Subsurface Tunnel Detection Using Electrical Resistivity Tomography and Seismic Refraction Tomography: A Case Study

Seismic and electrical methods are two geophysical techniques commonly used to detect rock property changes in the subsurface. Surveys are usually carried out with sources and sensors placed at or near the earth's surface. Various levels of data processing are applied to construct a map of the subsurface distribution of a physical property. Seismic methods are sensitive to velocity and density changes of the rock, while the electrical response is dependent upon the electrical resistivity of the rock. In this paper, we present an evaluation of using both seismic refraction and electrical imaging to locate a tunnel. Although the contrast in seismic velocity and electrical resistivity between the tunnel and its surrounding material can be large, tunnels remain difficult to detect. The difficulty primarily arises because the spatial resolution of these methods is less than or on the order of the size of the tunnels. Detection is further complicated by heterogeneity of the near surface materials within which many tunnels are constructed. In this paper, we present a case study using ERT dipole-dipole data and seismic refraction data at a tunnel site. Both seismic and electrical surveys were acquired at the same time with a lateral offset of about 3m. The results from both techniques show anomalies at the location of the tunnel. The confidence in predicting the location of the tunnel is increased by using data from both techniques.