Electrical Tomography Surveys Research Articles

Climate-driven versus anthropogenic induced erosion of the last 3000 years from an ancient lake in the Southern Phokis Plain (Desfina), Greece

The timing and causes for erosional events at the Kastrouli (Greece) archeological site – a Late Myceneaen with reuse in later periods – are presented. Two borehole sediment cores (max 6 m depth), collected from the footsteps of the settlement hill plain, were studied. Sedimentary analysis and luminescence dating techniques investigate and identify periods of soil aggradation in this record. Moreover, optically stimulated luminescence (OSL) dating confirmed the concept of an ancient lake during Kastrouli settlement times. Macroscopic overview of the stratigraphic structure for each core, included lithological and textural evaluation of the core sediments, assessments of grain size, and determination of the geochemical and mineralogical composition of the sediments. X-ray powder diffraction (XRPD), X-ray fluorescence (XRF), and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) techniques were applied for the mineralogical and geochemical analysis. OSL ages from seven sediment horizons spanned from the Ottoman period to archaic times. All lithological findings correlate with the results of the electrical tomography survey conducted in same area. Extrapolation of the Logarithmic fitting of the data back in time of prehistoric Late Mycenaean era of Kastrouli verifies a lake deeper than 10 m, with a considerable enhancement in soil aggradation of 1.2 cm/year. Attempts to dry out the area are evidenced by the hydraulic works found in two engineering sinkholes. Sedimentation changes had occurred in the local environment over the last 2500 years, and soil aggradation underwent significant fluctuations in the two studied cores. The Roman period (a wet period) and the Byzantine period experienced high aggradation rates. By contrast, soil aggradation rates in southern Phokis (Kastrouli region) have remained exceedingly low from the Ottoman era (circa 13th century) to the present. In the context of the Phokis case study, it appears that the sedimentation rates, driven by climate, have strengthened anthropogenic activities.

Fréchet Derivatives Calculation for Electrical Resistivity Imaging Using Forward Matrix Method

Frechet derivatives calculation or sensitivity matrix is an integral part of every non-linear inversion process. The sensitivity values indicate the variation of the forward response with respect to the variation of model parameters. Sensitivity patterns are also a criterion to assess the reliability of the inverted models and to design optimum resistivity surveys. In this study, a numerical approach based on the forward matrix calculation in the frame work of the 2.5D finite difference electrical resistivity forward modeling is presented. First, using the potential distribution in the Fourier space obtained from the forward calculation and the derivatives of the coupling coefficients with respect the conductivity distribution, the sensitivity values in the wave-number domain are computed. Then, these values are transformed into the space domain using an inverse Fourier technique. To verify and analyze the proposed numerical method, the sensitivity distributions assuming homogeneous and inhomogeneous media for commonly used electrical resistivity tomography configurations (e.g. pole-pole, pole-dipole, dipole-dipole, and Wenner) are computed. The numerical experiments reveal that the sensitivity patterns vary spatially throughout the model depending not only on the resistivity distribution but also on the electrode configuration. It is also concluded that the sensitivity analysis can be used as a supplementary tool for any optimum electrical tomography survey design.

Geostatistical Modeling of Electrical Resistivity Tomography for Imaging Porphyry Cu Mineralization in Takht-e-Gonbad Deposit, Iran

This work presents the application of a geostatistical-based modeling approach for building up electrical properties acquired from a geophysical electrical tomography survey deployed for the purpose of porphyry Cu exploration at the Takht-e-Gonbad deposit, in the central domain of Iran. Electrical data were inverted in 2D along several profiles across the main favorable zones of Cu-bearing mineralization to image electrical resistivity and chargeability properties. Upon tight spatial correlation of these geophysical properties and Cu mineralization (i.e. Cu grade), electrical models were constructed in 3D through geostatistical interpolation of 2D inverted data to provide insights into the geometry of probable ore mineralization. Anomalous geophysical zone that was coincident simultaneously with higher values of electrical chargeability and resistivity, was in accordance with the main body of high Cu grades generated from exploratory drillings. It reveals that the porphyry-type Cu mineralization system in this area has strong geophysical footprints controlled mainly by rock types and alterations. Note that these physical models supply valuable pieces of information for designing the layout of further exploratory drillings, constructing geological characteristics, separating non-mineralized form mineralized zones, and resource modeling.

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

Computation of optimized arrays for 3-D electrical imaging surveys

3-D electrical resistivity surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artefacts. Many 3-D surveys use a grid where the number of electrodes along one direction (x) is much greater than in the perpendicular direction (y). Frequently, due to limitations in the number of independent electrodes in the multi-electrode system, the surveys use a roll-along system with a small number of parallel survey lines aligned along the x-direction. The ‘Compare R’ array optimization method previously used for 2-D surveys is adapted for such 3-D surveys. Offset versions of the inline arrays used in 2-D surveys are included in the number of possible arrays (the comprehensive data set) to improve the sensitivity to structures in between the lines. The array geometric factor and its relative error are used to filter out potentially unstable arrays in the construction of the comprehensive data set. Comparisons of the conventional (consisting of dipole-dipole and Wenner–Schlumberger arrays) and optimized arrays are made using a synthetic model and experimental measurements in a tank. The tests show that structures located between the lines are better resolved with the optimized arrays. The optimized arrays also have significantly better depth resolution compared to the conventional arrays.

Contribution of electrical tomography methods in geotechnical investigations at Mavropigi lignite open pit mine, Northern Greece

In this paper, the application of 2D and 3D electrical resistivity methods in geotechnical investigations is explored through a case study in Northern Greece. These two methods were employed at a lignite surface mining operation where fracture zones and discontinuities have been recently observed close to the pit boundaries. The main aim of the geophysical survey was to estimate the inclination of the contact between the Neogene and Schist/Carbonate formations near the southern limits of the pit, as well as to estimate the thickness of the carbonate rocks on top of the Schist formations to evaluate the stability of the southern slopes. Synthetic data were initially generated to help plan an efficient electrical tomography survey, in a region with complex geology and irregular terrain. Three configurations (Wenner–Schlumberger and dipole–dipole or pole–dipole) proved essential in such conditions and helped improving the resolution of the resistivity section. The sections were then calibrated by boreholes. Finally, the geophysical survey provided invaluable data regarding the geometry of the bedrock and possible faults, which was essential for the slope stability calculations.

Applying of Electrical Imaging Survey (EIS) to Evaluate Leachate Pollution in Underground Area of Informal Landfill

An informal landfill is an open dump that pollutes the underground environment because it lacks an impervious liner. The leakage of such a landfill is unidirectional and thus difficult to directly test. This study uses electrical imaging survey to evaluate the pollution of the underground environment of an informal landfill for municipal solid waste in Beijing. We hypothesize that every location has a specific resistivity resulting from the leachate. We use the membership function of fuzzy mathematics to quantitatively represent the pollution of the underground environment in the sanitary landfill. The results are consistent with borehole data.

2D Electrical Imaging Surveys for Leachate Plume Migration at an Old Dump Site in Ibadan South Western Nigeria: A Case Study

The site surveyed is along a popular ring road in Ibadan. It is an old dump site which had been closed over a long period of time. Development into industrial and residential house schemes had crowded around the site. This development resulted in the exploitation of groundwater within the thick regolith that characterize the area. The investigation was carried out to ascertain leachate plume generation and migration and its impact on the surrounding soil and the groundwater. 2D electrical resistivity imaging using Wenner array was used to delineate the plume and probable trend of migration. The method involves the injection of low frequency DC current into the subsurface and measuring the potential difference set up by another pair of electrodes called potential electrodes. All the traverses show areas of very low resistivity values ranging between 4 ohm-m and 13.8 ohm-m; in addition to this, the water samples collected from hand dug wells that surround the dump site show high total dissolved solids (TDS) with an average value of 2033 mg/L, high conductivity with an average value of 2868 mg/L, and high nitrate value (63 mg/L); these values are above the World Health Organization (WHO) permissible values. This poses a health challenge to the residents living around this site.

Higher-order statistics correlation stacking for DC electrical data in the wavelet domain

DC (direct current) electrical and shallow seismic methods are indispensable to the near surface geophysical exploration, but the near surface areas are very difficult environments for any geophysical exploration due to the random noise caused by near surface inhomogeneities . As a new algorithm based on higher-order statistics theory, the higher-order correlation stacking algorithm for seismic data smoothing in the wavelet domain has been developed and applied efficiently to filter some correlation noise that the conventional second-order correlation stacking could not inhibit. In this paper, this higher-order statistics correlation stacking technology is presented for DC electrical data in wavelet domain. Taking into account the single section and multiple section data, we present two new formulations of correlation stacking for DC electrical data. Synthetic examples with Gaussian noise are designed to analyze the overall efficiency of the new algorithm and to determine its efficacy. Meanwhile, comparison with the traditional least-squares optimization inversion method for field examples from electrical imaging surveys and time-domain IP measurement in China shows its significant advantages. The quality of the new algorithm also has been assessed by physical simulation experiments. This new technology in DC electrical exploration measurements provides a new application in engineering and mining investigation. • The higher order statistics correlation stacking in wavelet domain • Considered the correlation in adjacent electrode resistivity data • Considered the full time decay of the induced polarization response • Presented two new formulations of correlation stacking for DC electrical data • The higher order statistics correlation stacking applied efficiently in DC data

Optimized arrays for 2D cross‐borehole electrical tomography surveys

ABSTRACTThe use of optimized arrays generated using the ‘Compare R’ method for cross‐borehole resistivity measurements is examined in this paper. We compare the performances of two array optimization algorithms, one that maximizes the model resolution and another that minimizes the point spread value. Although both algorithms give similar results, the model resolution maximization algorithm is several times faster. A study of the point spread function plots for a cross‐borehole survey shows that the model resolution within the central zone surrounded by the borehole electrodes is much higher than near the bottom end of the boreholes. Tests with synthetic and experimental data show that the optimized arrays generated by the ‘Compare R’ method have significantly better resolution than a ‘standard’ measurement sequence used in previous surveys. The resolution of the optimized arrays is less if arrays with both current (or both potential) electrodes in the same borehole are excluded. However, they are still better than the ‘standard’ arrays.

The 13 November 2007 rock-fall at Viale Tiziano in Rome (Italy)

Abstract. The aim of the study was to perform a study on the western slope of the Monti Parioli hill (Rome, Italy) affected by frequent rock-fall phenomena, such as the one that occurred on 13 November 2007. This goal was achieved by defining a detailed reconstruction of the stratigraphical, geological and geomechanical structure of the slope and by conducting a back-analysis of the rock-fall event using 2-D and 3-D modeling tools. The reconstruction of the slope's geological structure, characterized by the presence of two anthropogenic cavity systems, and the characterisation of geomechanical properties of outcropping terrains have been realized by means of a detailed geological survey and a campaign of direct and indirect investigations. Therefore, continuous rotary, coring boreholes up to 60 m, collecting undisturbed samples for laboratory tests and performing direct investigations such as SPTs and pressuremeter tests were carried out. The indirect investigations included electrical tomography surveys, linear surface seismic refraction surveys and seismic cross-hole tests. Using the reconstructed geological-technical model, it was possible to define the stability conditions of the slope at the time of collapse by using a computational two-dimensional explicit finite difference program (FLAC) and a 3-D finite element analysis (FEMLAB).

Electrical Imaging for Geohazard and Environmental Monitoring

This special issue represents a contribute to disseminate the results of novel applications of the electrical resistivity imaging. Recently, innovative algorithms for tomographic data inversion and modern technologies for the field surveying have rapidly transformed the electrical imaging in a noninvasive, cost-effective, and powerful tool for geohazard and environmental monitoring. To date, the scientific interest for the electrical imaging is constantly growing and the novel approaches based on 3D and time-lapse 4D tomographic methods are disclosing the way for new and interesting applications, as well as the time-continuousmapping andmonitoring of water content in a landslide body and the monitoring of weakly resistivity changes related to fluidmigration processes close to seismic faults. The papers published in this special issue mainly concern the study of hydrogeological problems typical of Mediterranean countries (i.e., landslide phenomena, saltwater intrusion) and the design of experiments for better evaluate the spatial and time-dependent changes of subsurface resistivity. Of course, these topics are not completely representative of all application fields of the electrical imaging. In the paper of the special issue titled “Geoelectrical tomography investigating and modeling of fractures network around Bittit spring (Middle Atlas, Morocco),” Qarqori and other coauthors describe the main results of a geoelectrical survey carried out for monitoring a strategic aquifer in the northern part of Morocco. The study gives a significant contribute for defining the water flow in karst environment and through subvertical fractures. The paper by T. Zhu et al., titled “Experimental studies on the changes in resistivity and its anisotropy using electrical resistivity tomography,” deals with laboratory estimation on electrical anisotropy using electrical resistivity tomography on dry magnetite samples under uniaxial pressure. The paper by A. Satriani et al., “Geoelectrical surveys for characterisation of the coastal salt water intrusion in Metapontum Forest Reserve (Southern Italy),” presents the results of a case study using resistivity imaging integrated with water and soil analysis to investigate salt water intrusion in a coastal aquifer, identifying large areas affected by intensive soil salinization, and delineating the saltwater intrusion front. In the paper “Geoelectrical tomography as an operative tool for emergency management of landslide: an application in Basilicata Region, Italy,” G. Colangelo and A. Perrone describe an electrical imaging survey, integrated with borehole data, to study a complex rototranslational slide, showing how the application of this indirect technique could be particularly useful for end users involved in the landslide risk management. Finally, the paper “A prototype system for time–lapse electrical resistivity tomographies” by Luongo et al. concerns the first experimental results obtained with a new system for time-lapse resistivity monitoring of a landslide. The system seems to be suitable for water content monitoring in shallow layers.

A synthetic model of cracking in Santos Morcillo Lake, central Spain

This article proposes a synthetic model to explain the cracking process observed in Santos Morcillo Lake, Ruidera Lakes Natural Park, central Spain. The model was obtained by means of a comprehensive review of the available data, supplemented with an electrical tomography survey and a field and laboratory investigation. The system’s structure has been condensed into a microscale model based on a sediment column connecting the crust of the lake bottom with the Mesozoic substrate. The deformation of both this column and the crust covering it allows for a consistent explanation of the behaviors observed. It also provides a simple reproduction of the crack opening data. Hence the model has been used to define guidelines for the safe use and management of this protected space.

Parallel computation of optimized arrays for 2-D electrical imaging surveys

Modern automatic multi-electrode survey instruments have made it possible to use non-traditional arrays to maximize the subsurface resolution from electrical imaging surveys. Previous studies have shown that one of the best methods for generating optimized arrays is to select the set of array configurations that maximizes the model resolution for a homogeneous earth model. The Sherman–Morrison Rank-1 update is used to calculate the change in the model resolution when a new array is added to a selected set of array configurations. This method had the disadvantage that it required several hours of computer time even for short 2-D survey lines. The algorithm was modified to calculate the change in the model resolution rather than the entire resolution matrix. This reduces the computer time and memory required as well as the computational round-off errors. The matrix–vector multiplications for a single add-on array were replaced with matrix–matrix multiplications for 28 add-on arrays to further reduce the computer time. The temporary variables were stored in the double-precision Single Instruction Multiple Data (SIMD) registers within the CPU to minimize computer memory access. A further reduction in the computer time is achieved by using the computer graphics card Graphics Processor Unit (GPU) as a highly parallel mathematical coprocessor. This makes it possible to carry out the calculations for 512 add-on arrays in parallel using the GPU. The changes reduce the computer time by more than two orders of magnitude. The algorithm used to generate an optimized data set adds a specified number of new array configurations after each iteration to the existing set. The resolution of the optimized data set can be increased by adding a smaller number of new array configurations after each iteration. Although this increases the computer time required to generate an optimized data set with the same number of data points, the new fast numerical routines has made this practical on commonly available microcomputers.

An evaluation of optimization strategies to automatically select the optimal set of array configurations for 2D electrical imaging surveys

This paper presents results of tests involving four different strategies used to automatically select the optimal set of array configurations that generate the maximum amount of information regarding a subsurface with 2D electrical imaging surveys invoking the same number of measurements. The first strategy (Compare R) calculates the improvement in the model resolution for each array configuration added to the set of measurements. This strategy generates the best results but requires the greatest amount of time to complete. The second (BGS) and third (ETH) strategies use linear approximations to calculate a goodness function. Both strategies are less time consuming than the Compare R strategy; however, the BGS strategy produces array configurations with model resolution values that are slightly less accurate than the Compare R strategy, while the ETH strategy produces results of the poorest quality. The fourth strategy (Combined BGS–CR) uses a combination of the BGS and Compare R strategies and produces results that are almost as accurate as the Compare R strategy but requires significantly less computing time. The four optimization strategies were tested using model resolution calculations, synthetic models and field surveys. The results obtained using the different optimization strategies were also compared with conventional arrays. The various tests show that arrays selected via the optimization strategies result in subsurface inversion models that have better resolution and identify more accurate depths and shapes compared to conventional arrays.

Fast computation of optimized electrode arrays for 2D resistivity surveys

Four different methods to automatically select an optimal set of array configurations that gives the maximum subsurface resolution with a limited number of measurements for 2D electrical imaging surveys were tested. The first (CR) method directly calculates the change in the model resolution for each new array added to the base data set, and uses this to select array configurations that gave the maximum model resolution. However this method is the slowest. The algorithm used by the CR method for calculating rank-one updates was optimized to reduce computational time by a factor of eighty. The sequence of calculations was modified to reduce the traffic between the computer main memory and the CPU registers. Further code optimizations were made to take advantage of the parallel processing capabilities of modern CPUs. The second (ETH) and third (BGS) methods use approximations based on the sensitivity values to estimate the change in the model resolution matrix. The ETH and BGS methods, respectively, use the first and second power of the sensitivity values to calculate approximations of the model resolution. Both methods are about an order of magnitude faster than the CR method. The results obtained by the BGS method are significantly better than the ETH method, and it approaches that of the CR method. The fourth method (BGS–CR) uses a combination of the BGS and CR methods. It produces results that are almost identical to the CR method but is several times faster. The different methods were tested using data from synthetic models and field surveys. The models obtained from the inversion of the data sets generated by the four different methods confirm that the models generated by the CR method have the best resolution, followed by the BGS–CR, BGS and ETH methods.

Electrical imaging and self-potential survayes to study the geological setting of the Quaternary, slope depositsin the Agri high valley (Southern Italy)

We present the results of a geophysical survey carried out to outline the structural modelling of Quaternary slopedeposits in the northern part of the Agri high valley (Basilicata, Southern Italy). Quaternary folding and brittle deformations of the subaerial slope deposits have been studied combining electrical imaging and self-potential surveys with geological structural analysis. This integrated approach indicates that the area underwent both transpressional and transtensional tectonics during Pleistocene times as testified by the existence of a push up structure in the basement buried by deformed Quaternary breccias. On this basis, the valley appears to be a more complex structure than a simple extensional graben, as traditionally assumed in the literature.

Geophysical-geochemical investigation of fire-prone landfills

This paper deals with the integration of electrical resistivity tomography and geochemical methods for studying four different fire-prone landfills. Landfill gas composition (CH4, H2S, O2, CO, CO2) and subsurface temperature were measured with the constant net 50 × 50 m from the depth 10–60 cm. 28 electrical resistivity tomography lines were surveyed, while Wenner and Sclumberger electrode arrays were employed for all measurements. At the studied sites the landfill gas and temperature measurements mapped gas and temperature anomalies over underground fire sources. 2D electrical resistivity tomography lines, performed over these anomalies, showed these fire sources as high-resistivity zones. The joint employment of the electrical imaging and geochemical survey seems to be a useful tool in carrying out diagnostic investigations at fire-prone landfills.

Multiple superimposed probability tomography on a second electrical field

Based on the requirement of probability tomography, we developed two-directional pole–pole measurement on a 2D electrical imaging survey which is able to extract all second electrical field profiles to participate in probability tomography. The concept of charge occurrence probability (COP) is defined as the average of cross-correlation of the space domain scanning (SDS) function with the second electrical field being generated by a whole set of point current sources. To quantitatively analyse the results of the tomography, the COP values are normalized. Finally, in order to test the effectiveness of the tomography, we use a finite element algorithm to synthesize the potential data for 2D models, and the profiles of the second electrical field are extracted by eliminating the primary field from the total field. Using the second electrical field of all profiles, the geometric patterns of normalized COP (NCOP) values are reconstructed by means of multiple superimposed probability tomography. The results are quite satisfactory.

An approach to construct the weathering profile in a hilly granitic terrain based on electrical imaging

For investigation of a granitic hillside at the Kata Noi beach in Phuket, Thailand, a common geotechnical site investigation might not be suitable because it is difficult to bring a drilling rig to make the boreholes on the hillside. As an alternative electrical imaging (EI) survey was proposed. The main issues addressed in this study are how to find the boundary between the weathered and unweathered granites in a gradually changing resistivity pseudo-section and how to assess the effect of hillside topography and the type of electrode array in locating such a boundary. To solve these problems an approach was adopted, including (i) selection of a suitable EI array in the field; (ii) data analysis with topographical correction; and (iii) the incorporation of a conceptual weathering profile in EI data interpretation. As a result, a geological–geoelectrical model was constructed for the weathering profile of granitic rocks at the study site, which can be applied for investigation of similar hilly terrains in Southern Thailand.