Geoelectrical Signals Research Articles

Geoelectrical signals of geologic and hydrologic processes in a fringing reef lagoon setting

Coastal groundwater may discharge into nearshore and offshore waters forced by terrestrial fluxes, controlled by local geology, and modulated by the hydrodynamics of littoral water. We investigated the electrical signature of these features with a dense, multiscale network of electrical resistivity tomography (ERT) surveys in the Muri Lagoon of Rarotonga, Cook Islands. The ERT surveys spanned from onshore to 400 m into the lagoon and used standard electrodes on land and across the foreshore, submerged electrodes in the shallow subtidal zone, and floating electrodes towed throughout the reef lagoon by a boat. ERT surveys on land mapped a typical freshwater lens underlain by a saltwater wedge, but with possible deviations from the classical model due to an adjacent tidal creek. Further inland, ERT surveys imaged a layer of lava flow deposits that is potentially a confining hydrogeologic unit; this unit was used to constrain the expected electrical resistivity of these deposits below the lagoon. ERT surveys across the intertidal zone and into the lagoon indicated fresh groundwater and porewater salinity patterns consistent with previous small-scale studies including the seaward extension of fresh groundwater pathways to the lagoon. Electrical resistivity (ER) variations in the lagoon subsurface highlighted heterogeneities in the lagoon structure that may focus submarine groundwater discharge (SGD) through previously unknown buried lava flow deposits in the lagoon. A transition to higher ER values near the reef crest is consistent with the ER signature of porosity reduction due to ongoing differential cementation of reef deposits across the lagoon. The imaged coastal hydrostratigraphic heterogeneity may thus control terrestrial and marine porewater mixing, support SGD, and provide the pathways for groundwater and the materials it transports into the lagoon. This hydrogeophysical investigation highlighted the spatial heterogeneity of submarine coastal geology and its hydrogeologic control in a reef lagoon setting, but is likely to occur in many similar coastal settings. Ignoring geologic complexity can result in mischaracterization of SGD and other coastal groundwater processes at many spatial scales.

Study on the Spatial Variation of the Geoelectric Field Signals Recorded at the Stations in the Eastern Huabei Area in China When a Heavy Current is Injected

AbstractIn the study of seismo‐electromagnetics, the relation between the flowing characteristics of electric current and the geological structure is a very important problem. The distribution of many geoelectric field stations and heavy current inflow into the underground offer us a golden opportunity to study the scientific problem. In this paper, by studying the signals originated from grounding electrode 2100∼3004 A current of Qingdao converter station and recorded at twenty one geoelectric field stations in the east Huabei area in China, we could have a better understanding of the variation of the geoelectric field. The study shows that (1) the telluric current signals present extremely big variation along the directions of the Tan‐Lu fault and the southwest beyond the fault zone as well as the northwest of it, where current signal could be recorded, and there are obvious directionality feature and the phenomenon of sensitive site; (2) the larger the current intensity of the source is, the higher the signal strength recorded at observatory stations is. When the current intensity of the source changed from 2100 A to 3004 A the increase rate of the signals recorded on the long electrode is different from that on short electrode at many stations; (3) the uniformity degree of the geoelectric field in the area does not follow the regular patterns of homogeneous medium, which illustrates that the characteristics of inhomogeneous medium and anisotropy are obvious at many stations in the area; (4) there are few stations that could be used to determine the direction of the source, but the azimuth obtained at Anqiu, Dashan and Xinyi stations could be used to locate the source site when a large current was injected. Considering the influence of the three factors, i.e., the device system consisting of a point source and the electrode distribution of a station, the heterogeneity of medium in a large area, as well as the fine structure of the medium underground at a station, this paper has made a deep analysis and explanation of the characteristics mentioned above by numerical simulation.

Investigation of the characteristics of geoelectric field signals prior to earthquakes using adaptive STFT techniques

Abstract. An earthquake is one of the most destructive natural disasters that can occur, often killing many people and causing large material losses. Hence, the ability to predict earthquakes may reduce the catastrophic effects caused by this phenomenon. The geoelectric field is a feature that can be used to predict earthquakes (EQs) because of significant changes in the amplitude of the signal prior to an earthquake. This paper presents a detailed analysis of geoelectric field signals of earthquakes which occurred in 2008 in Greece. In 2008, 12 earthquakes occurred in Greece. Five of them were recorded with magnitudes greater than Ms = 5R (5R), while seven of them were recorded with magnitudes greater than Ms = 6R (6R). In the analysis, the 1st significant changes of the geoelectric field signal are detected. Then, the signal is segmented and windowed. The adaptive short-time Fourier transform (adaptive STFT) technique is then applied to the windowed signal, and the spectral analysis is performed thereafter. The results show that the 1st significant changes of the geoelectric field prior to an earthquake have a significant amplitude frequency spectrum compared to other conditions, i.e. normal days and the day of the earthquake, which can be used as input parameters for earthquake prediction.

Detection and reduction of precipitation effects in geoelectrical potential difference data

AbstractGeoelectrical fluctuations are the end product of several geophysical phenomena. In particular, geoelectrical signals measured in seismically active areas can be attributed to stress and strain changes associated with earthquakes. In order to study the geoelectrical potential difference changes associated with the Earth's crustal activities, it is important to distinguish them from other noises because they are generally weak. The complexity of this problem has called for development of advanced statistical methods suitable for investigation of the heterogeneous nature of these fluctuations. In this paper we analyzed the time dynamics of short‐term variability of the geoelectrical field measured at Kiyosumi station located in the southern part of Boso peninsula, Japan. We tried to apply principal component analysis (PCA) and singular spectral analysis (SSA) to detect and reduce precipitation effects which are the most intense noise in geoelectrical potential difference data. The results show that an application of PCA to two orthogonal electric data can detect the precipitation and SSA has a potential to remove the precipitation effects. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(3): 1–8, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22301

MULTIFRACTALITY OF RADON CONCENTRATION FLUCTUATION IN EARTHQUAKE RELATED SIGNAL

Radon anomaly acts as a precursor to seismic events. Hence the fluctuation pattern of time series of radon bears a lot of significance. Multifractal detrended fluctuation analysis (MFDFA) has been applied successfully in past to various geo-electrical signals. The time series of earthquake related radon signal of duration three years (November 2005–November 2008) obtained from our experiment performed at Kolkata, India was analyzed in the framework of MFDFA. The study revealed that the fluctuation of radon concentration is multifractal in nature.

Discriminating geoelectrical signals measured in seismic and aseismic areas by using Ito models

Geoelectrical signals can be considered as the end-product of complex and collective interactions among system elements. Their dynamical behavior could be different if they are measured in seismic or aseismic areas. Using the Ito equations, which represent a good macroscopic model of phenomena in which microscopic interactions are adequately averaged, we show that geoelectrical data recorded in seismic areas are well discriminated by those measured in aseismic areas. Our findings contribute to a better dynamical characterization of geoelectrical signals.

The construction of an Ito model for geoelectrical signals

The Ito stochastic differential equation governs the one-dimensional diffusive Markov process. Geoelectrical signals measured in seismic areas can be considered as the result of competitive and collective interactions among system elements. The Ito equation may constitute a good macroscopic model of such a phenomenon in which microscopic interactions are adequately averaged. The present study shows how to construct an Ito model for a geoelectrical time series measured in a seismic area of southern Italy. Our results reveal that the Ito model describes the whole time series quite well, but it performs better when one considers fragments of the data set with lower variability range (absent or rare large fluctuations). Our findings show that generally detrended geoelectrical time series can be considered as approximations of Markov diffusion processes.

Entropy of geoelectrical time series in the natural time domain

Abstract. Seismic electric signals (SES) have been considered precursors of strong earthquakes, and, recently, their dynamics have been investigated within the Natural Time Domain (NTD) (Varotsos et al., 2004). In this paper we apply the NTD approach and the chaotic map signal analysis to two geoelectric time series recorded in a seismically very active area of Mexico, where two strong earthquakes, M=6.6 and M=7.4, occurred on 24 October 1993 and 14 September 1995, respectively. The low frequency geoelectric signals measured display periods with dichotomic behavior. Our findings point out to an increase of the correlation degree of the geoelectric signals before the occurrence of strong earthquakes; furthermore, the power spectrum and entropy in NTD are in good agreement with the results published in literature. Our results were validated by the analysis of a chaotic map simulated time series, which revealed the typical characteristics of artificial noise.

Detection and Reduction of Precipitation Effects in Geoelectrical Potential Difference Data

Geoelectrical fluctuations are the end product of several geophysical phenomena. In particular geoelectrical signals measured in seismically active areas can be attributed to stress and strain changes, associated with earthquakes. In order to study the geoelectrical potential difference changes associated with the earth's crustal activities, it is important to discriminate from other noises because they are generally weak. The complexity of this problem has suggested the development of advanced statistical methods to investigate the heterogeneous nature of these fluctuations. In this paper we analyzed the time dynamics of short-term variability of geoelectrical field measured at Kiyosumi station, located in southern part of Boso peninsula, Japan. We tried to apply principal component analysis (PCA) and singular spectral analysis (SSA) to detect and reduce precipitation effects which are the most intense noise in geoelectrical potential difference data. The results show an application of PCA to two orthogonal electric data can detect the precipitation and SSA has a potential to remove the precipitation effects.

GEO-ELECTRICAL SURVEY ON THE SOIL OF AN APRICOT ORCHARD GROWN UNDER SEMI-ARID CONDITIONS

A geo-electrical survey, combined with direct measurements on soil samples, was performed to study spatial relationships among electrical resistivity (p), hydro-pedological parameters, and root characteristics. The trial was carried out on the soil (WRB: Haplic Regosols) of an apricot orchard grown in a semi-arid climate, cover-cropped, and micro-irrigated. Three 2D geo-electrical tomographies were performed in the Wenner and dipole-dipole array (electrode spacing of 25 cm). Soil samplings were carried out from vertical trenches excavated directly below the respective geo-electrical profiles. Direct measurements of bulk density, grain size distribution, volumetric water content (θ), electrical soil conductivity (EC 1:1 , EC 1:2.5 ), stone content, and root biomass were performed on the soil samples in situ and in the laboratory. Hydraulic saturation (HS) and soil porosity (Φ) were also calculated. Statistical analyses performed between the geo-electrical signals (p) and soil data showed a univariate relationship with EC 1:2.5 . Multivariate relationships among p, EC 1:1 , and HS were also found. The significant relationship found between p and soluble salt content proves the usefulness of the geo-electrical technique in the medium-scale assessment of the spatial variability of salt accumulation processes in heterogeneous tree orchard systems.

Investigating the temporal fluctuations in geoelectrical and geochemical signals Jointly measured in a seismic area of Southern Apennine chain (Italy)

We analyse geoelectrical and geochemical time series jointly measured by means of a multiparametric automatic station close to an anomalous fluid emission in Val d'Agri (Basilicata, Southern Italy). In the investigated are some destructive seismic events occurred in past and recent years. We analysed the temporal fluctuations of the signals by spectral tools. We detected scaling behaviours in the power spectra of the time series recorded, that are typical fingerprints of fractional Brownian motions. The estimated values of the spectral indices reveal the presence of antipersistent behaviour in the time dynamics of all geoelectrical and geochemical data recorded. This work intends to improve our knowledge of the inner time dynamics of geophysical non-seismometric parameters.

Anomalous geoelectrical and geomagnetic signals observed at Southern Boso Peninsula, Japan

Geoelectrical and geomagnetic fluctuations are considered the end product of several geophysical phenomena. In particular these signals measured in seismically active areas can be attributed to stress and strain changes associated with earthquakes. The complexity of this problem has suggested the development of advanced sophysticated methods to investigate the heterogeneous nature of these fluctuations. In this paper, we analyzed the time dynamics of short-term variability of geoelectrical potential differences and geomagnetic fields obsereved at Kiyosumi (KYS), Uchiura (UCU), and Fudago (FDG) stations, located in the southern part of Boso Peninsula, one of the most seismically active areas in Japan. Anomalous changes in electric and magnetic fields are obeserved in mid-night on October 6, 2002. the anomalous signals observed on October 6, 2002 are different from those originated from the train and other cultural noises according to the investigation on preferred directions of geoelectric field. The investigation of simaltaneous geomagnetic field changes suggest that the source of the electromagnetic change might be generated by underground current because of the polarity pattern oberved at KYS, UCU and FDG. Therefore, electrokinetic assumption under the ground seems one of the possible solutions for the generation of anomalous signals. It is important to understand the ULF electromagnetic environment for the study on the preparation process of crustal activity and systematic understanding both electromagnetic and seismic phenomena.

Investigating correlations of local seismicty with anomalous geoelectrical, hydrogeological and geochemical signals jointly recorded in Basilicata Region (Southern Italy)

This paper presents the preliminary results analysing the correlation between local seismicity and geoelectrical, hydrogeological and geochemical signals concomitantly recorded in Basilicata Region, one of the most seismically active areas in Southern Italy. The signals were recorded by two stations: Tito and Tramutola. Tito station measures vertically the Self-Potential field (SP) by an array of five no-polarizable electrodes equally spaced with the common electrode at 20 m depth as well as water-level, water-temperature and electrical-conductivity. Tramutola station measures self-potential signals in soil surface, gas flow and water temperature in a thermal-water well, as well as atmospheric barometric pressure and ambient temperature. Correlations were found between the sharp variability of the signals recorded by both stations and the seismic sequence that occurred on September 3 to 4, 2004, allowing us to link these anomalies with the tectonic evolution of the investigated area.

The study of local sources of ULF geoelectric signals with steep fronts

The method for recognition and monitoring of ULF electrotelluric signals with steep fronts is presented. The method is based on the multicomponent measurements of the geoelectric field and recognition of the clusters of signals using the data of spatial-polarization filtering. This detects the local sources of the signals associated with the variation of mode of deformation of the medium. The method was applied to the study of geoelectric sources associated with aftershocks of the Racha-Djava earthquake. The field observations show with a fair degree of confidence that these signals are generated by a small number of local zones in the vicinity of the observer (at a range of tens-hundreds meters). The spatial distribution and the dynamic properties of the local sources are described. The analysis of observations leads to the conclusion that the dynamics of the local sources of ULF signals with steep fronts may be considered possible electrotelluric precursors of the earthquakes. Consideration is given to the feasibility of the well-known VAN method for earthquake prediction. The overall methodological aspects of the current studies of the electrotelluric precursors of the earthquakes are discussed.

Investigating the time dynamics of geoelectrical signals measured in two seismotectonic environments in the Mediterranean region: the Southern Apennine chain (Southern Italy) and the Hellenic arc (Crete Island, Greece)

In this paper we explore the time dynamics of geoelectrical signals measured in two seismic active areas in the Mediterranean region: the Southern Apennine chain (Italy) and the Hellenic arc (Crete Island, Greece). After apreliminary filtering procedure carried out to remove man-made and climatic noises, the geoelectrical time seriesmeasured in both the seismological environments show features that are typical fingerprints of stochastic processes.In particular the time fluctuations follow a dynamics well described by an autoregressive model of a first order(red noise). The model has been tested in the frequency and time domains applying advanced statistical methodologies. Taking into account these results, we propose an objective methodology to pick out from geoelectrical time series anomalous patterns from background noise and we study the possible correlation between the appearance of extreme events in the electrical signals and the local seismic activity. Finally an in dept analysis of results obtained in the two investigated areas has been performed.

Pattern synchrony in electrical signals related to earthquake activity

We apply the cross sample entropy method to geoelectrical time series collected from independent channels (North–South and East–West directions) monitored at two sites located in Mexico, to assess the presence of pattern synchrony between the signals, particularly in the proximity of earthquakes. To our best knowledge, this method has not been applied yet for the study of electrical signals related to earthquake activity. Moreover, we introduce the multiscale pattern synchrony analysis by extending the multiscale entropy technique to calculate the cross-entropy between two signals, which represents a novel approach to the study of pattern synchrony. The results obtained suggest that in the vicinity of an earthquake the geoelectrical signals exhibit pattern synchrony that persists for long sequences and through multiple scales, in addition to the presence of correlations in each channel.

Anomalous geoelectric signals possibly related to the 2000 Mt. Usu eruption and 2003 Tokachi-Oki earthquakes

Telemetric observations of pulse-like geoelectric signals induced in a vertical buried dipole at Erimo in Hokkaido, Japan reveal the occurrence of anomalous signals associated with precursor activity to the 2000 Mt. Usu eruption and 2003 Tokachi-Oki earthquakes. The anomalous signals, at least 10 times stronger than the usual background level, are free of contamination due to meteorological or anthropological noise. These geoelectric signals are hypothesized to occur due to electrostatic coupling as a synergetic effect of thermal stimulated exoelectron emission and the release of gas trapped in rock at depth in the precursor stage of major activity.

Multifractal variability in self-potential signals measured in seismic areas

Abstract Multifractal variability in the time dynamics of geoelectrical data, recorded in a seismic area of southern Italy, was studied by means of Multifractal Detrended Fluctuation Analysis (MF-DFA), which allows the detection of multifractality in nonstationary signals. Our findings show that the multifractality of the geoelectrical time series recorded in the study area is mainly due to the different long-range correlations for small and large fluctuations. Furthermore, the singularity spectrum has led to a better description of the signal, revealing a clear enhancement of its degree of multifractality (measured by the variation of the standard deviation of the generalized Hurst exponents h ( q ) or the Hölder exponents α) in association with the occurrence of the largest earthquake. However, in order to assess significant correlations between large earthquakes and patterns of multifractal parameters, an investigation of data sets covering longer periods and different seismotectonic environments is needed. The study also furnishes details on our approach for investigating the complex dynamics of earthquake-related geoelectrical signals.

Multifractal fluctuations in earthquake-related geoelectrical signals

Multifractal fluctuations in the time dynamics of geoelectrical data, recorded in a seismic area of southern Italy, have been analysed. We find that the multifractality of the signal depends mostly on the different long-range properties for small and large fluctuations. Furthermore, we quantitatively characterized the multifractality of the geoelectrical time series, on the basis of the characteristic parameters derived from the multifractal spectrum (maximum α0, asymmetry B and width W) (Shimizu Y, Thurner S and Ehrenberger K 2002 Fractals 10 103). We then analysed the time evolution of the multifractal parameters and we found that the multifractal degree of the signal, revealed by the variation of the width of the multifractal spectrum, is enhanced in association with the occurrence of the largest seismic event. This study aims to suggest another approach to investigate the complex dynamics of geoelectrical signals.

Multifractal variability in geoelectrical signals and correlations with seismicity: a study case in southern Italy

Abstract. Multifractal fluctuations in the time dynamics of geoelectrical data, recorded in a seismic area of southern Italy, have been revealed using the Multifractal Detrended Fluctuation Analysis (MF-DFA), which allows to detect multifractality in nonstationary signals. Our findings show that the geoelectrical time series, recorded in the seismic area of southern Apennine Chain (Italy), is multifractal. The time evolution of the multifractality suggests that the multifractal degree increases prior the occurrence of earthquakes. This study aims to propose another approach to investigate the complex dynamics of earthquake-related geoelectrical signals.