Application of comprehensive geophysical prospecting method in concealed karst area: A case study from Ning 2019H18 drilling platform in Changning shale gas area

Karst collapse is a common geological problem in the process of engineering construction in carbonate rock areas because of its concealment and sudden occurrence. The karst collapse is widely developed in Changning area, therefore, the exploitation and utilization of shale gas is hampered by extensive karst collapse. In this paper, the authors used micromotion, high-precision gravity, transient surface wave and equivalent anti-flux geophysical prospecting methods to investigate the subsurface geological condition of Ning 2019H18 drilling platform, and ascertained the development of earth cave and karst cave, which were tested by the drilling data. The main achievements are as follous. ①The microgravity is high in southeast and low in northwest, and the gravity response indicates that the southeast bedrock has large density and complete structure, and that the northwest bedrock has low density. ②The micromotion response profile is well corresponded with the microgravity, and the court page is high in southeast and low in northwest. And the wave velocity is positively related to its density. ③The distribution of underground soft strata was also verified, and the southeast bedrocks were completely preserved. A fractured zone exists in the northwest of the area, filled with cracked and soft completely weathered sandstane, which has low bearing capacity. The 3D visualized physical prospecting achievements could provide directions for the next establishment of the area, and some scientific references for similar engineering cases.