| In the process of petroleum exploration,logging while drilling technology is needed to transmit downhole parameter data to the surface for analysis,so that the downhole oil distribution can be known,so that the oil can be exploited efficiently.There are mature drilling mud pulse transmission technology used in logging technology,but this technology has low transmission rate and high cost.In order to improve the transmission rate,it is necessary to study a new transmission while drilling technology,so the electromagnetic wave transmission while drilling technology is proposed.The electromagnetic wave transmission system while drilling is a low-frequency signal as the carrier.After modulation and amplification,it is transmitted through the antenna and transmitted to the ground along the channel of the drill pipe in the form of electromagnetic wave.The ground receiving system can adjust and decode the transmitted signals to get the initial downhole data.At the same time,command signals can be transmitted from the surface to the downhole to remotely control the downhole launching system.According to the characteristics of underground electromagnetic wave transmission and the existing achievements of electromagnetic wave transmission while drilling technology,the barriers needed to break through in this technology are obtained.Firstly,the signal transmitted in the mine is greatly attenuated and mixed with a large amount of noise,so it is difficult for the receiving system to process and extract the weak signal under the large noise.Secondly,how to improve the transmission rate is another difficulty in the case that the electromagnetic wave transmission in the mine is carried by low-frequency signals.After analysis and comparison,an integrated digital power amplifier was adopted to design a transmission circuit with high conversion efficiency,and 2FSK with low bit error rate and OFDM with high frequency band utilization were selected as the modulation mode for experimental comparative study.Ground launch system separate amplifier circuit design for high-power,under the condition of the load of 200 ?,command signal can be amplified to 100 w,so as to reach the downhole receiving system.Through theoretical analysis of the source of the inherent noise of the circuit,the instrument amplifier circuit and filter circuit with low noise performance are designed,which can effectively amplify the weak signal and filter out the noise outside the band.Considering the power frequency noise of logging site,the power frequency notch circuit is designed to optimize the received signal.At the same time,the post-stage automatic gain amplifier circuit is designed,which can flexibly amplify the signal amplitude to meet the acquisition voltage amplitude demand of the acquisition circuit.In addition,in order to increase the transmission distance of electromagnetic wave in the mine,the transmission method of relay and transmission is adopted.After the design of the system circuit,the laboratory system cascading and field downhole tests were carried out.The downhole tests in Yanjiao test site verified the2 FSK and OFDM one-way transmission and surface remote transmission schemes.2FSK can achieve 100bit/s transmission in shallow Wells,and OFDM can achieve120bit/s transmission in shallow Wells.The downhole test at the test site in Xinjiang verified the relay transmission scheme,and 2FSK can achieve one-way transmission with a transmission distance of 1300 meters and a rate of 10bit/s. |
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