New Step for Broadband Seismic Observation on the Seafloor: BBOBS-NX

Essential information for clarifying geodynamic processes is obtained by imaging the Earth's interior through geophysical observations. Huge oceanic areas are important locations for conducting such observations. The broadband ocean-bottom seismometer (BBOBS) that we developed has been used since 1999 in several array observations, which gave us new information. But, the BBOBS's noise model in periods longer than 10 s indicates the high noise level in horizontal components above the new high noise model (NHNM), although the vertical one is between the NHNM and the new low noise model (NLNM). It makes it difficult to apply modern analysis methods using horizontal component waveforms even from the data of the one-year-long observation at a single station. Recently, we have developed a geophysical instrument to investigate the oceanic mantle, namely the next-generation broadband ocean-bottom seismometer (BBOBS-NX), operated by a remotely operated underwater vehicle (ROV). The BBOBS-NX provides data of much higher quality than the conventional BBOBS, because it shows a comparable noise model in horizontal components with that of land seismic stations in periods longer than 10 s. Comparison between bottom currents and horizontal particle motions of the BBOBS-NX and conventional BBOBSs clearly indicates the effective reduction of the noise due to the bottom current in this period range. A preliminary receiver function analysis also shows the advantage of the BBOBS-NX to the conventional BBOBS, even if the observation period of the former was shorter than a quarter of that of the latter.