Electrical Tomography Data Research Articles

Image Restoration of Electrical Well Logging Based on Fourier Convolution

Imaging logging is an important technical means in logging evaluation of complex reservoirs. Through imaging logging, a two-dimensional image of the resistivity distribution around the well can be obtained, which can be used to evaluate the development of well wall fractures and caves and the formation sedimentary structure. However, due to the characteristics of resistivity imaging logging instruments, blank strips will appear on the resistivity logging image, which increases the difficulty of computer processing of electrical imaging data. The current image repair method and the existing neural network image repair method are not effective enough when the part to be filled is large. Therefore, there is an urgent need for an intelligent repair method based on deep learning. Based on a fast Fourier convolution-based imaging logging image blank strip filling network, this paper constructs the electrical imaging logging images of the southwest oil and gas field as a data set, and trains a new deep learning algorithm for intelligent filling of blank strips in imaging logging images based on fast Fourier convolution. The time of various algorithms is compared. The results show that the algorithm has a better repair effect in imaging logging images with large strip widths, and the repair efficiency is significantly improved. This method can realize the rapid, accurate and intelligent repair of blank strips in imaging logging, achieve the rapid generation of full-borehole images, and solve the difficulties in obtaining full-borehole images.

Geoelectric model of the exposure of basement rocks to the surface in the Uimon depression of Gorny Altay based on three-dimensional numerical modeling

In the western part of the Uimon depression of the Altai Mountains, during the 2023 field season, additional measurements were carried out using electrical tomography along three profiles to clarify the structure of the area with basement rocks exposed to the surface. During the study, the results of three-dimensional inversion were obtained taking into account new electrical tomography data, and a refined geoelectric model of the study area was constructed. To verify the resulting model, 2D and 3D numerical modeling was performed with the help of which the geometric dimensions and electrical resistivity of bedrock outcrops were justified. The influence of the daytime surface topography on geoelectric sections in mountainous regions with significant differences in elevation on the earth's surface has been shown. Determining the structure of the depressions is necessary to understand the stages of neotectonic formation of the Uimon depression and the entire region, as well as to assess the seismic hazard of the Altai Mountains. In the course of further research, it is possible to trace the object to a greater depth (~100 m) using a three-electrode electrical tomography installation or alternative equipment with an increased size of the measuring-generator cable.

An intelligent identification method of interlayers in deep clastic rock – An example of Donghe Sandstone in Hade Oilfield, Tarim Basin

The identification of interlayer types is the key to improve the geological model to study the characteristics of oil and water distribution. The Donghe Sandstone in Hade Oilfield is used as the research object to calibrate the interlayer type and location by means of the outcrop, core, conventional logging curves and electrical imaging data. However, the logging response characteristics of different types of interlayers are similar, which is difficult to be accurately characterized by the conventional method. Therefore, a Random Forest (RF) intelligent method based on the Ensemble Empirical Mode Decomposition (EEMD) and Hilbert Huang Transform (HHT) is proposed to identify the type of intercalation layer. The model consists of two parts: firstly, EEMD-HHT is used to automatically identify the location of the interlayer, secondly, RF is used to classify the interlayer type. The results show that the EEMD-HHT-RF model corresponds to the results of core identification of interlayer, which can achieve rapid identification and accurate prediction in the block.

Structural features of the upper part of the section of the Gorniy Altai Uimon depression according to electrotomography data using 3D software

Determining the structure of intermountain depressions in seismically active regions at different depths is important for understanding the stages of neotectonic formation of the region and seismic hazard assessment. On the territory of the Uimon depression of the Altai Mountains in different years, measurements were carried out by deep methods of electrical exploration, such as sounding by the formation of the field and vertical electrical soundings. However, to clarify the structure of the upper part of the section, where bedrock outcrops are observed on the day surface, it is necessary to use shallow methods, for example, electrical tomography (ET). The paper discusses the results of ET data interpretation using 2D and 3D inversion when studying basement outcrops in the western part of the basin. Taking into account a priori information and the obtained models, the geological structure of the object of study has been refined. Based on the results of the interpretation of the data of electrotomography using programs of two- and three-dimensional inversion, the geoelectric structure of one of the outcrops of the basement rocks on the day surface has been determined. The immersion of the studied object in the western direction towards the adjacent outcrop of rocks of the Baratal Formation of smaller sizes located in the immediate vicinity has been established. It can be assumed that these two exits are connected at a depth of 15–20 m and, most likely, are one structure.

Evaluation method for in-situ stress orientation using wave anisotropy and electrical imaging logging data: A case study from the Longmaxi Formation in Yongchuan area, Sichuan Basin

In-situ stress orientation is a key parameter in studying oil and gas migration and accumulation, analyzing the stability of borehole wall during drilling, carrying out horizontal well design, fracturing reconstruction and well pattern layout in water injection development. Accurately predicting the in-situ stress orientation of reservoirs is of utmost importance for oil and gas exploration and development. It is the most effective and commonly used method to evaluate the in-situ orientation by using the induced fracture and breakout fracture of imaging logging data. However, in cases where effective induced fracture or breakout fracture cannot be formed during drilling, the in-situ orientation cannot be evaluated by imaging data alone. Taking the shale reservoir of Longmaxi Formation in Yongchuan area as an example, this paper presents a novel method for evaluating in-situ orientation. Firstly, the in-situ orientation is determined relative to the core sample marker line through the wave velocity anisotropy experiment. Secondly, the core sample is rotated to obtain an unfolded image of the core surface. The orientation of the core marker line in the electrical imaging is then determined by comparing the core image with the electrical imaging logging image. By doing so, the true orientation of the core can be obtained. This method aligns well with the in-situ stress orientation determined through induced fracture evaluation, addressing the limitation of electrical imaging data in evaluating in-situ stress orientation in plastic strata.

Evaluation of volcanic reservoir heterogeneity in eastern sag of Liaohe Basin based on electrical image logs

The volcanic rocks of the Es3 (third member of the Shahejie) formation in the eastern sag of the Liaohe Basin are composed of basalt and trachyte, and the reservoir characteristics indicate complex reservoir space and strong heterogeneity. The latter is closely related to oil and gas reserves, and is important for evaluating reservoirs and predicting their oil and gas production. Electrical imaging data are obtained during such analysis and are subsequently transformed into a porosity map to obtain the porosity spectra and bin distribution. The results can then be used to qualitatively study the heterogeneity of the reservoir. In the present study, however, quantitative analysis is applied to evaluate the reservoir diversity of these volcanic rocks by calculating four heterogeneity parameters: the sorting index and the porosity variation coefficient, porosity ratio, and the heterogeneity coefficient. Combining the porosity spectra and porosity bin features enables the reservoir heterogeneity to be divided into three categories. The first is weak heterogeneity, which has both a sorting index and variation coefficient of <0.15, a porosity ratio of <2, and a porosity heterogeneity coefficient of <1.5. The second is moderate heterogeneity, with both a sorting index and porosity variation coefficient of 0.15–0.25, a porosity ratio of 2–4, and a porosity heterogeneity coefficient of 1.5–2.0. The third is strong heterogeneity, with both a sorting index and porosity variation coefficient of >0.25, a porosity ratio of >4, and a porosity heterogeneity coefficient of >2.0. In these three cases, the porosity spectra are dominated by unimodal distributions, bimodal distributions with tails, or multimodal distributions with tails, respectively. The heterogeneity of the volcanic reservoir was controlled by the lithology. The trachyte, from which most oil and gas is presently produced, is characterized mainly by weak heterogeneity, and the basalt is characterized mostly by moderate or strong heterogeneity. Thus, exploration conducted in the basalt layer should consider reservoirs with moderate heterogeneity and large pore sizes.

AN APPROACH TO PROCESSING ELECTROTOMOGRAPHY DATA FOR ESTIMATING THE VOLUMES OF SUBSTANCES IN TAILINGS

Methodical recommendations for processing the data of electrotomography are given. Taking into account a priori information, in particular the results of the study of pits, can be carried out by carrying out a limited inversion of the ET data, while it is possible to limit either the resistance of some layers of the model or the position of the boundaries of some layers [1]. In the software used for processing electrical tomography data Res2dinv [2], it is possible to set the boundary of the assumed layer or to limit the resistance of the model section (to introduce local inhomogeneities with a given resistance). But it is difficult to fix the resistance of a particular layer with the available software tools. The proposed approach makes it possible to take into account the parameters of the model built from the pitting data using a preliminary one-dimensional inversion of the electrical tomography data in the Ip2win [1] program, followed by the export of the obtained assumed layer boundaries to the Res2dinv software for carrying out a limited two-dimensional inversion.

Integrated Geophysical Investigation in Delineating Extent of Pollution Caused by Poultry Waste at Ilora Area, Southwestern Nigeria

An integrated VLF-Electromagnetic and Electrical Resistivity survey was carried out in Odo Abata, Ilora area of Southwestern Nigeria in order to investigate the possible extent of groundwater contamination caused by an indiscriminate accumulation of poultry fecal droppings in a seasonal stream. The poultry fecal accumulation solidifies during the dry season and the percolating water plays a significant role in leachate generation and infiltration during the wet season. When in contact with the subsurface aquifer, the leachate forms a plume which diffuses into the pore spaces of aquifer with high rate of infiltration. Field data were collected using ABEM WADI VLF- EM equipment and campus Tigre Terrmeter. A total of eight VLF profiles, four electrical imaging profiles and two VES points were obtained in the vicinity of the study area. Control profiles and VES point were obtained at 500 m away from the study area. Field data were interpreted using KHFFilt program version 1.1a for VLF, Dipro software for electrical imaging and Winresist version 1.0 for VES data. Interpretation of VLF data showed conductive near surface layers and some fractures at depth. Electrical imaging data however revealed a near surface very low resistivity region which is due to the surface fecal accumulation. VES results showed that the overburden is clayey, and the basement is fractured. Depth to basement ranges from 9.3 – 12.8 m. Poultry fecal accumulation with leachate is mostly restricted to the topmost 6 m of the soil and if not controlled, leachate infiltration may reach the shallow basement with time and contaminate the groundwater system of Odo Abata area. Keywords: Very Low Frequency Electromagnetic (VLF-EM), Electrical Resistivity, Contamination, Leachate, Fracture DOI : 10.7176/JEES/9-10-09 Publication date :October 31 st 2019

Structural and sedimentological analysis of the Pliocene–Late Miocene deposits from the Romanian Black Sea offshore based on borehole electrical imaging

This study presents the results of the structural and sedimentological interpretation of borehole electrical imaging data recorded in a gas exploration well from the Romanian offshore area of western Black Sea. The well intercepted on the investigated interval Pliocene to Late Miocene deposits, including a gas reservoir hosted in Early Pliocene shallow marine sands and silts. A total of 824 features were picked on the electrical image, such as bedding, soft sediment deformations, open (conductive) fractures and closed (resistive) fractures. An analysis of the dip angles and azimuths data identified four structural domains within the study interval: two are quasi-horizontal, one shows slightly inclined bedding dips and one is dominantly deformed (slump features and possibly sediment creep). The open fractures show a preferential WNW–ESE strike, whereas the closed fractures show, besides a WNW–ESE strike, mostly ENE–WSW to N–S strikes. The opposite directions of the two fracture sets suggest a relationship to the regional tectonic stress, with open fractures being nearly parallel to maximum horizontal stressSHmaxdirection and closed fractures being nearly normal toSHmaxdirection. The variable bioturbation intensity observed on the electrical image may reflect salinity fluctuations in the Black Sea basin.

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.

Structural characterization of the metamorphic basement from North Dobrogean Promontory (Romania) using geophysical well logging

Metamorphic processes, leading to mineralogical and structural changes of the rocks in response to physical (pressure, temperature) and chemical conditions, can be associated with the development of sinmetamorphic or post-metamorphic fractures. The post-metamorphic ones are directly related to tectonic stress. In addition, tectonic stress may cause the reorientation of minerals on a direction perpendicular to the stress direction, generating foliations (schistosity). This paper presents a structural analysis of the pre-Alpine metamorphic basement and its relations with the sedimentary cover by using geophysical data recorded in an exploration well located in the North Dobrogean Promontory (Romania). The analysis was based on quad-combo wireline logs, spectral gamma ray, sonic cross-dipole and borehole electrical imaging data. The imaging analysis allowed the identification and characterization of metamorphic foliations, sedimentary bedding, natural fractures and the determination of tectonic stress orientation. A brittleness index was computed by means of elastic parameters derived from density and sonic compressional and shear logs. Also, a fracture intensity characterization by using fracture area per volume of rock was conducted. The integration of geophysical well loging with mud logging and drilling data allowed us to carry out a detailed analysis of the metamorphic basement in the studied area.

TWO-PHASE FLOW REGIME THREE-DIMENSONAL VISUALIZATION USING ELECTRICAL CAPACITANCE TOMOGRAPHY – ALGORITHMS AND SOFTWARE

This paper presents the software for comprehensive processing and visualization of 2D and 3D electrical tomography data. The system name as TomoKIS Studio has been developed in the frame of DENIDIA international research project and has been improved in the frame of Polish Ministry of Science and Higher Education Project no 4664/B/T02/2010/38. This software is worldwide unique because it simultaneously integrates the process of tomographic data acquisition, numerical FEM modeling and tomographic images reconstruction. The software can be adapted to specific industrial applications, particularly to monitoring and diagnosis of two-phase flows. The software architecture is composed of independent modules. Their combination offers calibration, configuration and full-duplex communication with any tomographic acquisition system with known and open communication protocol. The other major features are: online data acquisition and processing, online and offline 2D/3D images linear and nonlinear reconstruction and visualization as well as raw data and tomograms processing. Another important ability is 2D/3D ECT sensor construction using FEM modeling. The presented software is supported with the multi-core GPU technology and parallel computing using Nvidia CUDA technology.

Multiple geophysical surveys for old landfill monitoring in Singapore.

One-dimensional boring presents limitations on mapping the refuse profile in old landfills owning to waste heterogeneity. Electrical imaging (EI) and multiple-analysis of surface wave (MASW) were hereby deployed at an old dumping ground in Singapore to explore the subsurface in relation to geotechnical analysis. MASW estimated the refuse boundary with a higher precision as compared to EI, due to its endurance for moisture variation. EI and MASW transection profiles suggested spots of interest, e.g., refuse pockets and leachate mounds. 3D inversion of EI and MASW data further illustrated the transformation dynamics derived by natural attenuation, for instance the preferential infiltration pathway. Comparison of geophysical surveys at different years uncovered the subterranean landfill conditions, indicating strong impacts induced by aging, precipitation, and settlement. This study may shed light on a characterization framework of old landfills via combined geophysical models, thriving landfill knowledge with a higher creditability.

Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics.

Catastrophic collapses of the flanks of stratovolcanoes constitute a major hazard threatening numerous lives in many countries. Although many such collapses occurred following the ascent of magma to the surface, many are not associated with magmatic reawakening but are triggered by a combination of forcing agents such as pore-fluid pressurization and/or mechanical weakening of the volcanic edifice often located above a low-strength detachment plane. The volume of altered rock available for collapse, the dynamics of the hydrothermal fluid reservoir and the geometry of incipient collapse failure planes are key parameters for edifice stability analysis and modelling that remain essentially hidden to current volcano monitoring techniques. Here we derive a high-resolution, three-dimensional electrical conductivity model of the La Soufrière de Guadeloupe volcano from extensive electrical tomography data. We identify several highly conductive regions in the lava dome that are associated to fluid saturated host-rock and preferential flow of highly acid hot fluids within the dome. We interpret this model together with the existing wealth of geological and geochemical data on the volcano to demonstrate the influence of the hydrothermal system dynamics on the hazards associated to collapse-prone altered volcanic edifices.

Resolution of electrical imaging of fluid movement in landfills

This paper investigates the efficacy of time-lapse electrical imaging using surface electrodes to monitor the movement of leachate within a landfill. A laboratory-based study allowed direct comparison between observed physical dewatering and time-lapse electrical imaging data. In combination with a forward modelling study this demonstrated the limitations and advantages of this geophysical technique in terms of its applicability, resolution and complementarity to conventional dewatering monitoring techniques. It also confirmed the nature, likely magnitude and impact of artefacts created by the resistivity inversion process and highlighted the potential for misinterpretation of results. An 18-month study provided field results comparing hydrogeological and imaging data during the dewatering of a real landfill system. Insights gained from the laboratory study and forward modelling exercise greatly enhanced the interpretation of the field data, enabling the method to be applied with greater confidence in the future. The need to combine a forward modelling exercise with any interpretation of resistivity data is clearly demonstrated.

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

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

Identification of the Saline Zone in a Coastal Aquifer Using Electrical Tomography Data and Simulation

A novel approach that borrows methods commonly used in environmental geophysics was developed for obtaining the estimates of the aquifer parameters. Specifically, estimates of hydraulic conductivity were obtained from field measurements of the electrical resistivity while accounting for the karsticity of the geological formations in the area of study. Geophysically determined hydraulic conductivity estimates were introduced to a 3-D groundwater numerical simulator (Princeton Transport Code - PTC) to compute the hydraulic heads distribution of the area of interest. The calibration of the numerical model was obtained matching the hydraulic-heads predicted by the simulator with the hydraulic-heads measured at specific well locations. Simulated hydraulic-heads were used with the Chyben-Herzberg equation to approximate the position of the sharp freshwater/saltwater interface of the base of the water supply aquifer. The existence of the faults impacts the groundwater flow and the distribution of the freshwater/saltwater interface.

Level set reconstruction of conductivity and permittivity from boundary electrical measurements using experimental data

A shape reconstruction method for electrical resistance and capacitance tomography is presented using a level set formulation. In this shape reconstruction approach, the conductivity (or permittivity) values of the inhomogeneous background and the obstacles are assumed to be (approximately) known, but the number, sizes, shapes, and locations of these obstacles have to be recovered from the data. A key point in this shape identification technique is to represent geometrical boundaries of the obstacles by using a level set function. This representation of the shapes has the advantage that the level set function automatically handles the splitting or merging of the objects during the reconstruction. Another key point of the algorithm is to solve the inverse problem of finding the interfaces between two materials using a narrow-band method, which not only decreases the number of unknowns and therefore the computational cost of the inversion, but also tends to improve the condition number of the discrete inverse problem compared to pixel (voxel)-based image reconstruction. Level set shape reconstruction results shown in this article are some of the first ones using experimental electrical tomography data. The experimental results also show some improvements in image quality compared with the pixel-based image reconstruction. The proposed technique is applied to 2D resistance and capacitance tomography for both simulated and experimental data. In addition, a full 3D inversion is performed on simulated 3D resistance tomography data.

Joint Interpretation of Geophysical Data for Archaeology: A Case Study

Interpret-joints within geophysical data recorded in a complex area where ruins do not outcrop and only earthenware remains within the surficial layer are present. The study area, located in central Italy, consists of Roman, medieval and modern ruins that are included in reworked sediments. The geology is formed by inhomogeneous alluvial sediments (sand and gravel) several meters thick with diamagnetic character. To reduce the ambiguity in the subsurface reconstruction, a joint interpretation of georadar, magnetic and electrical tomography data was performed. The georadar was chosen to reconstruct detailed subsurface features, the electrical tomography to distinguish resistive bodies (stones, voids, etc.) from conductive (cavities filled by clay) and, because of the diamagnetic character of in situ sediments, the magnetic method was chosen to detect the earthenware ruins. The geophysical data were controlled by excavation, which detected silos of 1 m in diameter and a concrete layer at a few centimeters from the topographic surface. Time slices in the georadar data allowed us to detect the silos and to define the lateral edge of the concrete layer. Silos were also indirectly detected by the magnetic data because of the earthenware present in the filling sediments. Electrical tomography detected the concrete layer and an ancient anthropogenic surface of few centimeter depths. The study demonstrates that, because the geophysical methods are based on different physical characteristics, they can have different resolution and therefore detect different bodies.

A comparison of electrical and electromagnetic methods for the detection of hydraulic pathways in a fractured rock aquifer, Clare Valley, South Australia

Within fractured rock, the irregular and often unpredictable distribution and geometry of hydraulically conductive fractures produces large spatial variations in bore yield and groundwater quality. As fractures act as conduits for flow of both groundwater and electrical charge, methods which can efficiently detect the distribution of electrical pathways can be used to infer characteristics of significant hydrological parameters. This study compares the capabilities and limitations of electrical data obtained from direct current (DC) and electromagnetic (EM) surface azimuthal measurements, and from DC borehole-to-surface and cross-borehole measurements, for the interpretation of major hydrological structures in Clare Valley, South Australia. Electrical and EM surface methods are limited by poor depth sensitivity and the presence of conductive overburden, but provide useful tools for determining directional variations in resistivity at sites lacking bedrock exposure and boreholes. Application of borehole-to-surface methods yielded a better-resolved interpretation of sub-vertical fracture strike and was useful in identifying lateral variations in bedrock heterogeneity. Improved flexibility and sensitivity to measurements at depth permitted cross-borehole electrical tomography data to be used in reconstructing the spatial distribution of sub-horizontal, laterally extensive, electrically conductive zones. While the technique is restricted to small-scale sites with multiple boreholes, inferences can be made on fluid connections over a much larger regional scale. It is important to note, however, that while electrical methods provide valuable information about in-situ hydraulic pathways, they do not provide a complete hydraulic characterisation. Such a task requires integration of surface and borehole geophysics, geologic mapping, sampling and pumping tests of wells with packed-off intervals.