Research On Key Technology Of EM-MWD In Underground Coal Mine

Currently, gas drainage, reinforcing floor and detecting geological structures by borehole in underground coal mine are an effective method to ensure safe production of coal mine in China. Horizontal measuring directional drilling in underground coal mine is a basis to achieve long-borehole directional drilling, precise control of the drilling tracing, guaranteeing borehole tracing in the destination layer effectively extending. This technology can be used to finish multi-branch hole construction, increase effective lengths of gas drainage, implement gas drainage and reinforcing floor in advanced region, and detect accurate geological structure of coal seam. Horizontal measuring directional drilling in underground coal mine is a main technical scheme used to gas drainage in the world, which has been applied in geological structure detection, water detection and water release system in coal mine.Measurement While Drilling is a key element to achieve directional drilling. Currently the wired MWD in underground coal mine has many disadvantages over the wireless MWD, such as the strict demand of precision manufacturing process and high cost of usage. Wireless MWD is an effective way to solve this problem. At present, Mud-pluse MWD and Electromagnetic MWD are more mature in the technology and applications than other wireless MWD. Compared to mud-pluse MWD, electromagnetic MWD has obvious advantages in data rates and drilling fluid. Therefore, the development of electromagnetic MWD suitable for underground coal mine will lay the foundation for large-scale commercial applications of horizontal measuring directional drilling technology.This dissertation focuses on MWD technical problems of horizontal measuring directional drilling in underground coal mine, and concentrates its attention on the research of communication technology in electromagnetic MWD, including attenuation property of EM channel, digital modulation scheme, channel coding methods and weak signal processing technology. The instrument prototype of EM-MWD in underground coal mine has been developed and transmission experiments have been finished respectively on the ground and in underground coal mine. The main contents of this dissertation are as follows:1. Modeling an EM channel for MWD in underground coal mine with equivalent transmission line, and reveal the attenuation property of EM channel for MWD in underground coal mine. By calculating received signal voltage between the electrodes near the hole in underground coal mine to analyze how to impact received signal voltage amplitude with coal rock conductivity, electromagnetic communication signal frequency, drill stem resistivity and drill length. Give the conditions suitable to apply EM-MWD in underground coal mine:(1) electromagnetic communication signal frequency is not higher than 30Hz; (2) an electromagnetic communication signal transmission distance is no longer than 2000m; (3) coal formation resistivity is not less than 2.5 Ω·m.2. Study digital modulation method for EM-MWD in underground coal mine. The EM-MWD communication channel is approximately Gaussian white noise AWGN channel when simulating the EM-MWD communication system in underground coal mine after analysising EM-MWD communication channel characteristics according to attenuation property of EM channel for MWD in underground coal mine. By theoretical calculations and a comprehensive comparison of the binary line codes, MSK and PSK modulated signals transmitted in EM-MWD communication channel with bandwidth efficiency, bit error rate and power efficiency, OQPSK, MSK or IJF-OQPSK modulation is more suitable for EM-MWD communication channel in underground coal mine than BPSK or QPSK modulation.3. Propose linear block codes with best performance for EM-MWD communication channel in underground coal mine. According to data transmission requirements for MWD of horizontal measuring directional drilling in underground coal mine, it will divide transmission data into 10-bits short data and 94-bits length data. Choose linear block codes to research because of a small price to achieve good error performance, theoretical calculation and Matlab/Simulink simulation, system analysis and comparison of different lengths of Hamming code, BCH code and RS code, bit error performance and block error performance when hard decision decoding or soft decision decoding. Based on EM-MWD communication channel in underground coal mine, linear block codes with best performance for 10-bits short data and 94-bits long data are BCH code (31,16) and BCH code (127,99) respectively.4. Analysis noise characteristics of wellsite for horizontal measuring directional drilling in underground coal mine, suppression of natural potential and strong frequency interference signals from the received signal to extract a useful weak electromagnetic communication signals is proposed with multi-stage filter method. Determine the switched capacitor filter is not suitable for pre-filter in the received channel; give the high order low-pass filter design methods; design AGC algorithm; based on Altera FPGA chip, give three types of design methods for digital filter.5. Give instrument prototype designed in detail for EM-MWD in underground coal mine. Present a method to change the emission current by series connection into the transmitting end using slide resistor, this method without changing background noise conditions, but changing useful communication signal amplitude between the received electrodes, to test instrument prototype that handle weak electromagnetic communication signals. The test results are also obtained by transmission experiments on the ground and in underground coal mine respectively.