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Roadway transient electromagnetic method has been widely used in detecting mine water disaster ahead of roadway excavation face. Due to the strong mutual inductance between transmitting-receiving coils, the early-stage data obtained by this method are seriously distorted and the effective resolving time of transient electromagnetic (TEM) field is greatly lengthened, which leads to the shallow blind area of exploration. To improve the ability of transient electromagnetic testing system and to resolve short-range geoelectric abnormalities in front of the roadway excavation face, we provided the expression functions of the primary TEM field and the receiving system’s impulse response, restored the data recorded by instrument theoretically and compared the differences between the recorded data and the pure secondary TEM data. We found that the factors, including the increase in the current switch-off time, the decline in the receiving coils’ inherent resonant frequency, the enlargement in the mutual inductance between transmitting-receiving coils, and the mismatch in the receiving circuit’s damping resistance and so on, could significantly affect the early-stage TEM data, which results in a great delay in the effective resolving time. Based on the above findings, we improved the TEM testing system, compared the monitored data obtained using the original and improved testing system. It demonstrates that the improved testing system shortened the effective resolving time, which greatly enhanced the reliability of early-stage data and efficiently resolved the geoelectric abnormal zones closer to the roadway excavation face.

Roadway transient electromagnetic signal analysis and its testing system improvement