Development Of The SLF/ELF Observation System And Its Applications

After reading a lot of papers related to Super/Extremely Low Frequency (SLF/ELF)electromagnetic wave, the author finds out that the artificial SLF/ELF electromagnetic wavetechnique today has become a new research field attracting much attention of geophysicistsall over the world and can be used to earthquake monitoring and prediction, reconnaissance ofresources, nuclear waste disposing and environment protection. However, the SLF/ELFtransmitter facility just starts to be constructed in our homeland, and there is no correspondingSLF/ELF receiver system. Therefore, this thesis focuses on the study of developing thesystem. Some important results are obtained as follows.1. Summarizing the key techniques of the artificial SLF/ELF. The main function of thereceiver system is the receiving and detection of electromagnetic signal from SLF/ELFtransmitter facility in the presence of strong background noise. For receiving techniques, itis crucial to use the correlation detection technique and improve spectral resolution.2. Detecting the weak SLF/ELF signal of known frequency. Although the power of thesignal from SLF/ELF transmitter facility is very powerful, it will become very weak aftera long distance transmission and submerge in strong noise, which often causes very lowsignal-to-noise ratio (SNR). Because it is difficult to detect the weak SLF/ELF signal inthe presence of strong background noise using traditional methods, the correlationdetection technique is used to optimize the SNR and carried out by the correlationalgorithm without any additional hardware. The test results indicate that the weak signalcan be detected very well.3. Programming the Zoom FFT algorithm suitable for the receiver system. The FFTalgorithm is widely used in spectral analysis for digital signal, but if the high resolutionspectrum is desired, more FFT points must be used, which is bound to result in the sharpincrease of data storage and operation of arithmetic. In order to solve this problem andobtain the given high resolution spectrum, zoom FFT with the technique of complexmodulation is used in the system, and the corresponding codes are written. The test resultsshow that it has dealt with the problems well.4. Analyzing the effects of instability of signal frequency on observation precision. By thenumerical simulation method, the effects of frequency shift on observation precision havebeen studied in detail. The results show that in order to get desired signal observationprecision, the stability of the signal frequency must reach a certain level. Or when thestability of the signal frequency is given, what the signal observation precision can bedetermined.5. Designing and developing high sensitivity induction coil magnetometer (ICM). Becausethe normal ICM (the sensitivity about 100 μV/nT) can not satisfy the observationprecision of the SLF/ELF receiver system, high sensitivity ICM is needed to be developed.On the basis of the analysis of the requirements and functions of receiver system, thetheory of ICM is summarized and a series of design principles and methods are proposedin the thesis. Then high sensitivity ICM is developed. After testing, the results show thatthe major specifications of ICM have achieved the level of the same products in the world:frequency ranges from 10 to 4000Hz;expanded frequency ranges from 1 to 10Hz;andsensitivity is 15mV/nT;The sensitivity can reach 150 mV/nT when combining with thereceiver system。6. Designing and writing the data acquisition and data processing software. The dataacquisition software used by measurement part and data processing software used by thesystem controlling part are designed and written using the assembly language and the VBlanguage, respectively. After testing, the results suggest that the software can be operatedeasily and conveniently and runs stably, which indicates the software is able to satisfy therequirement of long time and continuous work of the receiver system.7. Developing and testing the receiver system. The design scheme is proposed based on theanalysis of the requirements and key techniques of the receiver system. And the techniqueproblems during the development of the system are analysed and solved step by step.Then the receiver system is developed. Through test in field, the results show that thesystem specifications have achieved the design requirement, its major parameters haveachieved the level of the same products in the world: frequency ranges from 4 to 300Hz;resolution of spectrum can reach 0.001Hz and the dynamic range is more than 80dB,theminimum magnetic variation which can be detected is 0.01nT and the minimum telluricvariation is 0.5mV/km. The system runs stably and can be used for long time andcontinuous work in field.In the system test, the source is a simulated signal but not a practical one. So in the future,in order to improve the system, it is needed to receive the signal from the SLF/ELFtransmitter facility for a long time.