Geoelectrical Signals Research Articles

Methods for Assessing the Layered Structure of the Geological Environment in the Drilling Process by Analyzing Recorded Phase Geoelectric Signals

Methods for Assessing the Layered Structure of the Geological Environment in the Drilling Process by Analyzing Recorded Phase Geoelectric Signals

APPLICATION OF PHASEOMETRIC COMPENSATION METHOD FOR DETECTION AND LOCALIZATION OF GEODYNAMIC PROCESSES DURING GEOELECTRICAL CONTROL

This work is devoted to considering the issue of using the compensatory method of geoelectric control using the phase characteristics of the field to detect and localize geodynamic processes using the example of the development of a karst-suffusion cavity. The high noise immunity of the phasemetric method for recording geoelectric signals was noted in comparison with the amplitude parameters of the anomalous component of the electromagnetic field, usually used to analyze observation results. A formalized approach to the use of field phase characteristics to interpret monitoring data and the related problem of localizing geodynamic processes has been developed. Within the framework of this approach, it is proposed to determine the section parameters by the minimum sum of the weighted mean square error of interpretation and the regularizing functional containing a priori information about the geoelectric section. To check the possibility of localizing a spherical karst-suffosion cavity, modeling of the amplitude and phase of the anomalous component of the field potential, as well as the root-mean-square error of heterogeneity localization, was carried out when the center of the sphere moves along the installation profile. Modeling showed good differentiation of the potential from the location of the inhomogeneity; the highest localization accuracy was achieved with the combined use of the amplitude and phase components of the field in the problem of localizing the inhomogeneity.

GEOELECTRIC PHASOMETRIC CONTROL OF THE RAILWAY TRACKBED

During the operation of the railway track, various defects and deformations may occur and accumulate in the earth bed and its soil base, which leads to a violation of their bearing capacity. The task of developing new technological methods for monitoring the condition of the ground foundation – roadbed – railway track system is becoming urgent. The article proposes a geoelectric phase-measuring method for monitoring the phase of recorded signals with changes in the load on the ground at the base of the railway track. The control of phase characteristics allows for greater sensitivity, accuracy and noise immunity compared to the control of the amplitude of geoelectric signals. The developed laboratory setup allows simulating various natural processes (changes in soil moisture, suffusion, karst dips, landslides, landslides, and others) and assessing their impact on the railway transport complex. The installation contains a model of the controlled object (a container with soil), generators of probing signals, a subsystem for recording signals in the environment, a subsystem for data processing. The proposed approach makes it possible to detect early geodynamic indicators of deformations of the railway trackbed.

Statistical analysis for biogeochemical processes in a sandy column with dynamic hydrologic regimes using spectral induced polarization (SIP) and self-potential (SP)

SUMMARYThe capillary fringe (CF) is characterized by transient and steep redox gradients and is thought to be a hot spot for biogeochemical processes. Understanding chemical fate and transport in the CF is significant, however, biogeochemical dynamics at the CF are poorly understood because of the difficulty to measure representatively with high spatio-temporal resolution at depths under dynamic hydrologic regimes. Hydrogeophysics is a developing field that uses minimally intrusive and quick response methods to monitor hydrological properties. Two geoelectrical methods [spectral induced polarization (SIP) and self-potential (SP)], which are sensitive to the solid–liquid interfaces (SIP) and biogeochemical processes (SP) can address the above difficulty. The challenge lies on linking the geoelectrical responses with biogeochemical processes, where many different processes contribute to the signals. We conducted a soil column experiment under five hydrologic regimes focusing on nitrogen transformations with SIP and SP measurements: (1) a static regime with a stable water level; (2) an infiltration regime with periodic pulse infiltration events with a constant water level and (3) fluctuating regimes with water level fluctuations under three drying-wetting frequencies (6/12/18-day-cycle). This is the first large lab-scale work in a well-controlled and highly instrumented soil column. The dynamic hydrologic conditions stimulated complex biogeochemical processes at the CF, and therefore the SIP and SP signals result from many physical and biogeochemical processes. Therefore, we relied on statistical analysis in this study for a novel interpretation. Spearman correlation analysis supported water content played the most important role in real conductivity (σ′) dynamics in the vadose zone, whereas fluid conductivity dominated σ′ in the saturated zone. Both correlation analysis and spatial moment analysis implicated that water content was the driving factor for both σ′ and imaginary conductivity (σ″). A multiple linear regression model indicated the gradient of redox potential, the gradient of soil matric potential and water content were the three main influencing factors for the SP signals. We proposed that the water level fluctuation can efficiently facilitate microbial electron transfer through ions transport between the different redox zones, and aggregate redox processes to create SP signal gradients. Depth zonation analysis, using six environmental indexes (Eh and nitrogen species; water content; real conductivity; imaginary conductivity; SP signal; microbial community composition), suggested that water content induced by soil hydrology was the most dominant factor, captured by all the indexes. In turn, it led to indirect inference on biogeochemical processes and resultant geoelectrical signals. Applying geoelectrical methods to such biogeochemical processes will not only lead to a better understanding of the mechanistic meanings of the geoelectrical signals, but also build relationships between geoelectrical signals and biogeochemical parameters to facilitate a novel way to monitor biogeochemical processes.

Unveiling Informational Properties of the Chen-Ouillon-Sornette Seismo-Electrical Model.

The seismo-electrical coupling is critical to understand the mechanism of geoelectrical precursors to earthquakes. A novel seismo-electrical model, called Chen–Ouillon–Sornette (COS) model, has been developed by combining the Burridge–Knopoff spring-block system with the mechanisms of stress-activated charge carriers (i.e., electrons and holes) and pressure-stimulated currents. Such a model, thus, can simulate fracture-induced electrical signals at a laboratory scale or earthquake-related geoelectrical signals at a geological scale. In this study, by using information measures of time series analysis, we attempt to understand the influence of diverse electrical conditions on the characteristics of the simulated electrical signals with the COS model. We employ the Fisher–Shannon method to investigate the temporal dynamics of the COS model. The result showed that the electrical parameters of the COS model, particularly for the capacitance and inductance, affect the levels of the order/disorder in the electrical time series. Compared to the field observations, we infer that the underground electrical condition has become larger capacitance or smaller inductance in seismogenic processes. Accordingly, this study may provide a better understanding of the mechanical–electrical coupling of the earth’s crust.

Spatiotemporal geo-electrical sensing of a Pluronic-coated cobalt ferrite nanoparticle slug in natural sand flow-through columns

Rising industrial interest in the application of nanomaterials for the remediation of contaminated sites has led to concern over the environmental fate of the nanoremediation agents used. A critical requirement in evaluating and understanding nanoparticle (NP) behaviour in porous media is the development of analytical methods capable of in situ monitoring of complex NP transport dynamics. Spectral induced polarization (SIP), a non-invasive geo-electrical technique, offers a promising tool for detecting and quantifying NPs in soil and aquifer media. However, its application for monitoring the spatial migration and attachment behaviour of NPs remains uninvestigated. Here, we present results from flow-through experiments where we monitored the transport of cobalt ferrite nanoparticles (CoFe-NPs) coated with Pluronic, an amphiphilic polymer, in natural aquifer sand columns. We coupled concentration breakthrough curve analysis with SIP monitoring and reactive transport modeling to relate spatiotemporal NP concentration distributions to geo-electrical signals. Changes in the real (σ′) conductivity at three different locations along the columns closely correlated with model-computed total (solid plus aqueous phase) NP concentrations during the propagation of a NP slug. The imaginary conductivity (σ″) correlated closely with the arrival of the NP-slug. However, during the receding front, bimodal σ″-signal peak behaviour was observed propagating through the columns, indicating the existence of complex in situ NP transport dynamics, potentially revealing the rupture of nanoclusters upon straining and their effect on bulk charge storage that may not be obvious from breakthrough curve data alone. Fitting of a double Cole-Cole relaxation model yielded distinct shifts in relaxation time (τ) associated with the polarization of smaller length-scale particles. Post-NP pulse τ and σ″ did not return to pre-injection values; these lingering signals were caused by retained NP concentrations as low as 8.8 mg kg−1. Our results support the applicability of SIP for spatial and temporal monitoring of NP distributions, with implications for the investigation of NP transport and nanoremediation strategies.

A paradigm for developing earthquake probability forecasts based on geoelectric data

We examine the precursory behavior of geoelectric signals before large earthquakes by means of a previously published algorithm including an alarm-based model and binary classification [H.-J. Chen, C.-C. Chen, Nat. Hazards 84, 877 (2016)]. The original method has been improved by removing a time parameter used for coarse-graining of earthquake occurrences, as well as by extending the single-station method into a joint-stations method. Analyzing the filtered geoelectric data with different frequency bands, we determine the optimal frequency bands of earthquake-related geoelectric signals featuring the highest signal-to-noise ratio. Based on significance tests, we also provide evidence of a relationship between geoelectric signals and seismicity. We suggest using machine learning to extract this underlying relationship, which could be used to quantify probabilistic forecasts of impending earthquakes and to get closer to operational earthquake prediction.

Coupled mechano-electrokinetic Burridge-Knopoff model of fault sliding events and transient geoelectric signals

We introduce the first fully self-consistent numerical model combining seismic micro-ruptures, occurring within a generalized Burridge-Knopoff spring-block model, with the nucleation and propagation of electric charge pulses within a coupled mechano-electrokinetic system. This model provides a general theoretical framework for modeling and analyzing geoelectric anomalies to earthquakes. In particular, it is able to reproduce the unipolar pulses that have often been reported before large seismic events, as well as various previously reported anomalies in the statistical moments of the ambient electric fields and the shape of their power spectra.

Global Earthquake Forecasting System (GEFS): The challenges ahead

We conclude this special issue on the Global Earthquake Forecasting System (GEFS) by briefly reviewing and analyzing the claims of non-seismic precursors made in the present volume, and by reflecting on the current limitations and future directions to take. We find that most studies presented in this special volume, taken individually, do not provide strong enough evidence of non-seismic precursors to large earthquakes. The majority of the presented results are hampered by the fact that the task at hand is susceptible to potential biases in data selection and possible overfitting. The most encouraging results are obtained for ground-based geoelectric signals, although the probability gain is likely small compared to an earthquake clustering baseline. The only systematic search on satellite data available so far, those of the DEMETER mission, did not find a robust precursory pattern. The conclusion that we can draw is that the overall absence of convincing evidence is likely due to a deficit in systematically applying robust statistical methods and in integrating scientific knowledge of different fields. Most authors are specialists of their field while the study of earthquake precursors requires a system approach combined with the knowledge of many specific characteristics of seismicity. Relating non-seismic precursors to earthquakes remains a challenging multidisciplinary field of investigation. The plausibility of these precursors predicted by models of lithosphere-atmosphere-ionosphere coupling, together with the suggestive evidence collected here, call for further investigations. The primary goal of the GEFS is thus to build a global database of candidate signals, which could potentially improve earthquake predictability (if the weak signals observed are real and false positives sufficiently uncorrelated between different data sources). Such a stacking of disparate and voluminous data will require big data storage and machine learning pipelines, which has become feasible only recently. This special issue compiled an eclectic list of non-seismic precursor candidates, which is in itself a valuable source of information for seismologists, geophysicists and other scientists who may not be familiar with such types of investigations. It also forms the foundation for a coherent, multi-disciplinary collaboration on earthquake prediction.

Application of phase-metric compensation method for geoelectric control of near-surface geodynamic processes

This work is devoted to the application of the compensation method of geoelectric control using the field phase characteristics for the detection and localization of geodynamic processes on example of the development of the karst-suffusion cavity. High noise immunity of the phase-metric registration method of geoelectric signals in comparison with amplitude parameters of the anomalous components of electromagnetic field, usually used to analyze the results of observations, is noted. A formalized approach to the use of phase characteristics of the field for the interpretation of monitoring data and associated problem of geodynamic processes localization is developed. In the framework of this approach, the section parameters are proposed to determine by the minimum sum of weighted mean square interpretation error and regularizing functional containing a priori information about the geoelectric section. To check localization possibility of the spherical karst-suffusion cavity, simulation of the amplitude and phase anomalous components of the field potential, as well as the standard error of heterogeneity localization, when moving the sphere center along the installation profile, is carried out. Simulation has shown a good potential differentiation from the inhomogeneity position, the highest accuracy of localization is achieved with combined use of amplitude and phase field components in the problem of inhomogeneity localization.

A Comparative Study of Geoelectric Signals Possibly Associated with the Occurrence of Two Ms > 7 EQs in the South Pacific Coast of Mexico

During past decades, several studies have suggested the existence of possible seismic electric precursors associated with earthquakes of magnitude . However, additional analyses are needed to have more reliable evidence of pattern behavior prior to the occurrence of a big event. In this article we report analyses of self-potential records during approximately two years in three electro-seismic stations in Mexico located at Acapulco, Guerrero; Petatlán, Guerrero and Pinotepa Nacional, Oaxaca. On 18 April 2014 an earthquake occurred near our Petatlán station. Our study shows two notable anomalies observed in the behavior of the Fourier power spectrum of for ultra low frequency ULF-range, and the transition of the -exponent of the detrended fluctuation analysis of the time series from uncorrelated to correlated signals. These anomalies lasted approximately three and a half months before the main shock. We compare this electric pattern with another electric signal we reported associated with an that occurred on 14 September 1995 at Guerrero state, Mexico. Our characterization of the anomalies observed in both signals point out similar features that enrich our knowledge about precursory phenomena linked to the occurrence of earthquakes of magnitude .

Relationship Between Seismic Electric Signals and Tectonics Derived from Dense Geoelectric Observations in Taiwan

Some researchers have suggested that significant electromagnetic changes that can be detected prior to strong earthquakes are associated with the tectonics and subsurface structures beneath monitoring stations. Numerical simulation seems to support this phenomenon. However, to date, few field measurement data are available to address this issue. In this work, we used the shifting correlation method to analyze 2-month recordings of 20 geoelectric stations in Taiwan and found at least two stations had recorded geoelectric signals significantly correlated with known seismic events. These two stations are installed on large reverse faults, which are probably characterized by high stress concentrations before earthquakes. In addition, they share the same resistivity variation pattern, namely, a conductor layer sandwiched between two high-resistivity layers. Moreover, both stations are above the electrical resistivity boundaries, which means they have an increased probability of recording the abnormal disturbance once EM signals have been emitted from EQs. Our analysis results support the viewpoint that lateral and vertical electrical resistivity variation and conductors in the subsurface may be able to amplify seismic electric signals.

Estimation of the probability of karst cavities collapse under technogenic cyclic loads below the fatigue limit

The paper deals with the phenomenon of fatigue formation in soils over karst cavities under cyclic loading near the fatigue limit. A method is proposed for determining the stress state of the integumentary earth arising during the considered technogenic cyclic loads. Currently, the assessment of the possibility of karst cavities collapse is performed on the basis of static models, without taking into account the dynamic technogenic load. Especially in the case of slow cyclic loads of low intensity, which do not directly lead to collapse of the soil above the karst cavities. However, these actions change the bearing capacity of soils that are not taken into account at the exploration stage and are not laid in the project documentation for the construction of facilities in karst-dangerous areas. A technique for estimating pre-crisis states based on joint processing of active geoelectric and seismoacoustic geodynamic control signals is proposed.

The method of the phase control of the electrical installation during geodynamic monitoring

The paper substantiates the application of the phase control of the multipolar equipotential electrical installation to improve the geodynamic sensitivity of the geoelectric control method in geodynamic and geotechnical monitoring systems. The principle of the control of the source of sounding signals and the detection of geodynamics of the geological environment is shown in article. In this work is proposed the device on base of the phase modulator to eliminate the shortcomings of existing devices of the controlling of the phase of sound geoelectrical signals. The developed device contains two amplitude limiters, the integrating unit, the adder, the master oscillator and the amplitude modulator. In practice the application of the proposed device allow to obtain the technical and the economic efficiency by simplifying of the circuit of the generation of the signal with the phase-modulation, by increasing the linearity of the modulation characteristic, and by generating of the signal with the phase-modulate on output of the device.

New Insights on Scale-dependent Surface-Groundwater Exchange from a Floating Self-potential Dipole

In south-central Texas the lower Guadalupe River has incised into the outcrop of the Carrizo-Wilcox aquifer. The river and the aquifer are hydraulically connected across the outcrop, although the connectivity is obscured at the surface by alluvium and surface-water and groundwater exchange dynamics are currently poorly understood. To investigate surface-water and groundwater exchange dynamics between the lower Guadalupe River and the Carrizo-Wilcox aquifer, a geophysical study was completed along a 14.86 km reach of the river by using water-borne gradient self-potential (SP) profiling and two-dimensional direct-current electric resistivity tomography. This paper explores the applicability of these water-borne geoelectric methods in delineating gaining and losing channel reaches, and demonstrates that geoelectric signals in the form of total electric field strength can be logged with an electric dipole and decomposed into component SP signals depicting regional and local groundwater flow patterns attributable to regional and localized hydraulic gradients. Localized SP anomalies of several tens of millivolts, indicative of hyporheic exchange flows, are observed and superimposed upon a 124 mV regional SP anomaly indicative of ambient groundwater exchange flows between the river and the aquifer. The observed SP signals are interpreted through two-dimensional finite-element modeling of streaming potentials attributable to ambient groundwater exchange and hyporheic exchange flow patterns. Variables of the channel environment such as temperature and concentration gradients, depth, and velocity are considered and subsequently eliminated as alternative sources of the SP signals that are presented.

Pore‐scale modelling of complex conductivity of saturated granular materials

ABSTRACTThe complex conductivity method has been frequently used in solving hydrogeological, engineering, and environmental problems in practice. Macroscopic geophysical responses are governed by pore‐scale rock properties; therefore, a clear petrophysical understanding of how the pore‐scale structure controls electric conduction and polarization in geomaterials is necessary to fully interpret laboratory‐/field‐scale geophysical observations. In this study, we have developed a pore‐scale numerical approach to calculate the complex conductivity of water‐saturated granular materials. This physics‐based model has advantage over phenomenological or empirical models such that the intrinsic relations between pore structure and geoelectrical signals could be revealed. In the modelling, the influence of electrical double layer, which is quantified by complex surface conductance, can be converted to an apparent volumetric complex conductivity of either solid particles or pore fluid. The effective complex conductivity of the sample is determined by directly solving the finite‐difference representation of the Laplace equation in the domain of a representative elementary volume. The numerical approach is tested on one synthetic sample and one glass beads sample. For the synthetic sample, the consistency between the numerical and the analytical solution confirms the accuracy of the approach. Comparison with experimental measurements of the glass beads sample reported previously indicates that the developed numerical approach can well reproduce the key characteristics of the complex conductivity of water‐saturated granular materials in the frequency range between 10‐1 and 104 Hz. Furthermore, the numerical examples also show that the proposed approach can capture the salinity‐dependent electric conduction and polarization in saturated granular materials.

CHARACTERISTICS OF THE SPATIAL VARIATION OF GEOELECTRIC FIELD SIGNALS RECORDED AT THE STATIONS IN HUADONG AREA (SOUTH‐EAST CHINA) FROM 4 HEAVY CURRENT SOURCES

AbstractThe relation between the flowing characteristics of electric current and the geological structure is one of very important issues in the study of geoelectric field in seismo‐electromagnetics. This paper deals with heavy currents from four different sources injected into the underground and their consequences in the observations from the network of geoelectric field stations associated with the currents, which provides us with a good opportunity to study the issues. The sources are the earth electrodes of some substations of HVDC transmission line systems in East‐south China around Shanghai area (Huadong area) with the currents of 1200 to 4780A injected. The recordings of the additional signals of geoelectric field corresponding to the currents have been collected at eight geoelectric field stations in the same area. The following features could be seen in the study. ➀ Some stations show the selectivity effect. The magnitude of the additional geoelectric field recorded at different stations varied from one source to another, for example Pudong station is near all the heavy current sources, but only the clear signals originated from Nanqiao earth electrode were recorded while no signals from other sources (Tongli, Huaxin, and Fengxian), which shows so‐called selectivity effect of the site; ➁ Station‐source distance effect. The maximum observation distance is 350 km, beyond that no additional geoelectric signals from the sources could be recorded; ➂ The source azimuth determination has complicated characteristics. For example, at Chongming station the errors are minimum, only 0.2° and 0.8°, respectively, between the actual and calculated source azimuths determined by the signals from Tongli and Xinhua earth electrodes. However, Qingpu station is the nearest station from the heavy current sources, these errors are larger than that obtained from Chongming station due to some unknown reasons; ➃ The effect of non‐uniformity of the geoelectric field. At the stations in the studied area, the ratios of signals from long dipole over that of short dipoles do not follow the regular patterns which appear in the cases of homogeneous medium structure underneath the observation stations, which illustrates a kind of non‐uniformity effect, which is discussed in detail in this paper and proved by the results from a 2‐D high density geoelectrical prospecting in the studied area. ➄ The peculiar reverse effect of polarization direction. For the additional geoelectric field signals recorded at Qingpu and Chongming stations the polarization directions on the long diploes and short diploes are reverse, which need to be further studied in the future to explain the phenomenon. The influence of the three factors, i.e., the configuration of a point source and the electrode distribution of station, the heterogeneity of medium in a large area, as well as the fine structure of the underground medium at a station, is discussed in this paper which might give a relatively clear explanation for the features mentioned above.

Demarcating saline water intrusion pathways using remote sensing, GIS and geophysical techniques in structurally controlled coastal aquifers in Southern India

Application of remote sensing and geographical information system (GIS) with the help of ground truth studies like vertical electrical soundings (VES) and water quality measurements gave a comprehensive picture on saline water intrusion pathways in the structural controlled aquifers of Uppanar and Cauvery River systems located in Sirkazhi and Mannanpandal coastal areas of Southern India. The detailed geomorphic units and lineaments were mapped using IRS LISS-III (1:50,000 Scale) and SRTM Elevation Digital data under GIS environment. The area is mainly alluvial deposits of Holocene Age and classified into three major geomorphic units (i.e., fluvial, fluviomarine, and marine geomorphic units). The identified lineaments are mostly trending North-northeast (NNE)–South-southwest (SSW), Northeast (NE)–Southwest (SW), Northwest (NW)–Southeast (SE), East–West (EW) and East-Northeast (ENE)–West-Southwest (WSW) directions. Some of them are potential for fresh groundwater occurrence and movement. The lineaments trending NE–SW, NNE–SSW, and ENE–WSW act as pathways for saline water intrusion to the inland. The lineament L1 (NNE–SSW trending) conformed as a sinisterly shifted fault played a significant role in saline water intrusion. The area within the parallel lineaments L2 and L3 oriented ENE–WSW direction indicates low topography, beach ridges, paleo-channel sinuosity, large aerial extent of tidal flats, and backwater inundation. All the above geomorphic units gave indirect evidence of land subsidence, which was confirmed with the help of sub-surface geo-electrical signals obtained through VES and in situ groundwater quality measurements. Integrating all the above results, the saline water spread and fresh water pockets were identified.

Origin of transient self‐potential signals associated with very long period seismic pulses observed during the 2000 activity of Miyakejima volcano

AbstractOrigin of the previously reported transient geoelectrical (self‐potential, SP) signals in the Miyakejima 2000 activity, that repeatedly occurred concurrently with very long period (VLP) seismic pulses, was investigated. SP waveforms stacked across repeated VLP events showed a step‐like rise followed by a gradual decay at all stations spread over the island of 8 km diameter. Within a realistic range of hydrological diffusivity, the short time constants of the SP signals cannot be explained by the electrokinetic effect caused by fluid flow within a limited volume, proposed earlier as a fluid injection hypothesis. On the other hand, poroelasticity predicts an island‐wide distributed flow field to occur almost instantaneously upon VLP events due to the step of strain field imposed by the mechanical event. We propose that the observed SP signals resulted from the streaming current by this island‐wide flow field. Our quantitative model, assuming a vertical tensile crack as a mechanical source, which has been suggested by preceding seismic studies, can explain the time constants and the amplitude of the SP signals (both spatial pattern and absolute amplitude), within a reasonable range of rock properties and the scalar moment of the mechanical source (VLP event). Location and attitude of the mechanical source were well constrained by grid search and are consistent with those estimated earlier from other types of data.

Using the informational Fisher–Shannon method to investigate the influence of long-term deformation processes on geoelectrical signals: An example from the Taiwan orogeny

The time dynamics of geoelectrical signals measured at three sites in Taiwan were investigated by using the Fisher–Shannon method in order to investigate the possible correlation between their properties with deformation processes. The three sites are located along an almost perpendicular direction to the orogenic collision zone where each site experiences different amounts of crustal deformation. Our findings point out to a clear discrimination of the three sites on the basis of the informational properties of the recorded geoelectrical signals. In particular a relationship is found between the differential strain intensity of the sites where the geoelectrical stations are located and the Fisher–Shannon quantities.