Electrical Resistivity Tomography Lines Research Articles

Electrical resistivity tomography of a gypsiferous subsurface soil: Geotechnical detection of a geoenvironmental phenomenon

The near-surface soils are generally exposed to environmental changes and can be influenced by water, humidity, temperature, and other factors related to erosion or physical changes. It has been reported that serious distress occurred in the ground surface and below ground for many areas subjected to seasonal weather changes. The gypsum rich material is often linked to the cavity formation, uplift, and cracking of road surfaces due to the high solubility of gypsum in water and acidic media. Electrical resistivity techniques along with the advanced method of identifications (Scanning electron microscope; SEM and X-ray diffraction: XRD) were utilized to detect cavities and gypsum rich subsurface soils. A site where many cavities were formed as a result of dissolving gypsum and anhydrite was considered for this study. Syscal R1 electrical resistivity equipment (IRIS instrument) was used for several cross-sections established within the site. Several electrical resistivity tomography lines were constructed and studied. The SEM (scanning electron microscope) was used to examine and compare forms and fabrics of gypsum and minerals containing sulfates. The chemical analysis (EDAX) using an electron beam was conducted to establish elements present within tested samples. Quantitative chemical tests for some parameters including sulfate ions were carried out. A prediction model based on statistical correlations between sulfate ion content and the electrical resistivity is presented. Electrical resistivity is found to be inversely proportionalto the sulfate content for concentrations of 0 to 5%.

Fault mapping and characterization of a coastal aquifer related to a mangrove ecosystem, using electrical resistivity tomography (ERT), ground penetrating radar (GPR) and hydrochemical data: The case of the Mangue de Pedra Aquifer, Armação dos Búzios, Brazil

The Mangue de Pedra is a rare and fragile ecosystem. It stands above a sandstone-conglomerate substratum of the Neogene Barreiras Formation, which also hosts the local coastal aquifer, known as the Mangue de Pedra Aquifer. The Pai Vitório Fault, a regional structure, which represents the south border of the São João da Barra Graben, separates the Paleoproterozoic crystalline basement from these sediments and limits the occurrence of both the mangrove and aquifer. Aside from hydrochemical data, geophysical surveys, including electrical resistivity tomography (ERT) and ground penetrating radar (GPR), allowed to investigate the local substrate and the role of fault structures affecting the aquifer-mangrove ecosystem, as well as to provide a better understanding on groundwater circulation and effects of seawater interface dynamics. The hydrochemical data for 11 water samples were used for interpolation to generate pH, Eh and electrical conductivity (EC) maps, and the results revealed a zone of less saline water flow and a discharge zone at the cliff's base. The survey of five ERT lines, using the dipole-dipole configuration, provided a picture of the saturated zone of the aquifer. The interpreted sedimentary units have been divided in three different layers, based on water content and variation of the water quality: 1) unsaturated zone; 2) good/intermediate-quality freshwater (34.4–137 Ω-m); 3) brackish/poor-quality freshwater (4.46–34.4 Ω-m). This method indicated the presence of at least five normal faults, including major and shallow structures along the mangrove, and the basement 5 m below sea level. The radargrams also confirmed the presence of most of these faults, recognized as complex fault zones at shallow levels. The interpretation on orientation of these structures was then enhanced by geoprocessing techniques, indicating a major NE-SW trend. This set of faults provides some of the requirements for the mangrove existence, including: a) the shape and materials of the beach and surrounding landscape, composed by chalcedony breccias blocks which are resistant to erosion, protecting the local environment from impact of sea waves; b) pathways for groundwater flow; and c) traps or barriers within the aquifer, which channel less saline waters to that specific site. This set of factors creates a unique ecosystem, which normally does not occur in beaches from open sea shorelines.

Forensic Study on Federal Road Pavement Failures at Rengit and Semerah, Batu Pahat

Forensic investigations in engineering term may be conducted to identify the causes of failure to facilitate the design of proper repairs, or to improve the performance or lifespan of a component, assembly, or structure. This paper presents the combination of geotechnical investigation and geophysical survey method as a forensic tool to predict the causes of pavement failure occurred along the coastal area of federal road FT005. The number and type of field testing are varying on each selected study area at Rengit and Semerah, Batu Pahat as this location constructed on soft soil area. Non-destructive testing (NDT) method using electrical resistivity tomography (ERT) were chosen to be applied at the worst severity of the road failure. Three number of resistivity lines of 100m and 200m were laid out using ABEM Terrameter LS2 with gradient method of electrode arrays covering up to 40-meters depth. Then, further testing of destructive testing (DT) method using borehole drilling conducted near the ERT lines to obtain soil profile with SPT N-value measurement. The disturbed and undisturbed samples were obtained to carried out laboratory test for soil classification. After that, inspection of road pavement layers were implemented at five number of trial pit tests which excavated up to subgrade layer to determine the thickness of the materials used in road construction. Three number of mackintosh probe test were also conducted on top of the subgrade layer after the trenching to obtain the strength of the subgrade layer. The results presented showed that severe longitudinal cracking were the predominant premature failures on the roads studied due to settlement effect of soft soil. The analysis obtained from RES2DINV program stated that the subsurface profiling was dominantly in saturated condition which the resistivity value was less than 100 ohm.m. From the SPT N-value it is observed that, the very soft soil layer is up to 8 m followed by the soft to stiff clays soil. Another cause of failure was resulted from the differential settlement due to the effect of different design of road construction with varies material used. The reliability and efficiency of the instruments used were also discussed in this study.

GEO-ELECTRIC INVESTIGATION OF GROUNDWATER POTENTIAL IN LADERIN HOUSING ESTATE, ABEOKUTA, SOUTHWESTERN, NIGERIA

Vertical Electrical Sounding (VES) and 2D Electrical Resistivity Tomography (ERT), with Schlumberger and Wenner electrode array configurations respectively were employed to investigate the groundwater potential of Laderin Housing Estate located at Oke-mosan, Abeokuta, Ogun state, southwestern Nigeria. The area is underlain by the basement complex of the southwestern Nigeria. The research aimed at determining the aquifer/groundwater zone and characterizes the lithology of the study area. The geophysical survey involving nine VES and four profile of 2-D ERT lines with lengths varying from 100 m to 150 m were carried out. The field data from both the ERT and VES were processed and interpreted using RES2DINV and WINRESIST software respectively. Geometrical effects from the pseudo-section were removed and an image of true depth and true formation resistivity were produced. Three to four geo-electric layers were revealed in all, which are; topsoil, weathered layer, fractured and fresh basement rock. The result of 2 D inversion provide lithologic unit, weathering profile and geological structure favourable for groundwater potential. The results show that the basement rock was delineated with resistivity values that range between 701.3 Ωm and 9459.3 Ωm. The bedrock topography has a variable thickness of overburden between 3 m and above 16.4 m, which is fairly shallow. The geophysical survey show that VES 2, VES 3, VES 4, VES 5, and VES 6 are possible location for groundwater extraction. The difficult terrain, where thick overburden are located are also promising target for groundwater development.
 

Identification of Seawater Intrusion in Kota Lama Semarang and Surrounding Based on Geoelectrical Resistivity Survey

Kota Lama Semarang is a tourism potential that causes a high demand for groundwater, which can cause seawater intrusion. Seawater intrusion can affect groundwater quality and soil conditions. This research aimed to determine the depth and distribution of seawater intrusion in the Kota Lama Semarang and surrounding. The research used electrical resistivity tomography (ERT) and vertical electrical sounding (VES). Geoelectrical resistivity surveyed consist of 4 ERT lines and 3 VES lines with a line length was 100 to 160 m. Lines were scattered north and south of the Kota Lama to determine the distribution of seawater intrusion. Inversion processing used Progress for VES and Res2DInv for ERT. The content of chloride as well in Kota Lama was measured as supporting data. At this location it was estimated that it has 3 layers: top soil, sand and sandy clay. The sand layer at a depth of 3 to 11 m was the layer affected by seawater intrusion. Seawater intrusion on the north side was higher than on the south side. This was indicated by the lower resistivity value of the groundwater layer on the north side. The resistivity value of the groundwater layer on the north side was 0.96 Ωm while the south side was 3.76 Ωm. This result was appropriate to the chloride content which indicates a higher concentration on the north side of the Kota Lama Semarang.

Practical considerations for shallow submerged archaeological prospection with 3‐D electrical resistivity tomography

AbstractDirect current electrical resistivity method has experienced significant breakthroughs during the last decades with the development of advanced instrumentation and sophisticated inversion algorithms. These have substantially benefitted the field of archaeological prospection to extract quantitative information for the buried archaeological material in a complete three‐dimensional (3‐D) context. The obvious continuation of past human occupation and associated settlements in the ultra‐shallow part of coastal zones generated the necessity to compile methodologies for mapping submerged cultural assets. This study investigates the efficiency of dynamic floating and submerged 3‐D Electrical Resistivity Tomography (ERT) for mapping archaeological relics buried beneath the sediments in ultra‐shallow marine environments. Extensive testing is performed through numerical simulation and 3‐D inversion of synthetic apparent resistivity tomographic data. The generic resistivity model including a complex resistive structure embedded in a stratified conductive environmental regime simulated typical scenario of an archaeological feature submerged below the sea water layer. Different array configurations including Dipole–Dipole (DD), Gradient (GRD), Reciprocal Wenner (RecWEN) and Wenner‐Schlumberger (WS), which can be appropriately programmed for continuous off‐shore measurements with associated multichannel resistivity instruments, are validated in order to determine the most efficient one for such surveys. Densely spaced multiple parallel two‐dimensional (2‐D) ERT lines along a single direction composed the survey layout to extract the 3‐D apparent resistivity data set for the different electrode arrays using a 3‐D finite element program. The synthetic tomographic data were corrupted with 3% Gaussian noise and inverted with an iterative smoothness constrained inversion algorithm. The comparative results from the various tested arrays manifest the superiority of the DD in reconstructing the optimum resistivity inversion model for both the floating and submerged survey modes. Additional tests were made concerning the resolving capabilities of ERT with variable seawater thickness and target characteristics. Although accurate ‘a priori’ information regarding the water resistivity and thickness are essential for constraining the 3‐D inversion, erroneous estimation of these parameters can result to misleading results, especially for submerged survey modes. The simulation of floating and submerged 3‐D ERT surveys through synthetic modelling documented the benefits of such approach in reconstructing structured cultural material in shallow off‐shore environments. Finally, the results of a 3‐D dynamic floating ERT survey from a submerged archaeological site in Greece verified the theoretical outcomes, proposing at the same time techniques to overcome problems that can occur due to the unique conditions of the ultra‐shallow marine environment. Overall, this work enhances the conclusion that 3‐D marine ERT is a robust method to reconstruct submerged archaeological structures related to ancient built environment in ultra‐shallow marine regions.

Investigation of incessant road failure in parts of Abeokuta, Southwestern Nigeria using integrated geoelectric methods and soil analysis

A flexible pavement devoid of discontinuities allows for smooth movement of a vehicle load on the roadway. This study involved the use of integrated geoelectric methods comprised of 1D and 2D Electrical Resistivity Tomography (ERT) as well as soil analysis to investigate causes of unceasing road failures along busy Camp—Alabata Road, Abeokuta, Southwestern Nigeria. Four road sections (two failed portions and one fair section and one good section) were identified along which four resistivity traverses were established along the investigated roadway. Four 1D Vertical Electrical Sounding (VES) points were also carried out on the 2D ERT lines. Apparent resistivity data were measured along the four traverses using Schlumberger and Wenner arrays with the aid of a Campus Ohmega resistivity meter. The VES and 2D resistivity data were processed and inverted using WinResist and RES2DINV softwares, respectively. Twenty soil samples at a sampling depth (0–1.0 m) with an interval of 0.2 m were also collected on all designated road sections and analyzed for selected hydraulic/geotechnical properties related to pavement durability. Penetration resistance (PR) was measured in situ by a penetrologger while other subgrade soil properties were evaluated in the laboratory. The VES results delineate three geoelectic layers comprising topsoil, weathered basement (clayey), and fractured/fresh basement with their corresponding resistivity values ranging between 71 and 282 Ωm, 12–76 Ωm, and 261–10,094 Ωm. The thicknesses range between 0.9 and 3.2 m for topsoil and 4.5–19.1 m for weathered basement. 2D resistivity inverted sections delineate two lithologic layers: topsoil with resistivity values > 200 Ωm, devoid of linear geological structures as competent topsoil on a stable road section while incompetent topsoil on failed road sections were characterized by resistivity values < 200 Ωm. The weathered layers as depicted by 2D inverted resistivity sections were generally of resistivity less than 100 Ωm while fractured/fresh basement were not depicted in the 2D model. Failed road sections are underlain by topsoil with a resistivity (< 200 Ωm), shallow weathered (clayey) layer, and differential settlement of saturated subgrade materials. Soil analyses results showed that the stable portion depicts lowest mean values of plasticity index (5.12%) and liquid limit (22.21%), uniform sandy-loam textural class, highest mean value (1.24 cm/h) of saturated hydraulic conductivity (Ksat); % sand content (73.2%) and Penetration Resistance (PR) values that increased within the sampling depth (0–40 cm). However, one of the failed sections had lowest mean value (0.81 cm/h) of Ksat, % sand content (64.9%) and PR values that decreased within the sampling depth (0–40 cm). Analysis of variance (ANOVA) showed that there were significant differences at the 5% level among all designated road sections with respect to percent silt and moisture content (MC). Adequate engineering construction approaches should be adopted considering the thick layer of incompetent formation (clay) observed across the area investigated. Integrated interpretations present a better resolution and detailed characterization of the substratum underlying the pavement.

Identification of Flow Zones Inside and at the Base of a Uranium Mine Tailings Dam Using Geophysics

Potential problems related to a tailings dam’s stability are a matter of concern, especially where structural failure might endanger nearby communities and the environment. The Osamu Utsumi mine, located in the State of Minas Gerais, is currently not operating. The rock-soil tailings dam has water upwelling downstream in the bedrock, with water flux confined to rock fractures. This research was conducted to identify possible flux zones in the base of the dam using DC resistivity and electrical resistivity tomography (ERT). The data acquisition consisted of five ERT lines with 6 m of spacing between electrodes, using a Schlumberger array. The results are presented by 2D and 3D geophysical models comprising measured and processed resistivity values. It was possible to identify a low resistivity zone (5–20 Ωm), whose structural continuity indicates water infiltration in the bedrock under the dam. Moreover, the results do not indicate that erosion is taking place in the interior of the dam, reducing the risk of geotechnical instability and failure of physical integrity.

Application of combined electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) methods to investigate Xiaoshan District landslide site: Hangzhou, China

Near surface geophysical methods have been used extensively as non-invasive and cost-effective methods to characterize subsurface geological properties over the past two decades. Since these approaches are able to illustrate the temporal and spatial changes of geological conditions, they are applied for the characterization of landslide zones. Landslides are one of many natural disasters that cause life and property losses. Landslide mechanisms are affected by topsoil materials, bedrock geometry, and subsurface water content. This paper presents study results for a landslide area located in the Xiaoshan district in Hangzhou City, China. The 2D seismic P-wave refraction tomography (SRT) and 2D electrical resistivity tomography (ERT) methods were used to characterize the landslides in this area. The ERT technique was applied using four lines over and outside of the landslide scar using the Wenner electrode configuration. The SRT study was performed along four longitudinal profiles coincident with the ERT lines. Samples from two boreholes, located on two different profiles, and SEM-EDS (Scanning Electron Microscopy-Energy Dispersive Spectroscopy) testing provided calibration for of ERT and SRT measurements. The results of the combined techniques have defined the layers associated with sand and high clay content as well as the geometry of failure surfaces.

Investigation of groundwater aquifer at Noborejo, Salatiga using Electrical Resistivity Tomography (ERT) and Vertical Electrical Sounding (VES) methods

Local Water Supply Utility (PDAM) of Salatiga, Indonesia planned to build three new wells to meet the needs for clean water and increase the number of customers in Salatiga. The plans for the construction of the new wells are located in Randuancir, Kumpulrejo and Noborejo Sub-Districts. The groundwater aquifer layer in Noborejo Village is detected using the Electrical Resistivity Tomography (ERT) method, while the determination of the depth of aquifer layer will be controlled by the results of Vertical Electrical Sounding (VES) data processing. 4 ERT lines and 4 VES points were acquired in 2017. The processing results showed that water-containing layer tends to have a lower resistivity value. In the study area, 2 types of aquifers were found, namely unconfined aquifer (usually exploited using dug wells), and confined aquifer which are generally located on the deeper depth. Unconfined aquifers were identified with a lower resistivity value than its surroundings (20-80 Ωm) with an average depth of 25 m on all lines. Whereas confined aquifer has a higher resistivity in the range of 90 Ωm. However, confined aquifers are only found with certainty in Line 1, while in other Lines the possibility of a confined aquifers existence is smaller.

3D interpretation of helicopter-borne frequency-domain electromagnetic (HEM) data from Rams\xe5 Basin and adjacent areas at And\xf8ya, Norway

The Ramså Basin is situated on the strandflat on the northeastern side of Andøya between Ramså and Skarstein. It is the only known Mesozoic sedimentary basin onshore Norway and therefore represents an easily accessible analogue to study the setting and development of the offshore Mesozoic basins along the Norwegian continental shelf. Basement rocks (100–10,000 Ωm) and sedimentary rocks (5–1000 Ωm) have distinct ranges of the electrical resistivity. Resistivity of the subsurface can be investigated by electric and electromagnetic (EM) methods to delineate the extent of the sedimentary basin. This paper presents the interpretation of helicopter-borne frequency-domain electromagnetic (HEM) data and their correlation with data obtained from borehole logging and two-dimensional (2D) electrical resistivity tomography (ERT). Resistivity data obtained from these methods show a good agreement. Spatially constrained inversion (SCI) of HEM presents a three-dimensional (3D) subsurface resistivity image of a larger area in comparison to what could have been drawn from few ERT lines and borehole logging. HEM interpretation together with other ERT and borehole resistivity data indicates the extent of Ramså Basin beyond the presently mapped boundary. This extension is also supported by interpretation of earlier collected gravity, magnetic and seismic data. The Ramså sedimentary basin shows a wide range of electrical resistivity between ca. 10 and 600 Ωm at different depths. Two highly conductive areas are newly identified by HEM data interpretation. In one of these areas, outcropping graphite and sulfide minerals were found by field observations.

Identification of leachate infiltration and its flow pathway in landfill by means of electrical resistivity tomography (ERT).

Sanitary landfills are a well-planned engineering work for final disposal of municipal solid waste in order to minimize the environmental impacts in soil and groundwater. Therefore, several control systems are installed such as liners and leachate and biogas collectors. However, the establishment of landfill in vulnerable areas, the inadequate operations, and failures in collectors and liner systems can cause subsurface contamination. The discovery of eventual leachate leakage usually is based on chemical analyses of groundwater using monitoring wells; which may not be representative in spatial terms. This work involves a geophysical monitoring of the leachate percolation in a landfill waste cell closed in 2014, in which geomembrane boreholes and aquifer contamination have been proved. The DC resistivity method was applied by means of electrical resistivity tomography (ERT) in order to detect eventual contrasts in electrical properties. Twelve ERT lines have been carried out in a steady mesh during three years of study (2016, 2017, and 2018). The results of 2016 allowed a clear contrast between zones of leachate percolation into the aquifer and the natural environment. The comparative analyses of the three-year monitoring results reveal a gradual increase in resistivity values in the areas of leachate percolation. The absence of replacement of biodegradable organic matter after 2014 conditions the gradual decrease of leachate salinity. In this sense, the increase in resistivity is an indicator of the tendency for chemical stabilization of the organic waste and existence of natural attenuation processes.

Aquifer characterization of some parts of Ijebu Igbo using electrical resistivity methods

Groundwater aquifer mapping is a scientific process, where-in a combination of geologic, geophysical and hydrologic analyses are applied to characterize the quantity, quality and sustainability of groundwater in aquifers. A total of ten (10) Vertical Electrical Sounding (VES) and six (6) two dimensional (2-D) Electrical Resistivity Tomography (ERT) lines were achieved. The 2-D ERT lines were carried out on a length of about 240 m – 320 m using Dipole-dipole electrode configuration. Acquired 2- D and VES data was processed using RES2DINV and WINRESIST software respectively. The VES results obtained showed that the study area has 3 to 4 geoelectric layers with the following lithology namely; the topsoil, partially weathered, fractured layer and the fresh bedrock. The topsoil has a resistivity value of about 609 Ωm. The thickness and the depth representation of weathered basement and fractured basement vary from one another. They have a resistivity value of about 128 Ωm and 1089 Ωm respectively. The 2-D inverse models obtained from the area showed the presence of vertical contacts and fractured zones that can serve as an aquifer for the surveyed area. The pseudosection of the inversion results unravel the two profiles with saturated fractured basement where adequate groundwater can be obtained. The Iso-Pach map showed high contour closures which are good signature for good to moderate groundwater potential zones on profiles four, five and six (VES 7,8 and 9). In contrast, profiles one, two, three (VES 2 and 6) displayed low closures which depicts low overburden thickness and categorically means poor groundwater potential zones.

Analysis of the Physical Integrity of Earth Dams from Electrical Resistivity Tomography (ERT) in Brazil

Dams are structures that barrier rivers and streams for a variety of purposes. The constant maintenance of these structures is essential, since a possible accident can lead to damage of catastrophic proportions. In November 2015 in the city of Mariana, Minas Gerais (MG), Brazil, a tailings dam burst and spilled 56 million cubic meters of tailings. In addition to victimize 19 people, the event caused material, economic and environmental damage—it is considered the largest environmental disaster of Brazil. This research presents a cheap and quickly applied alternative for investigation of seepage of water in earth dams, built with distinct embankment materials—one dam composed of predominantly silt and clay soil and the other predominantly composed of sandy soil. The research methods used were: geotechnical tests as gradation test, permeability test with the Guelph permeameter method and the application of the (DC) resistivity geophysical method from the electrical resistivity tomography (ERT) technique in Wenner array. Three ERT lines were positioned parallel to the longitudinal axis at each dam. The data are presented in 2D and 3D geophysical images with electrical resistivity values modeled. Based on the physical principle of electrolytic conduction, i.e., decrease in electrical resistance in materials or siliceous minerals in moisture conditions compared to the material in the dry state, the results revealed low-resistivity zones restricted to some points, associated with water infiltration in the transverse direction. The results indicate saturation restricted to some points and low permeability, at the present time, an installation of piping processes.

Hydrogeophysical characterization of shallow light non‐aqueous phase liquid contamination at a karst aquifer

ABSTRACTTwo wells at an industrial facility were recently removed from service due to signs of long‐term hydrocarbon contamination. Groundwater monitoring at the polluted wells documented the presence of a massive body of light non‐aqueous phase liquid materials that were accumulated at a depth of ∼50 m in the water table. We have used transient electromagnetic and electrical resistivity tomography to characterize the ground at the light non‐aqueous phase liquid contaminated wells. Additionally, analyses of hydrochemical samples and calculations of hydrogeological parameters have been implemented to interpret and describe the geophysical signatures of several anomalous hydrocarbon contaminations.Resistivity models, which are based on the three‐dimensional inversion of transient electromagnetic data and the two‐dimensional inversion of electrical resistivity tomography lines conducted at the source of leaks and contaminated wells, confirmed the presence of a hyper‐conductive zone (i.e. 0.5 Ω⋅m), 15 m deep and 20 m thick, that contained a mixture of light non‐aqueous phase liquid, hot water, and chemical acids. In addition, resistivity levels up to hundreds of ohm metre can be attributed to near‐surface cavernous structures. Furthermore, the resistivity models can resolve a 30‐m‐thick conductive layer underlying cavernous structures, yielding resistivity values ranging from 0.78 to 5 Ω⋅m. The ease of light non‐aqueous phase liquid migration is promoted due to zones of cavernous structures acting as preferential flow pathways down to the water table, where light non‐aqueous phase liquid materials follow the hydraulic gradient of the limestone aquifer, which has a calculated porosity of 20%. Chemical analysis of groundwater samples at contaminated well sites shows the depletion of sulphate and nitrate concentrations and an increase in bicarbonate at locations where the highest hydraulic conductivities are observed. These findings facilitates the construction of a conceptual model for contamination which is used to better understand the evolution of light non‐aqueous phase liquid pollution at the study area.

Geophysical investigation of earth dam using the electrical tomography resistivity technique

Dams are structures that dam rivers and streams for a variety of purposes. These structures often need to be sturdy to withstand the force of the impoundment and the high values of accumulated water load. The constant maintenance of these structures is essential, since a possible accident can lead to damage of catastrophic proportions. This research presents an alternative cheap and quick application for investigating water seepage in earth dams, through the application of the DC resistivity geophysical method from the electrical resistivity tomography (ERT) technique in Wenner array. Three ERT lines were placed parallel to the longitudinal axis of a dam formed by clay soil from the decomposition of diabase. The data are presented in 2D and pseudo-3D geophysical images with electrical resistivity values modeled. Based on the physical principle of electrolytic conduction, that is, decrease in electrical resistance in materials or siliceous minerals in moisture conditions as compared to the material in the dry state, the results revealed low-resistivity zones restricted to some points, associated with water infiltration in the transverse direction of the dam. The absence of evidence as water upwelling on the front of the dam together with geophysical evidence indicate saturation restricted to some points and low probability at the present time, for installation of piping processes.

Shallow Off-Shore Archaeological Prospection with 3-D Electrical Resistivity Tomography: The Case of Olous (Modern Elounda), Greece

It is well known that nowadays as well as in the past the vast majority of human habitation and activities are mainly concentrated in littoral areas. Thus the increased attention to coastal zone management contributed to the development and implementation of shallow-water mapping approaches for capturing current environmental conditions. During the last decade, geophysical imaging techniques like electrical resistivity tomography (ERT) have been used in mapping onshore buried antiquities in a non-destructive manner, contributing to cultural heritage management. Despite its increased implementation in mapping on-shore buried archaeological remains, ERT has minimal to non-existent employment for the understanding of the past dynamics in littoral and shallow off-shore marine environments. This work presents the results of an extensive ERT survey in investigating part of the Hellenistic to Byzantine submerged archaeological site of Olous, located on the north-eastern coast of Crete, Greece. A marine area of 7100 m2 was covered with 178 densely spaced ERT lines having a cumulative length of 8.3 km. A combination of submerged static and moving survey modes were used to document potential buried and submerged structures. The acquired data from the marine environment were processed with two-dimensional and three-dimensional inversion algorithms. A real time kinematic global navigation satellite system was used to map the visible submerged walls and compile the bathymetry model of the bay. The adaptation of ERT in reconstructing the underwater archaeological remains in a shallow marine environment presented specific methodological and processing challenges. The in situ experience from the archaeological site of Olous showed that ERT provided a robust method for mapping the submerged archaeological structures related to the ancient built environment (walls, buildings, roads), signifying at the same time the vertical stratigraphy of the submerged sediments. The inherent limitation of employing ERT in a conductive environment is counterbalanced by the incorporation of precise knowledge for the conductivity and bathymetry of the saline water in the modelling and inversion procedure. Although the methodology definitely needs further refinement, the overall outcomes of this work underline the potential of ERT imaging being integrated into wider shallow marine projects for the mapping of archaeological sites in similar environmental regimes.

Using geophysical surveys to test tracer‐based storage estimates in headwater catchments

AbstractHydrogeophysical surveys were carried out in a 3.2 km2 Scottish catchment where previous isotope studies inferred significant groundwater storage that makes important contributions to streamflow. We used electrical resistivity tomography (ERT) to characterize the architecture of glacial drifts and make an approximation of catchment‐scale storage. Four ERT lines (360–535 m in length) revealed extensive 5–10 m deep drift cover on steeper slopes, which extends up to 20–40 m in valley bottom areas. Assuming low clay fractions, we interpret variable resistivity as correlating with variations in porosity and water content. Using Archie's Law as a first approximation, we compute likely bounds for storage along the ERT transects. Areas of highest groundwater storage occur in valley bottom peat soils (up to 4 m deep) and underlying drift where up to 10 000 mm of precipitation equivalent may be stored. This is consistent with groundwater levels which indicate saturation to within 0.2 m of the surface. However, significant slow groundwater flow paths occur in the shallower drifts on steeper hillslopes, where point storage varies between ~1000 mm–5000 mm. These fluxes maintain saturated conditions in the valley bottom and are recharged from drift‐free areas on the catchment interfluves. The surveys indicate that catchment scale storage is &gt;2000 mm which is consistent with tracer‐based estimates. Copyright © 2016 John Wiley &amp; Sons, Ltd.

3D inversion of irregular gridded 2D electrical resistivity tomography lines: Application to sinkhole mapping at the Island of Corfu (West Greece)

ABSTRACTWe study the case of the combined interpretation of irregularly gridded 2D electrical resistivity tomography lines. A 3D inversion algorithm was specially modified to accommodate the full 3D processing of such lines. The chosen approach uses a dense structured mesh and a flexible parameter design scheme. Tests with synthetic data show that full 3D inversion results are significantly improved when compared with the equivalent quasi‐3D imaging of the 2D inversion results. It is also shown that 3D inversion results can provide correct location and shape information about targets that are not directly located under the measured lines. The scheme was tested with real data from a site in the northern part of the Corfu Island (West Greece) where a large number of irregular gridded 2D electrical resistivity tomography lines were collected in order to map sinkholes related to gypsum. The 3D inverted images provide important information about the structure of the existing sinkholes and the potential sinkhole generation mechanism. Overall, the possibility of 3D interpretation of irregularly gridded lines is not only important for obtaining better subsurface images but also allows a more flexible and practical design of electrical resistivity tomography field surveys.

Combined electrical resistivity tomography and magnetic resonance sounding investigation of the surface-water/groundwater interaction in the Urema Graben, Mozambique

This study focusses on the hydrogeology of Urema Graben, especially possible interactions between surface water and groundwater around Lake Urema, in Gorongosa National Park (GNP). Lake Urema is the only permanent water source for wildlife inside GNP, and there are concerns that it will disappear due to interferences in surface-water/groundwater interactions as a result of changes in the hydraulic environment. As the lake is the only permanent water source, this would be a disaster for the ecosystem of the park. The sub-surface geology in Urema Graben was investigated by 20 km of electrical resistivity tomography (ERT) and three magnetic resonance sounding (MRS) surveys. The average depth penetration was 60 and 100 m, respectively. The location of the ERT lines was decided based on general rift morphology and therefore orientated perpendicular to Urema Graben, from the transitional areas of the margins of the Barue platform in the west to the Cheringoma plateau escarpments in the east. ERT and MRS both indicate a second aquifer, where Urema Lake is a window of the first upper semi-confined aquifer, while the lower aquifer is confined by a clay layer 30–40 m thick. The location and depth of this aquifer suggest that it is probably linked to the Pungwe River which could be a main source of recharge during the dry season. If a dam or any other infra-structure is constructed in Pungwe River upstream of GNP, the groundwater level will decrease which could lead to drying out of Urema Lake.