Interpretation Of Resistivity Data Research Articles

Modeling and interpretation of electrical resistivity data in anisotropic media using a generalized regression neural network: A case study on the Lokbatan mud volcano, Azerbaijan

This study explores the electrical anisotropy effects in a vertical electrical sounding dataset acquired from the Lokbatan mud volcano in Azerbaijan. Vertical electrical sounding is a fundamental geophysical technique for mapping subsurface resistivity distributions; however, electrical anisotropy is a factor often ignored, can significantly influence field measurements, especially with respect to layer thicknesses. This study investigates electrical anisotropy effects on vertical electrical sounding data using generalized regression neural networks for estimating parameters of layered subsurface. Schlumberger electrode arrays were employed for these purposes with a 22-sounding dataset. In general, a classical approach for estimating electrical anisotropy coefficient fails. Vertical and horizontal resistivity values cannot be estimated using vertical electrical sounding data. Thus, generalized regression neural networks are considered. The generalized regression neural networks for appraising parameters of electrical anisotropy can be used with keeping vertical resistivity higher than horizontal resistivity in each layer as an assumption. Further, the proposed method applied how the surrounding clay-rich geological material and layered structure influence electrical properties of the Lokbatan mud volcano in Azerbaijan. As a conclusion, without considering electrical anisotropy makes interpretation erroneous especially with respect to layer thicknesses.

Improved geoelectrical resistivity monitoring for earthquake precursors near the Xinfengjiang reservoir of southern China, with a fixed Wenner array of downhole electrodes in four separated boreholes

Over the past five decades, a geoelectrical resistivity monitoring experiment using fixed electrode arrays has been conducted at Heyuan in the Guangdong Province of China for detecting possible earthquake precursors near the Xinfengjiang reservoir. A pair of surface quasi-Wenner arrays designed to monitor temporal resistivity variations have been used in two oblique directions of N27°E and N80°W with electrode spacings AB/MN of 997 m/315 m and 1032 m/328 m, respectively. To reduce the effects of near-surface environmental changes, an improved experiment has been introduced in August 1992, using a subsurface Wenner array in the direction of N80°W with electrode spacings AB/MN of 54 m/18 m, consisting of downhole electrodes in four horizontally separated boreholes at the same depth of 65 m. During the experimental period of August 1992 to December 1996, a continuously downward trend in apparent resistivity with the improved downhole array and a peculiar annual variation with the conventional surface arrays have been observed. For the same period, no notable earthquakes occur in and around the Xinfengjiang reservoir. The field tests indicate that the downward trend of apparent resistivity was due to the aging of one of the current electrodes. By using simple layered models, we develop a sensitivity analysis to design a monitoring array with minimum environmental effects and to explain the observed resistivity variations, such as a paradoxical resistivity behavior related with negative sensitivity. No significant resistivity variations, which could be attributed to earthquake activity of magnitude 3.4 or less in the study area, are detected. We prove that the two monitoring methods with the modified subsurface Wenner and the conventional surface quasi-Wenner arrangements can complement each other to efficiently estimate a change in the electrical properties of rocks and reduce the ambiguity in the interpretation of apparent resistivity data.

Hydrostratigraphy and hydrogeophysical studies to delineate fresh and saline aquifer boundaries in Lesser Cholistan of Pakistan.

The differentiation of saline water and fresh water interfaces is a key objective in ground water exploration and management. Bahawalpur is the twelfth biggest metropolitan area of Pakistan situated in south Punjab near the bank of River Sutlej and lies at 29°59'55" N latitude and 73°15'12" E longitude at an elevation of 521 ft AMSL in the Cholistan area close to the Thar abandon. The study area comprised of Lesser Cholistan experiencing acute shortage of water for inhabitants and livestock as well. The occurrence of fresh water is also challenging because of high salinity in groundwater. The present study is intended to identify hotspots of fresh groundwater zones. To achieve the goal, vertical electrical resistivity and borehole data are used to mark fresh and saline interfaces in groundwater. To achieve the results 230 vertical electric sounding were performed in the study area. A total of 3 to 5 geo-electric layers are identified with modeling along with the processing and interpretation of resistivity data. In the study area, resistivity values are classified as very high (>230 Ω-m), high (230-100 Ω-m), medium (100-40 Ω-m), low (40-20 Ω-m) and very low (<20 Ω-m). Borehole data is used to interpret subsurface lithologies and to calibrate the modeled resistivity curves. The electric resistivity data indicates that thick layers of Quaternary sediments is present in the subsurface that is primarily composed of clay, silt, sand, gravels and some kanker. Inversion technique is applied to generate 2D subsurface resistivity maps to delineate fresh and saline water zones. The generated 2D resistivity maps at variable depth above and below water table and formation resistivity maps are successfully utilized to differentiate fresh and saline water zones. The identification of a saline water aquifer within sediments rich in clay was made possible by the observation of very low resistivity measurements in the southern region. Conversely, the detection of relatively high resistivity values, coupled with the presence of sand and gravel deposits in the northern section of the lesser Cholistan area, provided compelling evidence of the existence of fresh groundwater. These findings have significant implications for the management of water resources in the region, as they provide valuable insights into the distribution and availability of groundwater resources for future use.

Application of Combined Local and Global Optimization Algorithms in Joint Interpretation of Direct Current Resistivity and Seismic Refraction Data: A Case Study of Dammam Dome, Eastern Saudi Arabia

The main geological structures in the Dammam Dome are defined by integrating geophysical measurements and applying new methodological approaches. Dammam Dome is characterized by a well-developed fracture/joints system; thus, high complexity of the subsurface is expected. Direct Current Resistivity (DCR) and Seismic Refraction (SR) geophysical survey aimed to map the Dammam Dome's near-surface features. The geophysical data were acquired along two profiles in the northern part of Dammam Dome. To maximize the results from conducting DCR and SR measurements over a complex area, a combined local and global optimization algorithm was used to obtain high-resolution near-surface images in resistivity and velocity models. The local optimization technique involves individual and joint inversion of the DCR and SR data incorporating appropriate regularization parameters, while the global optimization uses single and multi-objective genetic algorithms in model parameter estimation. The combined algorithm uses the output from the local optimization method to define a search space for the global optimization algorithm. The results show that the local optimization produces satisfactory inverted models, and that the global optimization algorithm improves the local optimization results. The joint inversion and processing of the acquired data identified two major faults and a deformed zone with an almost N-S direction that corresponds with an outcrop were mapped in profile one, while profile two shows similar anomalies in both the resistivity and velocity models with the main E-W direction. This study not only demonstrates the capability of using the combined local and global optimization multi-objectives techniques to estimate model parameters of large datasets (i.e., 2D DCR and SR data), but also provides high-resolution subsurface images that can be used to study structural features of the Dammam Dome.

Structural Evaluation for Groundwater Within the Crystalline Basement of Ilorin, Nigeria, Using Multisensory Data Techniques

Within the hard rock terrains, stream channels are suspected to be controlled by fractures and joints. In the same light, the topographic lineaments and the lineaments observed from the aeromagnetic anomalies are expected to reflect the structural trends within the complex. This study has therefore adopted multi-data sets results first on a regional scale to depict the structural fabric of Ilorin Sheet 223 and then at the chosen location of the University of Ilorin Main Campus, which is made up of the Basement Complex rocks. From the remote sensing map of Ilorin Sheet 223 structural trends have been divided into two: the NE-SW and the NW-SE structural trends. The aeromagnetic anomalies divide the rocks types into metamorphic (low-to-medium magnetic intensity) and igneous (high magnetic intensity) rocks. The resistivity data interpretation reveals the weathered basement at the University of Ilorin ranges from 5m to 20m. The radial sounding results reveal the structural trend to be NW-SE and NE-SW with the interconnection of joints. The radial sounding shows the rock type in the University of Ilorin Campus to be quartzitic, clayey and igneous as judged from the near hexagonal and elongated shapes of the polygons. The results reveal a good agreement between the trends of fractures derived from the radial vertical electrical sounding and that from the lineament maps. Areas with low bedrock resistivity, thick overburden, presence of two or more interconnected fractures, and high co-efficient of anisotropy indicate intense fracturing and thus, indicate areas of high groundwater potentiality and good water quality.

Pelatihan Akuisisi Data Geolistrik Dalam Identifikasi Subsurface di SMK Penerbangan Techno Terapan

School partners indicated that there were several problems, namely that there were still students who did not understand how to interpret resistivity geoelectrical data. This is due to the limited facilities and infrastructure as well as teaching staff in supporting the learning process in schools. The purpose of this PkM is to prepare students' abilities as early as possible so that they can understand how to interpret resistivity geoelectrical data before entering the world of work in geology and mining. Another thing is that school partners can more easily and effectively teach the interpretation of resistivity data using the geoelectric method in the teaching and learning process in schools.. The method used in achieving the objectives is a questionnaire (pretest and posttest) with descriptive statistical analysis, lectures (presentations) and discussions as well as field practice. The results of the analysis showed an increase in students' ability in the acquisition of geoelectric resistivity data after attending the training. Prior to participating in the training, 70% of students had the ability to acquire geoelectric resistivity data in the Poor category. However, after participating in the training, the students' abilities increased to Excellent (55%).

The effect of near‐surface azimuthal anisotropy on a joint interpretation of seismic and electrical resistivity data

ABSTRACTIn this paper, we study the anisotropic near surface with seismic refraction surveying and electrical resistivity tomography. We present a field data processing case study and show a significant dependence of the determined P‐ wave velocity and electrical resistivity on the acquisition line direction. We show that the anisotropy influences not only the determined velocities themselves, but also the estimated depths of the determined boundaries and can cause misinterpretation of the arrival time curves and lead to hidden layer issues. We justify our field data‐processing results with ray modelling and synthetic data processing. As a result of our case study, we suggest a consistent direction of anisotropy for electrical resistivity and seismic surveying for the particular site in the Novosibirsk region.

Experimental and modeling of electrical resistivity changes due to micro-spatial distribution of fluid for unconsolidated sand

Electrical resistivity measurements of reservoir rocks are usually applied to identify fluid type and saturation. Although it has been widely studied, the effect of fluid saturation on electrical resistivity is not yet fully understood, especially when considering the spatial distribution of fluids. Hysteresis and electrical resistivity jump in the imbibition process are two of the phenomena that remain unclear. Microscopical evaluation would be useful in understanding these phenomena. In this paper, we analyzed the influence of the micro-spatial distribution of fluids on electrical resistivity. In order to achieve the objective, laboratory measurements and computer modeling of electrical resistivity were carried out. Measurements were made on unconsolidated sand samples that were saturated with brine. Two categories of samples were prepared based on the position of the brine injection to obtain fluid spatial distribution variations in rock pores. Measurement results showed that the electrical resistivity varies depending on the brine injection position. Subsequently, 3-D microcomputed tomography images were acquired to analyze sample microstructure and model fluid spatial distribution. We implemented three different saturation models made up of varying fluid filling mechanisms onto digital images of samples, then used them to compute the electrical resistivity of the sub-sample using the finite element method. The measurement and calculation of electrical resistivity showed good agreement. These results indicate that the proposed model is capable of representing spatial fluid distribution in pore spaces of measured samples. This fluid distribution is most responsible for the variation of the electrical resistivity in the samples. Furthermore, the electrical resistivity jump phenomenon in the imbibition process was successfully explained through the proposed model. Based on this study's results, it can be concluded that the spatial distribution of fluids needs to be considered in the estimation of electrical resistivity. The research findings provide insight into how micro-spatial distribution affects electrical properties of rocks of importance to the interpretation of electrical resistivity data.

Designing Aquifer Model for the Banks of the Serayu River, Sokawera, Somagede, Banyumas, Indonesia by Means of 1D-Electrical Resistivity Data

A geoelectric survey using the 1D-electrical resistivity method was applied to design a groundwater aquifer model for the banks of the Serayu River in Sokawera Village, Somagede District, Banyumas Regency, Indonesia. The aim of this research was to identify the characteristics of aquifers in the research area based on resistivity log data. Acquisition, modeling, and interpretation of resistivity data were carried out and the results were lithological logs at seven sounding points. Correlation between the lithological logs resulted in a hydrostratigraphic model. This model is composed of several hydrological units, i.e. shallow aquifer, aquitard, and deep aquifer. The shallow aquifers are composed of sandy clay (10.81-18.21 Wm) and clayey sand (3.04-7.43 Wm) with a depth of groundwater from the water table to 27.51 m. The deep aquifers are composed of sandstone with variation of porosity (2.24-12.04 Wm) at a depth of more than 54.98 m. Based on this model, potential shallow aquifers were estimated to be at sounding points Sch-5, Sch-6, and Sch-7. This hydrostratigraphic model shows that the two types of aquifers are separated by an aquitard layer, allowing groundwater infiltration from the shallow aquifer to the deep aquifer and vice versa. Moreover, the Serayu riverbanks in this research area are estimated to be a groundwater discharge area.

A scalar-geometric approach for the probable estimation of the reserve of some Pb-Zn deposits in Ameri, southeastern Nigeria

The Cretaceous sedimentary sequence of the Asu River Group in Ameri, southeastern Nigeria play host to lead-zinc ore deposits in irregular mineralized veins of non-uniform geometry. Two deposits (designated DEPOSIT A and DEPOSIT B) were economically assessed using an integrated approach. This was performed by employing a geometry-based classical scheme involving the mathematical relationship between the three cardinal variables of ore reserve estimation namely: surface area, thickness and density. Firstly, detailed and extensive geological mapping was carried out to establish the geological attribute of the mineral deposits. Since reserve estimation extends beyond exposed faces, spontaneous potential traverse, vertical electrical sounding and horizontal profiling, being orthogonal to the trend of the enclosing veins were used to characterize the existing sub-surface stratification and create forward models that revealed the concealed ore geometry (limiting depth, lateral extents and thickness variations). Subsequent to delineation, the depth to the top of the target ore lodes was obtained by implementing the Peter's half slope technique on self-potential curves. Thickness values of the ore bodies of interest were applied in three numerical classical integration techniques, i.e. Trapezoidal, Simpson's and Simpson's third-eight rule, for volume estimation. To overcome the non-uniqueness problem of DC resistivity data interpretation, the available drilling data was applied. Samples retrieved from each lode were subjected to mass and volume analysis using the CS200 electronic balance and water displacement technique, to determine the average density of each lode. Results from 2D subsurface resistivity sections showed the lead-zinc deposits as zones of low resistivity in a background of high resistivity. The 1D stratigraphic models reveal a mean thickness estimate of 11.85 m and 10.50 m for DEPOSITs' A and B, respectively. Sample densities correlate well with the average density of pure galena. Applying the scalar-geometric rule, a combined estimate of 10,778.95tonnes was obtained, representing a relatively economically viable quantity worthy of medium scale extraction.

A scalar-geometric approach for the probable estimation of the reserve of some Pb-Zn deposits in Ameri, southeastern Nigeria

The Cretaceous sedimentary sequence of the Asu River Group in Ameri, southeastern Nigeria play host to lead-zinc ore deposits in irregular mineralized veins of non-uniform geometry. Two deposits (designated DEPOSIT A and DEPOSIT B) were economically assessed using an integrated approach. This was performed by employing a geometry-based classical scheme involving the mathematical relationship between the three cardinal variables of ore reserve estimation namely: surface area, thickness and density. Firstly, detailed and extensive geological mapping was carried out to establish the geological attribute of the mineral deposits. Since reserve estimation extends beyond exposed faces, spontaneous potential traverse, vertical electrical sounding and horizontal profiling, being orthogonal to the trend of the enclosing veins were used to characterize the existing sub-surface stratification and create forward models that revealed the concealed ore geometry (limiting depth, lateral extents and thickness variations). Subsequent to delineation, the depth to the top of the target ore lodes was obtained by implementing the Peter's half slope technique on self-potential curves. Thickness values of the ore bodies of interest were applied in three numerical classical integration techniques, i.e. Trapezoidal, Simpson's and Simpson's third-eight rule, for volume estimation. To overcome the non-uniqueness problem of DC resistivity data interpretation, the available drilling data was applied. Samples retrieved from each lode were subjected to mass and volume analysis using the CS200 electronic balance and water displacement technique, to determine the average density of each lode. Results from 2D subsurface resistivity sections showed the lead-zinc deposits as zones of low resistivity in a background of high resistivity. The 1D stratigraphic models reveal a mean thickness estimate of 11.85 m and 10.50 m for DEPOSITs' A and B, respectively. Sample densities correlate well with the average density of pure galena. Applying the scalar-geometric rule, a combined estimate of 10,778.95tonnes was obtained, representing a relatively economically viable quantity worthy of medium scale extraction.

Vulnerability assessment of hydrogeologic units in parts of Enugu North, Southeastern Nigeria, using integrated electrical resistivity methods

Aim/objectives: This research is geared towards integrating Vertical Electrical Soundings (VES) and Electrical Resistivity Tomography (ERT) profiles constrained with geoelectric sections and borehole logs to assess the vulnerability of the aquifer units employing Aquifer Vulnerability Indexing (AVI) method.Method: The dataset used in this study comprises of fifty VES, five ERT profiles surveyed in parts of Enugu north in Enugu state, two geoelectric sections and three borehole logs. The computer and manual interpretation of VES resistivity data using WinResist software gave values of resistivity, depth and thickness of each geoelectric layer using half current electrode spacing of 1.0 to 450.0m and maximum current electrode spacing of 900 m. 2D imaging data from the measured field resistance values were processed using RES2DINV32 version 3.71.115 software. The geoelectrical sections show the variation of resistivities with depth along transcent lines. The geohydraulic parameters were also estimated. Finding: Interpreted VES data revealed five to six geoelectric layers and fundamental parameters generated were used to estimate the values of hydraulic conductivity (s ) and hydraulic resistance (C) of the covering layers ranging from 0.010 to 0.769 mday􀀀1 and 40.47 to 8292.0 day􀀀1 respectively.This research revealed high hydraulic conductivity in the western part implying good groundwater potential with moderate to high protective capacity while areas with low hydraulic conductivity correspond to areas with high resistivity indicating little or no pore space and total devoid of water. The hydraulic resistance quantifies groundwater vulnerability using AVI and reveals that the area of study is characterized by low to high AVI with moderate AVI dominanting.Originality and novelty: The estimated geohydraulic properties from resistivity data and their spatial spread are promising and could increase the depth of knowledge on groundwater vulnerability within and around the study area. Keywords: VES; ERT; AVI; hydraulic conductivity; hydraulic resistance

Time‐intensive geoelectrical monitoring under winter wheat

ABSTRACTSeveral studies have explored the potential of electrical resistivity tomography to monitor changes in soil moisture associated with the root water uptake of different crops. Such studies usually use a set of limited below‐ground measurements throughout the growth season but are often unable to get a complete picture of the dynamics of the processes. With the development of high‐throughput phenotyping platforms, we now have the capability to collect more frequent above‐ground measurements, such as canopy cover, enabling the comparison with below‐ground data. In this study hourly direct‐current resistivity data were collected under the Field Scanalyzer platform at Rothamsted Research with different winter wheat varieties and nitrogen treatments in 2018 and 2019. Results from both years demonstrate the importance of applying the temperature correction to interpret hourly electrical conductivity data. Crops which received larger amounts of nitrogen showed larger canopy cover and more rapid changes in electrical conductivity, especially during large rainfall events. The varieties showed contrasted heights although this does not appear to have influenced electrical conductivity dynamics. The daily cyclic component of the electrical conductivity signal was extracted by decomposing the time series. A shift in this daily component was observed during the growth season. For crops with appreciable difference in canopy cover, high‐frequency direct‐current resistivity monitoring was able to distinguish the different below‐ground behaviours. The results also highlight how coarse temporal sampling may affect interpretation of resistivity data from crop monitoring studies.

Depth Estimation of the Aquifer Using Geoelectric Technique and It′s Comparison with Drilling Result in Pekuncen Village, Jatilawang District, Banyumas Regency

Pekuncen Village, Jatilawang District is one of the villages in Banyumas Regency which often experiences drought in the dry season. Exploration to obtain the groundwater sources needs to be done, especially to estimate the depth of the aquifer. The chosen exploration method was resistivity geoelectric using vertical sounding technique. Resistivity data acquisition is carried out on 3 trajectories with the length of the 1st and 3rd trajectories are 200 m, and the 2nd trajectory is 240 m. The sounding point on the 2nd trajectory was drilled, where rock samples were taken at every 5 m depth. The rock samples obtained were correlated with the resistivity data interpretation results, including the type and thickness of each depth. The aquifer in the research area consisted of two parts, namely unconfined aquifer and confined aquifer. The unconfined aquifer at the first sounding point is located at 6.80 – 34.38 m depth, the second sounding point at 7.00 – 28.89 m depth, and the third sounding point at 4.39 – 31.39 m depth. Then the confined aquifer at the first sounding point is located at less than 48.24 m depth, the second sounding point at less than 47.22 m depth, and the third sounding point at less than 47.36 m depth. The correlation between the resistivity data interpretation results at the second sounding point and the drill log data shows a slight difference in rock types and thickness. The difference of the rock types due to the drilling data interpretation results which divides the sand in the confined aquifers into fine, medium, and coarse grained sand. The difference of confined aquifer depth after being correlated is less than 2 m depth.Keywords: Aquifer, geoelectric survey, resistivity data, drill, Pekuncen, Banyumas.

Interpretation of resistivity data using 3D Euler deconvolution and Radially Averaged Power Spectrum

Different electrode configuration in resistivity measurements over the same geologic structures generally produce different anomaly patterns. This is related to the position of the structure concerning the electrodes. Given the above, a unified approach has been proposed using the concept of analytical signal to interpret the resistivity data as electric potential follows Laplace’s equation. We interpret the data sets using Euler deconvolution and Radially Averaged Power Spectrum (RAPS) to determine both lithological boundaries and units and compare the results with inverted resistivity section. We analysed the resistivity data using electrical resistivity tomography (ERT) technique over conducting dyke and vertical fault for Wenner and dipole–dipole arrays, respectively. The obtained structural indices for dyke and fault are 1.0 and 0.6, respectively. The results from Euler depth solutions match well with the inverted resistivity section. Subsequently, two field examples one each over ground water and mineral exploration were analysed. The delineated lineaments over ground water exploration matches fairly well with the available results. In addition, some additional lineaments are also mapped. These new features could be a zone of interest for a detailed survey. The depths from Euler depth solutions and RAPS are in agreement. However, no depth information was available earlier. The data for mineral exploration have been acquired by ERT technique with a profile length of ~500 m over Dhanjori Basin, Jharkhand, India. The Euler depth solution and RAPS indicated the presence of two interfaces at an average depth of 8 and 20 m and 7 and 21 m, respectively. The first interface is present all along the profile, whereas the second interface lies between a part of the profile. The location and depth of these are in broad agreement with the resistivity sections obtained from ERT/AMT and borehole data. A borehole in the vicinity of the survey area indicated the presence of two interfaces. The first interface coincides with the soil depth and the second interface coincides with the sulphide mineralization.

Delineation of contaminated aquifers using integrated geophysical methods in Northeast Punjab, Pakistan.

A decline in surface water sources in Pakistan is continuously causing the over-extraction of groundwater resources which is in turn costing the saltwater intrusion in many areas of the country. The saltwater intrusion is a major problem in sustainable groundwater development. The application of electrical resistivity methods is one of the best known geophysical approaches in groundwater study. Considering the accuracy in extraction of freshwater resources, the use of resistivity methods is highly successful to delineate the fresh-saline aquifer boundary. An integrated geophysical study of VES and ERI methods was carried out through the analysis and interpretation of resistivity data using Schlumberger array. The main purpose of this investigation was to delineate the fresh/saline aquifer zones for exploitation and management of fresh water resources in the Upper Bari Doab, northeast Punjab, Pakistan. The results suggest that sudden drop in resistivity values caused by the solute salts indicates the saline aquifer, whereas high resistivity values above a specific range reveal the fresh water. However, the overlapping of fresh/saline aquifers caused by the formation resistivity was delineated through confident solutions of the D-Z parameters computed from the VES data. A four-layered unified model of the subsurface geologic formation was constrained by the calibration between formation resistivity and borehole lithologs. i.e., sand and gravel-sand containing fresh water, clay-sand with brackish water, and clay having saline water. The aquifer yield contained within the fresh/saline aquifers was measured by the hydraulic parameters. The fresh-saline interface demarcated by the resistivity methods was confirmed by the geochemical method and the local hydrogeological data. The proposed geophysical approach can delineate the fresh-saline boundary with 90% confidence in any homogeneous or heterogeneous aquifer system.

3D Interpretation of Resistivity Data for Groundwater Potential Assessment of Pakhli Plain, Mansehra District, Pakistan

Abstract The present research describes a method of combining geostatistical analysis with geophysical inversion of electrical resistivity data conducted in Pakhli Plain, northwestern Himalayas, Pakistan. The raw data has been collected from the Technical Report VII-I on Ground Water Resources in Pakhli Plain, Mansehra District. Subsequently, the data has been deciphered and broadened from one dimensional resistivity data into a 2D model that can be entirely visualized and deduced in a spatial sense. Interpretation and calibration of the electrical resistivity curves with the lithologies and geophysical logs of boreholes suggests possible identification of distinctive sedimentary accumulations occurring within the Pakhli Plain. The 2D and 3D gridding and visualization is imperative to map the extents of the alluvial deposits within the Pakhli Plain formed during the periods of extreme tectonic activity. The coarser sediments are associated with lower levels of resistivity as measured in the electrical surveys, whereas the finer sediments exhibit characteristically lower resistivities. Therefore, the zones of low and high resistivity values are indicative of particles associated with coarser and finer sediments, respectively. It has been mentioned that the Pakhli Plain has remained a lacustrine zone during some time in the geological past as indicated by low resistivities representing finer sediments in the middle of the Plain. Consequently, the overall transmissivity of the sediments is low, which imply poor conditions for commercial groundwater production in the Pakhli Plain. Moreover, high resistivity zones of coarse material could be further investigated for groundwater potential areas. In particular, the prime objectives of the present study include 3D modeling of underground resistivity and its exploration in terms of groundwater potential on the basis of distribution of low resistivity zones.

A scalar-geometric approach for the probable estimation of the reserve of some Pb-Zn deposits in Ameri, southeastern Nigeria

The Cretaceous sedimentary sequence of the Asu River Group in Ameri, southeastern Nigeria play host to lead-zinc ore deposits in irregular mineralized veins of non-uniform geometry. Two deposits (designated DEPOSIT A and DEPOSIT B) were economically assessed using an integrated approach. This was performed by employing a geometry-based classical scheme involving the mathematical relationship between the three cardinal variables of ore reserve estimation namely: surface area, thickness and density. Firstly, detailed and extensive geological mapping was carried out to establish the geological attribute of the mineral deposits. Since reserve estimation extends beyond exposed faces, spontaneous potential traverse, vertical electrical sounding and horizontal profiling, being orthogonal to the trend of the enclosing veins were used to characterize the existing sub-surface stratification and create forward models that revealed the concealed ore geometry (limiting depth, lateral extents and thickness variations). Subsequent to delineation, the depth to the top of the target ore lodes was obtained by implementing the Peter's half slope technique on self-potential curves. Thickness values of the ore bodies of interest were applied in three numerical classical integration techniques, i.e. Trapezoidal, Simpson's and Simpson's third-eight rule, for volume estimation. To overcome the non-uniqueness problem of DC resistivity data interpretation, the available drilling data was applied. Samples retrieved from each lode were subjected to mass and volume analysis using the CS200 electronic balance and water displacement technique, to determine the average density of each lode. Results from 2D subsurface resistivity sections showed the lead-zinc deposits as zones of low resistivity in a background of high resistivity. The 1D stratigraphic models reveal a mean thickness estimate of 11.85 m and 10.50 m for DEPOSITs' A and B, respectively. Sample densities correlate well with the average density of pure galena. Applying the scalar-geometric rule, a combined estimate of 10,778.95tonnes was obtained, representing a relatively economically viable quantity worthy of medium scale extraction. • An integrated approach to the quantitative evaluation of some Pb-Zn deposits is discussed. • Arithmetic alliance between the three cardinal variables of ore reserve assessment is employed. • To validate the result of geophysical schemes, drilling information is applied. • Ore bodies are characterized by low resistivity values. • Evaluated deposits proved fairly viable sequel to qualitative analysis.

Vector analysis of pole–pole array for determining the 3D boundary of object

ABSTRACTAn algorithm is proposed for the interpretation of resistivity data that allows the 3D parameters (the boundary in plan view and the depth range) of an anomalous resistivity object in a heterogeneous medium to be determined. The proposed method is based on a vector analysis of the apparent resistivity of soil obtained by a pole–pole survey within the measurement window. In the first stage of the algorithm, in each measurement window, the radius vector is calculated, and as a result, vector images of the main directions of change in the resistivity of the medium are constructed. This allows us to estimate the location of a local anomalous object and to correlate the resistivity of the object with that of the background medium. With a consistent variation in the effective depth of investigation, a set of vector images is formed that characterizes the apparent resistivity distribution in the soil layers. Mathematical analysis of the vector images by using the scalar product allows us to estimate the depth range of the anomalous object. The effectiveness of the proposed algorithm has been proven on synthetic models and in comprehensive investigations of archaeological sites. The proposed method does not allow the true resistivity of an anomalous object to be determined, which is a disadvantage. However, this simple algorithm for processing and analysing shallow electrical prospecting data can be directly used at the preliminary data processing stage during a field survey. First, a pole–pole array can be rapidly operated by a single person; second, the user does not need special knowledge in the field of electrical resistivity data processing. The estimated 3D boundaries of local objects will make it possible to determine the area of interest for further detailed surveying and to justify all the parameters of the measurement technique (distance between electrodes, depth range etc.).

Three-dimensional DC Resistivity Modeling using Galerkin Finite Element Method Composed by Tetrahedral Elements

Successful interpretation of DC resistivity data depends on the availability of a proper forward modeling scheme. In this study, a three-dimensional DC resistivity forward modeling scheme was developed using the finite element method. The finite element equations were obtained using a weakened form of the weighted-residual method called the Galerkin method. Discretization of the modeling domain was carried out by dividing it into smaller three-dimensional blocks and subdividing each block into five tetrahedral elements. A linear interpolation function was employed and elemental linear equations were set up, followed by formation of global matrix systems of equation and incorporation of proper boundary conditions. The conjugate gradient method was applied to solve the global system of equations, which in this study was proven to be more efficient than a direct solver, contributing to a 67% time reduction. Using a Wenner array configuration, comparison with theoretical calculation of the electric potential for a homogeneous model yielded a relative error of 3.66%. To confirm the applicability of this forward modeling scheme, apparent resistivity profiles for several basic three-dimensional subsurface resistivity models were compared with the analytical profiles, yielding an acceptable level of fitting.