Mapping the concealed structure using high-resolution EM - A case history of the investigation of B Tunnel route
Lanfang He, Zhanxiang He, Xuben Wang, Lunkai Yang · 2004
A successful case history of mapping the concealed structure using high-resolution EM in the investigation of B Tunnel route is presented in this paper. The high frequency electromagnetic system named STRATAGEM EH4, with the frequency ranging from 90 KHz to 1Hz, is used for the data acquisition. The orthogonal components of the electromagnetic field are measured, and the relevant electromagnetic attributes are extracted on the basis of the characteristics of the electromagnetic profiles. Hybrid sources, including the natural source and the full tensor controlled-source, are utilized to collect high quality field data. B Tunnel lies in the western part of Hubei province, the south of central China. The tunnel is only buried at the depth of less than 200 meters, but the geologic structure is very complex because of the regional geological action. During the first time of the B Tunnel route investigation, the investigators were deceived by the outcrop of the rock fall, considering that it is from the bed rocks and the roof of the tunnel fell when tunneling was in operation. High-resolution EM and seismic refraction were used in the second time investigation. The “real” bed rock and the geologic structure were mapped this time and a hidden fault was also detected. The tunneling result fits closely with what we have predicted.