Development and Performance Tests of a Sensor Suite for a Long-Term Borehole Monitoring System in Seafloor Settings in the Nankai Trough, Japan

In the Integrated Ocean Drilling Program (IODP), the long-term borehole monitoring system (LTBMS) has been planned for installation into boreholes in seafloor settings in the Nankai Trough, Japan. The LTBMS sensors are extremely sensitive instruments for collecting broadband dynamics to elucidate the mechanisms of megathrust earthquakes, which occur repeatedly in plate subduction zones. However, during IODP Expedition 319, it became apparent that the strong ocean current “Kuroshio” causes vortex-induced vibration (VIV) that damages sensors during installation. Consequently, the LTBMS sensors must be not only highly sensitive but also robust to prevail against VIV. Therefore, sensors with antivibration mechanisms were developed by a Japan Agency for Marine-Earth Science and Technology (JAMSTEC, Kanagawa, Japan) project team. After development was completed, noise evaluation tests and vibration and shock tests simulating vibration and shock in the installation scheme were conducted to confirm that the antivibration mechanism was functional. Power spectral density analysis was conducted using background noise recorded in a low-noise location before and after the vibration and shock tests. Results show that the sensor response was not changed by the vibration or shock tests. Finally, all sensors were loaded onto D/V Chikyu for installation at the C0002 site during IODP Expedition 332.