[S-SS12] 海域地震津波地殻変動観測の最前線

[SSS12-P13] Laboratory experiment characterizing pressure gauges for the borehole observatory

*松本 浩幸¹、荒木 英一郎¹、町田 祐弥¹、梶川 宏明² (1.国立研究開発法人海洋研究開発機構、2.国立研究開発法人産業技術総合研究所)

A new borehole observatory will be constructed in the Nankai Trough in 2023, where mega-thrust earthquakes take place repeatedly. Strain and pore pressure in the borehole are measured by fiber-optic strain meters and pressure gauges, respectively. The pore pressure measurement system consists of three pressure gauges, in which two pressure gauges measure the in-situ pore pressure inside the borehole, and the other pressure gauge measures the seafloor pressure. We have tested three pressure gauges (modules) by the pressure balance in the laboratory to characterize all pressure gauges before assembling the pore pressure measurement system to determine which pressure gauges are appropriate where to be installed within the system. Calibration curves were obtained to confirm the hysteresis and the repeatability. The other existing pressure gauges calibrated by the pressure balance in the National Metrology Institute of Japan one month ago were also pressurized, confirming the present experiment was performed within the accuracy of less than 10 ppm of full scale. Hysteresis and repeatability of all pressure gauges show less than 50 ppm of full scale, satisfying the specification. Then 20 MPa (i.e., pressure equivalent to 2000 m water depth) were pressurized continuously for one month by the pressure balance. The pressurization was conducted in the laboratory thermal condition. Difference between the pressure output and the standard pressure by the pressure balance over time shows the sensor drift. Sensor drift is fitted by the function combining exponential and liner components. One-month pressurization suggested that three pressure gauges show that sensor drift varies from 10 to 90 Pa per day. Our study suggests that a series of pressurization test, i.e., pressure calibration and long-term pressurization in the laboratory is effective to characterize the pressure gauges. After assembling the measurement system, all pressure gauges will be pressurized again under the seafloor thermal condition to demonstrate the sensor drift under the more realistic ambient temperature.