In order to monitor the seafloor stability during methane hydrate productions, a monitoring system continuously measuring seafloor displacement is required. We have been developing a system that mainly consists of a 3-component servo-accelerometer and gimbaled mechanism in a pressure chamber. The observed acceleration waveform data are used to calculate displacements by double integrals with noise reduction. After testing the performance of the servoaccelerometer, a prototype model of the system was created and tested in laboratory with soil samples and at an onshore landslide site. As the results of those tests, we confirmed that the monitoring by servoaccelerometer is practical.


In order to monitor the seafloor stability during methane hydrate productions, it is required to measure seafloor displacement continuously. In the case of onshore landslide site, ground displacement monitoring of the landslide movement is usually done by such instruments as the slide sensors, borehole tilt meters, etc. These instruments measure the relative displacements between a fixed point and measuring points in the landslide mass. However, those can not be used for displacement monitoring at the seafloor where it is difficult to set a fixed reference point. In this study, we propose a new method to monitor the seafloor displacement by using a 3-component servo-accelerometer system. The basic concept of the proposed method is to calculate displacements from the acceleration waveform records obtained at the seafloor by a 3- component accelerometer (Fig.1). The basic calculation is simple double integrals applied to the acceleration waveforms to obtain displacement waveforms. One of the most important points for the data processing is to reduce noises, especially long period trends, which produce much error in the integral calculations. In this paper, we will first describe the outline of the prototype monitoring system.

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