1. Geology Group, Subsurface Evaluation Unit, Technical Division, INPEX Corporation, Akasaka Biz Tower, 5-3-1 Akasaka, Minato-ku; Tokyo 107-6332 Japan
2. Institute of Geology and Paleontology, Graduate School of Science; Tohoku University; Aobayama, Sendai 980-8578 Japan
3. Faculty of Science, Department of Physics and Earth Sciences; University of the Ryukyus, 1 Senbaru; Nishihara, Okinawa 903-0213 Japan