An effective exploration technology is to be newly developed in order to accurately explore the fault group which controls the fluid flow in the geothermal reservoir. Taking the past progress in study into consideration, the result in fiscal 1991 was reported. Yutsubo geothermal field in Oita Prefecture was selected as an experimental site. In the high-accuracy seismic reflection method with both P-wave and S-wave hypocenters, reflection events which were good in S/N ratio were obtained down to the depth of about 1.5sec in two-way travel time. Discontinuous reflection events judged to be a fracture were discriminated from the S-wave reflection recorded in profile. As a result of seismic wave propagation test, the initial motion of S-wave was clearly observed also by vertical seismic profiling (VSP). The reflection method is sufficiently effective in generally grasping the fracture structure. In a P-wave`s zero offset VSP, the reflected wave from the boundary between lava and tuff was clearly detected with a reflection surface which corresponded well to that recorded in logging. Pursuable as a continuous horizon in the imaging record, the reflection surface can serve as a reliable index to detect the fracture. 80 figs., 4 tabs.