1. Department of Mining, Kyushu University, Fukuoka, 812-81, Japan.
2. Geothermal Research Laboratory, Kyoto University, Beppu, 874, Japan.
3. Central Research Institute of Electrical Power Industry, Chiba, 270-11, Japan.
4. Dowa Engineering Co.Ltd., Shinbashi, Tokyo, 105, Japan.
5. Chiba Electronics Laboratory Co., Sakura, Chiba, 285, Japan.