Research On Method Of Low-frequency Alternating Geo-resistivity Observation

Geo-resistivity observation has important practical significance in detecting the electrical structure of the earth's interior and monitoring the earthquake precursor. Nowadays, our stations for geo-electrical are facing with a variety of interference. Therefore, anti-interference method has become an essential technology for the geo-electrical observation stations. According to the characteristics of the geo-electrical resistivity, this paper studied the low-frequency AC geo-resistivity observed in single-frequency signal extraction which has carried out some preliminary research for developing low-frequency AC geo-resistivity methods. The creative research jobs and results are as follows:Firstly, the current common interference and some anti-interference measures are introduced to bring forward the theory of low-frequency AC geo-resistivity methods. Investments showed that interference could be avoided by frequency-selection transmitting and synchronization receiving and that in some frequency range of low-frequency signal, alternating current field seemed to be Quasi-Stationary Field which could employ a Four-Pole method that had been used in DC geo- resistivity.Secondly, principles of FFT, adaptive filtering, wavelet analysis and multi-layer auto-correlation are discussed respectively to extract signals from strong-noising background. Comparison research on the numerical simulation showed that it is effective to get better results in a weak-noising background by means of wavelet analysis and multi-layer auto-correlation. When the noise began to accumulate, however, the wavelet analysis method would be of little use while the multi-layer auto-correlation could still get comparatively more ideal results. Considering wavelet analysis could do some pre-processing jobs to distinguish signal from strong-noising background, we propounded a method of Multi-layer auto-correlation detection based on wavelet transform and a de-noising method of Donoho threshold wavelet process to get preferable results and a higher S/N(signal to noise ratio) through multi-layer auto-correlation detecting.Finally, the anti-interference properties of field observation test data are analyzed, which aimed at extracting low-frequency signal of single frequency by high-sampling data acquisition system, to achieve its intended goals. And then its main application prospects and current problems are discussed in the end. Test result shows that it is helpful to apply the methods this paper developed in other weak electro-magnetic signal extraction analysis.