Analysis And Application To Extremely Low Frequency Electromagnetic Wave Using Time-domain Method

Electromagnetic (EM) waves in the bottom of the frequency band–especially below3000Hz(including ultra-low frequency, ULF and extremely low frequency, ELF)–are being focused in therecent years. Because the Earth-ionosphere system works like a waveguide to the EM waves in thisfrequency range, many applications are worth studying, such as: the anomalous EM phenomenaduring earthquakes, long range submarine or underground communication, etc. Traditional analyticalmethod cannot solve or provide enough accuracy to modern ELF problems, which requires highresolution modeling of geological details in the Earth-ionosphere system. In recent years,time-domain (TD) methods are proposed and becoming a hotspot to solve such problems as it has theinherent advantage of accurately simulating complex geological data and directly presenting wavepropagation processes.This dissertation discussed the TD solutions to ELF EM problems and its improvements, itsapplication in studying ELF EM wave propagation properties in the Earth-ionosphere system and inseismo electromagnetics. Contents are briefly introduced below:1) TD solution to ELF EM propagation problemsComparing to traditional analytic solution, the TD solution has advantages in both theory andapplications. In the TD solutions to such problem, the geodesic finite difference time-domain methodhas many advantages to other spherical finite difference time-domain (LL-FDTD) method, because:1)the model of the former method were used by geologist before, thus it is easier to introduce geologicaldata;2) the cell sizes of the former method are almost uniform, thus provides better stability relation.We proved the above two advantages in both theoretic analysis and experimental simulations.2) Improvements to ELF TD solutionsThe simulation of ELF EM propagation problems using the TD method always requires largecomputational time and computer memory, because the whole Earth-ionosphere system has to beconsidered. Especially in most applications, the request of high resolution results in computationalcost that exceeds the computer’s computing capability. Thus the improvement of computationefficiency is important. Based on the geodesic FDTD method, we proposed a subgrid technique inwhich the cells were divided into smaller cells to increase the resolution of local region, thusimproving the computation efficiency. At the same time, parallel computing techniques were alsoincluded in the simulations to increase the computation efficiency when the program was running on multi core workstations.3) Study of ELF EM wave propagation in the Earth-ionosphere systemTo prove the efficiency of the geodesic FDTD program and apply it to solve practical problems,we introduced EM parameters in different regions and altitudes, such as in the ionosphere, in the crustor in the ocean. Based on these introductions, we simulated ELF EM wave propagation and obtainedcharacteristic parameters such as the Schumann resonance (SR) to compare with analytic results.After that, we studied the propagation property of ELF EM waves in different geological regions.4) Study of anomalous ELF EM phenomena during earthquakesThe anomalous EM phenomena have been observed many times in the recent years, and thesephenomena usually begin several days or months before earthquakes. In this dissertation, we appliedthe TD method to simulate possible physical processes during earthquakes, and studied the seismicEM phenomena at different observing stations. Our simulation results successfully explained theobservation data in the recent years and provided possible earthquake prediction method. Accordingto our simulation results: in the frequency range0.01-10Hz, stations near the earthquake zone canobserved anomalous EM phenomena which are caused by electrokinetic effects or ionosphericanomalies; in5-40Hz, similar reasons can result in observed SR enhancement; in50-250Hz, the EMpropagation anomalies occur.Based on former results, we also discussed the possibilities of using the ELF EM waves to locatethe earthquake center before earthquakes. It was finally pointed out that such prediction can besuccessful if either electrokinetic effects or ionospheric anomalies exist.