Hydrothermal plumes imaged by high-resolution side-scan sonar on a cruising AUV, Urashima

An autonomous underwater vehicle (AUV) is an ideal platform that enables a closer approach to the seafloor than other platforms. The AUV provides remarkable advancement not only in imaging small-scale bathymetry with high resolution but also in imaging the spatial distribution of scattering signals within a water column. Acoustic echoes can be recorded continuously by AUVs, potentially providing information related to hydrothermal plumes within a water column. In May 2007, such an AUV, Urashima, of the Japan Agency for Marine-Earth Science and Technology, captured fantastic acoustic images within the water column above an active hydrothermal field, the Iheya-North field, off Okinawa. Numerous filamentous and hyperbolic echoes were recorded on the side-scan sonar images. According to the temperature and geochemical anomaly, the scattering signals within the water column were hydrothermal plumes upwelling from the vent area. This proof of concept has implications for long-range exploration for vent field with AUVs.