We have developed a Super-deep-sea self-popup Ocean Bottom Seismometer （SDOBS） that can be deployed to the ocean floor up to9,000 m depth. Because the maximum applicable water depth of a conventional self-popup Ocean Bottom Seismometer （OBS） is 6,000 m, some areas have remained inaccessible to seismic surveys, such as the deep part of Japan Trench, where the Great East Japan Earthquake occurred in 2011. Using a ceramic pressure-tight sphere, we were able to develop a SDOBS that has almost identical size, weight, and buoyancy to those of a conventional self-popup OBS using a glass sphere. Regarding the acoustic transponder, which is a key device for the development of SDOBSes. We heighten the transmitting acoustic level of an existing acoustic transponder to raise the positioning accuracy. Detailed results of sea tests conducted to evaluate the acoustic transponder performance are described herein. We used the same built-in seismometers, recorders, batteries, and other equipment as those used for conventional OBSes. We also report that by improving the test procedures, we were able to heighten the measurement accuracy of the uniaxial compressive strength of ceramics, which are important parameters to determine the applicable hydraulic pressure. We have developed seven SDOBSes to date, and have deployed 18 SDOBSes cumulatively for seismic surveys in the Japan Trench and Ryukyu Trench.
