The paper reports the results of the works done in fiscal 1991 for verification survey on geothermal exploration technology, development of a method to explore fracture type reservoirs, and development of an exploration method using elastic wave. As for manufacturing multilevel/digital type downhole receiving system, conducted were element tests on high temperature parts of downhole units and ground units, making basic specifications, system fabrication/temperature tests. Also, for the purpose of testing vibration followability and heat resistance of this system, VSP measurements were made in the field adopted to Yutsubo to study basic characteristics. For the tomography test to be conducted in fiscal 1992, a study of downhole energy source is made using the well in the Yutsubo field. In the experiment, elastic wave traveling between the holes is measured setting a detonator type energy source in No. 1 well and a downhole receiver in No. 2 well. As a result of the analysis, it is found that the detonator type energy source is one sufficient for the tomography survey in terms of both energy and frequency. 138 figs., 18 tabs.