Research On The Acquisition System Of Weak Magnetic Signal In Cross-well

Cross-well electromagnetic detection belongs to the category of MT(Earth Electromagnetic Detection).The transmitter uses dipole sources to generate signals,the signals are collected in the receiving well,which is located hundreds of meters to the transmitter.Based on the waveform information of the extracted signal,the calculation of formation parameters between wells is completed.The cross-well electromagnetic detection studied in this thesis uses a magnetic dipole as the signal emission source,and the frequency of the transmitted signal is a low-frequency signal near the audio frequency band.According to the theory of electromagnetic field,the magnetic field signals arriving at the receiving sensor are axial component and radial component,The magnetic field of the two components is affected by the formation complex parameters when propagating.Therefore,the calculation of the amplitude and phase of the received weak magnetic signal is equivalent to determining the real and imaginary parts of the received magnetic field,from which the parameters containing the formation information are obtained.In the actual detection,however,when the transmitted signal is transmitted in the formation,it will be greatly attenuated by the skin effect.As a result,the signal received by the sensor is so weak that can hardly be identified due to the attenuation.In addition,the signal transmitter and receiver of the electromagnetic detection system between wells are far from one another,so that the circuit crystals at both ends need to be designed with non-homologous crystals.The frequency difference between the two crystals will generate a phase difference over time.That means the signal phase value calculated by the receiving end deviates from the actual value.The two key issues above require that the cross-well electromagnetic receiver must realize high-sensitivity and high-accuracy reception and acquisition functions for weak signals,and ensure that the receiver can realize the synchronous trigger control function with the transmitter circuit.In order to achieve high-sensitivity receiver design,this thesis rationally designs the front-end analog signal conditioning circuit,uses a high-gain,low-noise pre-amplifier to reduce the overall noise figure of the receiver,and optimizes the signal filtering channel to reduce the signal frequency band noise.In the design of digital circuits,a digital phase sensitive detection(DPSD)algorithm is implemented based on a field programmable gate array(FPGA)to achieve the extraction of weak signals,andthe performance of digital phase sensitive detection is tested through simulation.In the design of the inter-well synchronization function,this thesis realizes the synchronous triggering of the transceiver circuit based on the GPS timing pulse signal.Finally,after completing the debugging of the power supply,filtering,communication and other modules in the designed receiver circuit,the receiver sensitivity is tested under closed-loop conditions;under open-loop conditions,the function verification and synchronization of the inter-well electromagnetic transceiver system are completed Trigger the test of the effect of phase cumulative error suppression.The final receiver with 24 bit high precision sampling designed on the PCB board of 280*32 mm can receive and collect 3nV weak signals under closed-loop conditions,and each module can work normally in the system alignment under open-loop conditions.