Research Of Imaging Logging Circuit Of Weak Signal Detection In Large Dynamic Range

China is rich in oil and gas reserves,and how to exploit it is a big problem to be solved.In the early stage of exploration and development of oil and gas reservoirs,logging plays an important role in reservoir location and evaluation.Micro resistivity electrical imaging logging plays an important role in identifying fractures,evaluating fractures and thin layers,analyzing lithology,sedimentary facies and formation structure,and has a very important reference value for exploration and development of key exploration areas and complex strata.The core module of imaging logging is the acquisition system of the circuit,which plays an important role in providing accurate resistivity distribution of the formation,and the importance of its research is self-evident.The transmitter circuit aims to provide good and stable excitation signal for the acquisition circuit,so it is also necessary to study it.Taking weak signal detection as the main core and micro resistivity electrical imaging logging technology as the main direction,this thesis expounds the overall design scheme of transmitting circuit and acquisition circuit in imaging logging tool,and forms the specific circuit design through theoretical analysis and necessary simulation.The goal of the acquisition circuit is to extract the current amplitude through the measurement circuit.There are two difficulties in the design of the acquisition circuit.One is to extract the weak signal,and the other is how to ensure that the weak signals of multiple channels can be collected in accordance with the accuracy requirements under sufficient logging speed.Therefore,it is necessary to design a multi gear gain circuit according to the characteristics of weak loop current signal and large dynamic range in underground measurement environment.After the amplitude of the collected current signal is extracted by digital phase sensitive detection,the gain of the acquisition circuit is automatically adjusted by the amplitude,and the noise simulation analysis of all levels of circuits is carried out for weak signal extraction.The difficulty of transmitting circuit design is how to obtain a reliable and stable sinusoidal excitation signal.In this thesis,the generation method of excitation signal in the transmitting circuit is described,and the excitation signal is driven and amplified by class D power amplifier,so that the generated excitation signal has enough driving ability,A transmitting system of SPWM wave generated by FPGA is realized,and the amplitude of excitation signal can be controlled by sending down instruction.At the end of the thesis,through the construction of the experimental platform,the test and analysis of the transmitting circuit and acquisition circuit are completed.Through the experimental test,the transmitting circuit can produce a stable excitation signal with the maximum peak to peak value of 30 V and the output amplitude can be controlled by the command;the acquisition circuit can complete the detection of the input current signal amplitude of 15 n A～ 1.5m A,and the detection dynamic range is 100 d B.