The shallow surface of the area is dominated by mixed fill, silty clay and coarse round gravel. There is a large amount of construction waste distributed on the surface. The soil is uneven, containing a small number of rust spots, and locally rounded gravel. The coarse round gravel soil is mainly basalt and angular. The lower basalt is mainly divided into strongly weathered basalt and weakly weathered basalt. The main components are feldspar, pyroxene, hornblende, joint fracture development, accompanied by cave development.

Fig.1 Field investigation site; 
Fig.2 Transient electromagnetic method inversion results;

The survey was arranged with two measurement lines, one in two boreholes, with a point distance of 2 meters. Hole 1 offset original position 12.4m, according to drilling data, 10~19.6m is molten cavity, clay soil filling; drilling hole 2 has no offset. According to the drilling data, 9.1~11.3, 14.8~16.1, 18.6~19.9m are molten cavities, and clayey soil is filled. The transient electromagnetic inversion results correspond to the drilling data.