Seabed benchmark system in Aira Caldera

This paper describes the performance of a seabed benchmark system for measuring ground deformation associated with underwater volcanism. The system is installed in the center of the Aira Caldera, Japan, on the caldera floor 30 m below the sea surface. This is the first attempt to directly measure the expected ground deformation within a flooded caldera in Japan. The system consists of an anchor, a universal joint, a rigid steel shaft, a float, and a deck. A global navigation satellite system antenna array, consisting of a primary antenna and auxiliary antennas, is installed at the top of the moving buoy to monitor the position of the benchmark at the caldera bottom. A post-processing procedure is used to reduce the tilt of the master antenna based on measurements made by the auxiliary antennas. The system was installed in March, 2023, and has since been operating successfully with an acceptable measurement standard deviation.