Seafloor Hydrothermal Deposits Exploration by Bathymetry and Backscattering Data Using Multibeam Echo-Sounder in the Higashi-Aogashima Caldera

: We develop an effective method for exploring seafloor hydrothermal deposits using acoustic apparatus. The important thing for seafloor hydrothermal deposits exploration is to carry out the efficiently detailed exploration after narrowing down a promising area from a vast area by wide-area exploration. Here, we aim to establish such a method, by examining methods for obtaining detailed bathymetric data and normalization of backscattering strength. Compared to conventional methods, data density is increased in both the transverse and traveling directions by narrowing the swath width and dropping the vessel speed. Also, normalization of submarine acoustic image is proceeded by applying the least square method from actual measurement focusing on the backscattering strength varying logarithmically with respect to the incident angle. Utilizing these techniques, we carried out a wide-area hydrothermal deposits exploration in the Higasih-Aogashima caldera. Accuracy of acquired wide-area bathymetry data had a difference of less than 0.1% against the area depth (about 700 m). In addition, normalized seafloor acoustic images were able to grasp the features of the sediment in the caldera. By narrowing the hydrothermal deposits candidate point, hydrothermal deposits were discovered from subsequent detailed exploration using an autonomous underwater vehicle and gravity corer. We are confident that these techniques will increase the efficiency of wide-area exploration of hydrothermal deposits.