1. Kawasaki Geological Engineering Co., Ltd., 2-11-15 Mita, Minato-ku, Tokyo 108-8337, Japan.
2. Corresponding author.
3. Japan Continental Shelf Survey Co., Ltd., 1-11-2 Kyobashi, Chuo-ku, Tokyo 104-0031, Japan.
4. Hydrographic and Oceanographic Department, Japan Coast Guard, 5-3-1 Tsukiji, Chuo-ku, Tokyo 104-0045, Japan.