Precise bathymetry with an underwater vehicle for seafloor crustal movement observation

Abstract

DONET deploys about 50 stations covering an area of 150 km from north to south and 270 km from east to west to observe earthquakes and tsunamis, and the pressure gauges implemented for tsunamis observation at DONET are expected to be applied to wide-area and long-term observation of vertical crustal deformation. However, it has been reported that the pressure gauge observation data contains not only pressure changes associated with vertical crustal deformation but also instrument-specific drifts equivalent to 10 cm of water depth per year. To extract the vertical crustal movement component from pressure data, which is assumed to be about 1 cm per year, it is necessary to remove the effect of drift. Since each sensor has different drift characteristics, it is necessary to calibrate DONET’s pressure sensing system individually. Therefore, we developed an in-situ calibration method for DONET’s bottom pressure gauges with the mobile pressure calibrator. This method is similar to geodetic surveying on land, which is performed on the seafloor to measure the accurate pressure (depth) and the height difference between the pressure gauge to be calibrated and the mobile pressure calibrator and to measure the desired pressure that the pressure gauge to be calibrated should indicate. We have demonstrated that this method can calibrate the pressure gauges with an accuracy of less than 1 cm in depth based on the results of several in-situ calibrations. In-situ calibration using a mobile pressure calibrator, equivalent to the method of geodetic surveying on land, is inefficient for visiting multiple observation points deployed over a wide area because in-situ calibration with the mobile pressure calibrator is seated on the seafloor to perform measurements. Therefore, to improve the efficiency of in-situ calibration in the deep sea, we propose to apply a method equivalent to aerial surveying to in-situ calibration on the deep sea floor. The proposed "fly-by calibration" method combines depth with a pressure sensor and altitude measurement using a 3D laser scanner. The precise pressure measurement unit and the 3D laser scanner are mounted on an underwater vehicle, which cruises above a bottom pressure sensor on the seafloor, and the precise Bathymetric data for the surrounding area including the pressure gauge to be calibrated are obtained. We confirmed whether the actual combined measurements can provide the same level of accuracy as the in-situ calibration with mobile pressure calibrator.