Apparent Resistivity Pseudosections Research Articles

Мониторинговые электротомографические исследования зоны Иссык-Атинского разлома (Северный Тянь-Шань)

In 2023, monitoring geophysical studies were carried out using the electrical resistivity tomography (ERT) method along three profiles that cross the strike zone of the Issyk-Ata fault in the transverse direction. A detailed study was carried out of the observed variations in apparent electrical resistivity, which characterize the change in the electrical properties of the upper part of the Earth's crust section with a 40-minute discretization in time. Using the analysis of difference pseudo-sections of apparent resistivity, constructed based on the results of ERT monitoring, the most informative pseudo-depth interval from the point of view of geoelectric activity was determined for all three monitoring profiles. Accordingly, for a pseudo-depth of 24 m on each monitoring profile, an assessment was made of the geodynamic activity of these sections of the Issyk-Ata fault based on an analysis of the magnitude of two parameters: the amplitude of apparent resistivity variations and the correlation coefficient of electrical resistivity variations with lunar-solar solid tides. High values of these parameters, in our opinion, determine the location of modern active fault zones.

Rapid imaging approach for marine controlled-source electromagnetic with deployed transmission system

The marine controlled-source electromagnetic (MCSEM) method is widely used in gas hydrate exploration. For the deployed transmission system of MCSEM, it is equipped with two groups of orthogonal transmission antennas to obtain more comprehensive information. The mainstream processing method involves using a single rotated field component for conventional 2D inversion to obtain the resistivity model. However, data preprocessing and inversion speed are relatively slow, making it difficult to quickly or nearly real-time identify gas hydrate anomalous areas. In order to leverage the benefits of multi-component data and obtain a potential real-time imaging method for the future, we have developed a rapid imaging technique for the deployed transmission system to obtain apparent resistivity pseudosections. The current and electromagnetic field data in different directions are replaced by rotational invariants, and an approximate formula is derived based on the half-space formula and test corrections. This formula maps the apparent resistivity and field data. Results from model tests demonstrate that the apparent resistivity pseudosections can qualitatively identify reservoirs, particularly in the lateral range. In 2015, the South China Sea gas hydrate exploration project acquired data using the deployed transmission system. The pseudosections reveal that the rapid imaging method can directly detect resistivity anomalies and serve as an approximate and effective display method for deployed transmission system of MCSEM, thanks to its simplicity, efficiency, and low computational cost.

Appraisal of Resistivity Inversion Models With Convolutional Variational Encoder–Decoder Network

Recovering the actual subsurface electrical resistivity properties from the electrical resistivity tomography data is challenging because the inverse problem is nonlinear and ill-posed. This paper proposes a Variational Encoder-Decoder (VED) based network to obtain resistivity model, which maps the apparent resistivity data (input) to true resistivity data (output). Since deep learning (DL) models are highly dependent on training sets and providing a meaningful geological resistivity model is complex, we have first developed an algorithm to construct many realistic resistivity synthetic models. Our algorithm automatically constructs an apparent resistivity pseudo-section from these resistivity models. We further computed the resistivity from two different neural architectures for comparison – UNet, and attention UNet with and without input depth encoding apparent data. In the end, we have compared our deep learning results with traditional inversion and borewell data on apparent resistivity datasets collected for aquifer mapping in the hard rock terrain of the West Medinipur district of West Bengal, India. A detailed qualitative and quantitative evaluation reveals that our VED approach is the most effective for the inversion compared to other approaches considered.

Ensemble-Based Electrical Resistivity Tomography with Data and Model Space Compression

Inversion of electrical resistivity tomography (ERT) data is an ill-posed problem that is usually solved through deterministic gradient-based methods. These methods guarantee a fast convergence but hinder accurate assessments of model uncertainties. On the contrary, Markov Chain Monte Carlo (MCMC) algorithms can be employed for accurate uncertainty appraisals, but they remain a formidable computational task due to the many forward model evaluations needed to converge. We present an alternative approach to ERT that not only provides a best-fitting resistivity model but also gives an estimate of the uncertainties affecting the inverse solution. More specifically, the implemented method aims to provide multiple realizations of the resistivity values in the subsurface by iteratively updating an initial ensemble of models based on the difference between the predicted and measured apparent resistivity pseudosections. The initial ensemble is generated using a geostatistical method under the assumption of log-Gaussian distributed resistivity values and a Gaussian variogram model. A finite-element code constitutes the forward operator that maps the resistivity values onto the associated apparent resistivity pseudosection. The optimization procedure is driven by the ensemble smoother with multiple data assimilation, an iterative ensemble-based algorithm that performs a Bayesian updating step at each iteration. The main advantages of the proposed approach are that it can be applied to nonlinear inverse problems, while also providing an ensemble of models from which the uncertainty on the recovered solution can be inferred. The ill-conditioning of the inversion procedure is decreased through a discrete cosine transform reparameterization of both data and model spaces. The implemented method is first validated on synthetic data and then applied to field data. We also compare the proposed method with a deterministic least-square inversion, and with an MCMC algorithm. We show that the ensemble-based inversion estimates resistivity models and associated uncertainties comparable to those yielded by a much more computationally intensive MCMC sampling.

A geostatistical Markov chain Monte Carlo inversion algorithm for electrical resistivity tomography

ABSTRACTElectrical resistivity tomography is an ill‐posed and nonlinear inverse problem commonly solved through deterministic gradient‐based methods. These methods guarantee a fast convergence towards the final solution, but the local linearization of the inverse operator impedes accurate uncertainty assessments. On the contrary, numerical Markov chain Monte Carlo algorithms allow for accurate uncertainty appraisals, but appropriate Markov chain Monte Carlo recipes are needed to reduce the computational effort and make these approaches suitable to be applied to field data. A key aspect of any probabilistic inversion is the definition of an appropriate prior distribution of the model parameters that can also incorporate spatial constraints to mitigate the ill conditioning of the inverse problem. Usually, Gaussian priors oversimplify the actual distribution of the model parameters that often exhibit multimodality due to the presence of multiple litho‐fluid facies. In this work, we develop a novel probabilistic Markov chain Monte Carlo approach for inversion of electrical resistivity tomography data. This approach jointly estimates resistivity values, litho‐fluid facies, along with the associated uncertainties from the measured apparent resistivity pseudosection. In our approach, the unknown parameters include the facies model as well as the continuous resistivity values. At each spatial location, the distribution of the resistivity value is assumed to be multimodal and non‐parametric with as many modes as the number of facies. An advanced Markov chain Monte Carlo algorithm (the differential evolution Markov chain) is used to efficiently sample the posterior density in a high‐dimensional parameter space. A Gaussian variogram model and a first‐order Markov chain respectively account for the lateral continuity of the continuous and discrete model properties, thereby reducing the null‐space of solutions. The direct sequential simulation geostatistical method allows the generation of sampled models that honour both the assumed marginal prior and spatial constraints. During the Markov chain Monte Carlo walk, we iteratively sample the facies, by moving from one mode to another, and the resistivity values, by sampling within the same mode. The proposed method is first validated by inverting the data calculated from synthetic models. Then, it is applied to field data and benchmarked against a standard local inversion algorithm. Our experiments prove that the proposed Markov chain Monte Carlo inversion retrieves reliable estimations and accurate uncertainty quantifications with a reasonable computational effort.

Quantifying seasonal 3D effects for a permanent electrical resistivity tomography monitoring system along the embankment of an irrigation canal

ABSTRACTIn this paper, we discuss the necessity of quantifying and correcting seasonal 3D effects on 2D electrical resistivity tomography (ERT) data measured along the embankments of rivers or artificial canals. A permanent ERT monitoring system has been continuously operating along the levee of an irrigation canal in Mantua province, Italy, since September 2015. To evaluate the importance of 3D effects and their dependence on seasonal variations, we first performed numerical simulations and also laboratory tests on downscaled levees of the study site. The results showed that 2D apparent resistivity pseudosections measured along the levee are significantly affected by 3D effects of the embankment geometry. Moreover, it was observed that 3D effects not only depend on the levee geometry, but they are also affected by seasonal fluctuations in the water level in the canal. This proved the importance of calculating 3D effects for the study site during dry and irrigation periods. Therefore, different synthetic models based on the levee geometry and water level in the canal in each period were constructed in RES2DMOD and RES3DMODx64 to quantify 3D effects for the study site. The ratios of apparent resistivity values calculated in RES3DMODx64 to the values calculated in RES2DMOD showed that 3D effects approach a maximum of 30% when the canal is empty during winter, and they arrive at a maximum of 10% when the canal is filled with water in summer. Using the graphs of the modelled 3D effects as a function of electrode spacing, apparent resistivity pseudosections measured by the permanent ERT system are corrected for 3D effects to obtain reliable resistivity sections after inversion. The final resistivity maps can be then converted into water content images using the empirical and site‐dependent relationship developed from core samples in the study area. Water content maps can be used to evaluate the stability of the levee and to detect possible seepage zones.

In situ DC electrical resistivity mapping performed in a reinforced concrete wharf using embedded sensors

Abstract Installation, calibration protocol and data analysis of multi-electrode sensors embedded in concrete are proposed in this study. Measurements of resistivity are performed in a port wharf and analyzed in parallel with measurements of humidity, temperature, and evolution of the mechanical strength of concrete. The correlation between the evolution of the compressive strength and the resistivity, allows to validate the measurement protocol proposed. To visualize the distribution of resistivities in the studied volume, pseudo-sections of apparent resistivity were displayed. A methodology to plot and use them as an indicator of corrosion adapted to civil engineering structures is proposed.

PHYSICAL MODELING OF ELECTRICAL RESISTIVITY TOMOGRAPHY ON GROUND SURFACE WITH A COMPLEX RELIEF

Currently geophysicists actively use the method of ERT for a wide class of geotechnical problems. Often work is performed in rough terrain where there are areas with significant elevation changes of the earth's surface commensurate with the size of the electrical array, and which lead to the distortion of the electric field. The main types of distortion electrical fields on different forms of relief are known, and in all inverse programs provides for the consideration of the influence of topography. But issues remain with the accuracy of the calculation of the direct problem, which is embedded in the inverse problem, not the known values of the critical angles of the terrain that can cause the strong distortion of the field. Therefore, the question about the influence of the topography of the land results two-dimensional inversion of electrical tomography data is the actual geophysical problem. To solve this problem, the authors performed physical modeling on large homogeneous sand embankment with artificial complex surface topography. The mound was located on a laterally homogeneous loamy earth's surface. This experiment allowed us to identify false anomalies that were the result of a two-dimensional inversion for geoelectric sections. In addition, the simulation in the laboratory in a rectangular tank filled with cellulose-hydro gel mixture. The positive slope of the terrain relative to the background surface was 30 degrees. The result has distinguished four types of severe anomalies of the electric field. Geoelectric sections, obtained through inversion, were considered. It is shown that on steep bends of the relief, 2D-inverse program does not complete this task sufficiently. As a result of abnormalities of a different type from the adjacent landforms are summed with each other and complex patterns appear on the pseudo-sections of apparent resistivity. Therefore, the anomalous zones are formed on the geoelectric section which can explain how some objects, which actually does not exist.

Correcting the distortion of apparent resistivity pseudosection produced by 3D effect of the embankment geometry

Two-dimensional electrical resistivity tomography data acquired on the crest of the embankment are distorted by the 3D effect of the embankment geometry, reservoir water level and abutment. The distortion affects seriously the final solution of the 2D inverse problem. By comparing the apparent resistivity pseudosections from a 3D and 2D electrical resistivity model of the embankment, the distortion degrees of the apparent resistivity pseudosections along the axis on the crest were estimated for the cases of reservoir and which does not contain water. The obtained results indicate that the distortion degree acquired in the case of a reservoir that contains water was much less than that in the case of the reservoir that does not contain water. In the case of reservoir that contains water, the apparent resistivity pseudosections of the P–P and ED–ED arrays had the largest distortion degree and of D–D, W–S and P–D arrays had the smallest distortion degree. In the case of the reservoir that does not contain water, the apparent resistivity pseudosection of P–P array had the smallest distortion among all arrays. Through modeling investigation, a correction process to reduce the distortion of the apparent resistivity pseudosection was proposed. The correction process was tested in the embankment model, and two field works were carried out in the To Lich River in Hanoi and Khuan Cat embankment in Lang Son province, Vietnam. It is possible to bring the distortion of the apparent resistivity pseudosection down to 2.8–13.9%, depending on the type of electrode arrays and the type of reservoirs, containing or does not contain water. The distortion correction of the apparent resistivity pseudosection is strongly recommended before doing the 2D inverse interpretation.

Improved Pseudo-section Representation for CSAMT Data in Geothermal Exploration

Controlled-Source Audio-frequency Magnetotellurics (CSAMT) is a frequency domain sounding technique employing typically a grounded electric dipole as the primary electromagnetic (EM) source to infer the subsurface resistivity distribution. The use of an artificial source provides coherent signals with higher signal-to-noise ratio and overcomes the problems with randomness and fluctuation of the natural EM fields used in MT. However, being an extension of MT, the CSAMT data still uses apparent resistivity and phase for data representation. The finite transmitter-receiver distance in CSAMT leads to a somewhat “distorted” response of the subsurface compared to MT data. We propose a simple technique to present CSAMT data as an apparent resistivity pseudo-section with more meaningful information for qualitative interpretation. Tests with synthetic and field CSAMT data showed that the simple technique is valid only for sounding curves exhibiting a transition from high – low – high resistivity (i.e. H-type) prevailing in data from a geothermal prospect. For quantitative interpretation, we recommend the use of the full-solution of CSAMT modelling since our technique is not valid for more general cases.

Corrections of surface fissure effect on apparent resistivity measurements

Electrical resistivity tomography (ERT) is a useful tool to detect and track water flow paths in the subsoil. However, measurements are strongly affected by subsurface heterogeneities such as fissures of different sizes and genesis (shrinking-swelling, macropores and deformation). In this work, we focus on surface fissures characterized by dimensions lower than the interelectrode spacing and correct their effect on apparent resistivity pseudo-sections by incorporating fissure geometry in the topography. We show that fissures with depths greater than 0.10 times the interelectrode spacing for a dipole–dipole array and equal to 0.16 for the gradient array and the Wenner–Schlumberger arrays create significant anomalies (greater than 5 per cent) in the pseudo-section. Surface fissure widths and dip angles have little effect with respect to the fissure depths which can increase the apparent resistivity up to 200 per cent. The clogging of the fissures with water or soil material decreases the anomaly effect linearly with the percentage of filling. The correction of apparent resistivity values is possible for relatively simple fissure geometries and only requires a manual survey of the surface fissures. It allows to improve the quality of the inverted resistivity section by mitigating the inversion artefacts and therefore a better interpretation.

가스 하이드레이트 부존양상 도출을 위한 해양 전자탐사 자료의 겉보기 비저항 계산

해양 전자탐사의 겉보기 비저항은 해수층으로 인해 지표탐사와 그 정의가 달라지게 되며, 이를 적절히 계산할 수 있는 알고리듬의 개발은 해양 전자탐사의 출발점이 될 수 있다. 이를 위해, 1차원 층서 가스 하이드레이트 수치모형과 해수층과 그 하부의 반 무한매질로 이루어진 수치모형에서 계산한 전자기적 반응을 비교분석하였다. 겉보기 비저항을 계산하기 위해서는 실수와 허수 성분보다는 진폭과 위상을 사용하는 것이 더 적합하였으며 해양 전자탐사 반응의 민감도를 정량적으로 분석하여, 근거리 영역에서는 위상이 원거리 영역에서는 진폭 성분이 더 안정적인 결과를 주는 것을 알았다. 또한 위상과 진폭의 선택기준으로써 유도상수의 값을 제안하였다. 이러한 분석을 토대로 격자 탐색법(grid search)을 사용하여 겉보기 비저항을 계산하는 수치알고리듬을 개발하였다. 개발된 알고리듬을 이용하여 1차원 층서 가스 하이드레이트 수치모형의 다양한 변수를 변화시켜가며 겉보기 비저항을 계산해봄으로써 알고리듬의 타당성을 검증하였다. 마지막으로, 계산한 겉보기 비저항 값을 이용한 가스 하이드레이트 부존양상 정보의 도출가능성을 살펴보았다. 동해 울릉분지의 가스 하이드레이트 부존양상을 모사한 2차원 가스 하이드레이트 수치모형에서 계산된 자료의 겉치레 단면도는 가스 하이드레이트 부존양상 정보 추출이 가능함을 보여주었다. The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventional definition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm, which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, we compared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model below the sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real and imaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phase component in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitude or phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, which computes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition, we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various model parameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numerical models simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide the information on the geometric distribution of the gas hydrate deposit.

Application of integrated geoelectrical methods in Marand (Iran) manganese deposit exploration

This paper addresses the application of integrated geoelectrical methods including induced polarization (IP), resistivity (RS), and self-potential (SP) for exploration and evaluation of the Marand (Iran) manganese deposit. The SP method was used for reconnaissance, in which five profiles were designed and surveyed with a reference electrode that was placed in a distance of at least 800 m far from the measurement points. For the conduct of RS and IP, a new array proposed by Ramazi (2005a), named combined resistivity profiling and sounding (CRSP) was used along nine profiles that made it possible to do sounding survey and produce apparent resistivity pseudo-sections. Some anomalies were detected by means of SP surveys along the profiles and some other were located from IP and RS pseudo-sections, and there was good consistency between anomalies detected by IP and RS and SP method. To check the efficiency of CRSP and in comparison with dipole–dipole array, in the location of profile no. 6, a profile with the dipole–dipole array was designed and surveyed. Finally, by integration of the results from the geoelectrical methods, some locations were suggested for borehole drilling. After drilling in those locations, cores were studied and compared with the results obtained from the geoelectrical methods that resulted in confirmation of the geoelectrical findings. The results obtained by the CRSP array have a match better than the drilling results.

Evaluation of soil water content in tilled and cover-cropped olive orchards by the geoelectrical technique

Electrical Resistivity Imaging (ERI) was performed in southern Italy, under semi-arid climate, on a Haplic Calcisol soil of two rainfed olive orchards managed according to different soil management (Tillage – T, and Cover Cropping – CC), applied for 8 years. The main aim was to test the usefulness of such geoelectrical method for the assessment of spatial and temporal variability of soil water content for water balance studies. For that purpose, the quality of univariate relationships between soil electrical resistivity (ρ) and volumetric soil water content (θ v) was tested. Three geoelectrical measurement campaigns were performed in the autumn–spring period of 2006–2007. For each campaign, 3 two-dimensional (2-D) geoelectrical tomographies were carried out in the pole–dipole array using a fixed electrode spacing of 0.25 m. 2-D apparent resistivity pseudosection data sets have been inverted using the software TomoLab®. Each covered soil section was 3 m in depth and 11.5 m long. On 14 November 2006 and 18 April 2007 campaigns the ERI were followed by soil samplings carried out with auger directly below the respective geoelectrical profiles. Direct measurements of soil electrical conductivity and soil water content were conducted on the soil samples in situ and in the laboratory. Volumetric soil water content, θ v (mm mm −1), was significantly related to soil electrical resistivity data, ρ (Ω m), by the exponential model θ v = 1.641 ρ − 0.599 (R = 0.866, n = 84). This relation allowed to assess the change of soil water content in the two orchards during the autumn–spring period and to reveal that the CC plot was more efficient to intercept and store rainwater. ERI technique allowed to highlight differences between the systems in the dynamics of water distribution along the soil profile as an effect of the adopted soil management. In particular, such technique consented to easily evaluate the extent of water content of the deepest soil layers (> 1.0 m) evidencing a significant water reserve in the CC plot, convenient for the root system of rainfed olive trees in the driest months. Finally, the soil water content evaluated by means of the ERI technique was used for the short-term application of soil water balance which showed interesting data on the progress of water consumption during the autumn–spring period, particularly useful as operative indications for cover crops management.

The practical application of 2D inversion to marine controlled-source electromagnetic data

An algorithm is presented for the inversion of marine controlled-source electromagnetic (CSEM) data that uses a 2D finite difference (FD) forward driver. This code is demonstrated by inverting a CSEM data set collected at Hydrate Ridge, Oregon, consisting of 25 seafloor sites recording a [Formula: see text] transmission frequency. The sites are located across a bathymetric high, with variations in water depth of [Formula: see text] along the [Formula: see text] profile. To model this complex seafloor bathymetry accurately, the FD grid was designed by careful benchmarking using a different 2D finite element (FE) forward code. A comparison of the FE and FD forward model solutions verifies that no features in the inversion are due to inaccuracies of the FD grid. The inversion includes the local seawater conductivity–depth profile as recorded by the transmitter’s conductivity–temperature-depth gauge, because seawater conductivity is known to have a significant effect on the CSEM responses. An apparent resistivity pseudosection of the CSEM data resembles the 2D inversion in general appearance. However, the inversion provides depth and geometric control of features that cannot be provided by the pseudosection and eliminates artifacts generated from the pseudosection projection.

Identification of the shallow subsurface succession and investigation of the seawater invasion to the Quaternary aquifer at the northern part of El Qaa plain, Southern Sinai, Egypt by transient electromagnetic data

ABSTRACTThe phenomenon of seawater invasion is a very common and widespread environmental problem that is present in the majority of Egyptian coastal aquifers. Saltwater intrusion may occur due to human activities and by natural events such as climate change and sea level rise. El Qaa plain is a structural depression trending NNW‐SSE, parallel to the main rift system of the Gulf of Suez. The Quaternary aquifer in El Qaa plain is the main source for water supply in this region. So, this study has been conducted to shed more light on the description of the subsurface section and marking out the invasion of seawater to this aquifer at the northern part of El Qaa plain. In this concern, transient electromagnetic data were measured at 52 stations arranged along 2 parallel profiles, 10 km apart, trending NE‐SW to cross the major course of the plain and to be approximately perpendicular to the coast line of the Suez Gulf. In addition, information from a nearby borehole was used to confirm the results of transient electromagnetic data interpretation. Two apparent resistivity pseudosections and two geoelectrical cross‐sections were constructed along the selected profiles. Investigation of these sections revealed that four geoelectrical layers could be detected in the shallow subsurface medium at the studied area and the Quaternary aquifer has been affected by seawater invasion at the western and middle parts. Besides, three faults striking approximately NW‐SE parallel to the major track of the plain were inferred from this study. These faults may be controlling the groundwater accumulation and flow in this area.

Fast representation of dipole-dipole geoelectrical data with pseudosections for regional surveys

I propose a fast method for constructing pseudosections of apparent resistivity from geoelectrical data collected for deep studies with continuous polar dipole-dipole arrays. Once a vertical section is fixed, each value of apparent resistivity is assigned to a point on the section and finally pseudosections are obtained by interpolation. This allows the geophysicist to represent a large amount of data in a fast and simple way, to perform a qualitative interpretation and to facilitate the quantitative interpretation.

Application of integrated geoelectrical methods in Khenadarreh (Arak, Iran) graphite deposit exploration

This paper is devoted to application of SP, resistivity and mise-a-la-masse method in Khenadarreh (Arak, Iran) graphite deposit exploration. Mise-a-la-masse method is one of the main geophysical exploration methods used to investigate conductive bodies with outcrops on the surface or in exploratory pits. By integrating mise-a-la-masse results with other geoelectrical methods, such as self-potential and resistivity surveys, useful results can be acquired. Powerful survey design, data processing and interpretation methods can make geoelectrical surveying results more reliable. Here, we applied integrated geoelectrical methods to explore the Khenadarreh-bala graphite deposit in the Sarband region of the Markazi province, Iran. We present a new and simple method for processing and interpreting mise-a-la-masse data, which corrects for the effect of distance from the current electrode in the conductive body. After correction, the residual potential values corresponded with the SP results and the apparent resistivity pseudosections. The geophysical results were confirmed by drill holes in the study area.

Two‐Dimensional Modeling of Complex Resistivity Using Finite Element Method

AbstractWith the wide application of complex resistivity method, searching for precise and rapid forward and inversion algorithms has become the focus of complex resistivity research. In the paper we have developed a finite element algorithm for computing the complex resistivity response of general two‐dimensional models. Triangular element is used in the finite element method (FEM). The simplified boundary condition on infinite boundary is employed to improve the speed of calculation. The modeling takes two steps. Firstly, we compute apparent complex resistivity values for four different frequencies by FEM. Then, apparent Cole‐Cole parameters are derived from these apparent complex resistivity values using a recursive algorithm. The algorithm has been checked with one‐dimensional algorithm. Two typical two‐dimensional polarization models are used by the forward approach. The apparent complex resistivity and the apparent Cole‐Cole parameters maps show that there are distinct anomaly features in the pseudo‐sections of apparent complex resistivity and apparent Cole‐Cole parameters.

O arranjo pólo-dipolo como alternativa ao dipolo-dipolo em levantamentos 2D de eletrorresistividade

This study shows the results of a near-surface investigation conducted to compare pole-dipole with dipole-dipole array. The survey was carried out in a single profile, 40 meters long. In the dipole-dipole array, the inter-electrode spacings used were 2 m and 1 m. In the pole-dipole array, spacing was 1 m. The data collected from the dipole-dipole array were combined into a single section and then compared with the results of the pole-dipole array. The investigation levels used in each array were selected in order to achieve, a priori , the same depth in the subsurface. The corresponding apparent resistivity pseudosection (field data) and the model (obtained by an inversion procedure) were very similar, proving that the pole-dipole array is a possible alternative to the dipole-dipole array, since it provided similar results for resolution and depth of investigation, as shown in the final models.