In tunnel construction, the information regarding rock mass quality and the distribution of weak zones is crucial for economical tunnel design and to ensure safety. Usually, the rock mass grade is estimated by observing recovered cores obtained by drilling or by physical parameters calculated in a laboratory using core samples. However, the high drilling cost limits the number of boreholes; furthermore, rough terrains can reduce the access of drilling machines to the survey sites. In such situations, surface geophysical methods such as electrical resistivity or controlled-source magnetotelluric (CSMT) can provide a rough estimate of the rock mass condition over the planned tunnel route. These methods can also map weak zones (faults, fractures, coal bearing zones, and cavities), which are characterized by a lower resistivity than the surrounding fresh rock mass.
We present two successful applications of the electrical resistivity and CSMT methods to the site investigation of tunnel construction over a rough terrain. The first example demonstrates that the boundary of the bedrock and weak zones related to the distribution of coaly shale and coal seams were estimated to extend beyond a few hundred meters below the rough surface. The second example shows that the developing direction and depth of cavities, which are mainly related to the weak zones in limestone, were successfully interpreted by a three-dimensional (3-D) electrical resistivity survey with the aid of borehole test results.