Design And Implementation Of Borehole Data Transmission System Based On OFDM

The integrated borehole seismic observation system is one of the most important means of seismic observation,with the advantages of wide frequency band and low interference from human activities and natural noise,which is integrated with a wide range of sensors such as wide-band seismometer,strong seismometer,strain meter,inclinometer,geomagnetic meter and temperature meter.The comprehensive observation of deep Wells integrates sensors with multiple measurement items,and there are many observation signals.Generally,multi-core cables are used for communication.Multi-core analog signals have cross-interference problems in long-term transmission.Aiming at the above problems,according to the requirements of comprehensive observation power supply,timing and bidirectional data transmission in borehole,this paper designs a data transmission system in borehole based on single-core logging cable and Orthogonal frequency-division multiplexing(OFDM)technology.Realize the coupling transmission of power supply current and communication signal.The system uses broadband Power Line Communication(PLC)module to transmit data acquisition data,and uses OFDM symbol composed of PPS mark,packet mark and transmitted data content to realize downhole timing and equipment control functions.Deep well timing adopts partial OFDM symbol frequency points to mark the second pulse(PPS),and transmits the UTC time corresponding to the whole second at the same time.Downhole time delay is calculated based on the phase shift of the second pulse(PPS)mark,and the corresponding time of the initial phase of the second pulse(PPS)mark is restored.Through MATLAB simulation,the time shift error calculated when SNR=26dB(near the actual noise of deep well comprehensive observation)is 7.7 μs,which is almost consistent with that without superposition noise.Even when SNR=3 dB,the time-shift calculation error reaches 5.8 μs,and reliable PPS moments can be obtained.The experimental system of timing and control module is built based on STM32H743 and STM32L4P5.After experimental tests,the PLC module on 1200 m twisted pair transmission rate can reach more than12 Mbps,timing accuracy reached microsecond level.