Detection of unfavorable geological bodies of underground space with opposing coils transient electromagnetic method

Ground collapses caused by unfavorable subsurface geologic bodies occur frequently, posing a significant threat to the safety of people’ s lives and property. The detection of unfavorable subsurface geological bodies by non-invasive geophysical method is an important scheme to reduce urban ground collapse. The transient electromagnetic method (TEM) is one of the important methods. The opposing coils transient electromagnetic method (OCTEM) applies small and weak coupling coils configuration which is free of grounding and requires limited operational space, making it suitable for shallow subsurface detection in urban areas. In this paper, theoretical calculation and actual tests were carried out to study the OCTEM responses of subsurface unfavorable geological bodies. According to the electrical characteristics, three types of unfavorable geological bodies were constructed, i.e., they were resistive, conductive, and complex types. The relative anomaly between the abnormal response and the background response was calculated. The results show that OCTEM is sensitive to the pure conductive and the complex types, but not sensitive to pure resistive type. Under the same other conditions, the more conductive of the abnormal body is, the greater the thickness and the shallower the burial depth, the larger the relative anomaly is. As the depth increases, the detection resolution of OCTEM decreases. Finally, a karst detection case in a town of Liuyang City of Hunan Province was presented to show the result of OCTEM detection in unfavorable geological bodies detection in urban area. © 2024, Central South University Press. All rights reserved.