Trial exploration for hydrothermal activity using acoustic measurements at the North Iheya Knoll

Abstract

A survey method using acoustic equipment has attracted recently interest of researchers for the research of gas bubbles and hydrothermal activity because gas bubbles and hot water exposed from hydrothermal vents produce acoustic impedance anomalies in cold seawater. We tried a new hydrothermal exploration using a hull-mounted Multi Beam Echo Sounder (MBES) and a high-frequency MBES system mounted on AUV URASHIMA as one approach in the first stage of a comprehensive hydrothermal deposit survey process including geophysical exploration. The surveys using the MBES system were conducted at the North Iheya Knoll. First, we discovered heretofore unidentified hydrothermal activity at two areas of the North Iheya Knoll using hull-mounted MBES. Then, MBES data obtained during the AUV dive provided detailed positions of new hydrothermal activity zone. The existence of previously unknown hydrothermal vent was identified through ROV dives using our estimated plume position data for the North Iheya Knoll. These achievements demonstrate clearly that the acoustic survey using shipboard MBES and a vehicle with high-frequency MBES are useful as one approach for hydrothermal exploration.