Structural Imaging of Acoustic Survey Using a Deep-Towed Sub-Bottom Profiler and Hydrophone Cable

High-resolution images are required for the exploration of mineral resources on and beneath the deep seafloor. Under the Strategic Innovation Promotion Program launched by the Council for Science, Technology, and Innovation in Japan, we developed a survey system using a deep-towed acoustic source and receivers that produces very high-resolution subseafloor images. Our system uses a subbottom profiler as the acoustic source and a hydrophone cable as receivers. These instruments are towed near the seafloor by a remotely operated vehicle (ROV). The subbottom profiler generates chirp signals in the frequency band of 700–2250 Hz, and 32 hydrophones with a receiver interval of 1 m record the reflected waves. The system also includes two recorder units that record the acoustic wavefield with a sampling interval of 0.1 ms. We carried out field surveys at two locations in Japan: one site is in the Sea of Japan where gas hydrates were discovered and the other site is near Okinawa Island where a plume from a hydrothermal smoker was recorded by an ROV camera. Pre-stack depth migration was applied to the processed data using velocity models with a constant velocity gradient below the seafloor. The vertical resolution was 0.5–1 m depending on the depth, and the horizontal resolution was about ten times larger than the vertical resolution. The resulting images show that detailed subseafloor structures can be clearly imaged by the deep-towed system, even at sea depths greater than 1000 m.