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.

Contribution of Geophysical Prospecting(Geoelectrical Method) for the Hydrogeological Reconnaissance of the Region of Ouled Youssef(Tadla's Basin,Morocco)

The Turonian aquifer of the Tadla’s basin shows at present a pronounced reduction of its hydraulic potential linked to overexploitation and deficiency of effective rains.In order to make an evaluation of the resources of water and implant the exploitation’s drillings of groundwater,a geophysical study by

Tidal wave anomalies of geoelectrical field before remote earthquakes

In this paper, geoelectrical field anomalies at Changli and Xingji station in Hebei Province were analyzed before five remote earthquakes. It was found that the anomalies mainly occurred two or three months before earthquakes, which is of importance to short-term and impendent earthquake prediction. There exhibited different characteristics in geoelectrical field anomalies, but they were closely related to tidal waves, for example, the increasing in ampli- tude at semidiurnal and semimonthly periods of tidal waves; the decreasing or disappearing in amplitude of tidal waves that should have been recorded normally at that time; while there accompanied incremental signals with high frequency, such as jump variations. It was thought that the formation mechanism of the geoelectrical anoma- lies before strong earthquakes resulted from stress-strain resonance effects when rock was weakened during the preparation process in seismic area, and then signals of electric field propagated to remote stations in free space or crust, and superposed on the geoelectric field tidal waves recorded at the stations, leading to increasing or decreasing in amplitude of geoelectrical tidal waves. The high frequency signals may be related to the variation of rock porosity, permeability and so on beneath the stations, as a result of the dynamic effects of remote earthquakes.

Diurnal characteristics of geoelectric fields and their changes associated with the Alxa Zuoqi M_S5.8 earthquake on 15 April 2015(Inner Mongolia)

In China, efforts are being made to monitor geoelectric fields through a large network of stations deployed and managed by the China Earthquake Administration. The diurnal variations in the geoelectric field waveforms were similar in the quiet magnetic periods when K〈5 （generally, K〈3 indicates a quiet time）. The arrival time points of the maxima in the geoelectric field waveforms exhibited differences in local time related to geographic longitude. The amplitude of diurnal variation was several to 16.6 mV/km and decreased with increasing latitude. Further, the amplitude of diurnal variation, which was related to seasonal changes, was larger in summer and autumn than in spring and winter. The periods of diurnal changes during quiet days were 24, 12, 8, 6, 4 hours and several minutes over large areas. Finally, the observed diurnal variations in geoelectric field prior to the Alxa Zuoqi Ms5.8 earthquake on 15 April 2015 were studied, and pronounced changes in the spectral values of the geoelectric fields were found to be associated with the Alxa Zuoqi earthquake in Inner Mongolia.

Study on the variation characteristics of the geoelectric field preceding earthquakes

Earthquake events occurred in Nantong, Jiangsu Province on Nov. 3, and Dec. 25, 2001 in which the biggest mag-nitudes were ML=3.8 and ML=4.1, respectively. This paper firstly explains the principle of the eliminating noise method by the multi-dipole observation system of geoelectric field. Then based on the observation data of the multi-dipole observation system obtained by ZD9A telluric current monitors installed in Chongming and Nanjing stations, we study the abnormal variation of the geoelectric field preceding the earthquakes. The study shows that: a) Eliminating noise method of multi-dipole observation is an excellent method by which many kinds of geoelec-tric field noises can be eliminated successfully and the geoelectric precursor information can be recognized; b) The geoelectric precursor signals for the events were recorded on the NS and NE dipoles in Chongming station 42 days, 20 days and 2 days before the earthquakes respectively, in which the station is near the epicenter, and the longest time of persisting period was 9 days. The abnormal variation signals of geoelectric field observed in Nanjing sta-tion are all the noises but not the seismic electric signals, in which the station is not near the epicenter; c) Dipole distribution method of common electrode is not good in the multi-dipole observation system of the geoelectric field.

A Hydrogeophysical Model of the Relationship between Geoelectric and Hydraulic Parameters, Central Jordan

Geoelectrical soundings using the Schlumberger array were carried out in the vicinity of 23 pumping test sites to determine aquifer parameters, central Jordan. On the basis of aquifer geometry, the area has been di-vided into two hydraulic units: the northern flood plain and the flood plain to its south. Field resistivity data are interpreted in terms of the true resistivity and thickness of subsurface layers. These parameters are then correlated with the available pumping test data. Significant correlations between the transmissivity and modified transverse resistance as well as between the hydraulic conductivity and formation factor were ob-tained for the two hydraulic units, in central Jordan are presented here.

Chemical grout diffusion in porous rock based on response of geoelectric feld

Porous rock chemical grouting simulation test was conducted using established large scale physical model.Chemical grout diffusing rule in porous media with different permeability was studied by monitoring the spontaneous potential and exciting current response during grouting.The results show that chemical grout spread evenly in all directions and diffusion areas are approximately concentric circles in the cross section of homogeneous transverse isotropic pore medium,the grout spread flling range can be quantitatively decrypted by the diffusion radius.The average diffusion speed and radius increase approximately as the square root with the permeability coeffcient in different permeability media under the same conditions.Calculation results using Maag cylindrical diffusion equation show that the calculated value of diffusion radius is in good agreement with the test value under different grouting pressures and permeability conditions.

Groundwater Investigation Using Geoelectrical Method: A Case Study of the Western Region of Cameroon

A geoelectrical survey using the electrical resistivity method was carried out in some villages in the western region of Cameroon to investigate the sub-surface layers and evaluate the characteristics of aquifers. The direct current electrical resistivity method was utilized for the present study. Applying the Schlumberger array, a total of twenty four (24) Vertical Electrical Soundings (VES) were conducted. Quantitative and qualitative interpretations of data were carried out to determine the nature and thickness of the aquifer zone combined with existing litho-logs aided correlation of geoelectric sections to litho-logs. Four to five geoelectric layers were delineated from the survey area. The first layer which is the topsoil has resistivity values ranging from 6 - 949 Ωm and the thickness is between 0.2 - 4.2 m. The second layer which is made up of clay and laterite has resistivity values ranging between 9 - 1862 Ωm and thickness range from 1.0 - 16.4 m. The third and fourth geoelectric layers are made up of clay and granite/basalts with thickness varying from 2.2 - 39.5 m which corresponds to an aquifer horizon. Resistivity values of the aquifer ranges from 10 to 70,506 Ωm. The resistivity map drawn from these measurements shows the presence of a low resistivity zone which indicates the reflection in the direction of ground water from northeast to southwest with the recharge concentrated to the south of the study area. This study has revealed for this area, an average depth of the aquifer of 32 m with the average thickness of the aquifer being 22 m. The geoelectric sections of some VES stations demarcated corroborate very well with the geological description of the area.

Evidence of Iron Mineralization Channels in the Messondo Area (Centre-Cameroon) Using Geoelectrical (DC &IP) Methods: A Case Study

A geophysical survey was conducted in the Kelle-Bidjocka village, Messondo subdivision, in the Centre Region, Cameroon. The data acquisition was made by combining Schlumberger profiling and electrical soundings along six (06) profiles of 1500 m in length for a total of 64 geoelectrical stations’ survey conducted through a variable mesh 100 m × 200 m, or 100 m × 300 m. The equipment used is the DC resistivimeter Syscal Junior 48 (Iris Instrument). Processing and modelling of field data are made by using the Res2Dinv, Qwseln and Surfer software. The investigation methods used are electrical resistivity (DC) and induced polarization (IP) methods. The analyses and interpretations have helped to highlight areas of weakness or conductive discontinuities (fractures, faults, shear zones, etc.) in Precambrian gneiss formations, sometimes undergoing weathering processes. They identify the weathering or mineralogical accumulation horizons, the most promising is a mineralization channel identified in the NE-SW direction. The highlighted mineralization is characterized by strong gradients of chargeability or polarization. Samples and other geological evidences observed in the area are used to associate the most polarizable structures with ferriferous formations. Weakly polarizable and particularly conductive backgrounds identified by the inverse pseudo-sections are thought to be sulphate minerals or groundwater targets for future hydrogeological studies.

Determination of Aquifer Properties and Groundwater Vulnerability Mapping Using Geoelectric Method in Yenagoa City and Its Environs in Bayelsa State, South South Nigeria

Nineteen Schlumberger vertical electrical soundings (VES) were carried out within and around Yenagoa city, South South Nigeria, using a maximum current electrode separation ranging between 300 - 400 m. The objectives of the study were 1) to evaluate the possibility of mapping Quaternary sediments to infer the geological structure from the electrical interpretation and identify formations that may hold fresh water with low concentration of conducting minerals such as iron 2) to evaluate the vulnerability of the aquifer in the study area. The interpretation of the data collected was by computer assisted iterative interpretation using 1-D inversion technique software (1X1D, Interpex, USA). The results of the interpretation revealed four distinct geoelectric layers. Sounding curve types obtained in the area are mostly of the form ρ1 > ρ2 ρ3 ρ4 (where ρ is the density) but fresh water lenses with low concentration of conducting minerals such as iron were obtained only in locations that exhibit ρ1 > ρ2 ρ3 ρ4 curve types. Depth to the aquifer ranges from 4.5 m in the vicinity of VES 05 to 27.0 m at the vicinity of VES 14. The resistivity of the aquiferous horizon varies between 60 - 2868 Ωm. High transverse resistance values obtained were associated with zones of high transmissivity which agrees with the geology of the Benin Formation (Coastal Plain sands) consisting of fine-medium-coarse sands. The aquifer vulnerability map illustrates the impermeability of the overburden clay layer. Values of >0.5 mhos indicate good protective capacity, while values <0.3 mhos indicate vulnerable zones with probable risk of contamination.

Predicting Soil Corrosivity along a Pipeline Route in the Niger Delta Basin Using Geoelectrical Method: Implications for Corrosion Control

The corrosivity of the top three metres of the soil along a pipeline route was determined using soil electrical resistivity for the emplacement of a conduit intended to serve as a gas pipeline. Fifty-six Schlumberger vertical electrical soundings (VES) were carried using a maximum current electrode separation ranging between 24 - 100 m at 2.0 km interval. The data was interpreted using a 1D inversion technique software (1X1D, Interpex, USA). Model resistivity values were classified in terms of the degree of corrosivity. Generally, the sub-soil condition along the pipeline route is non-aggressive but being slightly or moderately aggressive in certain areas due to local conditions prevailing at the measuring stations. Based on the corrosivity along the pipeline route, appropriate cathodic protection methods are prescribed.

Experiments on exploring and monitoring landslip-mass using geoelectric resistivity observations

In the experiments, a high-density resistivity method is used to explore the electric structure of landslip mass, and a resistivity-changing anisotropy method is used to monitor the orientation and speed of main fracture extending of landslip mass. The results are as follows. 1 The exploring experiments have verified a part of creep deformation borderline, the depth and thickness of groundwater horizon, and the property of superstrata in the landslip mass investigated formerly, which have proved that the landslip belts contain rich groundwater; 2 The main fracture extending orientation inferred from the resistivity-changing anisotropy accords with the strike of fracture belt of landslip mass deduced from GPS displacement. Moreover, the changing rates of resistivity-changing anisotropy coefficient matches with the changing speeds of deep displacement of landslip mass were measured by suing clinometer in the borehole.

GPR Wave Propagation Model in a Complex Geoelectric Structure Using Conformal First-Order Symplectic Euler Algorithm

Possessing advantages such as high computing efficiency and ease of programming,the Symplectic Euler algorithm can be applied to construct a groundpenetrating radar(GPR)wave propagation numerical model for complex geoelectric structures.However,the Symplectic Euler algorithm is still a difference algorithm,and for a complicated boundary,ladder grids are needed to perform an approximation process,which results in a certain amount of error.Further,grids that are too dense will seriously decrease computing efficiency.This paper proposes a conformal Symplectic Euler algorithm based on the conformal grid technique,amends the electric/magnetic fieldupdating equations of the Symplectic Euler algorithm by introducing the effective dielectric constant and effective permeability coefficient,and reduces the computing error caused by the ladder approximation of rectangular grids.Moreover,three surface boundary models(the underground circular void model,the undulating stratum model,and actual measurement model)are introduced.By comparing reflection waveforms simulated by the traditional Symplectic Euler algorithm,the conformal Symplectic Euler algorithm and the conformal finite difference time domain(CFDTD),the conformal Symplectic Euler algorithm achieves almost the same level of accuracy as the CFDTD method,but the conformal Symplectic Euler algorithm improves the computational efficiency compared with the CFDTD method dramatically.When the dielectric constants of the two materials vary greatly,the conformal Symplectic Euler algorithm can reduce the pseudo-waves almost by 80% compared with the traditional Symplectic Euler algorithm on average.

Geoelectric Sounding for the Determination of Aquifer Transmissivity in Parts of Bayelsa State, South South Nigeria

The application of geophysical methods in combination with pumping tests provides a cost-effective and efficient alternative to estimate aquifer parameters. In this study, nineteen Schlumberger vertical electrical soundings (VES) were occupied in parts of Bayelsa State using a maximum current electrode separation ranging beweeen 300 - 400 m with the aim of estimating the transmissivity of the alluvial aquifer in areas where no pumping test has been carried out. Four of the soundings were carried out near existing boreholes in which pumping test had been carried out. The VES data obtained was interpreted, and layer parameters such as true resistivities and thickness were determined. The geoelectric parameters were used to generate the Dar Zarrouk parameters. Correlating the Dar Zarrouk parameter (e.g longitudinal unit conductance) with transmissivity derived from pumping test data, a constant was found which translate longitudinal unit conductance to transmissivity in a hydrogeological setting where effective porosity is the primary control on resistivity and hydraulic conductivity. Transmissivity determined from the pumping test data range between 1634.0 - 5292.0 m2/day while transmissivity values estimated from the longitudinal unit conductance (Lc) range between 721 - 8991 m2/day. The transmissivity estimated from the pumping test (Tp) data and transmissivity estimated from the longitudinal conductance (Lc) on comparison show excellent correlation (R2 = 0.92). The high transmissivity values agree with the geology of the Benin Formation (Coastal Plain sands) consisting of fine-medium-coarse sands. The results give a useful first approximation of the transmissivity and could be used to site exploratory boreholes.

Characterization of Pan-African Aquifer Layers by the Least Squares Inversion Method Applied on Geoelectric Data

Geoelectric data obtained from forty (40) vertical electrical soundings collected with a Schlumberger device in the Adamawa plateau region, also known as the Cameroon water tower, have been treated by the least-squares inversion method. In order to study the nature and thickness of the aquifer and the necessary geoelectric parameters, quantitative and qualitative interpretations of the data were made. The results obtained showed that: about four to five geoelectric layers have been delimited in the study area with a dominance of the KH curve, which can be used as a reference for future studies. The first two layers constitute an association of clay and laterite with resistivity values ranging from 58 to 9122 Ω&middotm and whose thickness is between 0.6 and 13.4 m. The third layer is a potentially aquiferous laterite composed of clay, laterite and especially clay sand and cracked/good granite, with a dominance of sandy alteration whose resistivity values are between 81 and 960 Ω&middotm and its thickness between 12.2 and 26.8 m. The fourth and fifth layers are made up of cracked/good granite with a resistivity ranging from 12-10705 Ω&middotm with an average value of 1817 Ω&middotm. This study also shows that the North-East, South-West and South sectors could be the groundwater convergence zones and that the average depth of the basement aquifer roof is about 28.3 m. The geoelectric sections of certain demarcated vertical electrical sounding stations are consistent with the geologic description of the area.

Numerical Calculation of Geoelectric Fields That Affect Critical Infrastructure

One of the modern applications of geomagnetism is determining the effect of geomagnetic disturbances on critical infrastructure such as power systems and pipelines. Assessing the geomagnetic hazard to such systems requires calculation of the geoelectric fields produced during geomagnetic disturbances. Such geoelectric fields can then be used as input to system models to calculate the impact on the system. This paper describes what is involved in calculating the geoelectric fields produced during real geomagnetic disturbances. The theory of geomagnetic induction is presented and used to derive the Earth transfer function relating the geoelectric and geomagnetic field variations at the Earth’s surface. It is then shown how this can be used to make practical calculations of the geoelectric fields and how the calculation process can be verified by comparison with analytic solutions obtained with synthetic geomagnetic variation data. The accuracy of the calculated geoelectric fields for geomagnetic risk assessments is limited, not by the accuracy of the calculation methods, but by the availability of geomagnetic field measurements and Earth conductivity information over the whole extent of the affected infrastructure.

Application on Anomaly Detection of Geoelectric Field Based on Deep Learning

The deep learning method has made nurnerials achievements regarding anomaly detection in the field of time series.We introduce the speech production model in the field of artificial intelligence,changing the convolution layer of the general convolution neural network to the residual element structure by adding identity mapping,and expanding the receptive domain of the model by using the dilated causal convolution.Based on the dilated causal convolution network and the method of log probability density function,the anomalous events are detected according to the anomaly scores.The validity of the method is verified by the simulation data,which is applied to the actual observed data on the observation staion of Pingliang geoeletric field in Gansu Province.The results show that one month before the Wenchuan M_S8.0,Lushan M_S7.0 and Minxian-Zhangxian M_S6.6 earthquakes,the daily cumulative error of log probability density of the predicted results in Pingliang Station suddenly decreases,which is consistent with the actual earthquake anomalies in a certain time range.After analyzing the combined factors including the spatial electromagnetic environment and the variation of micro fissures before the earthquake,we explain the possible causes of the anomalies in the geoelectric field of before the earthquake.The successful application of deep learning in observed data of the geoelectric field may behefit for improving the ultilization rate both the data and the efficiency of detection.Besides,it may provide technical support for more seismic research.

Saltwater Intrusion in Jizan Coastal Zone, Southwest Saudi Arabia, Inferred from Geoelectric Resistivity Survey

This work focuses on the causes of water quality deterioration in the coastal plain of Jizan area, southwest Saudi Arabia using vertical electrical sounding (VES) surveys. Schlumberger electrode array is used in the study with the current electrode spacing ranging from 400 to 600 m to delineate the thickness of the shallow aquifer and its possible interaction with the sea water. The differences in resistivity are associated with the variations in lithology and groundwater saturation and salinity. The interpretation of VES curves reveals low resistivity zones characterizing the study area. These zones reflect saline water intrusion in the coastal aquifer. Generally, it is observed that the resistivity of saturated zone decreases towards the sea, indicating the influence of seawater. Based on the interpretation of the constructed resistivity pseudo-sections and 1-D sequential inversion models, three factors are identified to control the seawater intrusion into the shallow groundwater aquifers: 1) presence of faults that contribute extensively in the seawater intrusion as the seawater invades the coastal aquifers through the crushed rocks in fault zones related to the Red Sea rifting, 2) over-withdrawal of groundwater from the coastal aquifers, resulting in saline water intrusion from the sea into the freshwater aquifer, and 3) the lithological variation where the alluvial sediments of the ancient buried wadi (dry valley) channels provide potential pathways for saltwater intrusion and make a hydraulic connection between the aquifer and the sea water.

Geoelectrical Investigation of a Proposed Dam Site in a Sedimentary Terrain: Case Study of Aba River at Amapu-Ideobia, Akanu Ngwa Southeastern Nigeria

The geoelectrical investigation of the proposed Aba River dam at Amapu-Ideobia, Akanu Ngwa Southeastern Nigeria has been carried out. The objective of this study is to determine the downward and lateral trends of the rock layers or units along and near the proposed dam axis and deduce the possible structures that may enhance workable design of the dam. ABEM Terrameter SAS 4000 model was used and the symmetrical Schlumberger configuration was adopted. Twelve (12) Vertical Electrical Sounding stations were located and fully occupied along the dam axis. Preliminary input data from the field were fed into Zohdy software to generate real resistivities and depths to geoelectric layers. Five geoelectric layers were interpreted as Loamy Top soil, Alluvial matter, Pebble bed, Sandy lateritic and Gravely sand. Layer 1 (the top loamy soil) was encountered in VES 1, 2, 3, 4, 5, 9, 10, 11 and 12 locations with maximum thickness of 1.5 m in VES 3 and 4. Resistivity values ranged from 216 to 519 Ohm-m. The second layer (lateritic matter) had a maximum lower depth of 5 m in VES 3 and 4 points. This was not encountered in VES 6 point being replaced by alluvium. Resistivity values ranged from 101 to 6190 Ohm-m. Layer 3 was interpreted as a restricted pebble bed which occurred only at VES 6, 7 and 8 locations flanking the river course with thickness of about 3.5 m and resistivity values range of 182 415 Ohm-m. The fourth layer was modeled as the alluvial matter and restricted to the river course (VES 6, 7, 8) locations with base at between 12.5 m in VES 8 and 8 m in VES 6. The last modeled layer (Layer 5) was composed of gravely sandstone that underlined the whole study area apart from the restricted pebble bed at the NE crestal portion (VES 12). No structures like fractures, lineaments and faults that would be of deleterious effect were observed in all the VES points down to about 40 m. However, it was observed that the axial length had overriding sandy matter with high porosity and potentially rife for great infiltration;a condition that could facilitate seepage around the reservoir portion of the dam.

Geoelectric Structure of the Crust along the Mbere Trough (South Adamawa) from Audio-Magnetotelluric Data

The South Adamawa trough represents a major structure associated to the tectonic activity in the Adamawa region. The Mbere and the Djerem troughs represent the eastern and the western parts of the South Adamawa trough respectively. The audio-magnetotelluric (AMT) method is used to study the Mbere trough and to bring out tectonic features that affect the basement of this region during the Cretaceous. From analysis and interpretation of AMT data, based on pseudo-section and geoelectric section, two geologic models are constructed across the trough at Belel and Nyambaka. Those models bring more information on the sediments thickness and the borders faults of the trough compared to previous studies. The results shows that the Mbere trough is dissymmetric, and bounded on its northern side by an intra-gneissic fault and on its southern side by two faults with at least one considered as a faulted contact between the granitic and gneissic rocks. The sediments filling of the trough are consist mainly of conglomerates on which lay sandstones. The thickness of these sediments is about 2060 m at Belel and decreases to about 1800 m at Nyambaka, with a low sandstones thickness, which does not exceed 360 m. The basement consists of granite in the South of the trough and gneiss in the North, with resistivity more than 10,000 Ω.m. The Moho is located between 25 and 35 km in the South Adamawa region.