The application of the opposing coils transient electromagnetic method and microtremor survey method in shallow karst detection

Concealed karsts frequently lead to disasters such as ground collapse, water reservoir seepage, and tunnel water inrush. Identifying the distribution and development of karsts in advance holds great significance for guiding engineering design and construction in karst areas. Geophysical methods rank among the most effective methods for karst detection. Given the problems of karst development, limited space, and serious electromagnetic interference in the tank farms of the Longkou Port, Shandong Province, this study explored shallow karsts in these farms through the joint application of the opposing coils transient electromagnetic (OCTEM) method and the microtremor survey method. Firstly, surface scanning was conducted with a grid density of 10 m × 5 m using the OCTEM method. Based on this, 3D geoelectric modes were constructed. Then, three short sections for microtremor surveys were supplemented above the karst developmental areas that were determined using inversion and interpretation, aiming to counteract the OCTEM method’s inability to distinguish air-filled karst caves based on medium to high resistivity anomalies. The following results are obtained. The areas with low resistivity anomalies in the bedrock layer also exhibited low S-wave velocity anomalies. These two types of anomalies, which corresponded well, are inferred to be caused by karsts. Besides, there also existed some areas with low S-wave velocity anomalies in the bedrock layer that exhibited medium to high resistivity on resistivity profiles. As verified by late drilling, these anomalies are caused by air-filled karsts. Based on the joint inference and interpretation using the two methods, the late drilling yielded a success rate of encountering karst caves of up to 70%, demonstrating the effectiveness of the combined detection methods The results of this study can be used to effectively solve the challenge of detecting shallow karsts in a complex environment, thus warranting wide promotion and application.