共查询到20条相似文献，搜索用时 31 毫秒
1.
Jean H. Filloux 《Physics of the Earth and Planetary Interiors》1973,7(3):323-338
Electromagnetic fluctuations in the ocean have external sources above (ionospheric) and below (secular variation of the earth's magnetic field), and internal, purely oceanic sources associated with interaction between water velocity fields and the earth's field. Energy diagrams indicative of the electromagnetic activity in the sea are presented. From the latter, estimates of the resolution required in electromagnetic research at sea can be made. Absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively. Because the ocean shields overhead sources at frequencies above a few hundred c/h and because motional fields have weak signatures, a resolution at least 10 times higher would considerably enhance the scope of such research.The response of electric field instruments to motionally induced fields depends upon whether they are fixed or drifting, but both types respond similarly to fields of external origin.The most stringent limitation to electric field sampling in the sea is the difficulty in achieving low-noise electrical continuity between measuring circuits and sea water. Even the best matched silver—silver chloride electrodes introduce variable electrochemical signals hard to maintain below a millivolt. These mask very low frequency signals unless sophisticated techniques such as electrode switching are used. 相似文献
2.
5月12日汶川8.0级地震强余震观测的电磁同震效应 总被引：8，自引：2，他引：6
2008年5月12日汶川8.0级大地震发生后,在武都汉王地震台及其附近地区进行了为期22d的余震序列电磁异常连续监测,观测到多次余震事件的电磁同震现象。通过与汉王强震台的地震记录数据比较发现,同震信号存在于所有的电场和磁场记录分量中,它们与地震波的到达同步,而不是在地震发生的时刻出现。地震发生时的电磁辐射信号似乎在记录数据中有所显示,但是与地震波到达观测点时的电磁信号相比幅度要小得多 相似文献
3.
ELECTROMAGNETIC ANOMALOUS PULSES BEFORE AND AFTER EARTHQUAKES(M>6.0) IN CHINA CONTINENT IN 2008 
下载免费PDF全文
下载免费PDF全文 In this paper,we provided the electromagnetic anomaly phenomena prior to five earthquakes with magnitude MW>6.0 occurring in China continent in 2008.The electromagnetic data in frequency bands 800~0.1Hz are recorded at two stations located in Urumqi,Xinjiang Uygur Autonomous Region (URQ) and Tonghai,Yunnan Province (TH).The time series of four horizontal components of the electromagnetic field at the stations are converted to the spectra using Fast Fourier Transform.The relationship between the electromagnetic anomalous phenomena and the earthquake events is studied through analyzing the temporal variation of electromagnetic spectra and comparing them with earthquake events.This study provides the new examples for electromagnetic anomaly phenomena before the earthquakes.The following features can be found.①The obvious anomalous power spectrum density (PSD) of electromagnetic fields in frequency band of 128~0.5Hz appeared before earthquakes.The anomalous PSD is related to the earthquake magnitude and the epicenter distance.The anomaly size of magnetic PSD is about 1-3 orders of magnitude bigger than the background field before Wenchuan MS8.0 earthquake.② The changes of parameters are also related with frequency of the signal;The ELF signal strength observed at seismic stations is significantly related with distance between the stations or the measured magnetic and electrical field components and the source of earthquake.We also found that the field strength attenuated faster in area closer to the source than in far area.It is clear that the magnetic signals have higher signal noise ratio than the electric data.③ The amplitudes are also related with the azimuth of the earthquake to station and with the frequency of the data.④ The anomalous pulses showed cluster phenomenon in time. 相似文献
4.
为进一步了解井间地下介质结构及其电性特征,本文用时域有限差分方法(FDTD),实现了井间电磁场响应的数值模拟.激发源是探测深度相对磁偶极子更大一些的电偶极子,首先选择一个位于井轴上的垂直电偶极子作为场源,并假设参与计算的介质相对于发射井井轴是轴向对称的,这样可将研究区域作为二维问题处理.推导了二维井间电磁波传播时域差分公式.该方法适用于任何方向入射的电偶极子源,尤其善于解决频域差分方法所难以描述的宽频脉冲.给出了井间金属圆柱和矿体圆柱数值模拟2个例子,结果表明,时域有限差分方法能有效的模拟井间地下介质中电磁波的传播,揭示电磁波传播规律.该方法速度快、精度高、结果稳定,适合用于井间电磁场反演成像的正演响应计算. 相似文献
5.
B. V. Borovskii L. M. Bogomolov A. S. Zakupin V. A. Mubassarova 《Izvestiya Physics of the Solid Earth》2011,47(10):926-936
The combined impacts of the electromagnetic fields generated by two independent sources on the process of crack formation
in loaded heterogeneous materials (rocks) are studied. The interest in such experiments lies in the fact that in natural conditions,
the combination of several fields—the potential triggers of seismic activity—is rather the rule than the exception. It is
shown by examples that in the conditions of crossed electric and magnetic fields (CrEMFs) the enhancement in acoustic activity
can be triggered with a lower strength electric field than in case of the electric impact alone. 相似文献
6.
Zhang Ping Zhao Guoze Tang Ji Liu Feng Sun Weihuai Han Bing Wang Lifeng Zhang Li 《中国地震研究》2018,32(2):254-264
This paper summarizes the layout of the Yunnan seismic ELF electromagnetic observation network, site selection, ELF electromagnetic instrument system, data processing and other construction. The principle and method of using the ELF electromagnetic wave technique to monitor and predict earthquakes are expounded. The long term monitoring of ELF electromagnetic fields is carried out in the Yunnan earthquake prone area, and at the same time, the changes in electrical parameters and spatial electromagnetic fields of the regional crustal medium structure are monitored. The functions such as automatic, quasi real time, remote monitoring, network monitoring, data processing specialization, data service, data sharing and industrialization of the ELF electromagnetic observation data have been realized. In order to capture the deep electromagnetic precursory information of the earthquakes, service for earthquake prediction research, which has broad application prospects and development potential. Through the research of the seismicity of Yunnan in the trial run period of the project, the preliminary results of the extreme low frequency electromagnetic observation of the Yunnan earthquake in recent years are given. The electromagnetic precursors and the electromagnetic effects of the Yangbi earthquake are recorded. In the 3-month period before the earthquake, the power spectrum of the electric and magnetic fields, the apparent resistivity and the impedance phase in the observed signals are all abnormal, and gradually increased with time. The maximum value is reached 20 days before the earthquake, and an earthquake occurs when the change is restored to normal. 相似文献
7.
2009年利用大功率人工源极低频电磁技术(CSELF)在位于华北、华南、东北、西北和西南的12个地震台站和几个流动观测点进行了连续30天新的观测试验.结果表明,利用CSELF技术可以在1700 km之外测量到人工源电磁场信号,计算得到的电、磁场功率谱密度和视电阻率与天然源信号相比,抗干扰能力更强,观测信号更稳定,特别有利于识别和捕捉地震等诱发的电磁异常现象,在地震预测监测中具有很大的研究应用潜力.试验还发现,各地震台站和流动测点观测的CSELF信号的强度,与台站或者测量的电、磁场分量相对于发射源的距离、方位有明显的关系;发现在离开发射源的相对近区,场强随距离的衰减,比在相对远区更快;发现接收磁场信号比电场信号较易获得更高信噪比的数据. 相似文献
8.
Precursors to earthquakes: Seismoelectromagnetic signals 总被引：1，自引：1，他引：1
Stephen K. Park 《Surveys in Geophysics》1996,17(4):493-516
Field measurements in the past several years have documented electromagnetic signals which are attributed to precursory stress and strain changes which were followed ultimately by earthquakes. Precursory electric field changes observed in Greece on multiple dipoles have been used to issue earthquake predictions. While the source of these signals is still unknown, a sufficient number of predictions has been issued to allow some, but not all, statistical analyses to show this method is better than randomly sampling the earthquake catalog. Ongoing efforts to identify the sources of both these signals and the magnetic field variations prior to the Loma Prieta earthquake are focusing on electrokinetic coupling of fluid flow and transient electric fields. A mechanism related to local fluid flow appears to be best suited at this time of explaining the variety of purported precursors. However, much more work is needed to improve the observations and refine the models of precursor generation. Efforts to monitor magnetotelluric transfer functions at longer periods (T>10s) have been hampered by variability of the functions. The use of modern noise reduction techniques such as remote referencing should reduce this variability, but may not reduce errors to a level needed for monitoring. Monitoring of high frequency (81 kHz) seismoelectric emissions may be promising, but lack of simultaneous observations on multiple instruments hinders the utility of this technique. 相似文献
9.
地质雷达所探测的地球介质常常具有频散性．为了研究地质雷达在频散介质中的探测能力,提出了频散介质中时间域有限差分法计算麦克斯韦方程的方法,给出了满足Debye关系的频散介质中的电位移和磁场的迭代算法,以及由电位移计算电场的算法．只有在电场计算时才用到介质的物性参数．提出一种新的吸收边界条件的算法,通过增加假想的介电常数和磁导率,实现了吸收层中波的无反射衰减,克服了以往Berenger完全匹配层计算时对场进行分裂带来的麻烦,从而提高了计算效率．计算实例表明,频散介质中电磁波的衰减更快,测量信号变得很弱． 相似文献
10.
本文利用DEMETER卫星VLF频段电场和磁场频谱数据对DEMETER卫星运行期间2005年至2009年澳大利亚甚低频(Very Low Frequency)通信台NWC发射的通信信号造成的电离层电磁响应的日变化、季节变化及年变化特征进行了统计分析,统计结果表明电磁响应日变化显著,夜间电场强度明显增强可达40dB,磁场变化略小也可为15dB左右,而季节变化不显著,年变化主要受太阳活动的影响,太阳活动越强,电磁响应越小.为解释数据分析结果,对地-电离层电磁波传播过程采用传递矩阵方法进行了模拟计算,模拟结果与数据分析的结果一致.我们认为这种随时间变化的特点可能由250km以下电离层电子密度分布特征导致,因此研究250km以下的电离层电子密度变化可能对寻找地震电离层电磁异常有重要意义. 相似文献
11.
J. Vassal M. Menvielle Y. Cohen M. Dukhan V. Doumouya K. Boka O. Fambitakoye 《Annales Geophysicae》1998,16(6):677-697
In the framework of the French-Ivorian participation to the IEEY, a network of 10 electromagnetic stations were installed at African longitudes. The aim of this experiment was twofold: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field experiment will be discussed in this study. The electromagnetic experiment will be described. The analysis of the electromagnetic transient variations was conducted in accordance with the classical distinction between quiet and disturbed magnetic situations. A morphological analysis of the recordings is given, taking into consideration successively quiet and disturbed magnetic situations, with the results interpreted in terms of the characterization of external and internal sources. Particular attention was paid to the effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the SRE source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The other source (the SR*E source) was responsible for most of the electric diurnal variation, and was also likely to be an ionospheric source. Electric and magnetic diurnal variations are therefore related to different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field diurnal variation is not relevant. Furthermore, the magnetotelluric probing of the upper mantle at dip equator latitudes with the electromagnetic diurnal variation is consequently impossible to perform. In the case of irregular variations, the source effect related to the equatorial electrojet is also discussed. A Gaussian model of equatorial electrojet was considered, and apparent resistivities were computed for two models of stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences between daytime and night-time apparent resistivity curves. In particular, it was shown that electromagnetic probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric method is impossible with daytime recordings made at dip latitude stations. 相似文献
12.
13.
Experimental study on the mechanism of non-synchronism of seismo-electromagnetic radiation precursors 总被引：1，自引：0，他引：1
ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｒｏｕｇｈ２０－ｏｄｄｙｅａｒｓ’ｏｂｓｅｒｖａｔｉｏｎａｎｄｓｔｕｄｙａｆｔｅｒｔｈｅ１９７６Ｔａｎｇｓｈａｎｇｒｅａｔｅａｒｔｈｑｕａｋｅ,ｔｈｅｓｅｉｓｍｏ－ｅｌｅｃｔｒｏｍａｇｎｅｔｉｃｒａｄｉａｔｉｏｎｐｒｅｃ... 相似文献
14.
Makoto Uyeshima 《Surveys in Geophysics》2007,28(2-3):199-237
There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or
volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research,
the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state
is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate
in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments
have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent
healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic
signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports
about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained
by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of
detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals.
Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic
signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts
to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended.
At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary.
A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In
the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with
long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale,
near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical
conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes
caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities.
The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by
the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation
of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions. 相似文献
15.
16.
芦山MS7.0地震前的电磁异常信号 总被引：1，自引：0，他引：1
对南北地震带18个地电场及电磁扰动观测台站进行了跟踪与研究, 2013年4月20日四川芦山MS7.0地震前只有其中5个台站观测到了异常电磁信号. 研究表明, 地震前的这些异常信号时空分布非常不均匀. 在时间上表现出震前电磁异常信号在出现的台站上不是同步的, 且随着时间的推移有向震中方向迁移的趋势; 在空间上体现在成都以北的台站都没有记录到震前电磁异常信号, 主要出现在成都及其以南的一些台站. 芦山MS7.0地震前地震电磁信号出现的最早时间是震前21天, 其次是震前16天; 临震时间则为震前5天、 4天和1天. 通过接地极注入地下的大电流以及以往的震例分析, 认为仙女山台是又一个比较突出的电磁信号观测的敏感点. 相似文献
17.
汶川5.12大地震发生后,在武都汉王地震台及其附近地区布置了2个大地电磁测深点,进行了两期余震序列的地震电磁效应连续监测,共观测到Ms4.0及以上的余震事件35次.通过与汉王强震台的地震记录数据中15个强震记录比较发现,与地震波相对应的地震同震电磁信号存在于所有的电场和磁场记录分量中,它们与地震波到达同步.地震破裂发生时的电磁辐射信号在记录数据中相比地震波达到观测点的电磁信号幅度要小得多,无法从观测数据识别出来.通过分析对比地震位移和同震电磁信号的关系,提出了地震电磁同震信号产生的地震波驱动机理,认为观测系统在地球基本磁场中随地面运动产生了地震同震电磁信号,并从观测的地震振动位移和电磁信号进行了验证. 相似文献
18.
A. V. Kuvshinov 《Surveys in Geophysics》2008,29(2):139-186
Electromagnetic induction in the Earth’s interior is an important contributor to the near-Earth magnetic and electric fields.
The oceans play a special role in this induction due to their relatively high conductivity which leads to large lateral variability
in surface conductance. Electric currents that generate secondary fields are induced in the oceans by two different processes:
(a) by time varying external magnetic fields, and (b) by the motion of the conducting ocean water through the Earth’s main
magnetic field. Significant progress in accurate and detailed predictions of the electric and magnetic fields induced by these
sources has been achieved during the last few years, via realistic three-dimensional (3-D) conductivity models of the oceans,
crust and mantle along with realistic source models. In this review a summary is given of the results of recent 3-D modeling
studies in which estimates are obtained for the magnetic and electric signals at both the ground and satellite altitudes induced
by a variety of natural current sources. 3-D induction effects due to magnetospheric currents (magnetic storms), ionospheric
currents (Sq, polar and equatorial electrojets), ocean tides, global ocean circulation and tsunami are considered. These modeling
studies demonstrate that the 3-D induction (ocean) effect and motionally-induced signals from the oceans contribute significantly
(in the range from a few to tens nanotesla) to the near-Earth magnetic field. A 3-D numerical solution based on an integral
equation approach is shown to predict these induction effects with the accuracy and spatial detail required to explain observations
both on the ground and at satellite altitudes.
On leave from Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Russian Academy of Sciences, 142190
Troitsk, Moscow region, Russia. 相似文献
19.
We present a method to unify the calculation of Green’s functions for an electromagnetic (EM) transmitting source embedded in a homogeneous stratified medium. A virtual interface parallel to layer interfaces is introduced through the source location. The potentials for Green’s function are derived by decomposing the partial wave solutions to Helmholtz’s equations into upward and downward within boundaries. The amplitudes of the potentials in each stratum are obtained recursively from the initial amplitudes at the source level. The initial amplitudes are derived by coupling with the transmitting sources and following the discontinuity of the tangential electric and magnetic fields at the source interface. Only the initial terms are related to the transmitting sources and thus need to be modified for different transmitters, whereas the kernel connected with the stratified media stays unchanged. Hence, the present method can be easily applied to EM transmitting sources with little modification. The application of the proposed method to the marine controlled-source electromagnetic method (MCSEM) demonstrates its simplicity and flexibility. 相似文献
20.
V. A. Rakov 《Surveys in Geophysics》2013,34(6):731-753
Both cloud-to-ground and cloud lightning discharges involve a number of processes that produce electromagnetic field signatures in different regions of the spectrum. Salient characteristics of measured wideband electric and magnetic fields generated by various lightning processes at distances ranging from tens to a few hundreds of kilometers (when at least the initial part of the signal is essentially radiation while being not influenced by ionospheric reflections) are reviewed. An overview of the various lightning locating techniques, including magnetic direction finding, time-of-arrival technique, and interferometry, is given. Lightning location on global scale, when radio-frequency electromagnetic signals are dominated by ionospheric reflections, is also considered. Lightning locating system performance characteristics, including flash and stroke detection efficiencies, percentage of misclassified events, location accuracy, and peak current estimation errors, are discussed. Both cloud and cloud-to-ground flashes are considered. Representative examples of modern lightning locating systems are reviewed. Besides general characterization of each system, the available information on its performance characteristics is given with emphasis on those based on formal ground-truth studies published in the peer-reviewed literature. 相似文献