Research On The High Resolution Micro-resistivity Imaging Logging Acquisition Circuit

The resistivity measurements of open-hole wells in the early stages of exploration and development are significant for assessing the distribution of oil and gas.It's necessary to study the acquisition circuit,as it actually building a foundation for the microresistivity imaging tool.This paper describes the research and design of water-based mud micro-resistivity imaging logging acquisition circuit,aiming to provide accurate resistivity distribution information for well wall imaging.The circuit in this paper realizes the acquisition of multi-channel current signals with a dynamic range of 90 dB.The research in this paper mainly focuses on the acquisition of weak current signals,based on theoretical analysis,supplemented by necessary simulations,and finally forms a specific circuit design.High-resolution signal acquisition means high signal-to-noise ratio of the circuit.This paper organizes a noise simulation method by modeling and analyzing various noise sources of the circuit.The method can not only obtain the spectral density curve of the output noise of the circuit,but also integrate the density curve to obtain the effective value and peak-to-peak value of the output noise.It can quickly analyze the influence of op amp noise and resistance thermal noise on the output of the circuit,which is more intuitive and accurate than the conventional noise analysis method.The design goal of the acquisition circuit is to extract the current amplitude information in the measurement loop.In order to ensure the measurement accuracy,digital phase sensitive detection is used to extract the amplitude,that is,the acquisition circuit must realize the acquisition and quantization of the AC signal.The design difficulty of the project is to achieve the acquisition of multi-channel weak signals under the premise of satisfying the measurement accuracy and the logging speed.According to the design index requirements of the subject,the parallel hardware structure is proposed.Then the specific circuit design of each functional module is described,and the transfer function of each circuit is solved by the method of network analysis.Finally,the theoretical feasibility of the design scheme is verified by circuit simulation.In order to effectively realize the acquisition of multi-channel AC signals,the switching of the acquisition channels must be controlled according to strict timing,so the main control chip uses a non-volatile FPGA.The difficulty of programming is to comb the channel switching process during signal acquisition.In the article,the program control flow chart of the acquisition circuit is first given,and the function composition and execution sequence are briefly described.Then,the detailed design description and simulation verification of each function module are carried out.At the end of the paper,the test platform was built to test and analyze the output parameters of the acquisition circuit,including repeatability,linearity and standard deviation.It verified that the circuit meets the expected design specifications.