Variations In Apparent Resistivity Research Articles

Apparent Resistivity Variation Imaging Method Based on Magnetic Field Gradient by NGO-LSSVM for the Ground-Airborne Frequency-Domain Electromagnetic Method

Apparent resistivity imaging is a typical rapid imaging method in the ground-airborne frequency-domain electromagnetic method. At present, the apparent resistivity is typically calculated by the measured magnetic field, however, this imaging method exhibits limited capability in recognizing the center of three-dimensional anomalies. Therefore, this paper proposed the calculation of apparent resistivity using magnetic field gradients. To solve the problem of random artificial anomalies that existed during the calculated process, this paper presents a hybrid least square support vector machine (LSSVM) and Northern Goshawk optimization (NGO) to establish the mapping relationship between the magnetic field gradient and apparent resistivity variation. This approach enables accurate prediction of apparent resistivity variations and effectively resolves the challenge of correcting background resistivity. Furthermore, three typical theoretical models and field examples are used to predict the apparent resistivity variation, the imaging results demonstrate that the proposed NGO-LSSVM algorithm is a feasible and efficient tool for predicting the apparent resistivity variation with high accuracy. This study provides a novel and efficient imaging method, which facilitates the application of ground-airborne frequency-domain electromagnetics for high-resolution detection requirements, such as mineral exploration.

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.

Upgrading of a Technique for Visual Comparing for the Results of Azimuthal Magnetotelluric Monitoring with the Seismic Events Parameters

The paper presents a practical implementation of the task of visualizing additional geophysical information for analyzing the results of azimuthal magnetotelluric (MT) monitoring. Since some patterns of a sharp change in the sign of relative apparent resistivity variations (Δρ) for MT monitoring points depending on ongoing seismic events, as well as lunar-solar tides, were found in previous studies, it became necessary to give illustrative examples proving such cases. For this, we propose to improve the technique for constructing additional graphs containing such parameters of seismic events as: source depth, distance between the source and the observation point, azimuth and energy class, in accordance with the observation time. All parameters are obtained from published seismic catalogs. A method for displaying pseudosections Δρ, combined with a hypocenter distribution plot, in Surfer program is shown. An example of a graph containing simultaneously information about the distance and azimuth of the event relative to the monitoring point, in the form of rotated arrows, is given. Such graphical constructions are maximally automated, and allow the interpreter to confirm the arguments in favor of the spatial-temporal dependence of the variations of electromagnetic parameters on the events occurring in the observed earth's interior.

Principle of a low-frequency transient electromagnetic radar system and its application in the detection of underground pipelines and voids

The road collapse caused by tunnel construction was highly related to the leakage of the underground pipeline and the presence of voids. Thus, underground pipelines and voids should be clearly surveyed to prevent road collapse accident. The transient electromagnetic radar (TER) was developed and applied to the detection of underground pipeline and voids in this study. Variation of apparent resistivity in different materials underground pipelines were studied and summarized by the validation test. The application effect of TER in the inspection and recognition of different pipelines was verified by urban road detection before the subway station construction. Meanwhile, the application effect of TER in the detection of voids and loose soil area under the urban road subgrade was explored and analyzed in an urban road. The results show that: (i) different pipeline could be detected and distinguished by the variation of apparent resistivity; (ii) information of position, depth, size, and material of the underground pipelines and pipe trench could be discovered and located by TER; (iii) due to the difference of apparent resistivity, voids and loose soil area under the subgrade of the road could be detected and located by abnormal reflection signal of the lower apparent resistivity by TER. Thus, to prevent the road collapse effectively, the research and development of TER in the urban road detection provided a new idea for the inspection of underground pipeline and voids.

Stress-Induced Apparent Resistivity Variations at the Kalpin Observatory and the Correlation with the 2020 Mw 6.0 Jiashi Earthquake

Stress may induce apparent resistivity changes. Clarifying the deformation process of the source media is critical for determining the correlations between resistivity variations and earthquake occurrence. In this study, the stress state of a medium was analyzed by integrating GPS measurements, the spatiotemporal evolution of the load/unload response ratio (LURR), geochemical monitoring, and synchronous apparent resistivity changes preceding the 2020 Mw 6.0 Jiashi earthquake. The medium hosting the Kalpin Observatory underwent elastic deformation before 2019, and the synchronous decreases in the E–W and N–S apparent resistivities from 2015 can be attributed to N–S-dominated compressive stress. The microdamage stage occurred in 2019, with subsequent E–W apparent resistivity variation amplitudes that were ~0.4 Ωm higher than those in previous years. This difference is a result of microdamage to the medium owing to tensile stress during the seismogenic process. The spatiotemporal evolution of the LURR and gas seepage monitoring data also indicate that the medium was damaged prior to the earthquake. Variations in the apparent resistivity measured at the Kalpin Observatory indicate that the medium underwent elastic deformation, followed by microdamage, until stress triggered the earthquake.

Opportunistic magnetotelluric transects from CSEM surveys in the Barents Sea

Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.

A geoelectric study of aquifers in the Essaouira coastal region, Morocco

A geophysical study carried out in the framework of the water resources recognition within the Essaouira coastal area (Morocco) enabled to outline the spatial distribution of the Plio-Pleistocene and Cretaceous formations. It helped to determine the thickness and extension of the main aquifer used for irrigation purpose. The overall aim of this study is to highlight the hydrogeological structures of Essaouira Basin.A total of 45 vertical electrical soundings (VES) were measured using linear Schlumberger array configuration with a maximum half-length (AB/2) of 1000 m along eight sections of 126 km total length. The number of soundings constituting the eight VES profiles depends on the topography and the obstacles encountered. The VES data were processed and interpreted. According to the thematic maps of the study area, the results indicate the existence of three types of VES with apparent resistivity values ranging from 44 to 320 Ω m. In relationship with wells and mechanical drillings existing in the area, the analysis of the results obtained made it possible to derive from the quantitative and qualitative maps the different apparent resistivity variations of the aquifer and aquiclude geological layers.The careful examination of these maps shows that the region around the Qsob River is of interest from a hydrogeological point of view. It is represented by a significant thick layer given the large extension of the resistive Plio-Pleistocene and Cretaceous formations. Geophysical studies show that this region is partially protected from marine intrusion given its location between Tidzi Diapir and hidden diapir from Essaouira which little plays the role of barrier. It is crossed by the Qsob River which lends itself as the main source of both these layers. The electrical discontinuities emerging from the superposition of the different maps, E-W, NE-SW and NNE-SSW oriented, are abundant and dense in the north Haha region (Qsob River). Generally, the salt water circulates along these discontinuities from West to North and Northeast which explains the abundance of low apparent resistivity values recorded northeastwards. This geophysical reconnaissance is the most important for water supply of this area affected by increasingly long periods of drought.

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, its’ distance, energy class and position, relative to the DMT point.

ANALYSIS OF GEOPHYSICAL PARAMETERS VARIATIONS AND SEISMIC EVENTS AT THE POINT OF DEEP MAGNETOTELLURIC SOUNDING

The paper considers the results of processing and analysis of data from deep magnetotelluric soundings (DMT) performed in 2018. Comparison of variations in apparent resistivity, the endogenous component of the magnetotelluric field, lunar-solar tidal deformations and seismic events that were recorded during the research. The objective of the study is to detect the relationship between the appearance of variations in electromagnetic parameters of the data of magnetotelluric monitoring from the occurred earthquakes, their distance, energy class and position, relative to the DMT point.

Deep-Sea DC Resistivity and Self-Potential Monitoring System for Environmental Evaluation With Hydrothermal Deposit Mining

Environmental impact assessment has become an important issue for deep-sea resource mining. The International Seabed Authority has recently developed recommendations for guidelines on environmental assessment of resource mining effects. Several research and development groups have been organized to develop methods for environmental assessment of the seafloor and sub-seafloor under the “Zipangu in the Ocean program,” a part of the Cross-ministerial Strategic Innovation Promotion Program managed by the Cabinet Office of the Japanese government. One attempt planned for long-term environment and sub-seafloor structure monitoring uses a cabled observatory system. To support this observatory plan, we began development of a system to monitor the sub-seafloor resistivity and self-potential reflecting the physicochemical properties of ore deposits and the existence of hydrothermal fluid. The system, which mainly comprises an electro-magnetometer and an electrical transmitter, detects spatio-temporal changes in subseafloor resistivity and in self-potential. Because of the project’s policy changes, cabled observatory system development was canceled. Therefore, we tried to conduct an experimental observation using only a current transmitter and a voltmeter unit. Data obtained during three and a half months show only slight overall apparent resistivity variation: as small as 0.005 Ω-m peak-to-peak. The electrode pair closest to the hydrothermal mound shows exceptionally large electric field variation, with a semidiurnal period related to tidal variation. Results indicate difficulty of explaining electric field variation by seawater mass migration around the hydrothermal mound. One possibility is the streaming potential, i.e., fluid flow below the seafloor, in response to tides. However, we have not been able to perform rigorous quantitative analysis, and further investigation is required to examine whether this mechanism is effective. The system we have developed has proven to be capable of stable data acquisition, which will allow for long-term monitoring including industrial applications.

Application of transient electromagnetic radar in quality evaluation of tunnel composite lining

Quality evaluation of the composite lining structure is very important for the safe operation of the tunnel. This study applied the transient electromagnetic radar (TER) to evaluate the contact status behind the composite lining structure. The feasibility of TER is verified by the variation of the apparent resistivity in the composite lining based on the one-dimensional (1D) forward calculation and physical simulation. The results of TER in the detection of voids and insufficient thickness is validated by the drilling test. A comparative test is conducted at the same location with quality problems using ground-penetrating radar (GPR). The results show that: (i) the TER is able to detect the insufficient lining thickness and voids behind the tunnel composite lining, contact status between composite lining and surrounding rocks can be determined by the variation of apparent resistivity; (ii) due to the difference of apparent resistivity, test image of TER shows clearly the boundary between surrounding rocks, primary and secondary lining, and voids, which made TER easier than GPR to detect the area of insufficient thickness and voids; (iii) combined with the drilling test results, TER and GPR results are generally consistent with the actual situation. The research and development of TER provides a new idea for the detection and evaluation of the tunnel lining diseases.

The results of dipole sounding of the right bank of the dam of the Chirkei hydroelectric power plant

For the first time in the practice of full-scale studies in the area of the Chirkei HPP dam, the method of dipole electric sounding was used. The aim was to study the geodynamic processes in the rock mass located in the sides of the highrise dam to develop new methods of control of natural and man-made hazardous geological processes. The basis for the study of deformation processes in a large volume of the right bank of the dam was laid by the results of electric sounding of rocks by a small four-electrode in-well installation over the period 2010–2015, which showed that the change in the level of the reservoir caused seasonal deformation of rocks. The use of dipole sounding can significantly increase the size of the object under study. The volume of the probed rocks of the right bank of the dam is 6×106 m3, that is 2000 times larger compared to the four-electrode in-well installation. The equipment used is characterized by the continuity and high accuracy of measurements of apparent resistance of the mass, which enables to register the dynamic processes occurring in it. The results of observations by the method of dipole sounding over a short period of 2018 enable to conclude about the anthropogenic impact of the Chirkei hydroelectric power plant on the stress-strain state of rocks adjacent to the right side of the dam. For individual periods of observation, variations of apparent resistivity of large amplitude are detected, suggesting that the analyzed mass is in conditions of unstable equilibrium, i. e. displacements occur from time to time along the cracks in place. The movements can be triggered by starts of hydraulic units. Shifts in large tectonic cracks in the rock mass near the right side of the dam can lead to landslide processes and opening of cracks in the dam contact zone. The method of continuous dipole electric sounding can find its practical application for monitoring of hazardous geological processes in hydroelectric power plants.

New interpretation of the reduction phenomenon in the electrical resistivity of rock masses before local earthquakes

Based on long-term observational data from the area of Peter the Great Ridge (Pamir), a decrease in the electrical resistivity of the rock massif before local earthquakes has been observed. To explain the observed phenomenon, a hypothesis has been proposed: the structure of the pore space of a rock in the focal zone of an earthquake changes during the compression/tension associated with earthquake preparation. However, the models proposed have only qualitatively interpreted the experimental data. This article attempts to quantify this hypothesis based on 3D modeling of the observed electromagnetic effects. It is shown that variations in the apparent resistivity are due to changes in the electrical resistivity in a narrow fault zone located at the focus of the dipole-dipole sounding system. The changes occur at depths of <2–2.5 km and are due to low-resistivity fluid saturation of the highly porous fracture crushing zone. Small, smooth changes in the resistivity of the containing medium on both sides of the fault are associated with regional stresses in blocks of rocks and are not associated with the local earthquakes. An analysis of electromagnetic monitoring data at the Bishkek test site (Tien Shan) showed that the observed variations in apparent resistivity during pulse soundings are related to stresses and deformations of rocks in a wide region and cannot be used as a tool for predicting earthquakes. It is shown that, for studies of variations in the resistivity of rock massifs, measuring the response of the medium by grounded electrodes is ineffective due to the “static shift effect” generated by 3D inhomogeneities of the upper layer of the medium. To eliminate this effect, it was proposed that pulse technologies with magnetic field measurements made by ungrounded antennas be used.

Resistivity variations related to the large March 9, 1998 eruption at La Fournaise volcano inferred by continuous MT monitoring

The 2645m-high La Fournaise volcano, located in the Southwest of Réunion Island (Indian Ocean), is a shield basaltic volcano where effusive eruptions generally occur along long fissures starting from the summit, alongside major fractures that characterize the eruptions' dynamism and effusivity. Between 1992 and 1998, the volcano underwent a quiet period during which few earthquakes were recorded. Minor seismic activity returned after 1997 and picked up in March 1998 during the 35h preceding the March 9 eruption. From 1996, two autonomous stations (CSV and BAV) were installed on the volcano. CSV was located inside the Enclos Fouqué caldera while BAV was positioned 8.2km NW of the volcano summit. Horizontal components of the electric and magnetic fields were sampled every 20s. Continuous time-series were available from 1996 to 1999 at CSV, and from 1997 to March 1998 at BAV. Data have been processed using both single-station and remote-reference processing. Both results show apparent resistivity variations synchronous to the eruption. Time-lapse impedance estimates are computed on overlapping time windows of about two days at both stations. The only major decrease of the observed impedance coincides with the March 1998 eruption. At CSV, the resistivity started to drop about five days before the eruption, reached several local minima until April, and then slowly increased as the volcanic crisis reduced in activity. After the end of the crisis in September 1998, the apparent resistivity recovered its pre-crisis value.The time-lapse results also show variability in directionality: sharp and elongated phase tensor ellipse residuals appear during the eruption with a N105° orientation, suggesting the emergence of an almost NS-striking dyke. A 1D background model built from MT soundings performed during the quiet period (1996 to February 1998) on which a 3D NS-striking dyke was added shows a good agreement with phase tensor residuals and spatial distribution of the resistivity variations observed during the eruption.

Continuous magnetotelluric observations in Western Crete as a tool for the study of the Hellenic subduction Zone

In the frame of the “MagnetoTellurics in studying Geodynamics of the hEllenic ARc (MT-GEAR)” project, a new continuous monitoring Magnetotelluric (MT) station has been installed in Western Crete. The new permanent MT-station, designed and constructed incorporating contemporary technology, is solar powered, fully telemetric, and the data transfer can be performed alternatively through 3G network or a satellite link. The selected site for the installation resulted from an exploratory MT survey, among other prospective sites aggregating MT qualifications. The purpose of the operation of a permanent MT observatory is twofold. The primary objective is to act as remote reference station to the ongoing MT survey (at selected Islands of southern Aegean) of the geodynamics of the Hellenic Subduction Zone. In addition, possible apparent resistivity variations as well as electric and/or magnetic field transient anomalies are investigated, while it would be attempted to associate these variations with seismic activity.

Correlation dependences of electromagnetic and deformation parameters

In-situ monitoring observations are carried out, and correlation analysis aimed at estimating the dependences between the electromagnetic parameters (variations of apparent resistivity, the impedance phase, components of the impedance tensor, and deformations of the daylight surface on the territory of the Bishkek geodynamic polygon) is performed. A new approach to the explanation of the physical mechanism forming variations in electrical conductivity of the medium is proposed on this basis.

English

Fractured aquifer is considered as a main promising aquifer in northeastern boarders of Egypt where the Nubian aquifer is deep. The groundwater exploration and evaluation are the main objectives of the present study. Some localities were carefully selected for applying the geophysical techniques to achieve the goal of the study. Both the Vertical Electrical Sounding (VES) and horizontal profiling techniques of direct current resistivity method were applied. The hydrogeological information were analyzed and evaluated using the available software. The Direct Current (DC) resistivity method was applied for delineation the variation of apparent resistivity of sedimentary succession that depends on the density of fractures and cavities. The lateral and vertical distribution of resistivity was mapped to delineate the lateral extension of fractures. Calibration of the resistivity model with the lithologic information, some field measurements carried out for some clearly observed fractured areas helped in identifying the density and the direction of fractures controlling groundwater potentiality. Results indicated that the technique can be applied in similar areas where the carbonate rocks constitute the main reservoir. In the study area, the fractured zone was clearly detected to be extending downward from depth of 15 to 80 m and considered as good aquifer in the area comprising lower resistivity values. Consequently, two different types of water due to different recharge conditions are expected as a result of variation in fractures discontinuity. Key words: Water, flow, facture, resistivity, aquifer, Sinai.

Application of geoelectrical profiling in the delineation of shallow periglacial structures on the Drohiczyn Plateau

Abstract The paper describes the results of geoelectrical sounding application for delineation of periglacial structures in Wierzchuca Nagórna within the Drohiczyn Plateau (eastern Poland). The method records the variation of apparent resistivity of deposits along a selected horizontal direction. The survey utilizes a distinct geoelectrical contrast between a glacial till and structures composed of sand that have been recognized earlier in a gravel pit wall. The studies have been conducted in direct hinterland of the gravel pit. Rectangular and concentric arrays were applied in the survey. The results have been presented as graphs and resistance distribution maps. The obtained geological image is not complete, although it shows an oriented course of the largest lineaments and traces of a network with a small mesh size (1.5-2 m). The recognized pattern of structures points to harsh climatic conditions during their formation. General orientation of the structures corresponds to the basic elements of the contemporary landscape in the area (slope, valley axes). Geoelectrical sounding using the rectangular array gave better results than in the concentric array, whereas introduction of brine into the system did not enhance the resolution of the readings. The presented resistance distribution maps with a characteristic rhythm of variations may serve as a benchmark for recognizing periglacial structures in areas without exposures.

Application of an In Situ Electrical Resistivity Device to Monitor Water and Salt Transport in Shandong Coastal Saline Soil

In situ monitoring of water and salt transport in saline soil is a valuable tool for the rational exploitation and conservation of water and soil resources in coastal areas. In this paper, an automatic monitoring device was used to monitor the vertical variation of soil resistivity during leaching in laboratory and field experiments. The experimental results showed that the resistivity peak moved downwards and eventually disappeared during leaching. An equation describing the change in leaching speed over time was obtained. It was found that the desalination occurred from top to bottom and that the desalination depth was constant when the same volume of leaching fluid was applied. Resistivity indication and salinity content showed a significant exponential correlation (R (2) > 0.9). The field experiment demonstrated the stability of data transmission, and variations in apparent resistivity in one day and over a longer period were observed. Rainfall resulted in the downward migration of salt from surface soil, whereas evaporation led to migration of salt upward. The temporal and spatial resolution of this monitoring device can be adjusted to achieve dynamic monitoring, providing an easy means to manage soil and water conservation.

Field identification of groundwater flow systems and hydraulic traps in drainage basins using a geophysical method

AbstractGroundwater flow systems and stagnant zones in drainage basins are critical to a series of geologic processes. Unfortunately, the difficulty of mapping flow system boundaries and no field example of detected stagnant zones restrict the application of the concept of nested flow systems. By assuming the variation in bulk resistivity of an aquifer with uniform porosity is mainly caused by groundwater salinity, the magnetotelluric technique is used to obtain the apparent resistivity of a profile across a groundwater‐fed river in the Ordos Plateau, China. Based on the variations in apparent resistivity of the Cretaceous sandstone aquifer, the basin‐bottom hydraulic trap below the river has been detected for the first time, and its size is found to be large enough for possible deposition of large ore bodies. The boundaries between local and regional flows have also been identified, which would be useful for groundwater exploration and calibration of large‐scale groundwater models.