Dual-Waveform Combination of Electromagnetic Detection System Based on Small-Loop TEM Method for Shallow Refinement

The small-loop transient electromagnetic (TEM) method has made great contributions to urban underground space detection in recent years due to its small working space. For the problems of single sampling rate and low sampling rate of traditional receivers, a dual sampling rate receiver is designed to record electromagnetic response with high accuracy. In addition, this article employs the passive clamping technique and oscillation absorption technique to realize the falling edge linear fast shutdown of the triangle and trapezoidal wave emission currents and to improve the excitation capability of the shallow target. In order to avoid the saturation of the early response, the primary field intensity of the center point is reduced by a bucking coil. In order to solve the problem of high-precision data interpretation in a shallow layer, this article adopts the method of remaining primary field removal and response splicing to splice the pure secondary field response of triangle wave and trapezoidal wave current waveform and realizes the joint resistivity imaging of electromagnetic data. According to the laboratory test results, the transmitter can output triangle wave and trapezoidal wave with adjustable parameters; the sampling rate of the receiver is 64 and 256 kHz, which can effectively record the electromagnetic response and current waveform. The field experimental results exhibit that the small-loop electromagnetic detection system can effectively measure the shallow anomalies in urban underground space, which demonstrates the effectiveness of the dual-waveform combined small-loop detection system for improving the accuracy of shallow exploration and promoting the development of geophysical refinement detection process.