Research Of Special Code In Application Of Electrical Prospecting

The subject is derived from the supervisor's the national "863" sub-project "The Development of Distributed Ground-Hang Electromagnetic Receiver System" and the project of CNPC "The Design of Time-frequency Electromagnetic Acquisition System and the Research of Coding of Coherent Noise".Electrical prospecting investigates and assesses resources based on the difference in electrical property. It is widely used in industrial nondestructive examination (NDE), environmental protection, hazard prediction, engineering survey and etc. Whether use sinusoidal wave or square wave as man-made source of field, there will always be two difficult problems in electrical prospecting. The first one is that the SNR of signal is low so it's difficult to extract the useful signal which requires the system designed should have a strong capacity of resisting disturbance. The second problem is that it is necessary to increase the delivered power of the transmitter to raise the SNR. But it will make the transmitter becoming cumbersome and operating inconveniently. Special code(such as pseudo random code) is a sequence made up of different signals synthesized and coded according to a certain discipline,it contains approximately equal amplitude signals of various frequencies, and has strong capacity of resisting disturbance,it is suitable for adverse circumstances of lower SNR. Therefore, combined with the advantages of special code such as strong capacity of resisting disturbance, wide spectrum and the needs of electrical prospecting method to build detection model of special coding signal to detect geophysics information like resistivity, it will be significant as the anti-interference performance of machines will be improved and the delivered power of the transmitter will be decreased, the geophysical exploration equipment will thus become light and small.Based on the above ideas, resistivity parameters in the condition of m-sequence as the artificial field source was explored theoretically and studied experimental, the method based on the resistivity identification of m sequence was proposed, an experimental prototype model was designed to achieve coding signal detection and identification technology, finally the identification theory was validated by the prototype and it was achieved the desired results.At first, the thesis elaborates the principle for identification of resistivity on the basis of m-sequence. Relating with detection principle of resistivity method and m-sequence, this thesis put forwards the formula of identifying resistivity on the basis of m-sequence according to the testing principle of special code, meanwhile the situation on the complex impedance are briefly described, in addition to the capacity of resisting disturbance based on the identification theory was analysis; the correlation parameters of m-sequence were described for application.According to the principle for identification of resistivity, an experimental prototype was designed; the design of hardware and software of prototype was discussed. The hardware includes three parts:the PC 104 control board, data acquisition board and the H bridge transmitter which built the hardware platform for operating system and build environment required to achieve a pseudo-random signal m-sequence high-power, high efficiency transmission and data acquisition. The software consists of m-sequence generation, correlation detection, Fourier transform, based on MS-DOS 6.22 operating system and Turbo C++ Version3.0 build environment with C language programming to achieve different parameters of m-sequence coding control algorithm, correlation detection algorithm and the discrete Fourier series transform algorithm.Based on experimental prototype, the resistivity identification theory and the interference characteristics were verified experimentally. First, the technical indicators of the prototype and the method of experimental data processing were introduced; second, specific experimental scheme and results were given from three points:the resistivity experiments, impedance experiments and interference experiments respectively.The results show that:the resistivity identification theory on m-sequence can make the resistivity good identification results and better capacity of resisting disturbance, and thus demonstrates the special encoding used in electrical exploration that has great advantages and the feasibility of certain. The thesis has made innovation in the identification of resistivity and prototype of receiver and transmitter in the special code for electrical prospecting.At last, the thesis provides the achievements and conclusion of the study and proposes some practical suggestion for the future work.