Level adjust unit at seafloor for precise pressure measurement

The ocean bottom pressure measurements in DONET have been expected to detect long-term ocean bottom crustal deformations. However, the pressure values at ocean bottom contains the drifts of the pressure gauges in addition to the pressure changes associated with crustal deformations on the sea bottom. The sensors of DONET have been installed and operated on the seafloor for about eight years in the longest. It is reported that each pressure gauge has different drift characteristics and that it has a displacement of 10 hPa/year at maximum depend on long-term observation. In order to remove the instrumental drift of each pressure gauge which deployed wide area, we have been developing the in-situ calibration technology for ocean bottom pressure gauges of DONET. This calibration system is put near the pressure sensing system of DONET using a ROV. And then, it measure the desired pressure at the pressure gauge level. In order to measure precise pressure, it is necessary to adjust the difference of level between the mobile pressure calibrator and the pressure sensing system within 1 mm from the position up to 5 m away. The developed mobile pressure calibrator consists of the pressure measurement system and the level adjust system. The level adjust system adjusts the level of the pressure measurement unit to the level of the pressure sensing system of DONET using three motors, inclinometer, and cross line laser. The measurement range of level is 400 mm, accuracy is about 1 mm. This paper describe about the level adjustor for the mobile pressure calibrator and result of the experiments.