| Borehole seismic technology refers to the receiving system and seismic energy excitation system suitable for seismic data acquisition in borehole,respectively or simultaneously sinking into existing Boreholes for seismic data acquisition to obtain Borehole seismic data information.Because seismic data has many transmission parameters and large data volume,whether the data can be transmitted to the ground accurately and rapidly will directly affect the data processing and geological analysis in the later period.Therefore,it is very important that the logging data collected by the seismic data acquisition system be transmitted to the ground receiver at high speed and accurately.In this paper,in order to improve the transmission rate of data collected by downhole measurement equipment and the characteristics of anti-jamming distortion during transmission,the data transmission characteristics of logging cables are studied.Although well logging cables have been widely used in well logging equipment for their reliable and stable excellent transmission performance,when cables are used as logging data transmission media,their length often reaches several kilometers.Therefore,distribution parameters such as distributed resistance,distributed capacitance and distributed inductance inside the cable will have a great influence on the data.As the frequency of the signal increases,the distortion effect gradually increases.This feature severely limits the available bandwidth for data transmission and restricts the logging data transmission rate.Aiming at the limited available bandwidth due to the severe attenuation of high frequency signals in logging data transmission systems,This article proposes a pre-equalization technique that appropriately reduces the amplitude of low-frequency signals and increases the signal amplitude of high-frequency based on the low-pass high-impedance characteristics of the cable.In the case of limited transmit power,compromise the transmit power of the high-frequency and low-frequency signals to be transmitted.After equalization,the high-frequency data still has good amplitude characteristics,which effectively expands the available bandwidth and improves the signal-to-noise ratio of high-frequency signals,effectively improving the data transmission rate and stability.In this paper,a combination of orthogonal frequency division multiplexing(OFDM)modulation techniques and pre-equalization techniques is used.Designed and implemented transmit and receive parts of data transmission system on FPGA hardware development platform.In the data transmission system designed in this paper,the available data bandwidth is widened to 300 KHz,which effectively improves the data transmission rate.The three-kilometer long seven-core armoured logging cable was used as the transmission medium to test the entire data transmission system.When the effective bandwidth of the data is 300 KHz,tests show that the entire data transmission system can accurately demodulate the original transmission data.The bit error rate is as low as 10E-8 and the transmission rate can be 1.1Mbit/s.The data transmission system has achieved the expected goal and meets the requirements for accurate real-time transmission of logging data. |
