The Research Of Signal Acquisition Channel Noise Suppression Technology Under Cased Hole Formation Resistivity Logging

Through-casing resistivity logging method is applied in measuring the formation resistivity by surveying the voltage drop of the casing among three measuring electrodes. The ultra-low frequency sine-wave excitation current source is applied to gap the influences of the skin effect, contact electromotive force and cable coupling. The through-casing resistivity logging signals are very weak because of the low casing resistance and the effective signal amplitude is only10-9V, therefore the acquisition and inspection methods of ascertained frequency sine-wave signals become the key technical difficulty.This paper firstly describes the three electrodes resistivity logging principles and the overall framework of the corresponding system. Then, one introduces hardware design of the ultra-low noise preamplifier and high-resolution data acquisition system. Next, The basic methods of the weak signal detection and the key noise suppression technology in data acquisition are presented. Finally, the data processing algorithms and the system simulation results are provided. This paper not only deals with the hardware design but also the software filtering algorithm about the average median filter and phase sensitive detection of weak signal in thru-casing resistivity logging. The goal of noise suppression in weak signal extraction is attained and the output signal to noise ratio is improved. Therefore the detecting accuracy is improved to a higher level.In this topic, ultra-low-noise preamplifier, oversampling method, sampling integration and sampling average method, digital phase-sensitive detection technique are applied in detecting logging signals and30nV is achieved. The indoor calibration test and field experiment of through-casing resistivity logging model machine with those weak signal detection techniques were carried out. The result showed that the measurement range of formation resistivity is0～200Ω.m. The system of high precision and works stably meets the design requirements, reached the technological level of domestic and foreign similar equipment.