The application of ADCP to the measurement of current system at the hydrothermal active areas

Direct current measurement in the hydrothermal area is one of the important factors to understand the hydrothermal system. The application of ADCP is known to an effective to measure the general understanding of the ocean current. The RDI Workhorse Sentinel 300kHz ADCP was tried to use attaching on the JAMSTEC/Deep Tow Sonar System, the manned submersible Shinkai 2000 and deploying at the seafloor by ROV Dolphin 3K during a few scientific cruises at the hydrothermal active areas in the Western Pacific. The JAMSTEC/Deep Tow Sonar was carried out to survey for precise topographic at the hydrothermal area in the Southern Mariana Trough in January 2000. During the survey cruise, the ADCP was attached on the sonar with down looking. It was towed about 80-120m above the seafloor as a speed of less 1kts. According to the observation, acoustic backscattering was too weak to measure over 30m ranges; in addition vertical component was difficult to measure depending on the heaving effect from the ship. It is necessary to reduce the heaving motion at the fish such as using sub-fish system that is separated from main-fish via a secondary cable. In the hydrothermal area of the Mid Okinawa Trough, which is known as several active hydrothermal. areas with huge chimneys which height about 30m as temperature of over 230°C, the current measurement by ADCP was carried out using the Shinkai 2000 in June 2000. ADCP was mounted down looking on the Shinkai 2000 as a payload. The Shinkai 2000 was operated with 0.3-0.5kts keeping its depths from 870m to 900m. During this observation, it was obtained the current profile as about 100m range. It was measured about 10-20cm/sec upward current in the hydrothermal active area. In the same area, ADCP was deployed for 23 hours as up looking at beside of the hydrothermal chimney. It was settled and recovered by the ROV Dolphin 3K in June 2000. Current measurement condition was changed about every 6 hours; in a half period of the observation, acoustic backscattering signal is too weak to measure over 30m range. This change was considered to cause by the tide. The water column of hydrothermal fluid contains enough particles for acoustic backscattering; however the standard deep water of this area is too clear to ob- tain the enough acoustic backscattering. During measurable period, upward current was measured as its maximum current speed as over 20cm/sec.