A development of a mobile hydraulic pressure gauge for seafloor geodetic measurements and data evaluation of field studies conducted in the Nankai Trough

Abstract:

A precise hydraulic pressure measurement is important in order to obtain geophysical knowledge associated with the occurrence of large interplate earthquakes. However, pressure measurements contain instrumental drift in a sensor in addition to the pressure changes related to a crustal deformations. Therefore a correction of the drift using an absolute calibration system is necessary. We developed a mobile pressure gauge. The mobile pressure gauge has a quartz water pressure meter (Paroscientific Inc. 410K) as a pressure sensor with pressure holding system due to heating of a pressure piping. The system enables us to maintain a constant pressure of a target field pressure from a laboratory to a seafloor in order to eliminate effects of pressure hysteresis of the sensor. Laboratory experiments showed a temperature characteristics and hysteresis of the sensor in addition to the pressure hysteresis. Because a temperature of the sensor is affected by an outside temperature which ranges from 2 degrees (at the bottom of the sea) to room temperature, we need to calibrate outputs values of the sensor using calibration values estimated by laboratory experiments. These procedure would improve the measurement accuracy and contribute to obtaining a small amount of crustal deformations related to occurrence of large interplate earthquakes.

In the Nankai trough, we deployed a long-term borehole monitoring system with pressure gauges into the C0002 boreholes. Because the data are available in a real time, the observatory is good target for the calibration study. During the KY15-05 cruise, we deployed the mobile pressure gauge on the C0002 observatory using a ROV twice. A repeatability measurements between two dives indicates 2.55 hPa, which is equivalent to 2.55 cm converted into cm. And a repeatability measurements before and after the cruise indicates 3.56 hPa. In the presentation, we examine the calibration results with consideration of a characteristics of the pressure sensor based on laboratory experiments.