Electrical Resistivity Sections Research Articles

Using archived magnetotelluric data for geologic interpretation in the Transdanubian Region

We provide a key magnetotelluric section, composed of archived magnetotelluric data along a NW-SE profile in Transdanubia, Hungary. For the interpretation of the key section, observations from raw magnetotelluric data and inversion results were used. In addition, other geophysical-geological information was also considered to confirm the conclusions based on the electrical resistivity sections. All this information was combined to identify the main structural lines and geologic units along the profile. Main structural lines observed on the resistivity sections are the Alpokalja line, Rába line, Balaton line, Kapos line, and Mecsekalja line. Geologic units that can be delineated due to their resistivity contrast include the Lower and Upper Austroalpine Units, the Transdanubian Range Unit, the Mid-Hungarian Megaunit, the Tisza Megaunit and sedimentary rocks filling the sub-basins of the Miocene Pannonian back-arc basin. The inversion results of the transverse magnetic (TM) polarization mode and the phase-depth sections of the raw data were found to be the most suitable for detecting the morphology and identifying the depth of the Pre-Cenozoic basement along the profile.

Hydrological behavior of distinct types of ferricretes (“cangas”) of the southeast region of the iron quadrangle, Minas Gerais, Brazil

Ferricretes (“cangas”) in southeastern Brazil are hardened iron-rich layers that usually cover the itabirites of the Cauê Formation of the Iron Quadrangle – a world-class mineral province well-known for the intense iron-ore mining activities. Despite the significant porosity of ferricretes, there is still some controversy regarding the relationships between such parameter and hydraulic conductivity. Thus, the role of ferricretes in the recharge of underlying aquifers can only be assessed on the basis of hydrological studies. The Cauê Formation is a relevant groundwater resource of the Iron Quadrangle and the removal of ferricretes by mining activities may considerably affect the replenishment of aquifers and consequently water supply. In order to estimate the hydraulic conductivity of the Cauê Formation ferricretes, a topographic ramp located in the southeast portion of the Iron Quadrangle was selected for the physical and hydraulic characterization of two of the most common types of ferricretes – namely structured and detrital ferricretes. Ferricrete samples were selected for mineralogical, microstructural, chemical characterization and quantification of the apparent porosity in the laboratory. Infiltration tests were carried out in the field using sprinkle, double ring and mini-disk infiltrometers. The comparison of these results with other previous studies shows that the porosity and the hydraulic conductivity of ferricretes are variable, depending on their mineralogical, textural and structural characteristics. The apparent porosity and hydraulic conductivity of the ferricretes of this study are equivalent to those of silty sand or silt (10−5 to 10−7 m/s), with water predominantly flowing across pores larger than 1 mm in diameter. The monitoring of the infiltration and percolation of a NaCl tracer solution through the ferricretes via multitemporal electrical resistivity sections obtained using dipole-dipole array configurations indicated the predominance of horizontal flows that are controlled by the transition horizon. Interferences in the flow patterns were caused by a meter-sized cavity present in the transition horizon. Karst-like features are common at greater depths and, as previous studies have already indicated, karstification in this medium is strongly conditioned by geomorphology. Therefore, the recharge of the underlying aquifers tends to be significant but variable in space and time.

Improving tailings dam risk management by 3D characterization from resistivity tomography technique: Case study in São Paulo – Brazil

Tailings dams are traditionally monitored by routine visual inspections combined with the use of instruments such as piezometers, topographical landmarks, and water level indicators. As they are structures that have high potential damage associated, the use of robust investigation methods is essential to ensure the operational safety of these dams. Geoelectrical methods have been successful in mapping humid zones inside the tailings dam structures, using the proper acquisition parameters and data sampling that these methods allow. This is a promising approach to investigating a tailings dam with a heterogeneous embankment made of limestone tailings, that has been in operation for 50 years in São Paulo state, Brazil. Considering its structural complexity, electrical resistivity sections were acquired with a tight profile spacing – of 10 to 15 m, processed, and modeled by applying a specific workflow. The Electrical Resistivity Tomography (ERT) operational workflow applied to the dataset obtained from two field campaigns, conducted in the embankment and the right abutment of the dam, performed well in mapping possible high water content zones inside the structure, defining a relevant element for the risk assessment. For this purpose, compartmentalization of the resistivity ranges, and three-dimensional modeling techniques were used, both constrained by the geotechnical instruments of the structure. The embankment campaign mapped zones of attention in the right abutment driving the decision to perform a follow-up survey dedicated to this region. The right abutment campaign mapped zones of high resistivity, associated with the natural terrain, composed of sandy soils, saprolite, and rock, as well as zones of low resistivity related to high moisture content regions. The geometry of these conductive zones made it possible to infer the depth of the water table in the region between the direct measurements from the instrumentation and to model zones with low resistivity values. The 3D modeling revealed that part of the conductive zones, present in the right abutment and extending to the embankment indicate a possible contribution of moisture from the natural terrain towards the embankment of the dam. Thus, the achieved results demonstrated that 3D modeling applied to the combination of the geotechnical and geophysical datasets contributes to expanding the knowledge of the internal structure of the dam and consequently the safety of the operations, especially where there is no instrumentation, improving the tailings dam risk management.

Geoelectrical integrated models for determining the geometry of karstic cavities in the Zarrinabad area, west of Iran: combination of fuzzy logic, C-A fractal model and hybrid AHP-TOPSIS procedure

The main aim of this study is to determine the location of the cave and cavities in Zarrinabad area, Lorestan province, west of Iran. For this purpose, a combination of hybrid AHP-TOPSIS procedure and content-area (C-A) fractal method has been conducted on three types of geoelectrical data derived from electrical resistivity, self-potential and mise-a-la-masse methods to detect karstic zones and determining the geometry of the cavities. Initially, the raw values of the data were transformed to [0–1] domain using the MSlarge fuzzy function, and then the fuzzified values were discretized applying C-A fractal model to separate different geological populations (e.g., conglomerate and limestone layers as well as the geometry of the cave). Subsequently, the analytical hierarchy process (AHP) as the most widely used multiple criteria decision making (MCDM) technique was applied for calculating meaningful weights and then assigning them to different fractal-based classes of investigation criteria (here, triple geoelectrical data) based on the expert knowledge. A decision matrix based on the calculated weights were constructed and TOPSIS as another well-known MCDM technique was then employed for generating cave potential model of AHP-TOPSIS procedure. In addition, a fuzzy Gamma model (gamma = 0.9) based on three fuzzified electrical layers is implemented to assess the AHP-TOPSIS model. Finally, for the evaluation of generated models, electrical resistivity sections along profile 1, 2 and 3 were prepared. The results exhibit a strong spatial association between final prospectivity models, electrical resistivity sections and the outcrop of the cave on the surface. According to the obtained results, a cave with dimensions of several meters (for example, its length in one direction is up to 100 m) is appeared to be expanded from the north of the study area to the south. Besides, it is possible that the karstic zones contain a number of small cavities which are linked together like a beehive. Thus, this area can be considered as a good source of underground water due to the presence of these holes which often contain fresh water. In addition, this area is prone to convert into a tourism area in the future as this cave contains high level water especially in rainy seasons.

Stiffness and Cavity Test of Concrete Face Based on Non-Destructive Elastic Investigation

A non-destructive testing (NDT) method was used in a concrete face rockfill dam (CFRD) to identify the condition of the concrete face slab and detect any existing cavities between the concrete face slab and the underlying support layer. The NDT for the concrete face slab was conducted using the impulse response (IR) method and the electrical resistivity tomography (ERT) method with the application of non-destructive electrodes. Information regarding the dynamic stiffness and average mobility of the concrete was obtained based on the mobility-frequency of the IR method, and cavity detection under the plate structures was analyzed using the two-dimensional (2D) electrical resistivity section of the ERT method. The results of the IR method showed that zones with low dynamic stiffness and high average mobility were expected to be found in concrete of poor quality and in cavities beneath the concrete face slab. The results of the ERT method showed that zones with high resistivity were expected to be cavities between the concrete face slab and the underlying support layer. As a result, the tendency toward low dynamic stiffness, high average mobility, and high resistivity in both methods implies unstable concrete conditions and the possible occurrence of a cavity. The results of the two methods also showed a good correlation, and it was confirmed that the NDT method was reliable in terms of cavity estimation.

Detection of subsurface cracking depth using electrical resistivity tomography: A case study in Masjed-Soleiman, Iran

Soil cracks affect the engineered earth structures, being known to contribute to failures in road surfacing. Electrical resistivity imaging technique is a generally appealing method to investigate such phenomena. Cracks deviate the resistivity distribution of the soil, since resistivity of the air-filled cracks is significantly higher than the intact soil. In this study, the non-invasive 2D electrical resistivity imaging survey was conducted along two parallel lines using a Wenner-Schlumberger configuration to investigate vertical and horizontal extension of the existing cracks in the structure of a road covered by asphaltic sandstone near Masjed-Soleiman city. From the inverted results, 17 different cracks were detected, which their depths ranged from 0.5 to 5.5 m. However, there is not any considerable crack progressing toward the axis of the road. Borehole drilling confirmed the results of electrical resistivity sections to detect the cracks. The results show that the 2D electrical resistivity imaging survey using the Wenner-Schlumberger array in combination with an appropriate 2D inversion method can be highly useful to delineate vertical extension of the existing cracks in road structures.

Imaging Quaternary glacial deposits and basement topography using the transient electromagnetic method for modeling aquifer environments

Aquifer formations along the northern shore of the Saint-Lawrence River in Quebec (Canada) mainly consist of glacial and coastal deposits of variable thickness overlying Precambrian bedrock. These deposits are important because they provide the main water supply for many communities. As part of a continuing project aimed at developing an inventory of the groundwater resources in the Charlevoix and Haute-Côte-Nord (CHCN) regions of the province of Quebec in Canada, the central loop transient electromagnetic (TEM) method was used to map the principal hydrogeological environments in these regions. One-dimensional smooth inversion models of the TEM soundings have been used to construct two-dimensional electrical resistivity sections, which provided images for hydrogeological validation. Electrical contour lines of aquifer environments were compared against available well logs and Quaternary surface maps in order to interpret TEM soundings. A calibration table was achieved to represent common deposits and basements. The calibration table was then exported throughout the CHCN region. This paper presents three case studies; one in the Forestville site, another in the Les Escoumins site and the other in the Saint-Urbain site. These sites were selected as targets for geophysical surveys because of the general lack of local direct hydrogeological data related to them.

Subsurface investigation on Quarter 27 of May 15th city, Cairo, Egypt using electrical resistivity tomography and shallow seismic refraction techniques

Geophysical tools such as electrical resistivity tomography (ERT) and shallow seismic (both P-wave seismic refraction and Multi-channel Analysis of Surface Waves (MASW)) are interesting techniques for delineating the subsurface configurations as stratigraphy, structural elements, caves and water saturated zones. The ERT technique is used to delineate the contamination, to detect the buried objects, and to quantify some aquifer properties. Eight 2-D (two dimensional) electrical resistivity sections were measured using two different configurations (dipole–dipole and Wenner). The spread length is of 96m and the electrodes spacing are 2, 4 and 6m, respectively to reach a depth ranging from 13 to 17m. The results indicate that, the subsurface section is divided into main three geo-electrical units, the first is fractured marl and limestone which exhibits high resistivity values ranging from 40 to 300ohmm. The second unit is corresponding to marl of moderate resistivity values and the third unit, which is the deeper unit, exhibits very low resistivity values corresponding to clayey marl. The fourth layer is marly clay with water. The presence of clay causes the most geotechnical problems. Fourteen shallow seismic sections (both for P-wave and MASW) were carried out using spread of 94m and geophone spacing of 2m for each P-wave section. The results demonstrate that the deduced subsurface section consists of four layers, the first layer exhibits very low P-wave velocity ranging from 280 to 420m/s, the second layer reveals P-wave velocity ranging from 400 to 1200m/s, the third layer has P-wave velocity ranging from 970 to 2000m/s and the fourth layer exhibits high velocity ranging from 1900 to 3600m/s. The ERT and shallow seismic results, reflect the presence of two parallel faults passing through Quarter 27 and trending NW-SE.

Effects of feedlot manure on soil and groundwater assessed with electrical resistivity tomography

The expansion of feedlot activities in the NW zone of Buenos Aires Province (Argentina) has increased the risk of groundwater contamination, due to a higher concentration of manure on the soil surface. Groundwater is shallow and the soils are sandy loam, which are factors that make this area vulnerable to pollution. The heterogeneous distribution of manure in the pens and the hard compacted layers in the soils makes it difficult to make a representative soil and groundwater sampling. The purpose of this work was the use of electrical resistivity tomography (ERT) to aid subsequent sampling campaign for the delineation of the spatial distribution of nitrates, in soils and groundwater at pens of a feedlot, with different times of animal confinement. The soil sampling, up to 3 m depth was performed taking into account the anomalies of bulk resistivity. The sites of lower resistivity than background values were potentially associated with different levels of soil salinization. Volumetric water content and concentration of N–NO3− of the soils were determined. The electrical resistivity sections indicated low values in the saturated and unsaturated zone, near waterers and feedbunks and a decrease in the sense of decreasing topography. This reduction was also observed at sites of higher water content, in Pen 1 (4 months of animal confinement) associated with the proximity of the water table. The values of N–NO3− contents were much higher at Pen 2 (7 years of confinement) with values between 436 and 877 kg N–NO3 ha−1, while at Pen 1 the range was from 43 to 129 kg N–NO3− ha−1. At both pens, the lower resistivity was associated with sites of higher contents of water and N–NO3−. The presence of an impermeable hard soil layer, below the surface of Pen 1, enhanced superficial runoff toward the lower zone outside the pen, where nitrates were finally able to leach. On the other side, greater values of nitrates were found up to 3 m depth in Pen 2, which were directly related to rainy events. Resistivity may be regarded as a marker of soil salinity produced by manure and an indicator of variations of water content, organic matter and N–NO3− concentrations. The geophysical exploration by electrical resistivity tomography was successful in the design of soil sampling in a nonconventional application, such as the study of the distribution of nitrates in pens of animal feedlots.

Electrical resistivity tomography revealing the internal structure of monogenetic volcanoes

AbstractEruptive activity of individual monogenetic volcanoes usually lasts a few days or weeks. However, their short lifetime does not always mean that their dynamics and structure are simple. Monogenetic cones construction is rarely witnessed from the beginning to the end, and conditions for observing their internal structure are hardly reached. We provide high‐resolution electrical resistivity sections (10 m electrode spacing) of three monogenetic cones from northeastern Spain, comparing our results to geological observations to interpret their underground continuation. The 100 m maximum depth of exploration provides information on almost the entire edifices, highlighting the relationships between Strombolian and hydromagmatic deposits in two multiphase edifices. A main observation is a column of distinct resistivity centered on the Puig d'Adri volcano, which we interpret as the eruptive conduit. This method can provide valuable information on the past volcanic dynamics of monogenetic volcanic fields, which has real implications for the forecast of future activity.

ANN based inversion of DC resistivity data for groundwater exploration in hard rock terrain of western Maharashtra (India)

Inversion of vertical electrical sounding (VES) data, especially from the crystalline hard rock area, assumes a special significance for groundwater exploration. Here we used a newly developed algorithm based on the Bayesian neural network (BNN) theory combined with Hybrid Monte Carlo (HMC)/Markov Chain Monte Carlo (MCMC) simulation scheme to invert the Direct Current (DC) VES measurements obtained from 30-locations around Tenduli-Vengurla, Sindhudurg district, Maharashtra, India. The inversion results suggest that the top layer is mostly comprised of laterites followed by mixture of clay/clayey sand and garnulites/granite as basement rocks. The source of groundwater appears to be accessible in weathered/semi-weathered layer of laterite/clayey sand that exists within the depth of 10–15 m from the surface. The NW–SE trending major lineaments and its criss-crosses are also identified from the apparent and true resistivity surface map. The pseudo-section at different depths in the western part of the area, near Nivti, shows extensive influence of saltwater intrusion and its impact reaching up to the depth of 30 m from the surface along the coastal area. Our results also show that intrusion of saline water decreases from the western part to the eastern part of the region. Two dimensional modeling of four resistivity profiles from the study region identified two potential groundwater reservoirs; one lying between Path-Tenduli and another in between Mat and Zaraph. The deduced true electrical resistivity section against depth correlates well with available borehole lithology in the area. The results presented here would be useful for interpreting the geological signatures like fractures, major joints and lineaments, which in turn will be helpful for identifying groundwater reservoirs and drainage pattern in the crystalline hard rock area. The newly developed HMC-based BNN method is robust and would provide insights for constraining the geophysical models and criteria for modeling resistivity data.

Delineation of shallow resistivity structure around Malvan, Konkan region, Maharashtra by neural network inversion using vertical electrical sounding measurements

Modeling resistivity profiles, especially from hard rock areas, is of specific relevance for groundwater exploration. A method based on Bayesian neural network (BNN) theory using a Hybrid Monte Carlo (HMC) simulation scheme is applied to model and interpret direct current vertical electrical sounding measurements from 28 locations around the Malvan region, in the Sindhudurg district, southwest India. The modeling procedure revolves around optimizing the objective function using the HMC based sampling technique which is followed by updating each trajectory by integrating the Hamiltonian differential equations via a second order leapfrog discretization scheme. The inversion results suggest a high resistivity structure in the north-western part of the area, which correlates well with the presence of laterites. In the south-western part, a very high conductive zone is observed near the coast indicating an extensive influence of saltwater intrusion. Our results also show that the effect of intrusion of saline water diminishes from the south-western part to the north-eastern part of the region. Two dimensional modeling of four resistivity profiles shows that the groundwater flow is partly controlled by existing lineaments, fractures, and major joints. Groundwater occurs at a weathered/semi-weathered layer of laterite/clayey sand and the interface of overburden and crystalline basement. The presence of conduits is identified at a depth between 10 and 15 m along the Dhamapur–Kudal and Parule–Oros profiles, which seems to be potential zone for groundwater exploration. The NW–SE trending major lineaments and its criss-cross sections are indentified from the apparent and true resistivity surface map. The pseudo-section at different depths in the western part of the area, near Parule, shows extensive influence of saltwater intrusion and its impact reaching up to a depth of 50 m from the surface along the coastal area. Further, the deduced true electrical resistivity section against depth correlates well with available borehole lithology in the area. Present analyses suggest that HMC-based BNN method is robust for modeling resistivity data especially in hard rock terrains. These results are useful for interpreting fractures, major joints, and lineaments and crystalline basement rock and also for constraining the higher dimensional models.

A multidisciplinary study for mining landscape reclamation: A study case on two tailing ponds in the Region of Murcia (SE Spain)

The aim of this work was to evaluate the mobility of Pb, Cu, Zn and Cd in two tailing ponds (Brunita and San Cristobal) from mine activity in the Southeast of Spain before applying reclamation measures for reducing the risk for human and environment. To achieve this objective, five profiles of electrical resistivity imaging (ERI) in Brunita, three profiles in San Cristobal, a drill-hole in each pond and undisturbed columns amended with marble waste for leaching experiments were taken. Results showed that all layers of the two ponds exceed the maximum Pb and Zn concentrations allowed by the selected European legislations. In both ponds, the mixture of tailings with natural soil is the main process involved in soil pollution under the ponds. However, due to high pH in the substrate the metals are precipitated and their solubility reduced, therefore there is not a risk of leaching to groundwater. At Brunita tailing pond electrical resistivity sections displayed some lower electrical resistivity region into the bedrock which has been described as some fault occurrence due to breaking events suffered by the tailing pond. At San Cristobal tailing pond geochemical results were consistent with data obtained by the electrical resistivity imaging (ERI) method where no evidence of faults or cracks into the bedrock and the ponds were observed. Therefore, no preferential pathways of acid mine drainages containing heavy metals were reported. Leaching experiments indicated that, after 11 weeks of leaching, amended and control columns showed comparable values of pH (∼2.3) and similar trends for Cu, Cd and Zn, with a very drastic decrease up to week 3 and thereafter the tendency was to reach near steady-state conditions. In contrast, the evolution of Pb showed that marble amended reduces significantly its concentration over time compared with the control. In accordance with the results, future reclamation action should be based on the reduction of heavy metal mobility by means of long-term and large-scale marble waste application, which increases the pH and prevents the formation of acid mine drainage.

Study of flow in vadose zone from electrical resistivity surveys

Among the indirect methods applied to environmental monitoring, electrical resistivity has played an important role in accessing water flow in the non-saturated zone of the soil and, consequently, aquifer recharge. This work evaluates the influence of some pedo-geomorphologic properties in the infiltration and percolation of water through the superficial soil horizons by means of multitemporal electrical resistivity sections. The experiments were carried out in two areas of distinct pedo-geomorphological characteristics developed on the crystalline basement of southeastern Brazil. The first is situated in a hollow -shaped slope where lixisols predominate; the second, in a nose -shaped slope where well-developed soils (ferralsols ) predominate. The results proved the efficiency of the electrical resistivity method to delimit percolation of tropical soils, even when subtle differences are found in the hydraulic properties of the soil. It was also possible to establish that in the nose slope percolation is faster than in the hollow.

Electrical conductivity images of Quaternary faults and Tertiary detachments in the California Basin and Range

Comparison of an electrical resistivity section derived from magnetotelluric (MT) data to a geologic section extending eastward from the Sierra Nevada near latitude 36°20′N shows that the crust is dominated by steeply dipping conductive features that correlate with active strike‐slip faults. While there is a subhorizontal conductor at a depth ∼20 km beneath some of the profile, it is broken by vertical structures associated with the active strike‐slip faults. The continuous subhorizontal anomalies in the lower crust typically observed in extensional regions are therefore absent in the resistivity section. The present‐day strike‐slip tectonic regime as indicated by geodetic data in this part of the Basin and Range is not producing features that could be inferred to indicate subhorizontal shear zones resulting from lateral crustal flow during extension. Because the Miocene tectonic regime resulted in the formation of metamorphic core complexes and thus was accompanied by such flow, the present regime appears to represent a fundamental transition in the mode of crustal deformation in the region. A serendipitous result of our study was the identification on resistivity sections of carbonate aquifers in the upper crust. Comparison of resistivities from the MT section to measured fluid resistivities from springs and boreholes suggests that the aquifers must be heterogeneous, with more saline brines occupying the deepest portions of the carbonates.