Electrical Resistivity Array Research Articles

Comparison of machine learning and electrical resistivity arrays to inverse modeling for locating and characterizing subsurface targets

This study evaluates the performance of multiple machine learning (ML) algorithms and electrical resistivity (ER) arrays for inversion with comparison to a conventional Gauss-Newton numerical inversion method. Four different ML models and four arrays were used for the estimation of only six variables for locating and characterizing hypothetical subsurface targets. The combination of dipole-dipole with Multilayer Perceptron Neural Network (MLP-NN) had the highest accuracy. Evaluation showed that both MLP-NN and Gauss-Newton methods performed well for estimating the matrix resistivity while target resistivity accuracy was lower, and MLP-NN produced sharper contrast at target boundaries for the field and hypothetical data. Both methods exhibited comparable target characterization performance, whereas MLP-NN had increased accuracy compared to Gauss-Newton in prediction of target width and height, which was attributed to numerical smoothing present in the Gauss-Newton approach. MLP-NN was also applied to a field dataset acquired at U.S. DOE Hanford site.

Comparison of Three Electrical Resistivity Arrays to Investigate Weak Zones in Soil, Along a Profile Southeast Baghdad City, Iraq

The electrical resistivity method is one of the geophysical methods for detecting weak subsurface zone. The 2D resistivity data were used to compare three electrode configurations, Wenner, Dipole-dipole, and Schlumberger, to detect weak subsurface zones along a profile south of Baghdad near the Bismayah pumping station. The results show many zones of low resistivity that may be weak zones. A dipole-dipole array is a large number of measurements and is more sensitive than others. The Wenner-Schlumberger array has a depth also higher than other arrays. Wenner array has higher signal strength than other arrays. Because it is more sensitive to horizontal and vertical structures, the dipole-dipole array is the optimum for mapping subsurface weak zone.

Assessing the suitable electrical resistivity arrays for characterization of basement aquifers using numerical modeling

Numerical modeling analysis was used to assess the suitable electrical resistivity arrays for the characterization of geological structures, including dyke, horst, graben, sub-vertical, and vertical structures. These geological structures usually make up the aquifers interested in the hydrogeological evaluation of crystalline basement terrains. Six electrode configurations, including Wenner alpha (α), Wenner beta (β), Wenner gamma (γ), Schlumberger array, dipole-dipole array, and pole-dipole array, were used to assess the geological structures for groundwater exploration. The synthetic models of the geological structures were generated using RES2DMOD code, and 5% noise was added to all the models. The generated models were inverted using the RES2DINV code. The results show that the most suitable arrays for dyke and graben structures are Wenner alpha, while Wenner beta is the most suitable for the horst structure. The Schlumberger array was the best for both sub-vertical and vertical structures. This study has demonstrated the efficacy of numerical modeling in assessing the best resistivity arrays for 2D electrical resistivity imaging for groundwater exploration prior to geophysical field investigation.

Using Electrical Resistivity Tomography (ERT) to Delineate Subsurface Structures at Siloam Hot Spring in the Soutpansberg Basin, South Africa

Electrical resistivity tomography (ERT) was used to delineate subsurface structures at Siloam hot spring in the Soutpansberg Basin of South Africa. The spring water was reported to be hottest in South Africa. Groundwater with temperature ranging between 63.3 °C and 67.5°C was observed flowing from the spring. Relatively wet areas were noted to the south and north of the spring. Electrical resistivity arrays with Dipole-dipole and Wenner-Schlumberger configurations were used. Current injection and potential dipole separations of 5 m and 10 m were applied across the spring. Inversion results for arrays with the shorter dipole separation of 5 m had better resolution. Resistivity sections were generated from inversion of ERT data. The resistivity values for the low and high resistive zones were ρa < 10 and ρa > 70 Ωm respectively. Sub-horizontal resistivity zones were obtained from the inversion of Wenner-Schlumberger array survey data. High resistivity zones associated with dykes or sills were delineated on Dipole-dipole derived depth sections. Near vertical structures associated with water bearing faults or fractures were delineated at the Siloam hot spring.

Application of Three Electrical Resistivity Arrays to Evaluate Resolution Capacity of Fractured Zones at Apatara Farms, Iwo, Osun State, Nigeria

The study applied three different electrical resistivity arrays (Wenner, dipole-dipole, and Pole-dipole) based on their resolution capacity to delineate fractured zones at Apatara Farm in Iwo, Osun State, Nigeria. Theoretical apparent resistivity data were computed for each model and contaminated with 5% Gaussian noise as a further concession to real field conditions. The simulated results revealed that the Wenner array gave the least error in trying to reconstruct the true model when the fractured zone is placed near the subsurface. However, when the fractured zone is placed at a depth beyond 5 m, the Dipole-Dipole array gave a better resolution than Pole-Dipole and Wenner array in decreasing order of resolution. The study further revealed that the Wenner array is less susceptible to edge effect at shallow depth while Dipole-dipole is more susceptible to edge effect at deeper depth followed by the Pole-dipole array. 2D electrical resistivity field measurements were carried out to confirm the results of the numerical simulation in the same field using the same parameters. The inverted resistivity images showed that the fractured zones are well delineated by the Dipole-dipole and Pole-dipole arrays but poorly resolved by the Wenner array. The study has demonstrated the usefulness of numerical modelling for imaging of fractured zone necessary for hydrogeological purpose and through modelling, the user has unlimited power to image or simulate a real-world scenario seamlessly before carrying out the actual field survey.Keywords: Electrical resistivity array; fractured zones; finite element method; 2-D models; resolution, mean absolute error

Modified electrical survey for effective leakage detection at concrete hydraulic facilities

Three original electrode arrays for the effective leakage detection of concrete hydraulic facilities through electrical resistivity surveys are proposed: ‘cross-potential’, ‘direct-potential’ and modified tomography-like arrays. The main differences with respect to the commonly used arrays are that the current line-sources are separated from potential pole lines and floated upon the water. The potential pole lines are located directly next to the facility in order to obtain intuitive data and useful interpretations of the internal conditions of the hydraulic facility. This modified configuration of the array clearly displays the horizontal variation of the electrical field around the damaged zones of the concrete hydraulic facility, and any anomalous regions that might be found between potential poles placed across the facilities. In order to facilitate the interpretation of these modified electrical surveys, a new and creative way of presenting the measurements is also proposed and an inversion approach is provided for the modified tomography-like array. A numerical modeling and two field tests were performed to verify these new arrays and interpretation methods. The cross and direct potential array implied an ability to detect small variations of the potential field near the measurement poles. The proposed array showed the overall potential distribution across the hydraulic facility which may be used to assist in the search of trouble zones within the structure, in combination with the traditional electrical resistivity array.

CRSP, numerical results for an electrical resistivity array to detect underground cavities

AbstractThis paper is devoted to the application of the Combined Resistivity Sounding and Profiling electrode configuration (CRSP) to detect underground cavities. Electrical resistivity surveying is among the most favorite geophysical methods due to its nondestructive and economical properties in a wide range of geosciences. Several types of the electrode arrays are applied to detect different certain objectives. In one hand, the electrode array plays an important role in determination of output resolution and depth of investigations in all resistivity surveys. On the other hand, they have their own merits and demerits in terms of depth of investigations, signal strength, and sensitivity to resistivity variations. In this article several synthetic models, simulating different conditions of cavity occurrence, were used to examine the responses of some conventional electrode arrays and also CRSP array. The results showed that CRSP electrode configuration can detect the desired objectives with a higher resolution rather than some other types of arrays. Also a field case study was discussed in which electrical resistivity approach was conducted in Abshenasan expressway (Tehran, Iran) U-turn bridge site for detecting potential cavities and/or filling loose materials. The results led to detect an aqueduct tunnel passing beneath the study area.

Imaging Fractures beneath a Residential Complex Using Novel 3-D Electrical Resistivity Arrays

Typical 3-D electrical resistivity tomography sampling schemes, which require a grid of electrode lines to be deployed, are limited by physical conditions of the area under study. New array techniques are needed to characterize the subsoil beneath anthropogenic or natural structures to define hazardous zones. Use of multiple L-shaped arrays overcome the need for a grid of electrodes by surrounding an area in a square of electrode lines; however, in some instances, the physical environment does not allow closure of a square of electrodes. An alternative array introduced in this investigation is termed the horseshoe array. The horseshoe array combines the L-shaped arrays with equatorial and minimum coupling arrays to overcome array closure problems. Three synthetic examples were investigated to establish the limitations of the horseshoe array, and to describe the geological conditions of the subsoil, e.g., building foundations and fractures.The first two examples represent two resistive cubes initially located in the southern and northern positions of the array, and then are moved to a diagonal of the array. In both examples, the cube located near the electrode lines was well defined, while the cube located near the line with no electrodes was not detected. On the other hand, a weak signal for the cube located along the diagonal was observed, but only when located near the electrode line. This alternative array revealed a low tri-dimensional resolution zone possessing an inverted triangular-shaped geometry towards the line with no electrodes.A third example consisted of a low resistivity thin fracture embedded in a highly resistive infill. The solution computed demonstrated that the horseshoe array can resolve the infill close to the surface; however, the thin fracture is masked by the infill.The above-mentioned methodology was applied on a residential complex named La Concordia. Several buildings within the residential area suffered strong structural damage caused by fractures and subsidence within the subsurface. The residential complex, consisting of six four-story buildings in an area 33 × 80 m2, is located towards the eastern region of Mexico City. The horseshoe geometry, combined with Wenner-Schlumberger, dipole-dipole, equatorial-dipole, and minimum-coupling arrays, was used to investigate the subsurface beneath the buildings.A maximum depth of 8 m was investigated. A pattern of elongated resistivity anomalies (northwest-southeast direction) were associated with possible fracturing or differential compaction. Such features are caused by intense water extraction of the poorly consolidated clays that cover most of the central portion of the Mexican Basin.

The Influence of Electrical Resistivity Array on its Soil Electrical Resistivity Value

Electrical resistivity technique has become a popular alternative tool by the geotechnical engineers in subsurface investigation. This study presents the influence of soil electrical resistivity value (ERV) due to the different types of electrical resistivity array used in practice. The dissimilarity of ERV was become a popular debate by the engineers which posses less fundamental knowledge in this area. In the past, the theory of electrical resistivity technique was less being discovered by the engineer which creates lots of black boxes during the utilization of electrical resistivity method (ERM) in engineering purposes. Hence, the result which produced from the ERM was difficult to deliver in a sound of definitive ways due to lack of knowledge and experienced of most engineers. Hence, this study presents the influence of soil ERV due to the different types of array used with particular reference to as Dipole-dipole and Pole-dipole. A line of electrical resistivity imaging was performed on small embankment of sandy and lateritic soil with different types of array using ABEM SAS (4000) equipment. Three in line of soil samples were tested for moisture content (w) test immediately after the electrical resistivity data acquisition was completely measured. Moreover, particle size distribution test also was performed for all soil samples in order to support the findings. It was found that the ERV was never be the same for each types of array used even on the same particular location of the survey line. However, it was found that there was a consistent relationship between ERV and moisture content for both types of soil tested which can be represent by ERV 1/w. Hence, it was found that ERV produced was relative to the types of array used during the field measurement. Each types of array were applicable to be used in subsurface profiling and its selection was subjected to the target of interest.

The Ancient Roman Aqueduct of Karales (Cagliari, Sardinia, Italy): Applicability of Geophysics Methods to Finding the Underground Remains

ABSTRACTDetection of the buried antique Roman aqueduct, which supplied fresh water to the ancient town of Karales (Cagliari, Italy), is not a trivial problem because of the small size of its cross section, its depth (about 10 m), and of the presence of shallow conductive layers. In order to determine the best geophysical method to use in the research of the conduit, a test was carried out over a well‐known section of the underground aqueduct in its extra‐urban part. Taking into account the geological features of the site, time‐domain electromagnetic (TEM), very low‐frequency (VLF) and electrical tomography methods were chosen. The test was conducted over several profiles. The results showed that among the electrical resistivity arrays, the Wenner–Schlumberger is the most suitable for the detection of the conduit some metres in depth, whereas the other electrical methods did not have enough resolution to distinguish between the aqueduct and other non‐related anomalies at the requested depth. The TEM method also showed good capability to detect the presence of the aqueduct in spite of very conductive superficial layers. With the exception of one profile, all VLF profiles showed anomalies that could be correlated to the aqueduct. Copyright © 2014 John Wiley & Sons, Ltd.

Geoelectric imaging properties of traditional arrays and of the optimized Stummer configuration

In this paper a systematic, semi‐empirical comparison is presented between two‐dimensional geo‐electric models and their inversion images, obtained by using five different electrical resistivity arrays and an optimized Stummer configuration. Eight different models (more or less in order of growing complexity) are studied and both noise‐free and noisy data cases are considered. The results show that (1) the quality of the inversion images obtained with traditional arrays depends significantly on the model and on the noise level, (2) among the traditional arrays it is definitely the dipole‐dipole array that provides inversion images mostly similar to the geoelectric models, (3) the inversion images obtained by using the optimized Stummer configuration are even more similar to the original geoelectric model than those obtained by the dipole‐dipole array. It means that the optimized Stummer array is even better than the best traditional array, the dipole‐dipole array, especially in the deepest part of the inversion images. We conclude that in a general field situation the Stummer configuration is good enough for not being forced to search specific configurations. As presented, optimization procedures, involving null arrays could even further improve the quality of the inversion images obtained by using the Stummer configuration.

Application of a Vertical Electrical Resistivity Array (VERA) for monitoring CO2 migration at the Ketzin site: First performance evaluation

Abstract At the CO2 storage site Ketzin, near Berlin (Germany), an integrated monitoring concept has been applied for characterization of the reservoir state before and during CO2 injection. Electrical Resistivity Tomography (ERT) is an essential part of this multi-method concept in order to image the spatial extent of the CO2 plume. ERT is shown to be sensitive to saturation changes caused by propagation of supercritical CO2 within a brine-filled reservoir. A permanent downhole electrode installation is placed behind casing in the three Ketzin wells (one for CO2 injection and two for observation). The so-called Vertical Electrical Resistivity Array (VERA) is deployed as crosshole configuration at a depth ranging from 590 to 735 meters, penetrating the Stuttgart formation, which hosts the target storage reservoir. For more than two years after finishing drilling and installation work the electrode array has been operating at a temperature of about 35 °C, an average reservoir pressure of about 75 bar, and a formation fluid pH of 5.5. These conditions have an effect on the life time of the components of the electrical monitoring system. Therefore, the performance of the VERA system has to be evaluated thoroughly during its long-term application. From the first results of the ongoing project we can conclude that crosshole ERT is indeed valuable as part of the CO2 monitoring program. Further use of the time-lapse resistivity data is in progress, e.g. support of CO2 distribution mapping in conjunction with results of seismic measurements. In the frame of an integrated monitoring concept, crosshole ERT can support the regulator’s work concerning safe reservoir operation and efficient risk management.

Geoelectrical methods for monitoring geological CO 2 storage: First results from cross-hole and surface–downhole measurements from the CO 2SINK test site at Ketzin (Germany)

The feasibility of monitoring CO 2 migration in a saline aquifer at a depth of about 650 m with cross-hole and surface–downhole electrical resistivity tomography (ERT) is investigated at the CO 2SINK test site close to Ketzin (Germany). The permanent vertical electrical resistivity array (VERA) consists of 45 electrodes (15 in the injection well Ktzi201 and 15 in each of the two observation wells Ktzi200 and Ktzi202), successfully placed on the electrically insulated casings, in the depth range of about 590–740 m with a spacing of about 10 m. The three Ketzin wells are arranged as perpendicular triangle with distances of 50 and 100 m. First synthetic modelling studies indicate an increase of the electrical resistivity of about 200% caused by CO 2 injection, corresponding to a bulk CO 2 saturation of 50%, which is in good agreement with laboratory studies. Finite difference inversion of field data delivers three-dimensional resistivity distributions between the wells which are consistent with the reservoir modelling studies. To increase the limited observation area provided by the cross-hole measurements, additional surface–downhole measurements were deployed. A main CO 2 migration in SE–NW direction is deduced from surface to downhole resistivity experiments. The first cross-hole time-lapse results show that the resolution and the coverage of the electrode array in the Ketzin setting are sufficient to resolve the expected resistivity changes on the characteristic length scale of the electrode array. Significant resistivity changes could be measured, however, detailed information on the CO 2 plume could not be resolved yet by VERA under the existing geological circumstances.

Diesel transport monitoring in simulated unconfined aquifers using miniature resistivity arrays

The principal aim of this study is to assess the scope of monitoring diesel plume migration in a scaled aquifer model with a miniaturised electrical resistivity array. Respectively 1000 and 500 ml of diesel were injected in both the unsaturated and water-saturated zones of a sand body overlying a clay aquitard, and diesel migration was monitored with a miniature electrode array and an off-the-shelf resistivity meter. Inverted time-lapse electrical resistivity tomography (ERT) data reflect downward and lateral spreading of the diesel plume away from the injection point in the unsaturated zone. Diesel was also imaged to spread upwards and laterally away from the injection point in the saturated zone, as controlled by capillary rise. In both cases later-time ERT images reflected preferential pooling of diesel on the water table, as well as vertical smearing of pooled diesel in response to simulated water-table fluctuations. Repeat fluid electrical conductivity (EC) and dissolved oxygen (DO) measurements validate the observed changes in bulk resistivity caused by both diesel injections. Artefacts introduced by 2D inversion of 3D contaminant transport were abound. Time-lapse ERT imaging of diesel transport is therefore inferred to be feasible and well-suited to complementing conventional techniques of intrusive site investigation, although time-lapse 3D or 4D ERT imaging is strongly advocated.

Assessing 3D volume inhomogeneity using a noninvasive diver-deployed resistivity array

A noninvasive electrical resistivity array imaging system (RAIS) has been developed to measure sediment volume inhomogeneities in three dimensions. Novel measurements of electrical fields, due to controlled galvanic current-flow, have been inverted into electrical resistivities, in three dimensions beneath the seafloor. The limitations of conventional 3D resistivity inversions are explored, and additional a priori constraints are shown to significantly improve performance in the seafloor environment. From these 3D resistivity distributions, correlation lengths in x, y, and z dimensions are calculated to support the modeling of acoustic scattering from the seafloor. Examples obtained during November 1999 in the Gulf of Mexico for the ONR High-Frequency Sound Interaction in Ocean Sediments (SAX99) experiment are described. New technology has reduced the acquisition time for a 3D dataset of 3840 measurements from approximately 2 h to 90 s, enabling a suite of locations to be investigated during a single dive. Converting to formation factors and combining with porosity/density data enabled to tortuosity of the electrical current-paths to become a pore morphology descriptor. Formation factors were found in the range 2 to 5, being consistent with substantial bioturbation in sandy sediments, with heterogeneities identified up to 100 mm.

Non-Destructive Measurement of Corrosion State of Reinforcing Steel in Concrete

This paper presents a new nondestructive method that uses a multi-electrode electrical resistivity array to measure the complex impedance along the surface of a concrete structure to determine the position of the reinforcing bars and their corrosion state. A laboratory demonstration of the new method was conducted on a concrete block with four embedded steel reinforcing rods, each with a different surface preparation to simulate a variety of conditions: corroded, clean, coated bar, and gold-plated. The gold-plated bar was intended to represent a condition of complete chemical inertness and the painted bar was intended to represent the condition that should, in principle, offer only capacitive coupling to the concrete. The surface spectral resistivity method was able to locate the bars and distinguish between the different surface impedances of the bars; therefore, it can be a useful tool in assessing corroded reinforcement in concrete structures.

Mapping and Monitoring of Toxic Wastes with Subsurface Electrical Resistivity Arrays

Ground water contamination is a major problem facing industrial nations. Electrical methods seem particularly promising in mapping and monitoring ground water regimes since the electrical conductivity of rocks depends almost entirely on the fluid saturation, salinity, and distribution. The most important recent developments in resistivity include the use of numerical modelling and resistivity mapping using subsurface electrodes. The latter yields far greater accuracy and resolution than can be obtained with surface arrays. To illustrate the power of subsurface-surface arrays we have studied an idealised two dimensional model of a contaminated zone. Since we are interested in emphasising the anomaly caused by the repository, or subsequent changes over time in its vicinity, we have discovered that it is very useful to express the apparent resistivity results as percentage differences from either the background (for surface arrays) or from the apparent resistivities observed at a particular depth of the current source (for subsurface arrays). Percent differencing with respect to data at the repository depth dramatically reduce near-surface and topographic effects that usually confound quantitative interpretation of surface surveys. Thus, dc resistivity appears to have great potential for mapping and monitoring zones of impaired ground water.

11 - Fibrinolytic Agents

Numerical modeling analysis was used to assess the suitable electrical resistivity arrays for the characterization of geological structures, including dyke, horst, graben, sub-vertical, and vertical structures. These geological structures usually make up the aquifers interested in the hydrogeological evaluation of crystalline basement terrains. Six electrode configurations, including Wenner alpha (α), Wenner beta (β), Wenner gamma (γ), Schlumberger array, dipole-dipole array, and pole-dipole array, were used to assess the geological structures for groundwater exploration. The synthetic models of the geological structures were generated using RES2DMOD code, and 5% noise was added to all the models. The generated models were inverted using the RES2DINV code. The results show that the most suitable arrays for dyke and graben structures are Wenner alpha, while Wenner beta is the most suitable for the horst structure. The Schlumberger array was the best for both sub-vertical and vertical structures. This study has demonstrated the efficacy of numerical modeling in assessing the best resistivity arrays for 2D electrical resistivity imaging for groundwater exploration prior to geophysical field investigation.

ARRAYS AND NOMOGRAMS FOR ELECTRICAL RESISTIVITY EXPLORATION*

AbstractA number of electrical resistivity arrays are available to the exploration geophysicist in the conduct of vertical or horizontal profiling. The advantage of using central‐type arrays which produce large potential drops, such as the Wenner or the Schlumberger, must be weighed against the ease of acentral arrays such as the polar and equatorial arrays.A series of nomograms has been designed to provide a means of rapid calculation of the potential drop to be obtained by any of the various central and acentral arrays, as a function of apparent resistivity, electrode spacings and available transmitter power. The same nomograms may also be used for approximate computation of the apparent resistivities in routine surveys. However, the accuracy of resistivity calculation is directly related to the accuracy of drawing lines between the scales and hence is rather limited in reduced‐size nomograms in this paper.