Development of Buried Sensor for Mobile Ocean Bottom Broadband Seismic Observation of the Next Generation

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Since 1999, we have already developed the mobile broadband ocean bottom seismometer (BBOBS), and many practical observations in the northwestern Pacific Ocean and French Polynesian Sea have been conducted. But, through the evaluation of the seismic data, the noise level of horizontal components in long periods, those are important in data analyses, is rather high in average and its variation in time is also large. The reason of this high noise level is assumed as the small tilt variation of the large housing sphere due to a bottom current with time variation. To clear this problem, one idea of observation without the tilt variation is the use of small and low profile broadband sensor that enables to intrude into the sediment. Now, we developed a prototype of this next generation BBOBS (BBOBS-NX). In June 2008, we made a first step for it during the cruise of the R/V Kairei (JAMSTEC) with the ROV, Kaiko 7000II. It is a practical test to know achievement of the noise reduction with the idea above under support of the ROV. The prototype of BBOBS-NX was loaded on the ROV, and was located nearby the BBOBS that has already deployed in 2007. The sensor unit of BBOBS-NX was separated from the recording unit, and the sensor was pushed into the sediment. The OBDC had been dropped before the dive of the ROV, and was moved to nearby of them. The distance from the BBOBS to the BBOBS-NX is about 10m. We will recover these instruments several months later to get enough length of data to evaluate correcty