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For Selected: Download Citations EndNote Ris BibTeX Toggle Thumbnails Select PROBING THE SUBSURFACE ELECTRIC STRUCTURE FOR CSELF NETWORK IN CAPITAL CIRCLE REGION DONG Ze-yi, TANG Ji, ZHAO Guo-ze, CHEN Xiao-bin, CUI Teng-fa, HAN Bing, JIANG Feng, WANG Li-feng SEISMOLOGY AND GEOLOGY    2022, 44 (3): 649-668.   DOI: 10.3969/j.issn.0253-4967.2022.03.006 Abstract （850）   HTML （32）    PDF（pc） （13890KB）（359）       Knowledge map    Save The first control source extremely low frequency(CSELF)electromagnetic observation network through the world, consisting of 30 fixed stations located in the Beijing captical circle region(15 staions)and the sourthern secton of the north-south earthquake belt(15 stations), China, has been established under the support of the wireless electromagnetic method(WEM)project, one of the national science and technology infrastructure construction projects during the 11th Five-year Plan period. As a subsystem of the WEM project, the CSELF network is mainly to study the relationship between elctromagnetic anomalies and mechanisms of earthquake, and further improve our ability to monitor and predict earthquakes by monitoring real-time dynamic changes in both electromagnetic fields and subsurface electric structure. Carrying out the detection of the underground background electric structure in the CSELF network area/station is an important part of this project and of great significance to play its role in the study of earthquake prediction and forecast. In this paper, we elaborate how to acquire the subsurface electric structure of the CSELF network in the Beijing captical circle region and make a simple explanation for the structure. Firstly, a short magnetotelluric(MT)profile, almostly perpendicular to the regional geological strike, was deployed at each station of the CSELF network in the capital circle region during the 2016 and a total of 60 broadband MT sites was collected using ADU -07e systems. Then, all the time series data were processed carefully using the robust method with remote reference technique to MT transfer functions. MT data quality was assessed using the D+algorithm. In general, data at most sites are of high quality as shown by the good consistency in the apparent resistivity and phase curves. Different impedance tensor decomposition methods including the phase tensor analysis, Groom and Bailey(GB)tensor decompositon, and statistical image method based on multi-site, multi-frequency tensor decompositon were used to analyze data dimensionality and directionality. For data inversion, on the one hand, one-dimensional(1-D)subsurface electrical resistivity structures at each station and MT site were derived from 1-D adaptive regularized MT inversion algorithm. On the other hand, we also imaged the 2-D electric structures along the short MT profile by the nonlinear conjugate gradients inversion algorithm at each station. Robustness of all 2-D structures along each short profile were verified by sensitivity tests. Although fixed stations and MT sites are limited and distributed unevenly, the 3-D inversion of 15 stations was also performed to produce a 3-D crustal electrical resistivity model for the entire network using the modular system for 3-D MT inverson: ModEM based on the nonlinear conjugate gradients algorithem. Intergrating 1-D, 2-D and 3-D inversion results, the resistivity structure beneath the CSELF network in captical circle region revealed some significant features: The crustal electrical structures are mainly characterized by high resistivity beneath the Yinshan-Yanshan orogenic belt in the northern margin of North China, the Taihangshan area in the middle, the Jiao-Liao block in the east, while the North China Plain and Shanxi depression areas have relatively lower resistivity in the crust; There are obvious electrical resistivity difference on both sides of the gravity gradient of Taihang Mountains and the Tanlu fault zone, which indicates they could be manifested as an electric structure boundary zone, respectively. Overall, the electric structure characteristics of the entire network area shows high correspondence with the regional geological structure and earthquake activity to some extent. In summary, implementing the detection of underground electrical resistivity structure in the CSELF network of the capital circle region will provide important foundations for the researches on the regional seismogenic environment, the generation mechanism of seismic electromagnetic anomaly signals, and earthquake prediction and forecast. Table and Figures | Reference | Related Articles | Metrics Select THE ELECTROMAGNETIC ANOMALY OF TANGSHAN GUYE MS5.1 EARTHQUAKE ON JULY 12, 2020 FAN Ye, TANG Ji, MIAO Jie, YE Qing, CUI Teng-fa, DONG Ze-yi, HAN Bing, SUN Gui-cheng SEISMOLOGY AND GEOLOGY    2022, 44 (3): 669-685.   DOI: 10.3969/j.issn.0253-4967.2022.03.007 Abstract （1215）   HTML （28）    PDF（pc） （6006KB）（173）       Knowledge map    Save On July 12, 2020, an MS5.1 occurred in Guye, Hebei Province, and as the largest earthquake in the capital circle in recent years, its unique geographical location has attracted more attention. During an earthquake, the electromagnetic properties of underground media will change, so dense electromagnetic observation stations were arranged in the capital circle. In this study, the data of geoelectric resistivity, geoelectric field, and extremely low frequency(ELF)observation within 400km of the Guye earthquake are analyzed using a combination of time-domain waveform analysis, sliding Fourier analysis with annual variation removed, normalized variation rate method(NVRM), and geo-electric azimuthal method. After eliminating the influencing factors such as operation status, observational environment, and the spatial electromagnetic effect, we analyzed the characteristics of electromagnetic phenomena that may be related to the Guye earthquake preliminarily and found that there was a variation process of “trend decrease—accelerated decrease—postseismic recovery” observed in 6 geoelectric resistivity stations and that the normalized variation rate exceeded the threshold value of ±2.4 in 7 stations within one year before the earthquake. In Luanxian station, the intensity of the geoelectric field in the north-south and north-western directions decreased and then rose back before the earthquake. In addition, the azimuth shifted to the direction of the Guye earthquake in the preseismic period, and then returned to the direction of the Luanxian-Laoting Fault. The ELF stations in Wen'an and Fengning precisely recorded the coseismic change of the 16Hz natural magnetic field, in which the variation of the vertical component is twice larger than that of the horizontal component. Under the condition of large subsurface structure difference beneath the stations, the observed electric values from the two stations are distinctively different; moreover, the coseismic disturbance is submerged by the background noise. The subsurface electric structure was obtained by interpolating and inversing the data collected from the ELF stations in the capital area, which indicates that the Guye earthquake occurs near the boundary of the electric property changes. Meanwhile, it shows high electric resistivity in the northern area, low electric resistivity in the southwestern area, and partially low electric resistivity in Baodi and Wen'an, which is consistent with the location of the abnormally stronger ground motion. Regarding the spatial selectivity of the anomalies, we believe it may be related to the direction of the two main conjugated structures in the capital area, which lie in NEE and NW direction, respectively. And the study also enlightens researchers that the investigation of the mechanism of seismic electromagnetic anomaly should start from the coseismic phenomenon, and then focus on the aspects of seismic signal source and propagation path, because the extremely low-frequency observation band is wide and the coseismic electromagnetic signals can be clearly recorded. There are many effective ways to extract electromagnetic signals related to earthquakes from strong interference background, such as making retrospective analysis of moderately strong earthquakes in time, summarizing the electromagnetic anomaly characteristics of different earthquake events, densifying the electromagnetic observation layout appropriately, so that the abnormal information can be mutually corroborated and a variety of means for fusion and comparative analysis can be developed. Table and Figures | Reference | Related Articles | Metrics Select THE SEISMOGENIC STRUCTURE OF THE 1303 HONGTONG M8 EARTHQUAKE INFERRED FROM MAGNETOTELLURIC IMAGING ZHAO Ling-qiang, ZHAN Yan, WANG Qing-liang, SUN Xiang-yu, HAN Jing, CAO Cong, ZHANG Song, CAI Yan SEISMOLOGY AND GEOLOGY    2022, 44 (3): 686-700.   DOI: 10.3969/j.issn.0253-4967.2022.03.008 Abstract （1197）   HTML （33）    PDF（pc） （4532KB）（235）       Knowledge map    Save In the early Autumn of 1303AD, a large earthquake with a tremendous impact occurred in the northeast of Hongtong County, Shanxi Province, and this earthquake was the first major earthquake of M8 identified by seismogeologists through the study of historical records. The magnitude of the earthquake was large, and the isoseismal line was distributed in the NNE direction. The meizoseismal area was mainly located in the densely populated Fenwei fault-depression zone, so it caused great economic and property losses and casualties at that time, and left a lot of historical data. Most scholars have identified the seismic rupture of this earthquake as the Huoshan piedmont fault, but the current research methods are focused on geological methods such as seismogeological surveys and trenching. At present, in addition to seismogeological investigation and research, there is an urgent need for detailed geophysical exploration of the fine structure and seismogenic environment of the 1303 Hongtong earthquake area and the deep structure of the Huoshan piedmont fault. The phase tensor decomposition techniques and NLCG three-dimensional inversion were used to process the data of a MT profile, which is 160km in length and across the 1303 M8 Hongtong earthquake area, combined with the present-day crustal vertical motion data(including GPS and leveling data)and the latest geological and geophysical survey results in and around the study area. The results show that the Huoshan piedmont fault is an obvious large electrical boundary zone in the study area. In the middle and deep part, it is a low resistivity belt, which runs through the whole scale of the crust. The fault is a NNE-trending dextral normal fault, which may be the basement fault dividing Ordos block and North China block, extending from the surface to 40km underground. The Lishi Fault also shows as an obvious electrical boundary zone, which may be a large-scale fault system in the study area. With the Huoshan piedmont fault as the boundary, the Ordos block and North China block on the east and west sides of the fault show different electrical structural characteristics. The Ordos block in the west shows a stable tectonic environment, while the lithosphere in the North China block in the east is seriously damaged and has a trend of thinning. The results of magnetotelluric survey support the point that the Huoshan piedmont fault is the seismogenic fault of Hongtong earthquake in 1303. The earthquake might occur in the low resistivity zone under the Huoshan piedmont fault, and the focal depth may be between 10~20km. We believe that the seismogenic environment of the 1303 Hongtong earthquake may be controlled by multiple factors, such as the northeastward extrusion of the Qinghai-Tibet Plateau and the possible overall counterclockwise movement and uplift of the Ordos block, which led to an obvious right-slip movement of the Huoshan piedmont fault near the Linfen Basin. The upwelling of soft fluvial material in the lower and middle crust of the eastern part of the Linfen Basin caused the regional extension of the North China craton, leading to dip slip of the Huoshan piedmont fault, which may be the main controlling factor for the generation of this earthquake. Table and Figures | Reference | Related Articles | Metrics Select APPARENT RESITIVITY VARIATION OF TONGWEI SEISMIC STATION BEFORE THE MINXIAN-ZHANGXIAN MS6.6 EARTHQUAKE IN 2013 XIE Tao, YU Chen, WANG Ya-li, LI Mei, WANG Zhong-ping, YAO Li, LU Jun SEISMOLOGY AND GEOLOGY    2022, 44 (3): 701-717.   DOI: 10.3969/j.issn.0253-4967.2022.03.009 Abstract （785）   HTML （13）    PDF（pc） （4997KB）（193）       Knowledge map    Save A MS6.6 earthquake occurred at the junction area of Minxian and Zhangxian, Gansu Province, on July 22, 2013. Before the earthquake, the apparent resistivity observed at Tongwei station showed abnormal anisotropic changes. Electrical resistivity is an important physical property for sedimentary rock-soil. The continuous load of compressive stress, by causing crack growth and directional alignment, would tend to increase the connectivity of these crack films. Build-up of strain at the locked fault segment and its vicinity area before an earthquake ought to be accompanied by change in resistivity. Laboratory measurements of resistivity on rock specimens under deformation to failure under uniaxial and triaxial compression show that resistivity of water-bearing rocks declines as the stress exceeds about half of the fracture stress. The decline rate increases considerably near the stage of final fracture. The magnitude of resistivity change in axial direction is usually greater than that in the transverse direction. In-situ experiments taken on field soil using Schlumberger arrays also showed decline change in apparent resistivity under compression stress loading. Monitoring arrays in different directions at the same set of array usually have different magnitudes of change, i.e. anisotropic changes. The array perpendicular to or near perpendicular to the P axis has the maximum magnitude of change, while the magnitude of change is the minimum or even unnoticeable when the array is parallel to or sub-parallel to the P axis. It can be expected from the above experiment results that absolute stress level is often needed to discuss the relationship between crack variation and stress. However, it is difficult to obtain successive absolute stress-strain measurement at present for a large tectonic region. On the other hand, the general quantitative mathematic relationship between the stress level and micro-crack activity is not clear. One alternative compromise way is to obtain the qualitative spatial distribution characteristic of the stress-strain accumulation required to produce the coseismic slip using the fault virtual dislocation model. In this paper, we use the fault virtual dislocation model to analyze the changes in the apparent resistivity data of Tongwei station before the earthquake. In the model, the coseismic sliding displacements of the earthquake are loaded in the same magnitude but opposite directions, in order to calculate the stress-strain distribution required to generate these coseismic dislocations before the earthquake. The areas of compression enhancement or relative expansion before an earthquake can be displayed. It should be noted that results from the virtual dislocation model are the changes of stress or deformation, not the absolute state of stress-strain. Northeast margin of Tibetan plateau is in compressive tectonics as a whole. The compression areas from the virtual dislocation model can be seen as areas with compression enhancement before the earthquake. However, for the extension areas from the model, we cannot distinguish them between true extension areas and compressive areas. They can be regarded as relative extension areas where the original tensile effect is strengthened or the original compressive effect is released to some extent. The results show that the Tongwei station is located at the compression stress and strain accumulation area before the occurrence of the earthquake, which coincides with the decreases of the apparent resistivity data. On the other hand, the focal mechanism solution shows that the azimuth of the principal compressive stress of this earthquake is 65°. The angle between the P axis and the N20°W direction of Tongwei station is 85°, and the angle from the EW direction is 25°. Before the earthquake, the decrease amplitude of the N22°W is 1.04%, and the decrease amplitude of the EW' is 0.37%. The anisotropic changes observed in the two directions are consistent with the results given by the experiment results, theoretical models and the summary of earthquake examples. Therefore, it can be considered that there may be a mechanical relationship between the changes in the apparent resistivity of the Tongwei station and the seismogenic process. Table and Figures | Reference | Related Articles | Metrics Select THE INFLUENCE OF HVDC TRANSMISSION ON GEOELECTRIC FIELD AND LOCATING THE GROUNDING POLES ZHANG Xin, FAN Ye, YE Qing, QIAN Yin-ping SEISMOLOGY AND GEOLOGY    2022, 44 (3): 718-735.   DOI: 10.3969/j.issn.0253-4967.2022.03.010 Abstract （1264）   HTML （33）    PDF（pc） （6755KB）（155）       Knowledge map    Save The grounding current of HVDC(high voltage direct current)converter station causes the most significant interference in the observation of geoelectric field, which usually causes a step change as 0.5~100mV/km within a range of hundreds of kilometers near the grounding pole. However, it is difficult to determine which converter station’s grounding current causes the step change. Taking Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line as examples, we obtain the response data of three typical disturbances, and calculate the step change using the data of 58 geoelectric stations around these three lines. To compare the interference situation, we referred the extremely low frequency data of Dashan station for comparison with their original curves. First, we explained the different response modes of the stations at different locations to the ground current. This means that when the stations locate near a ground electrode, in the middle of the two poles and near to one side of the ground electrode between the two poles, the corresponding three response types are: step response, pulse response and pulse+half step response, respectively. In particular, stations located in the middle of two poles but close to one pole are mainly affected by the near pole and less importantly affected by the other pole. Consequently, step response appears in the near pole stations, step recovery appears in the far pole stations, and the final result is in the form of step+pulse. Then, we use daily variation amplitude to correct the order variable of HVDC interference and locate the position of grounding pole by the principle that the potential difference of multiple stations has directivity, and then determine the source of grounding current and the approximate location of converter station. The step change after diurnal change correction shows a certain trend, which is shown as the quadratic attenuation of the source-station distance. The fitting of the step change observed by a wide range of geoelectric stations confirms this trend. The locating results have good directive effect on the grounding poles’ positions of the Hainanzhou-Zhumadian line, Zhalute-Qingzhou line and Baoji-Deyang line, and by combining the step change synthesis vector of multiple stations, we can simultaneously determine the approximate location of the converter station. In addition, the amplitude of step change after daily variation correction can suggest the site of the ground electrode, which can supplement the locating results. Furthermore, we build the quantitative diffusion model of the grounding current to show the law of potential distribution of large input current, and determine the interference range and the variation trend. The simulation results show that the potential difference decreases rapidly within 50km near the grounding pole; the potential difference reduction effect is not strong in far-field exceeding 200km and basically maintains a gentle trend. Based on observation data of 58 geoelectrical stations and another station of extremely low frequency, the response characteristics of grounding current to the surrounding stations are identified, which may serve for the data correction of HVDC interference in the future. Results of the influence of grounding current on geoelectric and geomagnetic field can be further extended to the study of seismic electromagnetic signal. Electromagnetic stations are usually set up near the active fault zone in an attempt to detect electromagnetic signals generated by strong earthquakes. Relying on the observation data, researchers can present a preliminary prediction of strong earthquakes under certain conditions, and provide a spatial range and time scale of the earthquakes. However, the explanation of how the electromagnetic signal near the source propagates to the observation stations is not very satisfactory. In particular, there are anomalies appearing in some distant stations, while no anomalies appear in the nearby stations. It means the differential response is obvious. Moreover, some prediction is generally not logical and physical, which means the abnormal signal may not come from earthquake activity but some other sources. Therefore, it is necessary to study how the signal propagates from the source to the station and why it causes differential response. Table and Figures | Reference | Related Articles | Metrics Select CHARACTERISTICS AND PROCESSING OF MAGNETOTELLURIC DATA UNDER STRONG ELECTROMAGNETIC INTERFERENCE ENVIRONMENT HAN Jing, ZHAN Yan, SUN Xiang-yu, ZHAO Guo-ze, LIU Xue-hua, BAO YU-xin, SUN Jian-bao, PENG Yuan-qian SEISMOLOGY AND GEOLOGY    2022, 44 (3): 736-752.   DOI: 10.3969/j.issn.0253-4967.2022.03.011 Abstract （900）   HTML （27）    PDF（pc） （15760KB）（359）       Knowledge map    Save With the development of national economic construction, high-speed railway, wind power stations, and photovoltaic power stations, large-scale high voltage power grids are widely distributed. Under these strong electromagnetic interference environments, obtaining high-quality magnetotelluric(MT)observation data is a practical problem. We carried out MT observation in Yinchuan, Yuncheng, Hebi, and Zhangjiakou in the past two years, and based on the data acquisition and processing results of around 500 MT stations in these four survey areas, 45 typical MT stations under strong electromagnetic interference environments are selected. Based on the nearest interference source, we sorted out these stations into seven kinds of strong electromagnetic interference environment. The seven kinds of strong electromagnetic interference environment are high-speed railway(0.5~1km), electrified railway(1.3~3.7km), wind power station(0.1~3.7km), photovoltaic power station(2~9km), large-scale high voltage power grids(0.06~0.4km), colliery(0.15~1km), and city(0.05~0.8km). The apparent resistivity curve obtained from processing of the typical MT station’s original data shows that the electromagnetic interference near the high-speed railway, electrified railway, and photovoltaic power station is mainly near-field interference. The mid-band frequency apparent resistivity curve of MT stations under near-field interferences rises along an angle of 45° while the impedance phase curve tends to 0. The electromagnetic interference of wind power generation facilities on MT data is relatively small. Large-scale high voltage power grids, colliery, and urban integrated electromagnetic interference are reflected in the apparent resistivity curve as discrete “outlier” with single or multiple frequency points. The curve does not have a stable shape at all. For the 45 typical MT stations listed in this paper under the strong electromagnetic interference environment, the data collection time covers two nights. The use of remote reference, non-robust processing, and fine spectrum selection for the full-time time series data improves MT data quality. The process of obtaining effective spectrum data and the results show that to get effective magnetotelluric data in a strong electromagnetic interference environment, the MT data observation time should include at least two nights(41h). Secondly, when the seven types of strong electromagnetic interference cannot be avoided, the MT stations should be placed at a distance of no less than 0.5km from high-speed railways, 1.3km from electrified railways, 2km from photovoltaic power stations, 0.2km from large-scale high voltage power grids, and 0.3km from colliery. It is also recommended that the distance of MT station shall be no less than 0.2km from electric wires, no less than 0.3km from transformers, and no less than 0.5km from thermal power stations in the comprehensive urban disturbance. The wind power stations have little effect on magnetotelluric data. Finally, a high-quality remote reference shall be used in the data processing. The use of this data can effectively suppress the influence of electromagnetic near-field interference by performing remote reference processing and estimating the spectrum data with the non-robust method. Table and Figures | Reference | Related Articles | Metrics Select ANALYSIS OF ELECTROMAGNETIC CO-SEISMIC PHENOMENA OBSERVED IN CSELF STATIONS HAN Bing, TANG Ji, ZHAO Guo-ze, WANG Li-feng, DONG Ze-yi, FAN Ye, SUN Gui-cheng SEISMOLOGY AND GEOLOGY    2022, 44 (3): 753-770.   DOI: 10.3969/j.issn.0253-4967.2022.03.012 Abstract （920）   HTML （143）    PDF（pc） （8002KB）（7306）       Knowledge map    Save With the support of the wireless electro-magnetic method(WEM)project, the control source extremely low frequency(CSELF)continuous observation network, which includes 30 electromagnetic stations in Beijing capital area(BCA)and the southern section of the North-South Seismic Belt in China, was built for recording the artificial and nature source singles. The natural source observation of the network was started in July 2013 and December 2013 in batches and the electromagnetic field was recorded continually with a sampling rate of 16Hz. Until now, the co-seismic electromagnetic signals have been recorded repeatedly in several stations. In this paper seven co-seismic electromagnetic signals recorded at Jinggu station and co-seismic electromagnetic signals associated with two strong earthquakes recorded at different stations surrounding the epicenter are studied. It is found that the variation of the EM filed is similar to the seismogram, and the amplitude of the co-seismic EM signal is much larger than the background signal generated by earth induction, and the intensity of the vertical magnetic field is about ten times as big as the horizontal electromagnetic field. For co-seismic EM signals recorded at the same station, the relationship between the amplitude of electromagnetic field and the magnitude of the earthquake is basically linear in logarithmic domain. Meanwhile, the amplitude of electromagnetic field is also affected by focal depth of the earthquake and distance between the stations and the epicenter. When the epicenter distance is close, the amplitude of the co-seismic signal caused by the earthquake with shallow focal depth is higher. When the focal depth is similar, the amplitude of electromagnetic co-seismic signal caused by the earthquake closer to the station is larger. For the co-seismic EM signals associated with a same earthquake recorded by different stations, the larger the epicenter distance is, the later the signal appears and the longer the duration is. However, the signal amplitude is not only affected by the epicenter distance, but also related to the near-surface medium at the observation point. The electromagnetic co-seismic signals observed at Dali station which is the farthest away from the epicenter of Jinggu earthquake show the characteristics of large amplitude, long duration, and low dominant frequency. This may be related to the electrical structure near the surface of Dali Platform. The electromagnetic field signals of the 5 components of Jinggu, Muding and Dali stations before and after the Jinggu earthquake of magnitude 5.9 were transformed by wavelet transform. Finally, the wavelet spectrum with the horizontal axis as time and the vertical axis as frequency was obtained to indicate the time-frequency changes of the abnormal electromagnetic signals of the same seismic wave. According to the wavelet analysis and combining with the time series before and after the Jinggu earthquake of MS5.9, the energy enhancement mainly occurs in the shear wave and surface wave periods, while the P-wave is not obvious in the wavelet energy spectrum due to its small amplitude, and only some weak enhancement with scattered frequency can be observed. The main frequency of electromagnetic co-seismic signal is between 1Hz and 2Hz. At the beginning of the co-seismic signal, there are high frequency components, and the high frequency gradually decreases with the increase of epicenter distance. Moreover, compared with electric field, magnetic field can record more abundant high-frequency information. This may have to do with different dominant mechanisms for electric and magnetic field generation. In this paper, several earthquakes recorded at Jinggu station and electromagnetic co-seismic phenomena caused by two strong earthquakes at Jinggu station are summarized and analyzed. The results show that the variation of co-seismic electromagnetic signal is very complicated, and its starting time, duration, amplitude, and frequency range have some rules, but some stations show their particularity under multiple seismic events, so it is difficult to discuss the mechanism of its generation. However, in terms of observation phenomena, the electromagnetic field variation data observed continuously by extremely low frequency stations give us a more comprehensive understanding of the Earth’s electromagnetic field itself and the electromagnetic signals related to earthquakes. The accumulation of more seismic-related electromagnetic phenomena and the support of theoretical simulation can deepen the understanding of electromagnetic field variation before, during and after the earthquake. Table and Figures | Reference | Related Articles | Metrics Select CALCULATION OF SPATIAL DISTRIBUTION OF CSELF ELECTROMAGNETIC FIELD YANG Jing, CHEN Xiao-bin, ZHAO Guo-ze SEISMOLOGY AND GEOLOGY    2022, 44 (3): 771-785.   DOI: 10.3969/j.issn.0253-4967.2022.03.013 Abstract （862）   HTML （15）    PDF（pc） （4743KB）（152）       Knowledge map    Save The electromagnetic(EM)method using controlled-source extremely low-frequency(CSELF)waves is a new technology based on the large-power alternating electromagnetic field generated by an artificial procedure. The biggest advantage of this technology is that it has a long transmitting antenna(tens to hundreds of kilometers)and a large transmitting current(hundreds of amps)and can emit strong and stable electromagnetic waves, covering millions of square kilometers. It can be applied to earthquake monitoring, surveys for mineral resources and treatment of waste nuclear material as well as marine and land communication and detection to ionospheric structure in space. At present, domestic theoretical research on CSELF is not mature enough. This paper has carried out a more detailed study on the spatial propagation characteristics of the electromagnetic(EM)of controlled-source extremely low frequency(CSELF). The large-power CSELF EM waves cover almost all sections of space which can be divided into near, far and waveguide zones according to their propagation characteristics. The propagation of electromagnetic waves in the near and far zone is mainly manifested as the distribution and induction of the conductive currents, and the displacement current and effects of the ionosphere and spheric structure of the Earth can be neglected. The propagation theory of CSELF EM wave is similar to CSAMT in the near and far zones, and it can be described by the theory of quasi-stable field which is analogous to that of the classical theory of EM sounding. In this paper, we collated and verified the field strength calculation formulas in the existing literature. While in the waveguide zone, EM waves appear mainly as the displacement current, and the displacement current and effects of the ionosphere and spheric structure of the Earth must also be considered. The electromagnetic field is mainly the radiation field, and it runs in a way completely different from what the classic theory describes. Using the achievements of communication technology for reference, this paper presents the approximate calculation formula of CSELF EM wave of the earth-air-ionosphere spherical cavity model. Based on the field strength calculation formulas of the three regions obtained above, this paper has designed a piece of visualized software for calculation of the CSELF EM field in three coordinate systems(Cartesian, cylindrical and spherical coordinates). Finally, according to the calculation results, the spatial propagation characteristics of CSELF in the near area, far area and waveguide area are analyzed. The results show that the decay of CSELF EM field intensity is rapid in the near and far zone, but slightly slow in the far zone, which reflects the spatial distribution characteristics of the induced field in the lossy medium and the radiation field in the dielectric medium. The electric field enters the waveguide zone earlier than the magnetic field. Under the earth model, there is an increase in the field strength in the waveguide area near the antipole of the dipole source which shows completely different EM waves propagation characteristics in horizontal formation model. According to the calculation results of the CSELF EM field in near and far zones under the three coordinate systems, it is found that in the Cartesian coordinate system, the horizontal components have two zero lines and are distributed in four quadrants. While the vertical component field has only one zero line and are distributed in two half planes. In the cylindrical and spherical coordinate systems, all field components have merely one zero line and are characterized by half-plane distribution. The location of the zero line should be avoided as much as possible in the layout of field observation stations. We can choose different coordinate systems to solve this problem. In addition, it is also recognized that in the frequency domain EM sounding based on the horizontal electric dipole source, the far-field sounding mainly depends on the magnetic field rather than the electric field. Furthermore, it is recognized that in the frequency domain electromagnetic sounding method based on the horizontal electric dipole, the horizontal component of the electric field in the near zone is proportional to the resistivity of the medium, and has nothing to do with the frequency; the vertical component is proportional to the frequency and has nothing to do with the dielectric resistivity; the magnetic field has no relationship with the frequency and the dielectric conductivity. In the far zone, the horizontal component of the electric field is basically independent of frequency, and the vertical component of the electric field is related to both frequency and earth conductivity. However, due to the difficulty of observation, it is generally not used in the actual sounding. The three components of magnetic field in the far zone are all related to the frequency and the earth’s conductivity, so the far-field sounding mainly depends on the magnetic field rather than the electric field. Since CSELF antennas are generally very long(tens to hundreds of kilometers), the antenna can no longer be regarded as an electric dipole when measuring in the near and far zones, but should be regarded as a long wire source composed of multiple electric dipoles. In this paper, the electric dipole theory is still used for analysis, which has certain limitations that need to be overcome by further in-depth research. Table and Figures | Reference | Related Articles | Metrics Select MAGNETOTELLURIC TIME SERIES PROCESSING IN STRONG INTERFERENCE ENVIRONMENT ZHANG Yun-yun, WANG Pei-jie, CHEN Xiao-bin, ZHAN Yan, HAN Bing, WANG Li-feng, ZHAO Guo-ze SEISMOLOGY AND GEOLOGY    2022, 44 (3): 786-801.   DOI: 10.3969/j.issn.0253-4967.2022.03.014 Abstract （948）   HTML （26）    PDF（pc） （6871KB）（231）       Knowledge map    Save Magnetotelluric(MT)is a method of detecting electrical structures. The natural field source signal is weak, and there are many factors that affect the impedance estimation results, such as dead band, near-field interference, and random noise, so it is difficult to obtain accurate electromagnetic response in strong interference area. The stable and reliable impedance estimation is the premise for the follow-up inversion and interpretation. In order to suppress noise and improve the accuracy of impedance estimation, researchers have proposed various new data processing methods. However, these data processing methods are not widely used due to insufficient stability and poor applicability. The classic remote-reference method and robust estimation method are still the most widely used methods. This paper analyzes the characteristics of the strong interference data and the applicable scope of various data processing methods, combined with the processing effect of the measured magnetotelluric data in the strong interference area in eastern China, and summarizes a set of data processing strategies suitable for the strong interference area. The remote-reference method can effectively suppress coherent noise. It is essential in data processing in strong interference areas. Usually, the results will be improved after processing by remote reference. The remote-reference site should be selected at a place far enough away from the measuring point without interference. Robust estimation can highlight high-coherence signals and suppress low-coherence signals. In the dead band, the coherence of the natural field signal is higher than that of the background noise signal, so the robust estimation processing can improve the data processing result of the dead band. The intensity and coherence of the long-lasting near-field interference signal is higher than that of the natural-field signal. The robust estimation process will treat the near-field interference as the desired signal and suppress the natural source signal. Therefore, data containing long-term strong near-field interference is not suitable for using robust estimation but non-robust estimation. For data that is not well processed by the two methods, we can try a combination of the two. By carefully selecting the power spectrum obtained by the two methods, it is possible to improve the processing result. Increasing the number of data segments can provide more sets of power spectra for selection, and also increase the probability of obtaining higher quality power spectra. Through careful selection of multiple power spectra, it is more likely to obtain better processing results than when the number of segments is smaller. During the day when there is a lot of human activity, the interference signal is strong. And at night, the interference signal is weak. The measured data well proves this point, so we should extend the acquisition time at night as much as possible, and the data processing should also focus on the night data. In general, it is more likely to obtain better data with longer acquisition time. Research on synthetic data shows that the maximum valid period of magnetotelluric theoretical data is 1/8 of the data duration. The measured data results of Fengning Station also support this conclusion. The longer the data acquisition time is, the more effective power spectra can be obtained, and the more likely it is to select a better quality spectrum from them, and obtain a stable impedance estimation result. Therefore, the data collection time should be adjusted reasonably according to the interference situation during the observation to ensure the stability of the impedance estimation result. Magnetotelluric data processing methods are not invariable, and different data processing methods should be adopted according to the actual situation. When the better data processing method is not yet mature, flexible application of existing method is the necessary means for magnetotelluric data processing. Table and Figures | Reference | Related Articles | Metrics Select THE DESIGN AND APPLICATION OF TOPEAK: A THREE-DIMENSIONAL MAGNETOTELLURIC INVERSION CLOUD COMPUTING SYSTEM LIU Zhong-yin, CHEN Xiao-bin, CAI Jun-tao, CUI Teng-fa, ZHAO Guo-ze, TANG Ji, OUYANG Biao SEISMOLOGY AND GEOLOGY    2022, 44 (3): 802-820.   DOI: 10.3969/j.issn.0253-4967.2022.03.015 Abstract （995）   HTML （30）    PDF（pc） （9259KB）（231）       Knowledge map    Save Magnetotelluric(MT)three-dimensional inversion has the advantages of simple data preprocessing, the model is close to actual situation, and the inversion result is more reliable and stable. It is one of the most advanced research topics and would take the place of the dominant two-dimensional inversion definitely. With the improvement of computing capability of computers and the breakthrough in inversion methods, great progress was made in MT three-dimensional inversion in recent years, from the theoretical research and test of this method at the beginning to the current application to practical data interpretation. For the great computation amount of MT three-dimensional inversion, current MT three-dimensional inversion algorithm programs are all implemented in parallel way and it is recommended to do three-dimensional inversion calculations on supercomputing system to make better use of computing resources and improve the inversion efficiency. Different from the MT three-dimensional inversion algorithm programs which have basically realized the utility function, the practical application of MT three-dimensional inversion is still in an early stage. Users should be familiar with the use of multiple software and fulfill the function manually with the help of the software as follows: generating the files required for the inversion program, connecting to the supercomputer to upload data, inputting the command to perform the inversion, etc. The process of manually connecting and operating calculations is the most primitive cloud computing. All processes need to be done manually, which would cause not only heavy workload and the complicated operation, but also the problems for the long-term effective preservation and management of complex inversion data. To conquer this, we develop independently a three-dimensional magnetotelluric inversion cloud computing system, toPeak, using Delphi language. This paper introduces some main features of toPeak. To begin with, system design and analysis are carried out in combination with the current situation and system structure and functions are realized. The main idea is to realize a set of cloud computing system platform based on server-client(C/S), on the basis of perfect inversion data management, integrate the most advanced three-dimensional magnetotelluric inversion algorithm program in the cloud, and connect through the Internet to realize all the system functions of three-dimensional magnetotelluric inversion. Then, the different parts of toPeak are introduced separately, including design structures and designs. The server is deployed on the supercomputer system(supercomputing)to receive the data for inversion tasks, configure and manage the storage of the inversion result data. Combined with the Internet connection, the server and the Internet together constitute a computing cloud. The client is deployed on the users’ windows operating system, including Windows visual data integration processing software and Internet operation middleware. The client is designed on the basis of object-oriented programming ideas, with data as the core, using data engineering objects to encapsulate and store all MT data, process and interpret the results, realize data processing inversion and other operations around this data project, and display the process and results of these processing and inversion in graphics using visualization technology. Internet operation middleware connects the client and server based on the SSH protocol to realize data processing and inversion, transmission and command sending and receiving. Furthermore, the whole work flow of inversion using toPeak and parts of procedure of it are shown. At last, some inversion results from toPeak are displayed. toPeak has realized the full functions require for implementing three-dimensional inversion and can grid, process and select, inverse and explain the data. It is a good tool for the practical use of three-dimensional inversion. Table and Figures | Reference | Related Articles | Metrics Select REALIZATION OF MONITORING AND MANAGEMENT METHOD FOR EXTREMELY LOW FREQUENCY ELECTROMAGNETIC OBSERVATION IN HUAILAI CHI Hai-jiang, WEN Jia SEISMOLOGY AND GEOLOGY    2022, 44 (3): 821-830.   DOI: 10.3969/j.issn.0253-4967.2022.03.016 Abstract （629）   HTML （10）    PDF（pc） （2125KB）（121）       Knowledge map    Save The extremely low frequency(ELF)ground exploration project is one of the major national science and technology infrastructure construction projects approved by the National Development and Reform Commission in the Eleventh Five-year Plan. It uses the new artificial source extremely low frequency electromagnetic technology(CSELF)to carry out resource exploration, earthquake prediction and other cutting-edge scientific research. After several years of active preparation, 30 extremely low frequency seismic electromagnetic monitoring stations have been preliminarily built in the capital circle and the north-south seismic belt in Sichuan-Yunnan region to jointly record MT data from natural field sources and CSMT data from artificial sources. The latest generation of ADU-07e magnetotelluric observation system produced by the company Metronix of Germany is selected for observation. Its output data format, transmission mode and analysis method are quite different from the precursory instruments commonly used in seismic stations in the Tenth Five-year Plan. This paper analyzes the problems that have occurred since the installation of the instrument at Huailai seismic station. According to the actual situation and experience, practical software is developed, which has significantly improved the observation method. The main problems include: 1)The observers cannot grasp the operation status of the instrument in time; 2)The amount of data generated by observation is large and there are a large number of files; 3)The instrument and server are highly secure and professional, which is difficult for station personnel to master; 4)Log filling and submission are cumbersome and error prone. The development of practical monitoring software, which can monitor the observation status and observation data in real time on site or remotely, has important practical value for the long-term operation and maintenance of the station. The software functions mainly include: quasi real-time inspection of the working conditions of observation instruments, fault alarm, inspection, transmission and generation of preprocessed EDI files in the server, automatic filling and submission of logs, etc. This paper introduces the parameter configuration, function application and application method of each module in detail. The paper introduces the key programming statements such as control, dynamic library, macro language command, reading meteorological instrument data and alarm service, which realize software functions by VB language. The practical application effect of the software in Huailai seismic station is introduced, and it is found that the observation data quality and operation effect of the station is improved and the expected design purpose is achieved with the software. Extremely low frequency observation is a transformation from magnetotelluric sounding to long-term continuous monitoring to meet the need for earthquake prediction. It is a transformation from short-term manned observation to long-term unattended observation, which is a test of instrument performance and maintenance. Using software, the complicated instrument inspection is changed into “one key” threshold judgment, which improves the anomaly processing ability and response speed of the instrument. The file naming, data supplementary transmission, preprocessing, log submission and other operations under Linux system are turned into shortcut buttons under windows, which are standardized and convenient, suitable for the working conditions and popularizing and application in the stations. Table and Figures | Reference | Related Articles | Metrics