ABSTRACT

The Bayonnaise knoll, an active submarine volcano belonging to an actively rifted part of the Izu-Bonin volcanic arc, exhibits hydrothermal ore deposits on its caldera floor in a region known as the Hakurei Sulfide Deposit (HSD) area. We observed the HSD area using high-resolution acoustic observation equipment consisting of multibeam echo sounder (MBES), sidescan sonar (SSS), and sub-bottom profiler (SBP) systems, on the AUV Urashima. We used visual and acoustic results to examine the consistency of the HSD area extent and to consider possibilities of other ore areas within the caldera. The resultant high-resolution acoustic imageries suggest expansion of the HSD area to the northeastern caldera wall and the southwestern sub-seafloor of the caldera floor. The SBP data show a thick sediment layer on the western part of the caldera floor where many high-backscattering signals were observed. Small chimney-like features were acoustically observed in the HSD area and also at the central cone and along the rim of the caldera. However, most are remnant features of ancient volcanic activity of the knoll, and thus may not indicate current hydrothermal deposits. Acoustic investigations such as this, along with appropriate interpretation, are very useful to determine the detailed distribution of ore on the seafloor and at the shallow subsurface, and should be an effective tool for regional site surveying before seabed mineral mining.