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Borehole transient electromagnetic(BTEM)scanning detection method uses the advanced exploration boreholes in the roadway to scan and detect the hidden geological anomalies outside the borehole wall by rotating the transceiver coil inside the borehole. Because of the transceiver device is located in the borehole, it not only avoids the electromagnetic interference of the electrical equipment and metal in the roadway, but also improves the utilization rate of the borehole and reduces the drilling workload. However, the BTEM is affected by the whole space effect. In the scanning and detection process, the normal direction of the transceiver coil is used as the detection direction to receive the secondary field signal of the abnormal body, it is also susceptible to the superposition interference of signals in other directions. If the interference signals cannot be effectively suppressed, it will directly affect the positioning of the geological anomaly body around the borehole wall. Using the shielding method, the shielding cover is installed on the receiving coil, and the response signal in the detection direction where the coil normal line is consistent with the opening of the shielding cover is retained as much as possible, while the signals in other directions are shielded, so as to realize the directional detection of the radial direction of the borehole wall within a certain range. First of all, the influences of transceiver coils with different turns and sizes on transient electromagnetic response characteristics are discussed through physical simulation experiments, and the parameters of transceiver coils suitable for drilling are optimized. Subsequently, aiming at the electromagnetic wave to be shielded, different kinds of shielding materials are selected, and the shielding effect experiments of different materials on electromagnetic waves in different directions are carried out. By comparing the induced electromotive force of the detection direction and other directions, it is proved that the arc-shielding made of nickel foam can effectively suppress the response signal in non-detecting direction. The transient electromagnetic field response characteristics of different shapes, permeability and other physical parameters of the shield are compared and analyzed through experiments. Relationship between the opening angle size of the shield, the distance between the shield and the receiver coil and the directional shielding effect is discussed. The parameters of the directional shielding are optimized to enhance the shielding effect. Finally, the parameters optimized transceiver coil and directional shielding device are used to carry out the borehole transient electromagnetic method radial section scanning detection field experiment. The inverse method based on a mixed distribution chaotic quantum particle swarm optimization and a combined Levenberg-Marquarat and Occam algorithms is used for resistivity images. The research shows that the arc-shaped shielding cover installed on the receiving coil with foam nickel as the shielding material has a better directional shielding effect, which can effectively suppress the signal interference in the non-detection direction during the scanning detection process of BTEM and can retain the abnormal response signal in the detection direction. Therefore, the precise positioning of the hidden geological anomaly body around the borehole wall can be realized. |
