreted as zones of relatively high moisture content. The two long SP profiles conducted along the dam embankment displayed similar spatial correlations with these low-resistivity zones, suggesting potential preferential seepage pathways. The SP map generated from a suite of parallel SP profiles conducted over the abutment depicts a pattern of positive background and negative potential anomalies, which may suggest fluid movement or seepage potential. The MASW profile along the top of the dam characterized an upper low shear-wave velocity layer corresponding to the top dry section of the embankment underlain by a higher shear-wave velocity layer, interpreted as saturated zone. The utilized geophysical methods successfully characterized the different materials of the embankment and identified zones of potential seepage.

Geophysical methods reveal the soil architecture and subsurface stratigraphic heterogeneities across land-lake interfaces along Lake Erie

Abstract Purpose The land-lake interface is a unique zone where terrestrial and aquatic ecosystems meet, forming part of the Earth’s most geochemically and biologically active zones. The unique characteristics of this interface are yet to be properly understood due to the inherently high spatiotemporal variability of subsurface properties, which are difficult to capture with the traditional soil sampling methods. Geophysical methods offer non-invasive techniques to capture variabilities in soil properties at a high resolution across various spatiotemporal scales. Methods We combined electromagnetic induction (EMI), electrical resistivity tomography (ERT), and ground penetrating radar (GPR) with data from soil cores and in situ sensors to investigate hydrostratigraphic heterogeneities across land-lake interfaces along the western basin of Lake Erie. Results EMI revealed high spatial heterogeneities in ECa distribution across the land-lake interfaces, with higher values in the wetland and transition zones compared to the upland zone. Soil ECa maps matched soil maps from a public database with the hydric soil units delineated as high conductivity zones (ECa > 40 mS/m). ERT and GPR showed vertical variation in soil properties with clear stratigraphic boundaries, and correlation of ERT profiles with lithologs from piezometers revealed the stratigraphic units of silt–clay and till sequence down to 3.5 m depth which are consistent with the surficial geology of the study area. Conclusions These results validate the use of multiple geophysical methods for extrapolating soil properties and mapping stratigraphic structures at land-lake interfaces, thereby providing the missing information required to improve the earth system model (ESM) of coastal interfaces.

Applying Deep Electrical-Resistivity Tomography Techniques for the Exploration of Medium- and Low-Geothermal Energy Resources

The growth of the geothermal industry demands the constant search of techniques with the aim of reducing exploration efforts whilst minimizing subsurface uncertainty. The exploration of geothermal resources is fundamental from the exploitation point of view, especially in those regions where this energy is not as widespread as the rest of renewable sources. This research shows how geoelectrical methods can contribute to the investigation and characterization of medium–low enthalpy geothermal resources until about 800 m of depth. A 2000 m long electrical-resistivity tomography profile was performed in a region of Southern Spain with previous evidence of moderate geothermal potential. Results of this geophysical campaign (together with a preliminary geological characterization) allowed for the obtainment of a 2D profile and a pseudo-3D model with extensive information about the subsoil in terms of geological composition and formations. The interpretation of geophysical results denotes the existence of a potential formation constituted by carbonate materials with thickness greater than 300 m, crossing different fractures. Once the ideal location for the geothermal exploitation is defined, the research evaluates the contribution of the possible energy source, deducing that the energy extraction in the potential fracturing area would be double that of the one in the vicinity of the site.

A high resolution detection approach combining probe drilling and horizontal cross-hole resistivity tomography to interpret water conducting channels ahead of the tunnel: A case study in Yunnan, China

The geological conditions in Yunnan Province, China, are intricate, and numerous challenges are anticipated during tunnel excavation. The goal of this study was to develop a high resolution detection approach combining probe drilling and horizontal cross-hole resistivity tomography to interpret water conducting channels ahead of the tunnel. A complex section of the Xianglu Mountain tunnel was taken as an example. Drilling data from 10 boreholes are used to establish the initial resistivity model ahead of the tunnel face. The utilization of drilling data as a priori information refines the horizontal cross-borehole resistivity tomography imaging results, providing a more accurate depiction of the subsurface conditions. In practical applications, the use of multiple boreholes for high resolution detection via probe drilling analysis and horizontal cross-hole resistivity tomography is relatively uncommon in complex underground environments and represents an engineering innovation. At the Xianglu Mountain tunnel site, 13 electrical resistivity tomography (ERT) profiles were executed to thoroughly assess the geological conditions. A 3-D resistivity model was constructed via interpolation of this dataset. The 3-D resistivity model not only facilitated precise estimation of the 3-D hydrogeological environment at the Xianglu Mountain tunnel site but also enhanced the accuracy of identifying the water conducting channels by revealing low-resistivity interconnected zones. This approach offers a precise guide for determining the location of the critical water conducting channels and for optimizing grouting treatment strategies based on the characterization, geometry, resistivity spatial distribution, and hydrogeological conditions of the studied zones. The acquired results, which are in good accordance with the reality of the studied area in terms of locating the water conducting channels, can be utilized to develop preventative techniques for water and mud inrush disasters during tunnel construction.

Imaging leachate runoff from a landfill using magnetotellurics: The Garraf karst case

Electrical and active source electromagnetic geophysical methods have been traditionally employed to approach and tackle environmental problems, such as those caused by landfills. However, since these problems are more consequential and cover broader areas, it is necessary to use deeper penetration methods, such as magnetotellurics. In the Garraf Massif (Catalan Coastal Ranges, NE Spain), an urban waste disposal landfill had been in operation from 1974 to 2006, during which >26 million metric tons of garbage had been deposited. This landfill overlies karstic terrain, thus principally impacting groundwater circulation. Previous electrical resistivity tomography profiles had partially imaged the infill but were not able to penetrate below the base of the original landfill. During 2019 and 2020 we performed a magnetotelluric study over the landfill and its surrounding with the goals of characterizing the electrical resistivity of the infill and below it. The 2D and 3D resistivity models confirmed the highly conductive nature of the leachate and allowed us to identify its presence below the landfill base, which we quantified with maximum thicknesses of 90 m. This proved that landfill leachate had filtered through the original impermeable layer, enhanced by the karstic drainage structure.

Integrating seismic refraction and electrical approaches in determining geophysical properties of near-surface cavities in Calabar-Ikom highway, Odukpani, Cross River State, Nigeria

Integrated seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) were used to study near-surface cavities with the aim of determining their geophysical properties. Five seismic refraction profiles around the study area were carried out using a 24-channel seismograph (ES-3000) while four ERT and ERT profiles were also conducted with IGIS resistivity meter and PLOTREFA software respectively. The data was processed using the RES2DINV software. Travel-time curves and velocity models were generated from the processed SRT data for each survey line, and 2-D inverted apparent resistivity models along the same lines were also generated for the purpose of comparison respectively. The results as obtained from the profiles showed SRT 1 (with a modelled velocity of 1,216 m/s in layer two at a depth of approximately 11 m - 20 m beneath the subsurface and an estimated cavity diameter of 11 m) and ERT 1 (with an apparent resistivity of approximately 826 Ωm and a depth of approximately 12.5 m – 16 m) indicates that the near-surface cavity outcrop links with a close-by mountainous structure in the EW direction. Profiles SRT 2 (with an approximately modeled velocity layer of 980 m/s in layer two at an approximate depth of 3.5 m – 7.0 m) and SRT 3 (with a modeled velocity layer of approx. 1,000 m/s in layer two at an approximate depth of 4.0 m – 7.0 m and 3.0 m – 9.0 m); ERT 2 (with an apparent resistivity of approximately 80 Ωm); and ERT 3 (with apparent resistivities of 116 Ωm and 182 Ωm at depths of approximately 3.5 m to 6.4 m and 4.0 m to 15.8 m) indicates that the surface cavity outcrop extends underneath the road network (Odukpani Central Section of Calabar-Ikom Highway) with a width of approximately 12 m. Profiles SRT 4 and SRT 5 (with an average velocity layer of 1,000 m/s in layer two at an approximate depth of 6.0 m – 13.2 m), and ERT 4 (with apparent resistivity ranging between 121 Ωm and 172 Ωm) at a depth of approximately 9.5 m to 20.0 m reveal that the near-surface cavity extends up to about 11 m across the highway and about 120 m away from the edge of the road. The above result will serve as reliable technical information to Transport and Building Construction Engineers on the presence of cavities along road networks and settlement areas in Odukpani Local Government Area. The recommendation is also made for the use of other geophysical techniques like Ground Penetration Radar (GPR) in conjunction with SRT and ERT to get higher-resolution imagery of the study area.

Investigation of the geological and hydrogeological structure of chalk cliffs with visible, thermal infrared and electrical resistivity imaging

This study is focused on assessing the surface and subsurface properties of a coastal cliff undergoing erosion. The investigation takes place along the chalk cliffs located in Sainte-Marguerite-sur-Mer, within the region of Normandy, France. To achieve this, a combination of Electrical Resistivity Imaging (ERI), visible and Thermal Infrared (TIR) imagery, and geotechnical methods (specifically, piezometers) were employed.ERI was chosen due to its high sensitivity to changes in water saturation and salinity within subsurface formations. The research involved two transverse ERI profiles originating from the plateau, crossing the cliff, and extending to the shore platform. Additionally, two longitudinal profiles were conducted on the plateau, running parallel to the coastline, all using Wenner configurations. The primary findings unveiled the distribution of subsurface resistivity, revealing insights into cracks, potential karst formations, and weathered chalky areas. A significant discovery was the identification of a conductive zone at depth, which was associated with saltwater intrusion.The integration of piezometers, intersecting the ERI profiles, allowed for the correlation of resistivity values with the measured salinity data. Areas characterised by brackish water exhibited an apparent resistivity ranging between 4 and 75 Ω.m, commencing at the front of the saline wedge.

Site-Specific correlation between geotechnical and geoelectrical properties of soil deposits in India

The combined application of geotechnical and geophysical methods provides an effective means of subsurface exploration. The insufficient correlation between geotechnical and geophysical methods prevents the conversion of geophysical data to geotechnical information and vice versa. In the present study, a correlation has been proposed between the geotechnical and geoelectrical properties of two soil deposits in India. Eleven boreholes along with a Standard Penetration Test (SPT) and different laboratory tests have been conducted to obtain the different engineering and physical properties of the soil layers. Twelve Electrical Resistivity Tomography (ERT) profiles have also been conducted to obtain the resistivity profile of the subsurface layers. The variation of the resistivity of the soil layers is analyzed and explained. Results show a strong linear correlation between resistivity and SPT blow counts (N′) of the soil layer for one site while a weak correlation for another site. A strong linear correlation between transverse resistivity (T) and N′ values has been observed for both sites. A comparison with the other existing correlation between resistivity/transverse resistivity and N′-values has also been presented. The range of resistivity values and engineering properties of soil layers are mentioned. The study is useful for the development of similar correlations and provides an approximate estimation of different geotechnical parameters for infrastructure design.

Near surface assessment of New capital city using geophysical techniques

ABSTRACT The Egyptian government announced the creation of a brand new capital city to address Cairo’s rapid population expansion and improve the quality of life for citizens, which is thought to have a strong demand for housing. In this study, we focused on the application of geophysical techniques, specifically Electrical Resistivity Tomography (ERT) and Very Low Frequency Electromagnetic Method (VLF-EM), to investigate the shallow subsurface in the New Capital city in Egypt. The study area is located in the eastern part of Cairo and is characterised by Oligocene and Miocene rocks. The ERT profiles reveal four geoelectrical layers based on resistivity values, with the first layer (Resistivity >300 Ω.m) interpreted as sand and conglomerate on the surface or basalt below. The second layer (100–300 Ω.m) is identified as sandstone, while the third layer (15–100 Ω.m) is interpreted as humid sandstone. The fourth layer (1–15 Ω.m) is characterised as shale. The abrupt changes in resistivity values horizontally indicate the probability of subsurface structures, including faults. The VLF-EM method is used to identify faults and cracks in the earth’s crust. The results provide information on the size, shape, and depth of both shallow and deep subterranean conductors. Anomalies of high conductivity suggest fault or fracture zones influencing underlying strata.

Geospatial and geophysical insights for groundwater potential zones mapping and aquifer evaluation at Wadi Abu Marzouk in El-Nagila, Egypt

This study employs an integration of remote sensing, geographic information systems (GIS), and geophysical techniques to delineate groundwater potential zones (GWPZ). This multidisciplinary approach significantly enhances the precision and reliability of groundwater potential zone mapping. Eight criteria were introduced into the analytic hierarchy process (AHP) to identify GWPZs in Wadi Abu Marzouk by applying a weighted overlay analysis of all criteria layers. The criteria weights were derived by using a pairwise-comparison technique, while the accuracy of the weightage process was calculated by the consistency ratio as 0.06. As revealed in the GWPZ analysis, the northern and northwestern regions have the highest groundwater potentiality. Thirty-five vertical electrical soundings (VES) and four electrical resistivity tomography (ERT) profiles were measured across the study area. The interpretation of geophysical data revealed the presence of a water-bearing layer at depths varying between 27 and 51 m from the ground surface. Its resistivity values indicate the existence of freshwater in the northern and northwestern parts, which highly matches the GWPZ analysis results. However, the southern parts of the area are dominated by saltwater. The integration between geospatial techniques, geophysics, and hydrogeology forms a robust scientific approach, with invaluable insights for sustainable water resource management.

Assessing the dam's stability of the pond at the ‘El Bosque’ renaissance garden (Béjar, Spain)

AbstractThe renaissance botanical garden of ‘El Bosque’ in Béjar (Salamanca, Spain) presents a pond bounded by a dam in its western part. The latter is formed by two masonry walls interconnected by buttresses. Cubic spaces in between are filled with a variable grain‐size material (silty sand) that allows limited water flow. In recent years, the southern part of the dam has experienced localized and random subsidence that jeopardizes the entrance to part of the garden. To regain access, a proper and reliable diagnosis of the origin, magnitude and relevance of the subsidence must be made. In this regard, we have undertaken a microgravity survey in the dam area to identify places with an anomalous distribution of the filling material in order to foresee further sinking or potential collapsing areas. The precise positioning (2 mm resolution) and accurate terrain correction needed in this kind of high‐resolution gravity surveys (points every 1.5 m) were achieved by creating a detailed digital terrain model (cm resolution) with a remotely piloted aircraft. In addition, we performed three electric resistivity tomography (ERT) profiles at different levels of the garden: (i) on the dam itself; (ii) right on the foot of the dam and parallel to it (5 m below and ∼17 m to the W); and (iii) a bit farther, but also parallel to the dam (8 m below and ∼27 m to the W). The ERT profiles identified high conductivity in water‐saturated areas and determined the paths that rainfall and pond's seepage water follow in the dam and its underground, formed by granites. The geophysical studies were paired with geotechnical analyses of the sunk materials. The study concluded that the thinnest fraction of the dam's filling material (i.e., silts) is being washed away, leaving behind sand with less density and stability, susceptible to collapse. Thus, the observed sinking is related to soil piping, that is to soil internal erosion and compaction issues that force the soil material to re‐adjust geometrically and volumetrically.

Unveiling a hidden fortification system at “Faraglioni” Middle Bronze Age Village of Ustica Island (Palermo, Italy) through ERT and GPR prospections

We carried out a geophysical research project in the Middle Bronze Age village of Ustica (Palermo, Sicily, Italy), named “Faraglioni Village” after the stack formations which detach from the coast north of the archaeological site. The investigation, which comprised Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques, allowed us to discover the buried foundations of an outwork fortification system never evidenced by previous archaeological studies, only hypothesised from the observation of aerial photography and partially outcropping boulders, which align roughly parallel to the main defensive wall of the Village.Our geophysical prospection involved the entire 250 m-long arc of the outward village defensive wall, with the acquisition of eleven ERT profiles and 27 GPR scans. The techniques were selected based on both favourable logistics and methods applicability: ERT sections allowed us to trace a series of high-resistivity anomalies arranged to form an arc-shaped structure along the perimeter of the defensive wall. GPR investigation was localised in the most accommodating patch of terrain of the site, with the effort of intercepting clear enough sections of the target, to determine more accurately its shape, depth, and overall dimensions. Our discovery paves the way for new investigations, mainly aimed at defining the timing of construction of the fortification system, as well as the function of the remains of other architectural structures identified close to the wall, which could represent the target of further geophysical investigations.

Geophysical and hydrochemical analysis of saltwater intrusion in El-Omayed, Egypt: Implications for sustainable groundwater management

This study investigates groundwater resources in the El-Omayed area on the northwestern coast of Egypt. The aim is to assess the extent and severity of saltwater intrusion in the shallow aquifer system, as well as its potential impacts on groundwater resources. A combined geophysical and hydrochemical approach was adopted in this investigation. Across the study area, 32 vertical electrical soundings were conducted, along with two electrical resistivity tomography profiles. Sixteen water samples were collected, and then chemically analyzed to evaluate water quality, seawater intrusion, and source of salinity. The results of geophysical interpretation revealed the existence of a thin, shallow freshwater-bearing layer (42–95 Ω.m), predominantly situated in the central parts of the area, followed by saltwater saturated layers of (0.9–2.1 Ω.m) affected by the Mediterranean Sea invasion. The eastern portion of the aquifer also recorded a relatively high seawater intrusion. As evidenced by the actual data gathered from drilled wells, the study area can be divided into three distinct zones: fresh, brackish, and saltwater zones. The study identified the most suitable sites to establish new groundwater wells, in addition to identifying the source of salinity through the investigation of the geochemical properties of water samples.

Geophysical and Geochemical Pilot Study to Characterize the Dam Foundation Rock and Source of Seepage in Part of Pensacola Dam in Oklahoma

Pensacola Dam, operated by the Grand River Dam Authority (GRDA), is a multiple-arch buttress dam constructed in 1940. The dam has little or no existing geophysical reports on the integrity of the dam foundation rock and even less knowledge at depth. Visual inspection indicated evidence of seepage at some arches of the dam. As a pilot study, we conducted a suite of geophysical surveys inside two arches (Arch-16 and Arch-17) and a part of the downstream berm to characterize the dam foundation rock, delineate seepage zones, and identify the most appropriate geophysical methods for temporal monitoring of the dam’s conditions. The geophysical methods included electrical resistivity tomography (ERT), self-potential (SP), multichannel analysis of surface waves (MASW), compressional (P)-wave refraction, and shear (S)-wave reflection. Water samples were collected for geochemical analysis to investigate the source of the seepage flow inside Arch-16. The geophysical results characterized the dam foundation rock into an unsaturated limestone and chert overlying a water-saturated limestone and chert. The ERT profiles indicated that groundwater is rising inside the arches and significantly dropping under the downstream berm, which can be due to the uplift pressure beneath the dam base. Zones of high seepage potential were detected near the buttress walls of the two surveyed arches, which may be related to previous blasting, excavation of the dam foundation, concrete placement, or improper grouting. The geochemical analysis of water samples taken from the artesian wells inside Arch-16 and the Grand Lake revealed different chemical compositions, suggesting that the source of water could be a mixture of groundwater and lake water or lake water interacting with rock and reaching the surface through fractures; however, more sampling and further analysis are required to ascertain the source of the seeps. This study showed that the ERT, SP, and S-wave reflection methods have effectively characterized the dam foundation rock and seepage zones beneath the arches. The study provided a better understanding of the conditions of the dam foundation rock, evaluated the utilized geophysical methods, and determined the optimum geophysical methods that can be used for the characterization and monitoring of the subsurface conditions along the entire length of the dam. In this study, we have demonstrated that the integration of effective geophysical surveys and geochemical analysis yielded optimum results in solving a complex dam safety problem. This strategy promotes the best practice for dam safety investigation.

Setting the basis for a high-resolution record of the late Quaternary to present climate variability from Castiglione maar, central Italy: First results from AMUSED project

The AMUSED (A MUltidisciplinary Study of past global climatE changes from continental and marine archives in the MeDiterranean region) project aims at improving knowledge of late Quaternary climate variability and its expressions in different geological settings of the Mediterranean region. In this framework, the Castiglione maar, in the Colli Albani Volcanic District, central Italy, was selected for acquiring a high-resolution and geochronologically well-constrained multi-proxy record by drilling the entire lacustrine succession. Electrical Resistivity Tomography (ERT) profiles were acquired across the central portion and the SW crater edge to depict the geometry of the sedimentary infilling and select the best drilling site. Two parallel cores (C1 and C2), 116 m- and 126.5 m-depth respectively, were recovered from the central sector of the Castiglione basin, where, according to ERT profiles, the sedimentary succession reaches the maximum thickness. The sedimentary infilling consists of fine-grained sediments: mainly fine sand, silt and clay, with minor gravel intervals and numerous tephra layers and volcaniclastic lenses. Specifically, more than 60 tephras were identified and used, alongside other lithostratigraphic features, to correlate the C1 and C2 cores and to assemble a composite section. The variability in magnetic susceptibility, led by glacial-interglacial cycles, and the geochemical fingerprinting of key tephra layers allowed to establish a preliminary chronological framework for the Castiglione succession which certainly spans the last 365 ka, with a mean sedimentation rate of 0.33 mm/yr. The relatively long time span of the Castiglione maar succession arises as a new potentially meaningful node of the network of Mediterranean records for better reconstructing the late Quaternary climate dynamics on a regional and extra-regional scale.

Detection of Fractured Limestone Landslides Using Electrical Resistivity Tomography: A Case Study in the Mokattam Plateau, Egypt

Landslides occurred in the Ain Moussa area, north of Mokattam, Egypt. The study aims to detect landslide causes in fractured limestone using electrical resistivity tomography. The result of the electrical resistivity tomography profiles showed that there are two main layers: the first is a high-geoelectrical resistivity layer of fractured limestone covered in some places by backfill sediments, and the second is a low-geoelectrical resistivity layer of shale below the limestone. In between the two layers is a zone that is holding the weight of water leaking from the surface; it has a medium resistivity value. These results were compared with available satellite images. Based on the results, it was found that there is a layer of fractured limestone on the surface. The top part of it is dry, but the bottom part has water leaking from the surface, which has the biggest effect on landslides. The shale layer's surface is irregular, as it was discovered that the most influential part is the low part of the shale's surface, as it contains a high-water content and thus is more affected, and it is where the landslide occurred. It is possible to identify the deepest points of the shale layer, on which the water leaking from the surface collects, along the plateau, as these places are more likely to cause landslides.

Groundwater Investigation through Electrical Resistivity Tomography in the Galhareri District, Galgaduud Region, Somalia: Insights into Hydrogeological Properties

Electrical resistivity tomography (ERT) was conducted to delineate groundwater potential zones in villages located in the Galhareri district of the Galgaduud region, central Somalia. A total of four ERT profiles were examined using the gradient configuration, chosen for its practical advantages over other configurations. The study revealed that all profiles were situated within similar geological environments, characterized by comparable rock types. However, notable disparities were observed in lithological variations, particularly in the texture of rocks encountered at different locations and in the thicknesses of the encountered geo-electric layers. The two-dimensional inversion results derived from the electrical resistivity data unveiled the presence of four geo-electrical layers. The first layer was interpreted as sand dunes. The second layer exhibited relatively higher resistivity values, indicating the presence of compact limestone and sandstone. The resistivity of the third layer suggested the existence of a lower resistivity layer, interpreted as weathered limestone, while the fourth layer demonstrated very low inverted resistivity, interpreted as sandy clay with sandstone. The ERT models constructed for the survey area effectively delineated the aquifer zone, represented by layer 3, which likely consists of weathered limestone, sandy clay, and sandstone. The resistivity values obtained for the aquifer zone, specifically at depths ranging from 200 to 300 m, were relatively low, suggesting that the groundwater quality is brackish in nature.

Integrated Geophysical and Geochemical Analyses for Assessment of Potential Coal Prospects in Tirah Area, Khyber Pakhtunkhwa, Pakistan

This study presents a comprehensive investigation of the Paleocene coal in the Tirah area, District Khyber, Khyber Pakhtunkhwa, Pakistan, utilizing onsite Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) experiments combined with geochemical analyses of obtained coal-rock samples. ERT and GPR profiles, along with geochemical data, were collected and meticulously analyzed to assess the potential and quality of coal seams based on coal ranking, aiming to delineate their spatial distribution and future exploitation prospects. The study reveals a wide range of resistivity values, spanning from 8.93 Ωm to 2472 Ωm. The uppermost layer, comprising silt and clay with water saturation, exhibits resistivity values ranging from 8.93 Ωm to 50 Ωm. The subsequent stratum, characterized by wet sandstone with minor shale and clay, has resistivity values between 50 Ωm and 95.3 Ωm. Significantly, the zone with resistivity values between 95.3 Ωm and 800 Ωm is identified as the probable host of the coal seams. Based on the results, the coal seams are estimated to exist at depths ranging from 14 to 23 m, with resistivity values between 95.3 Ωm and 800 Ωm. GPR results further corroborate these findings, revealing coal-bearing strata at various depths, thus confirming the ERT results. Additionally, proximate and ultimate analysis provided insights into the quality of the coal, with average concentration values of moisture content (6.95%), ash content (22.12%), volatile matter (28.55%), fixed carbon (42.40%), carbon content (55.35%), hydrogen (4.7%), nitrogen (0.95%), sulfur (4.82%), and oxygen (10.06%). Comparative analyses of the obtained results with local coals from the Salt Range in Punjab, Pakistan, and coal from Ogboyaga, Nigeria, confirm that the studied coal ranks as sub-bituminous. This comprehensive assessment offers valuable insights into the potential and quality of Paleocene coal in the studied region and is applicable to other areas with similar geological settings.

Frequency domain electromagnetic calibration for improved detection of sand intrusions in river embankments

Sand intrusions pose significant risks to river embankments due to potential flow pathways that can lead to instability during flood events. Visual inspection is a first step to recognize critical segments, but it does not deliver information about the subsurface. In this context, the electromagnetic induction (EMI) technique is a useful method for preliminary zoning at regional scale while the electrical resistivity tomography (ERT) method, widely used for hydrological purposes, is considered among the most reliable techniques for local subsurface imaging. A major sand intrusion within the levees of the Brenta River, located near Venice (northern Italy), resulted in water seeping during seasonal floods and posed severe threats to embankment stability. ERT and EMI techniques, along with geotechnical investigations, were the best survey choices to address the problem. Resistivity profiling successfully imaged the sand body geometry within and underneath the levee, and results correlated nicely with borehole stratigraphy. A first multiarray EMI device, which represented a faster and less expensive survey, was deployed to map further anomalies along nearby levees, but results were not satisfactory because the inverted profile failed to image the known intrusion. A second multiarray EMI device, with larger coil spacing, was also tested. Although it performed better in detecting the intrusion, results were still below expectations. A calibration procedure based on Pearson's coefficients and using ERT as a reference was then devised and implemented to correct the EMI data prior to carrying out inversion. The procedure was successful for both EMI data sets, leading to realistic subsurface resistivity in the inverted sections. EMI measurements could then be recovered and interpreted correctly to estimate subsurface textures. The possibility of calibrating EMI data and obtaining subsurface resistivity images comparable to standard ERT profiling is an important improvement for cost-effective EMI surveying of river embankments to mitigate flood hazards.

Investigation of subsurface karst in an opencast mine in southwestern China via surface and cross-borehole electrical resistivity tomography

Subsurface karst features are significantly developed in Guangxi Province, China. This area mainly contains fractured subsurface rock, abundant karst channels, and widely distributed underground fissure networks. Such adverse geological conditions could potentially create hydrogeological hazards such as collapses, water inrush, and mud inrush during infrastructure construction. The Hejing limestone mine is an opencast mine in Pingnan County, Guangxi, that produces cement. Mining activities have altered the seepage fields in this area, causing large amounts of groundwater to flood into the mining pit; this has caused many ground collapses while severely reducing limestone production. More than 24 km of surface electrical resistivity tomography (ERT) profiles have been previously recorded in the region to identify potential karst positions and explore groundwater inrush paths. In this study. we employed surface and cross-borehole ERT surveys to delineate specific groundwater inrush paths on the eastern side of the mine and characterise karst distribution in the study area. Resistivity imaging results revealed some low-resistivity anomaly distributions and provided reliable geological information about the distribution of subsurface karst for future grouting work.