Horizontal Resistivity Research Articles

Upper mantle flow and crustal deformation patterns beneath the Dangerous Grounds and Borneo where multiple plates converge in South China Sea revealed by 3-D anisotropic magnetotelluric imaging

SUMMARY A large-scale magnetotelluric (MT) study was carried out in offshore Borneo to understand the lithospheric structure in this geologically complex region where three tectonic plates converge and past crustal studies generated long-standing debates. Marine MT data were acquired at 1416 stations with 3 km spacing along 13 regional lines (covering 4677 line-km) with periodicities of 0.1 to 10 000 s. These were inverted in 3-D incorporating electrical anisotropy with cross-gradients constraints between vertical and horizontal resistivities and the results were validated with resistivity well-logs from several exploration wells. The models reveal widespread presence of electrically resistive upper crustal and uppermost mantle layers, each underlain by a laterally varying conductive and anisotropic layer. The geometries of the anisotropic layers suggest large-scale ductile flow, thrusting and folding forming belts of alternating deep roots and thin lithosphere, consistent with multiple underthrusting/subduction. Our results are in agreement with the seismologically detected lithospheric variations in northern Borneo suggesting onshore–offshore continuity and a common lithospheric–asthenospheric origin for the deformation patterns. Over thinned lithosphere, we found consistent low resistivity and high anisotropy anomalies in the mantle above the spatial locations of fast shear-wave bodies imaged by recent seismological workers which we interpret to indicate post-subduction lithospheric–asthenospheric ductile flow in response to multidirectional regional compression. We demonstrate that these zones are spatially correlated with the distribution of mainly Cretaceous ophiolitic rocks and melanges exposed onshore and suggest that serpentinization of mantle peridotite can explain some of the anisotropic conductivity anomalies. The taper zones of these deep anisotropic conductors are also associated with Neogene sub-basins, high thermal gradients, and intrasedimentary magmatic bodies indicating a link between lithospheric thinning and magmatism. We propose that such anomalies could be important pathfinders for geothermal and natural hydrogen systems in the ongoing global drive for carbon-free energy resources.

Detection of shallow underground targets using electrical resistivity tomography and the implications in civil/environmental engineering

Applying the electrical resistivity tomography (ERT) technique in detecting very near-surface targets is quite challenging in geophysical investigation, especially in civil and environmental engineering for adequate planning and designing of structural foundations, contributing to the overall safety and efficiency of construction projects. However, locating the exact position and depth of underground targets such as faults, underground utilities, and contaminants is more challenging. Therefore, this study is aimed at examining the geophysical response of various buried targets and evaluating the ability of ERT to detect buried targets in terms of locations and depths of occurrence in the context of engineering investigation and environmental studies. A laboratory test was conducted on the targets to determine their electrical conductivity and resistivity before burial. The two-dimensional (2D) ERT survey was performed on thirteen targets buried at the site using both Wenner and dipole–dipole (DD) arrays. Both arrays captured the metallic targets with a low electrical resistivity contrast (< 0.1 Ωm) corresponding to the laboratory results. In comparison, the positions of the non-metallic buried targets were found to have a high resistivity contrast greater than 3000 Ωm, matching the laboratory results. The modelled pipes and the car engine block captured by both DD and Wenner arrays on 1.0 m electrode spacing were relatively smeared and poorly resolved in shapes, sizes and geometries, while some were not captured. The electrode spacing of 0.25 m and 0.50 m was explored on undetected targets, which provide a better resolution with sizes and depths compared to 1.0 m spacing but did not produce satisfactory results in some cases. The success and failure of ERT to detect a few targets were discussed alongside the environmental and engineering implications. The effectiveness of both arrays was assessed by their sensitivity in mapping the change in subsurface resistivity values. The DD array shows sensitivity to horizontal variations in resistivity values with low signal. In contrast, the Wenner array shows a good signal strength with a good change in the horizontal and vertical resistivity values. In addition, both arrays show capacity in mapping the geophysical signature of the buried targets and subsurface structures, which has significant application in engineering and environmental investigations.

Detection of low-contrast objects by electrical prospecting methods

Many geological problems cannot be solved by electrical prospecting methods. Often the reason for this is a slight difference in the electrical resistivity of target objects from that of the host medium. At the same time, the use of the transverse magnetic (TM) type field makes it possible to identify low-contrast objects and delineate their boundaries with high accuracy. TM-type prospecting is more efficient since its signal is much more strongly affected by changes in resistivity and other electrodynamic parameters. The article examines one of the difficult cases – a search for kimberlite pipes in Yakutia. Such objects differ very little from the host medium in terms of the horizontal resistivity. Exploration works to search for kimberlite pipes in Yakutia, carried out by conventional electrical prospecting methods, have many problems. The article considers the results of successful TM-type prospecting field surveys where kimberlite pipes stood out very prominently. Also presented are the results of modeling an even more complex situation in which the pipes are located at depths of over 140 m and overlain by traps; there are many objects in the area that create additional anomalies.

APPLICATION OF ELECTRICAL RESISTIVITY METHOD FOR SAFETY EVALUATION OF ASEJIRE DAM IBADAN, SOUTHWESTERN NIGERIA

Dam construction provides economic importance to the environs through, water supply and flood control. Post-construction investigation is however necessary for maintenance stability to avoid irreversible environmental changes. In view of this, horizontal resistivity profiling and vertical electrical sounding (VES) were conducted along the embankments and the downstream of Asejire dam to determine its integrity. The methods used were horizontal resistivity profiling and vertical electrical soundings conducted along the embankments and the downstream. Thirty-one VES and Dipole-Dipole Profiling were occupied along the embankments and downstream using Resistivity meter at 20 m intervals for both the VES and Dipole-Dipole. The Dipole-Dipole data were inverted into 2-D Resistivity Images using DIPPRO™ 4.0 Inversion Software while the VES data were quantitatively interpreted using the partial curve matching technique and Winresist 1.0 Version Software. Results of dipole-dipole image and geo-electric section identified three geo-electric layers; comprising topsoil presumably clayey sand and laterites with resistivity and thickness between 59-760m and 0.7-5.2m respectively. The second layer is weathered layer attributed with clay/clayey sand having resistivity and thickness 18-766m and 3.1-36.7 m. The third layer suspected to be fresh bedrock with resistivity range 121-3672m and 18-766 m thick. The Dipole-Dipole results displayed resistivity less than 500m in the first layer with structural evidence of discontinuous observables, but the underlying sequences displayed no indicative of structural weakness. This significant features play a major role in seepage processes from the dam, therefore lithological composition obtained from these results shows the dam has a good integrity.

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.

A high resolution detection approach combining probe drilling and horizontal cross-hole resistivity tomography to interpret water conducting channels ahead of the tunnel: A case study in Yunnan, China

The geological conditions in Yunnan Province, China, are intricate, and numerous challenges are anticipated during tunnel excavation. The goal of this study was to develop a high resolution detection approach combining probe drilling and horizontal cross-hole resistivity tomography to interpret water conducting channels ahead of the tunnel. A complex section of the Xianglu Mountain tunnel was taken as an example. Drilling data from 10 boreholes are used to establish the initial resistivity model ahead of the tunnel face. The utilization of drilling data as a priori information refines the horizontal cross-borehole resistivity tomography imaging results, providing a more accurate depiction of the subsurface conditions. In practical applications, the use of multiple boreholes for high resolution detection via probe drilling analysis and horizontal cross-hole resistivity tomography is relatively uncommon in complex underground environments and represents an engineering innovation. At the Xianglu Mountain tunnel site, 13 electrical resistivity tomography (ERT) profiles were executed to thoroughly assess the geological conditions. A 3-D resistivity model was constructed via interpolation of this dataset. The 3-D resistivity model not only facilitated precise estimation of the 3-D hydrogeological environment at the Xianglu Mountain tunnel site but also enhanced the accuracy of identifying the water conducting channels by revealing low-resistivity interconnected zones. This approach offers a precise guide for determining the location of the critical water conducting channels and for optimizing grouting treatment strategies based on the characterization, geometry, resistivity spatial distribution, and hydrogeological conditions of the studied zones. The acquired results, which are in good accordance with the reality of the studied area in terms of locating the water conducting channels, can be utilized to develop preventative techniques for water and mud inrush disasters during tunnel construction.

A Universal Data Format for Wellbore Logs

This paper describes a proposed high-specification standard format that is ideally suited for the data management of definitive records of wellbore logs. For this reason, it is a good standard for data exchange between applications. The format is suitable for complex three-dimensional (3D) data, including those generated by deep azimuthal resistivity (DAR) and ultradeep azimuthal resistivity (UDAR) tools, acoustic borehole reflection images, vertical seismic profiles (VSP), borehole imaging tools, multifingered caliper logs, and array data with multiple depths of investigation. It is applicable for use with logging-while-drilling (LWD) and wireline-conveyed logging tools. The format also naturally collapses down when utilized to store simple conventional logs that contain one value per depth in the wellbore. The proposed format provides spatial details of every data point collected by or interpreted from a wellbore-logging tool. The position of each data point is defined by reference back to the measure point of the sonde, which in turn is defined by the wellbore deviation survey and its coordinate reference system (CRS). Each data point in space may have an unrestricted number of parameters. An example might be most likely horizontal and vertical resistivity, maximum value based on uncertainty, minimum value based on uncertainty, and flags indicating the data position with respect to depth of detection (DOD). The new proposed format is so versatile. It is suitable as an Open Group Open Subsurface Data UniverseTM (OSDUTM) standard to store and exchange all data measured by logging tools in a wellbore and can possibly be extended to include all well data (for example, core, cuttings, and more). The proposed format requires a detailed definition so that computer scientists can implement it in applications used for subsurface modeling. The OSDU will also require this detailed definition in order to adopt it as a standard.

Petrophysical interpretation of logging-while-drilling borehole measurements in the presence of electrical anisotropy, mud-filtrate invasion, noise, and well deviation effects

Petrophysical interpretation of logging-while-drilling (LWD) borehole measurements in the presence of electrical anisotropy, mud-filtrate invasion, noise, and well deviation effects remains a challenge in formation evaluation. In high-angle and horizontal (HAHZ) wells penetrating electrically anisotropic sandstones, phase and attenuation apparent resistivity logs often exhibit values larger than resistivity parallel to bedding planes. Consequently, traditional interpretation methods often fail to accurately estimate hydrocarbon saturation when using only the long-spacing phase apparent resistivity log (P40H) in petrophysical calculations. We implement two approaches (analytical and Bayesian) to estimate horizontal and vertical resistivities from apparent resistivity measurements. The first analytical approach is based on the numerical simulation of LWD resistivity measurements using synthetic models under various conditions of electrical anisotropy and well inclination to derive analytical models. The relative error of analytical approximations is less than 6%. In the presence of measurement noise and shoulder-bed effects, we implement a Bayesian method for well-log inversion and uncertainty quantification. The relative error of Bayesian inversion is less than 2%. Estimates of horizontal and vertical resistivities then can be used for petrophysical analysis. A challenging set of synthetic and field examples of HAHZ wells penetrating electrically anisotropic formations is selected for examination and verification of the new methods. For radial lengths of invasion less than 20 cm (8 in), sensitivity analysis indicates that the effects of conductive mud-filtrate invasion on long-spacing resistivity logs are negligible. The long-spacing phase (P40H) and attenuation (A40H) apparent resistivities are used to estimate horizontal and vertical resistivities. In the field examples under study, electrical anisotropy is caused by intercalated laminations of coarse- and fine-grained sandstones; the volumetric concentration of shale is negligible. Considering that Archie’s parameters are direction dependent, sensitivity analysis indicates that the effective value of the saturation exponent parallel to bedding planes is less sensitive to the volumetric concentration of fine-grained sandstone layers compared with the saturation exponent perpendicular to bedding planes. Thus, water saturation of grain-laminated sandstones in HAHZ wells can be estimated using horizontal resistivity with Archie’s parameters that are consistent with petrophysical interpretations performed in vertical wells. Estimates of water saturation in the field examples using horizontal resistivity agree with saturation-height models. Compared with conventional interpretation methods that use P40H as the formation resistivity, the new approach improved the estimation of hydrocarbon saturation by 10%.

A novel evaluation scheme of resistivity anisotropy in near-tight sandstones using conventional geophysical logs: A case study of the Triassic Chang 8 oil layer, Zhenjing area, Ordos Basin

The resistivity anisotropy characteristic of near-tight sandstone reservoirs is important for reservoir evaluation, but it is a challenge to obtain it by using conventional logging curves in vertical boreholes. Thus, a novel evaluation scheme is established. Firstly, in the Chang 8 Formation of Zhenjing area, Ordos Basin, China, horizontal rock samples (parallel to bedding) and vertical rock samples (perpendicular to bedding) are collected for designed rock electricity experiments and analyzed to obtain the corresponding horizontal and vertical Archie's parameters. Secondly, based on the detection characteristics of acoustic logging instrument and the transformation form of the Archie's equation, an estimation method of vertical resistivity is established. Combined with the horizontal resistivity obtained from conventional deep resistivity logs in vertical boreholes, an estimation method of resistivity anisotropy coefficient is proposed. Thirdly, the relationships between resistivity anisotropy coefficients and water saturations and porosities are analyzed using experiment results, and four adjacent vertical wells in the study area are processed using the estimated method. The processed results demonstrate the feasible of the estimation method. In addition, the cross well profile of resistivity anisotropy coefficient in the target formation is characterized, which shows the target formation is slightly resistivity anisotropic and slightly heterogeneous. The relationships between the resistivity anisotropy coefficient and commonly used logs are discussed, which contribute to rapid analyze the resistivity anisotropy of the target formation. Based on the above methods and processes, the novel evaluation scheme of resistivity anisotropy is established, which contributes to quantify and qualitatively analyze the resistivity anisotropy characteristics of formations in and between vertical boreholes. The application results show that it provides an alternative scheme for continuously evaluating the resistivity anisotropy in boreholes and cross well profiles by using conventional logs and experiments.

Study of the anisotropy of frozen rocks at the Khanovei test site (Komi Republic) based on the data of controlled source radio-magnetotelluric soundings and electrical resistivity tomography

The results of a joint inversion of the data of controlled source radio-magnetotelluric soundings (CSRMT) and electrical resistivity tomography (ERT), obtained at the Khanovei permafrost test site near Vorkuta, are presented. The geological section in the work area is composed of Quaternary fluvio-glacial, lacustrine-glacial and alluvial deposits up to 60 m thick, overlying a sequence of sandstones, siltstones and mudstones of Permian age. The CSRMT survey was carried out using a grounded electrical line 480 m long as a source in the frequency range 1-1000 kHz. The results of various inversion options were compared: 1) isotropic one-dimensional (1D) inversion of CSRMT data; 2) isotropic separate two-dimensional (2D) inversion of CSRMT and ERT data; 3) joint anisotropic 1D inversion of CSRMT and ERT data. Based on the results of a joint anisotropic inversion of CSRMT data in the far and intermediate zones of the source and ERT data, a vertical anisotropy associated with horizontal layering of frozen Quaternary clay deposits in the depth range from 2 to 15-20 m was revealed and its parameters (horizontal and vertical resistivity and anisotropy coefficient) were determined. On the geoelectric section obtained from the ERT data, the thickness of the anisotropic layer of frozen clayey deposits is significantly overestimated. The middle part of the section, in the depth range from 15-20 to 60-70 m, is characterized by relatively low resistivity values (tens of ohmmeters). This is consistent with the results of previous studies, which established that the upper part of the Permian sediments is in a thawed state and is fractured and water-saturated. A high-resistivity layer (hundreds of ohmmeters) was revealed at the base of the geoelecric section according to CSRMT data. A significant increase in the resistivity of Permian rocks with depth is confirmed by the data of vertical electrical soundings previously performed in the Khanovey area.

Use of structure-guided 3D controlled-source electromagnetic inversion to map karst features in carbonates in offshore northwest Borneo

Fractured and/or karstic carbonates (FKCs) in the subsurface are commonly associated with significant drilling risk or reservoir effectiveness issues. Understanding the distribution of karsts will help avoid drilling into possible voids and also reassess risks associated with reservoir effectiveness. A 2020 3D seismic acquisition in offshore northwest Borneo provided data for identifying new carbonate prospects and an opportunity to revisit a 2010 marine controlled-source electromagnetic (CSEM) survey that was not designed specifically for FKC mapping but can serve to test new inversion capability for imaging karsted carbonate bodies. A 3D CSEM data were used to detect and characterize the nature (i.e., fractured and/or karsted) of carbonate prospects mapped using 3D seismic data. In one approach, structure tensors computed from these seismic data were used to constrain the 3D CSEM inversion on the Azure cloud platform. In another approach, cross gradients between vertical and horizontal resistivities were used as a structural guide obviating the need for seismic data. The resulting anisotropic resistivity models were validated using resistivity logs from a nearby well; the resistivity results matched the well-logs satisfactorily, providing geologic justification to interpret the presence of karst features in the resistivity volumes. Lateral boundary of the carbonate prospects was mapped based on the resistivity characteristic, in seismic- and nonseismic-guided structure-coupled inversion. Resistivity-depth cross section of the area revealed the presence of resistivity discontinuity at the seismic-inferred boundary of the main carbonate prospect. Also, resistivity depth slices of the models clearly demonstrated that the areal distribution of low and high resistive zones can be mapped and useful deductions such as the presence of FKC and basement characteristics can be made using the 3D inversion of the CSEM data.

Three-dimensional magnetotelluric inversion considering electrical anisotropy with synthetic and real data

Electrical anisotropy increases the non-uniqueness of magnetotelluric (MT) inversions. However, it is still useful to develop MT anisotropic inversion to provide new knowledge for this field. In this study, we developed a three-dimensional (3-D) MT inversion code considering electrical anisotropy. The forward procedure is based on a staggered-grid finite-difference method and solves the governing equation for the electric fields with a direct method. The values of a traditional objective function and its gradient could then be calculated. The inversion procedure was completed with a well-developed subroutine of non-linear conjugate gradient method. Synthetic tests for different isotropic models indicate the validity of this inversion code. Furthermore, we performed 3-D anisotropic inversions for synthetic and real data. The results indicate that vertical resistivity is still indistinguishable in the anisotropic inversion results. In our synthetic tests, it becomes more difficult to recover low-resistivity components of anisotropic anomalies as the depth increases, and the vertical sizes of anisotropic anomalies are reduced, as characterized by a much shallower bottom boundary. The anisotropic strike angle is not as stable as the principal resistivities in the inversion process. Under the same changes in model parameters, the strike angle may cause larger deviations of the responses than the principal horizontal resistivities. With this code, we revisited the field data collected at the northern margin of the Tibetan Plateau. The inversion results indicate a possible anisotropic anomaly exists below the boundary between the plateau and its northern neighbor. This anisotropic anomaly could represent an area of concentration of shear deformation.

Comparison between conventional and high-resolution measurements for unlocking the thin laminated potential reservoir, a case study

In Low resistivity low contrast (LRLC), also known as low resistivity pay (LRP), one of the factors is, the existence of alternating thin sedimentary layers of various rock types in hydrocarbon reservoirs, commonly consist of sandstone, siltstone and claystone and have been widely reported today. This phenomenon causes the potential intervals to be misinterpreted as non-hydrocarbon bearing zone due to its ability to reduce the measured apparent resistivity which result in overestimated water saturation calculation and due to its low contrast, in comparison with water bearing interval, this phenomenon always make well analyst to by passed this interval. As a result, from falsified measured resistivity, the final reserves volume tends to be underestimated. A huge number of studies have proven this kind of interval able to produce oil with very low water-cut therefore this paper would like to present the case study of solving this complex lithology by using sophisticated petrophysical modeling with a combination of Thomas-Stieber plot. Measured vertical (Rv) and horizontal (Rh) resistivity is available for this study and will be used as comparison. In general, this paper will present two case studies for final saturation computation, a) The sand resistivity will be estimated through inversion using measured resistivity anisotropy (Rv - Rh), b) The sand resistivity is reconstruct based on identified volume fraction of clay, whereas the shale resistivity estimated from conventional resistivity (RDEEP). Both of the cases will be calibrated using measured saturation from core by using Dean-Stark technique. Selected interval in this study (X1 zone) is a proven oil interval with a standard volume production rate 698 bbl/day. The final comparison of saturation computed using measured resistivity anisotropy yield a high correlation in contrast, the inversion using conventional resistivity yields less correlation with saturation from core. Nonetheless, by comparing with conventional computed saturation i.e. Waxman Smith, the final saturation of these two cases is significantly increased and unmasked the hydrocarbon-bearing interval. Therefore, case b indicates the best approach to be used whenever the advance measured resistivity anisotropy is unavailable.

Failure properties and stability monitoring of strip and column cemented gangue backfill bodies under uniaxial compression in constructional backfill mining.

The strip and column cemented gangue backfill bodies (CGBBs) are the main supporting components in the design of constructional backfill mining for coal mining, which determines the stability of goaf. Previous researches have mostly focused on the mechanical properties of column CGBB, but the mechanical properties of strip CGBB are still unclear. Herein, the uniaxial compression experiments for strip and column CGBBs were conducted to compare the failure properties. The acoustic emission (AE) and two types of resistivity monitoring were used to monitor the damage evolution. The effect of the length-height ratio on the mechanical characteristic of strip CGBB was analyzed by discrete element simulation. The results show that the strength and peak strain of strip CGBB under uniaxial compression is higher than those of column CGBB and the strip CGBB shows better ductility. The stress of column CGBB decreases significantly faster than that of strip CGBB at the post-peak stage. The strength and ductility of strip CGBB increase with the increase of length-height ratio. The strip CGBB is destroyed from both ends to the middle under uniaxial compression, and the core bearing area is reduced correspondingly. The AE signal evolution of CGBBs under uniaxial compression before the peak stress contains three stages, and the AE signals of strip CGBB at the peak stress will not rise sharply compared with column CGBB. The resistivity monitoring effect of the horizontally symmetrical conductive mesh is better than that of the axial. The horizontal resistivity increases gradually with the increase of stress under uniaxial compression, and increases sharply at the peak stress, and then drops after the peak stress. The damage constitutive models and the stability monitoring models of the CGBBs are established based on the experimental results. This work would be instructive for the design and stability monitoring of CGBB.

Geophysical Assessment of Freshwater Intrusion into Saline Aquifers Beneath Plain Reservoirs

Reservoir leakage can cause a waste of precious water resources and even severe environmental consequences. In this study, we use continuous resistivity profiling to evaluate the leakage problem of the Shuangwangcheng reservoir along the east route of the South-to-North Water Diversion Project. A numerical example was first built to validate the method's effectiveness in a saline aquifer environment. Thirty-five waterborne survey lines were then conducted with a total length of 74 km, and two ground survey lines had a length of 1.27 km each. We evaluated the quality of the overall data with the apparent resistivity of intersection points. Based on ground survey results, the resistivity value larger than 2 Ω.m at the bottom of the reservoir is regarded as leakage areas. Therefore, we divide resistivity survey results into three zones: freshwater reservoir, leakage zone, and saline aquifer. The distribution of freshwater intrusion is evaluated by fence diagram and interpolated horizontal resistivity contour maps. The delineated leakage zone is consistent with the lack of a low permeable loam layer on the north and east parts of the reservoir. The results prove that the waterborne resistivity survey method can efficiently and effectively assess leakage distribution inside a reservoir.

Electrical anisotropy of terrigenous deposits: a brief overview of approaches to its determination from electrical logging data in vertical wells

The article briefly reviews a number of publications on the problem of determining rocks electrical resistivity anisotropy (primarily of sedimentary genesis) using downhole resistivity logs. We provide the information on the main causes of sandy-clay sediments micro- and macroanisotropy according to Russian papers and the history of the model-based approaches to the resistivity anisotropy description according to Russian and foreign articles. We analyze the main approaches to the vertical resistivity estimation. Both new hardware solutions aimed at direct measurement of electromagnetic field components sensitive to anisotropy and methodological techniques for extracting this information from the signals of "conventional" resistivity logging tools are considered according to published studies. The hardware solutions are multicomponent measurements with inclined and perpendicular (to the tool axis) coils, as well as with coaxial toroidal coils. Methodological techniques include combining data measured by tools with different types of medium excitation, for example, induction and galvanic. A number of papers suggest a two-step scheme: the first step is to determine the horizontal resistivity using induction arrays data, and the second step is to determine the vertical resistivity using focused lateral logs. Such schemes are implemented using numerical inversion algorithms and in some cases by inventing transformations of measured signals. Adding anisotropy to the interpretational model is based on a priori data (core studies, microresistivity logs). In the absence of a priori data, anisotropy is added when it is impossible to reconcile the parameters of isotropic models built independently based on logs with different type of excitation. A special focus of the review is on the development of the theory of unfocused lateral logging (BKZ) method by Soviet and Russian scientists and on the history of research on the electrical anisotropy influence on the signals of gradient probes. Due to the widespread use of the method in the USSR, this topic is presented in a number of papers revealing the results of anisotropic geoelectric models theoretical studies, numerical and physical modeling of BKZ logs and methodological developments for their interpretation.

Electrical anisotropy of terrigenous deposits: a brief overview of approaches to its determination from electrical logging data in vertical wells

The article briefly reviews a number of publications on the problem of determining rocks electrical resistivity anisotropy (primarily of sedimentary genesis) using downhole resistivity logs. We provide the information on the main causes of sandy-clay sediments micro- and macroanisotropy according to Russian papers and the history of the model-based approaches to the resistivity anisotropy description according to Russian and foreign articles. We analyze the main approaches to the vertical resistivity estimation. Both new hardware solutions aimed at direct measurement of electromagnetic field components sensitive to anisotropy and methodological techniques for extracting this information from the signals of "conventional" resistivity logging tools are considered according to published studies. The hardware solutions are multicomponent measurements with inclined and perpendicular (to the tool axis) coils, as well as with coaxial toroidal coils. Methodological techniques include combining data measured by tools with different types of medium excitation, for example, induction and galvanic. A number of papers suggest a two-step scheme: the first step is to determine the horizontal resistivity using induction arrays data, and the second step is to determine the vertical resistivity using focused lateral logs. Such schemes are implemented using numerical inversion algorithms and in some cases by inventing transformations of measured signals. Adding anisotropy to the interpretational model is based on a priori data (core studies, microresistivity logs). In the absence of a priori data, anisotropy is added when it is impossible to reconcile the parameters of isotropic models built independently based on logs with different type of excitation. A special focus of the review is on the development of the theory of unfocused lateral logging (BKZ) method by Soviet and Russian scientists and on the history of research on the electrical anisotropy influence on the signals of gradient probes. Due to the widespread use of the method in the USSR, this topic is presented in a number of papers revealing the results of anisotropic geoelectric models theoretical studies, numerical and physical modeling of BKZ logs and methodological developments for their interpretation.

Frequency domain electromagnetic and electrical resistivity geophysical investigation of tar sands deposits in the Ijebu Waterside area, Eastern Dahomey Basin, southwestern Nigeria

The use of electromagnetic conductivity and electrical resistivity geophysical techniques has been employed in this study to investigate the occurrence, thickness, and spatial distribution of bitumen deposit around Makun-Omi in the Ijebu Waterside area of southwestern Nigeria. Subsurface apparent conductivity distribution data obtained using Geonic 34-3 equipment along fifteen traverses which sounded from different depths of 7.5, 15, 30 and 60 m at inter-coil separation of 10, 20 and 40 m in vertical and horizontal coupling modes and ground resistivity distribution measurements, obtained using Geopulse Tiger Allied resistivity meter at fourteen Vertical Electrical Sounding (VES) stations, were processed and employed to characterize the subsurface in terms of tar sands distribution. The conductivity and resistivity distribution in the form of a 3D earth model, iso-depth maps, 2D sections generated from the processed conductivity and resistivity data indicate relatively low conductivity &lt;20 mS/m and high resistivity &gt;1300 Ωm values around tar/bitumen impregnated sands which mostly occurred in the southern part of the study area. Clearly defined conductivity and resistivity anomalies which delineate the lateral and vertical occurrence of tar impregnated sands underscore the efficacy of integrating electromagnetic and electrical resistivity geophysical techniques to identify occurrence of economic deposits of tar sands in parts of southwestern Nigeria.

Investigation of Dam Integrity from Electrical Resistivity Methods: A Case of Erelu Dam, Southwestern Nigeria

DOI:10.17014/ijog.8.2.265-274Erelu dam is geologically within the Precambrian Basement Complex of southwestern Nigeria, which serves a principal source of water supply to an increasing population; currently at about 779,318. In an attempt to investigate the integrity of the dam, horizontal resistivity profiling (HRP) of dipole-dipole and Schlumberger’s vertical electrical sounding (VES) approaches were explored to delineate the lithologic layers, facies, and geologic structures in the subsurface of the dam. The HRP revealed changes in subsurface conductivity that depicts lateritic hard pan and saturated rock blankets at shallow depth. The vertical geo-electric section delineated predominant four lithologic units. The regolith is 7 m thick and constitutes the vertical rock fill impervious cores of the dam, which is underlained by saprolitic and fresh bedrock. The region of weathered rock depression coincided the shallow saturated anomalous zone and showed surface manifestation of fractured or unconsolidated terrain within the embankment. The fracture serves as conduit for seepage which could be responsible for possible loss in the reservoir water and increases its susceptibility to failure. It is recommended to keep monitoring the seepage with reservoir levels, by periodical geophysical and geotechnical measurements for the two (dry and wet) seasons.

First LWD Co-Located Antenna Sensors for Real-Time Anisotropy and Dip Angle Determination, Yielding Better Look-Ahead Detection

Electromagnetic (EM) resistivity tools measure the electrical properties of downhole formations that are critical in determining the hydrocarbon saturation of a reservoir. In complex and heterogeneous reservoirs, both horizontal and vertical formation resistivities are required to obtain an accurate hydrocarbon saturation. For decades, wireline multicomponent induction type measurements have provided reliable determination of formation anisotropy, structural dip, and dip azimuth in wells with any orientation relative to the bedding planes. Logging-while-drilling (LWD) multi-array propagation resistivity tools have also demonstrated similar capability in deviated wells where the relative dip angle is between 45° and 90°. However, measuring anisotropy and dip in wells with a low relative dip angle still poses difficulties for LWD propagation resistivity systems because of the antenna structures employed. This paper describes the development of a new LWD EM sensor equipped with an innovative, co-located, tilted antenna structure. The tool, along with a unique processing scheme, enables the determination of horizontal and vertical resistivity as well as the dip angle and the azimuth of the formation based on an assumption of transversely isotropic (TI) formation models (Graciet and Shen, 1998) while drilling in real time. The co-located sensor design is capable of acquiring multicomponent signals that are sensitive to formation anisotropy and structural dip in wells at any orientation. Modeling studies and several field trials have proven that the design concept can detect these formation properties at any arbitrary wellbore deviation. This paper presents test results from the new technology, together with reference measurements from azimuthally compensated LWD and fully triaxial wireline resistivity measurements. A good comparison is observed in these trials, providing an independent verification of the tool performance. The azimuthal measurements of the new sensors allow for determining formation anisotropy and dip at any wellbore deviation (Bittar et al., 2011b; Bittar et al., 2012), as well as providing 360° azimuthal resistivity and geosignals and allowing a three-dimensional (3D) resistivity mapping technique for real-time decisions. Integrating the co-located antennas with deep-reading antennas in a near-bit collar further provides both anisotropy measurements and ultradeep signals very close to the bit and enhances look-ahead detection ranges for LWD applications.