An Evaluation of Seismic Response of a New Generation Ocean Bottom Seismometer For Large-Scale Operation
Authors: Takahashi Narumi , Shimizu Satoshi , Watanabe Yoshitaka , Hatakeyama Kiyoshi , Maekawa Takuya 
Published: June 2014

Ocean Bottom Seismometers (OBSs) or Ocean Bottom Nodes (OBNs) are effective instruments for earth crustal studies, earthquake observations, and hydrocarbon explorations. We have developed a compact and lightweight OBS which is suitable for large-scale operation. The new OBSs have been tested on several sea-trials. These has been demonstrated excellent performance on operational features and data quality. On the recent sea-trial in January 2013, the two sets of new OBSs were deployed on the seafloor of 7,000 m water depth of Japan Trench, where giant 2011 Tohoku earthquake was generated. We have successfully acquired the seismic data and recovered the new OBSs from the ultra-deep ocean. The data demonstrate that the amplitude response is comparably effective with that of our conventional OBSs, and the vector fidelity is better than that of the conventional one. The new OBS showed availability of large-scale operation with high signal fidelity.

Introduction

OBSs have been developed for earthquake observation and crustal study since 1960s. Many types of OBSs have been produced and contributed to the earth science. In hydrocarbon exploration, seabed seismic acquisition using OBN are getting popular in this 10 years for multi-component and wide-azimuth seismic acquisition technique. An OBS/OBN is basically stand-alone instrument which contains at least three-component seismic sensors, a data recording system, and batteries. Typical OBS used for academic study is fully autonomous system; it is released from sea-surface by free-fall, recording seismic signal, and pop-up by releasing anchor. Such free-fall type OBS has an acoustic transponder, a ballast releasing mechanism, a flasher, and a radio-beacon in addition to the main housing containing seismic components. Commercial-based OBNs which usually doesn't have such peripheral devices are carefully set on the seafloor and retrieved by a Remotely Operated Vehicles (ROV). Although the OBN is placed on accurate position and sensor direction on the seabed by ROV, the deployment and recovery task require large-scale operating system and cumbersome water-works with possibly adverse seabed condition; for example, water can easily is turbid by ROV movement, sometimes OBN can be buried, ROV operator meet visible difficulty to find or manipulate OBN. ROV operation in deep water makes higher the cost, which is the biggest disadvantage.

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) experienced deployment of 3,500 units of OBS in this 15 years for academic studies. These OBSs are composed of 17-inch glass-sphere, and several cylindrical pressure cases, that rated hydrostatic pressure of 6,000 m (Figure.1). This compactly assembled glass-sphere type OBS makes the quantity of OBS units for an experiment be more than a hundred. However, maintenance and set-up procedure for such pressure cases, data recovery, and battery charge/replacement are very time-consuming and require us experienced skills. These disadvantage make difficult to expand the operation.