The crosshole seismic CT is a high-resolution response method for geological exploration. This paper describes the theory and field techniques of crosshole seismic CT, and presents an example to illustrate the application of the crosshole seismic CT to the detection of fissures in the foundation of a bridge. The means for obtaining good seismic data based on the authors' experience are given in this paper. Only by performing joint inversion of crosshole seismic data and geological data can good interpretation result be achieved.