Seismic prediction of unfavorable geobodies in tunnels using the borehole-roadway transient electromagnetic method

Tunnel detection in complex environments requires fine-scale detection of small unfavorable geobodies like karst caves and fissures. Hence, this study designed a tunnel construction model with a small karst cave in front of the tunnel face. A borehole was drilled at the center point of the tunnel face towards the construction direction, and then an electrical source was put into the borehole for excitation. Array data acquisition was conducted on the tunnel face. The 3D forward modeling based on transient electromagnetic data was performed using the time-domain finite element method. As indicated by the results, the electromagnetic sounding of the target was achieved through the movement of the electrical source, and the planar position of the unfavorable geobody was determined based on the distribution patterns of the electromagnetic field on the tunnel face. Therefore, electrical source excitation in a borehole can enhance the detection ability of the transient electromagnetic method on small karst caves, serving as a feasible method for improving the accuracy of tunnel seismic prediction.