Geoelectric Variations Research Articles

Frequency Considerations in GIC Applications

AbstractGeomagnetically induced currents (GIC) are a phenomenon well known for its negative effects on the operations of power systems. To efficiently mitigate them requires different types of power system modeling, from GIC to alternating current harmonic generation, to three‐dimensional finite element models of transformers. GIC are initiated by variations of the geomagnetic field in the presence of the conductive Earth, that is, the geophysical variables characterized by continuous frequency spectra, making GIC also exhibit continuous spectra. In order to adequately estimate their variations and peak values for mitigation purposes, an analysis is required of how sampling rate and spectral frequency content impact the measured characteristics of GIC and harmonics. The study is based on the geomagnetic measurements and the power network data (i.e., GIC and harmonics) with high sampling rates recorded during two geomagnetic storms, March 31, 2001 and July 26–27, 2004. Availability of data covering both the source and the result of geomagnetic storm impacts on power grid allows (a) analysis of the influence of spectral content on adequate representation of both geomagnetic and geoelectric variations during the intervals with significant increases in GIC and harmonics and (b) identifying the sampling rate sufficient to usefully represent the network response presented as GIC and harmonics variations. In summary, the adequate sampling rate is suggested and the deficiencies associated with undersampling of the geoelectric and GIC variations are identified and discussed.

Geoelectrical investigation of foundation failure in Akowonjo, Ogbomoso, Nigeria

An electrical resistivity investigation was carried out in Akowonjo, Ogbomoso, Nigeria with the aim to obtain various lithological units and subsurface geological features within the study area. Fifty-four sounding points adopting Schlumberger array, with 100 m as the maximum value for half-current electrode spacing were occupied in the study area. Partial curve matching technique and automated platform (known as WinResist) were used to interpret the obtained data qualitatively and quantitatively. The resulting final layers parameters involving resistivity and thickness of each layer were used to map the two-dimensional geoelectric variations beneath the study area. The curve types obtained from iterations of WinResist, which is based on 1-D forward modelling approach include H, KH, HA and QH. Four subsurface layers were identified. These layers are topsoil, laterite, weathered/partly weathered layer and fractured/fresh bedrock. The topsoil had resistivity and thickness values ranging from 76 – 1858 Ωm and 0.4 – 4 m respectively. The lateritic layer lies under the topsoil with resistivity and depth values ranging from 649 – 2021 Ωm and 0.7 – 1.9 m, followed by a weathered/partly weathered layer with resistivity and depth values ranging from 17 – 880 Ωm and 1.9 – 25.2 m, respectively. The fourth layer is the fractured/fresh bedrock which depict varying resistivity from 260 - 33385 Ωm. It is revealed that thick clayey materials dominate the first and second subsurface layers within the study area. This is known to be the major cause of cracks experienced in the buildings in Akowonjo community, Ogbomoso, Nigeria. It is recommended that further foundations for civil engineering works in the study should be extended beyond the zone of water content fluctuation.

The Diurnal Variation of Geo‐Electric Field Along the Longitude and Latitude Chains in China Mainland

AbstractThis paper studied the time and frequency characteristics of the geo‐electric field diurnal variation along two longitude chains and two latitude chains in China mainland, using the data of geo‐electric field observed at 37 stations along the chains. The following significant results are obtained. The diurnal variation at most stations appears as semi‐diurnal waves twice a day, near before and after the noon; in descending order of the FFT amplitude spectrums, the main periods of the diurnal variation are 12.4 h/12 h, 8 h, 24 h, etc., which are in accordance with the periods of the tidal harmonic components; the diurnal variation changes with latitude as the wave crest‐to‐trough amplitude gradually becomes bigger/smaller with reduced/increased latitude of the station, and also varies with longitude as the phase difference agrees with the local‐time difference between the stations along the same latitude. Furthermore, the amplitude of the diurnal variation is related with the Loyd season, which reduces orderly in J‐, E‐ and D‐seasons. The possible reasons which cause the main periodic components of the diurnal variation have been discussed, as a result, it is believed that the variation of the ground eddy current intensity caused by lunisolar tidal forces and the space electromagnetic activity aroused by the solar wind jointly produce the semi‐diurnal period components of the geo‐electric diurnal variation.

Annual variations in the electromagnetic field of the earth and electrical conductivity of the geological environment

Synchronous annual variations in the geoelectric and geomagnetic field are studied on the basis of long-term electromagnetic monitoring. It is shown that the annual geoelectric variations have intraterrestrial origin and are not related to the annual geomagnetic variations. Temporal variations in the magnetotelluric impedance and magnetic tipper, which characterize the electrical conductivity of the geological environment, are analyzed. It is established that annual variations in the magnetotelluric impedance mainly describe the variations in the electrical conductivity of surface crustal layers and are less sensitive to the deep electrical conductivity of the Earth. The annual variations in the imaginary magnetic tipper at the periods of 1000–3000 s probably reflect the changes in conductivity of a deep transversal low-resistive zone (the fault). It is suggested that annual variations in the geoelectrical and geomagnetic fields, as well as in the electrical conductivity of the geological environment, arise as a response to the changes in the geodynamical processes caused by the revolution of the Earth around the Sun.

Derivation of characteristics of the relation between geomagnetic and geoelectric variation fields from the surface impedance for a two-layer earth

The surface impedance is defined to give the ratio between horizontal geoelectric and geomagnetic variation fields at the earth’s surface in the frequency domain. Studying the properties of the surface impedance enables conclusions about the corresponding relation between the surface electric and magnetic variation fields in the time domain. In particular, it is possible to perform an investigation about assumptions that lead to a proportionality between the geoelectric field and the time derivative of the geomagnetic field and about situations that make the electric field and the variations of the magnetic field proportional. The results are directly applicable to the research of geomagnetically induced currents (GIC) driven by the geoelectric field in technological networks at the earth’s surface. Thus, the main objective of this paper is not in traditional magnetotellurics but in studies of ground effects of space weather. We use a two-layer earth model, which is simple enough to have a precise analytic formula for the surface impedance. It is shown in this paper that a poorly-conducting upper layer above a highly-conducting bottom is favourable to the electric field (and GIC) being proportional to the magnetic time derivative at the earth’s surface whereas a thin highly-conducting upper layer above a less-conducting bottom results in a surface electric field (and GIC) proportional to magnetic variations.

An electromagnetic sounding experiment in Germany using the vertical gradient of geomagnetic variations observed in a deep borehole

SUMMARY We have recorded for 13 d, geomagnetic variations simultaneously on the Earth's surface and in a borehole at 832 m depth straight below, with a sampling rate of 1 Hz. In addition, geoelectric variations were observed at the same site near Bad Konigshofen in Frankonia, Germany. The penetrated moderately conductive Triassic sediments lie above highly resistive Permian deposits. A presumably crystalline basement begins at 1500–1900 m depth. The purpose of the experiment is to determine the skin effect of geomagnetic variations and to derive from it the equivalent to the magnetotelluric (MT) surface impedance, using the vertical gradient (VG) method of electromagnetic (EM) sounding. In this way, we were able to reproduce all four elements of the MT impedance tensor, except for an unexplained but consistent downward shift of VG phases against MT phases by roughly 15° for the two off-diagonal elements. Hence, our tensor evaluation goes beyond the common practice, to express the skin effect by a single VG transfer function in response to a layered structure. The otherwise good agreement of VG and MT results implies that at our test site, the MT impedance tensor is largely distortion-free and that, for example, its pronounced anisotropy should be regarded as a genuine characteristic of the EM response for a laterally non-uniform or possibly anisotropic deep structure. The drilling site lies within the range of a widespread induction anomaly. We have observed the resulting variations of the vertical magnetic component at the surface and in the borehole and found them to be identical. The thus established absence of a skin effect for the vertical component allows us to treat the sedimentary layer down to the depth of the borehole instrument as a thin sheet, and the pertinent thin-sheet approximation for EM induction forms the basis of our analysis. We have derived the required estimate of conductance from the skin effect of horizontal components, noting that this estimate has to be real valued and the same for all frequencies. We were unable, however, to verify the resulting value of 76 S with independent geoelectric direct current (DC) measurements in the borehole and at the Earth's surface. A model study shows that the thin-sheet conductance, which is relevant for the skin effect, may be substantially higher than the depth-integrated conductivity from DC data. Robust estimates of transfer functions were derived for 20 frequencies from 5.625 to 0.007 cpm, which corresponds to periods from 10 to 10000 s. Squared skin effect coherencies are above 0.9 for periods longer than 20 s and thereby comparable to MT coherencies.

Spectrum characteristics of geoelectric field variation

The spectrum characteristics of geoelectric diurnal variation and geoelectric storm have been identified by maximum entropy method, based on geoelectric data from seven stations in the Chinese mainland, including Jiayuguan, Changli and Chongming. The study shows that, in geoelectric diurnal variation, the amplitude of the 12 h semidiurnal wave is the largest, followed in turn by the 24∼25 h diurnal wave and the 8 h periodic wave; Geoelectric storm usually occurs in a large-scale space, whose spectrum values are higher than those of geoelectric diurnal variation by 2∼3 orders of magnitude. A preliminary interpretation is presented for the generative mechanism of predominant waves in geoelectric field variation.

Application of statistical models to geoelectric variations with a long electrode span to detect anomalous changes

We examined two techniques for eliminating components induced by variations of the geomagnetic field to accurately and quickly detect anomalous changes in the geoelectric field, which would enable us to identify crustal signals,. One method is based on a multiple regression model, where any geoelectric variation is presented as a linear combination of the past, present, and future values of geomagnetic variations. The induced geoelectric variation is estimated as a convolution of the geomagnetic variations and the parameters of the obtained regression model. The other method uses BAYTAP-G, by which the observed geoelectric data are separated into four components: 1) tidal, 2) electromagnetically induced, 3) trend, and 4) irregular components. Signals due to volcanic and/or seismic activity may be detected since the trend and the irregular component indicate variations of the self-potential of the crust and/or noise, provided that the amplitudes exceed that of the typical variation. The geomagnetic field at the Kakioka Magnetic Observatory was used as associated data for both methods. These methods were applied to geoelectric variations in the Numazu group, which are contaminated by artificial noise but show anomalous changes. The multiple regression method can apparently eliminate daily variations and telluric storms and can clarify anomalous changes that are not obvious in the original data. However, the gain and phase characteristics calculated from the estimated parameters of the model do not yield information on the underground resistivity structure since the estimated impulse responses do not correctly reflect relationships between the inducing geomagnetic field and the induced geoelectric field due to the presence of excessive artificial noise. BAYTAP-G separates daily variations due to artificial noise as the tidal component and the responses to geomagnetic variations. Anomalous changes are identified in the trend and the irregular component.We investigated the relationship of the tidal component of geoelectric variations estimated by BAYTAP-G with the tidal current and the tide. The correlation between the tidal component of geoelectric variations and tidal currents could not be clarified since simultaneous data of tidal currents with geoelectric variations were not available. Temporal variations of the amplitudes of geoelectric variations of KSM-MTO and tides at OAR for O1 and M2 constituents were not correlated despite their high coherencies.

Estimating apparent resistivity and phase at low frequencies using telegraphic facilities

We propose a time-domain method for estimating the apparent resistivity and phase. The method utilizes a multiple regression model where the order is determined by minimizing the AIC (Akaike Information Criterion). The method is superior to conventional frequency-domain methods based on spectral analysis, since the data length necessary for the latter methods is several times longer than that for the former in estimating the apparent resistivity at the same low frequency. Therefore, applying this method to shorter length data having long wavelengths like those in a magnetic storm, we can estimate apparent resistivities and phases at low frequencies with little effect of the finite wavelength of the inducing geomagnetic field. This method was applied to geoelectric field data observed by means of telegraphic facilities and geomagnetic field data at the Kakioka Magnetic Observatory, which is the standard observatory in Japan, located in the observation network of the geoelectric field. The geoelectric field data are very stable over a long time period because the electrodes are buried to a depth of more than 5 meters and have a contact resistance of less than 2 ohms. Furthermore, the geoelectric data had a high signal-to-noise ratio (signal is induced variations), because the lengths of observation lines ranging from 18.8 to 27.4 km were much longer than that of ordinary observations and the greater part of geoelectric variations were induced by geomagnetic variations. Apparent resistivity and phase in the low-frequency range from 3.3 × 10-5 to 10 × 1.0-3 Hz (corresponding to periods of 512 to 16 minutes) were estimated. Since the time span of the data used in the analysis was within the period of a large-scale magnetic storm, the estimates were unlikely to be affected by the finite wavelength of the inducing geomagnetic field.

Geoelectric variations related to earthquakes observed during a 3‐year independent investigation

We present observations of the geoelectric field prior to some earthquakes. The data were collected during a three year (1992–1994) independent experimental investigation of VAN at the University of Patras Seismological Center. The recorded signals were: a) Gradual Variations of the Electric Field (GVEF), b) Periodic Variation of the Electric Field (PVEF), and c) Seismic Electric Signals (SES).

The character of the geoelectric field observed with a long electrode span

Observations of the geoelectric field with a long electrode span by making use of facilities of the Nippon Telegraph and Telephone Company (NTT) have been carried out in two areas. Mito and Numazu, since 1985. Observed geoelectric variations with periods longer than several hours are more reliable, because the contact potential of electrode used in these observations to the ground is very stable compared with that of ordinary observations. Precipitation does not have an adverse effect on these observations. Geoelectric variations of the Mito group are mainly those induced by geomagnetic variations, and are very similar to those observed at the Kakioka Magnetic Observatory, which is 30 km distant from Mito, except for the amplitudes and the predominant direction. On the other hand, the greater part of the short-period geoelectric variations of the Numazu group is attributed to leakage currents from railways supplied with direct current power, and from industrial electrical facilities. Nevertheless, there are some anomalous changes in the geoelectric variations of the Numazu group which seem to relate to near-tectonic activities.

Poynting vectors in geomagnetic and geoelectric variations.

Poynting vectors in geomagnetic and geoelectric variations.

Effect of Terrestrial Electromagnetic Storms on Wireline Communications

Effects of natural electromagnetic phenomena on wireline circuits are discussed. An important factor causing degradation of system operation or damage to equipment is the occurrence of earth currents, or geoelectric variations, associated with electromagnetic storms. Three to six earth-current storms occur yearly, during and after the maximum of the sunspot cycle. Peak disturbances last 10 to 30 minutes and return to normal within 6 to 48 hours. Typical disturbances are 3 to 20 v/km in the auroral zones, 0.5 to 5.0 v/km in middle latitudes, and less than 100 mv/km near the equator. Current flow is generally N-S, except near the geomagnetic equator where it is E-W. Undisturbed diurnal variations exhibit the same general characteristics, but attain values in the range of 5 to 300 mv/km. Cases are cited of adverse effects on commercial circuits; open wire, buried cable, and submarine cable systems are compared. Geophysical processes underlying geoelectric variations are discussed briefly.