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The Sea-Bottom Benchmark system in Aira Caldera and its current performance

Tomoki Tsutsui1 , Daisuke Miki1, Masato Iguchi2

Abstract

The Sea-Bottom Benchmark system (SBB) was installed in 2.2 kilometers northeast off Sakurajima on March 2023.  SBB comprises an anchor of 19 tons weight and a universal coupled buoy of 40 meters length and 21 tons weight.  The measuring system comprises four GNSS antennas, two GNSS receivers, a mobile router. The power supply system includes three iron phosphate lithium-ion batteries, three photo-voltaic cells, and a charge controller. Three of the antennas are placed at the end of 1 meter beams and form the apex of a right triangle.  Data in the receivers are pulled up once a day through a LTE commercial network, and are processed in the machine in the headquarters automatically.      The position at the top of the anchor is calculated through the attitude reduction with using a GNSS antenna array at the top of the buoy, which represents ground deformation at the seabed.  The reduced location result show a simple normal distribution with the standard deviations of location 0.17 m for lateral components and 0.017 m for the vertical component. The system operation has been running successfully for almost two years after the establishment without any day-long break and stood through strong wind of typhoon storms.  Although almost flat trend are observed in the surrounding station, the result from SBB shows on-going local subsidence at one year after the installation.  The reduction of the local subsidence is necessary to resolve exact ground deformation.  The reduction will be performed through continuing operation of SBB.