Introduction Crosshole electromagnetic techniques have been studied for detection and imaging subsurface anomalies. The use of higher frequency, i.e., radar, has high resolution, and has been used for imaging anomalies having high contrasts with the host rock. For example, Litle et al. (1979) and Lee et al. (1989) used crosshole radar to detect voids or tunnels, Lager and Lytle (1977) used tomography to visualize trace flow, and Park et al. (1998) used tomography for imaging voids. Borehole radar tomographic analysis has been applied to various other fields (Olsson et al., 1992; Mathisen et al., 1995; Lane et al., 1996; Wright et al., 1998). In most cases, crosshole tomography enables us to characterize subsurface media between two boreholes very well.