HR: 10:20h
AN: OS42A-01    [Abstracts]
TI: Improved performance of a broadband ocean bottom seismograph connected to off-shore submarine cable off-Tokachi, Hokkaido, Japan.
AU: Kawaguchi, K
EM: kawak@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AU: * Araki, E
EM: araki@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AU: Matsumoto, H
EM: hmatsumoto@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AU: Otsuka, R
EM: riyo@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushimacho, Yokosuka, KN 237-0061 Japan
AB: A seismograph deployed in the seafloor is subject to seafloor current which results in severe noise especially at frequencies below 0.1Hz. Burial of seismographs below seafloor to reduce the effect of seafloor current was suggested, and has been tried in several seafloor deployments. There is, though, not clear indication of how much reduction can be achieved by surficial burying. We compared the background noise levels of a broadband ocean bottom seismograph (OBS) before and after burial operation. In July 2004, we deployed a Guralp CMG-1T broadband seismograph in off-Tokachi, Hokkaido, Japan seafloor. This is where three ocean bottom seismographs (OBS) using submarine cable are installed. The Tokachi earthquake of magnitude 8.0 occurred in September 26, 2003 below the submarine cable OBS network. By installing additional broadband OBS, we aim to monitor postseismic activity of the large earthquake. The broadband OBS, as well as a pressure gauge, was connected to the submarine cable via the branch module of the cable to recover data. The burial of the ocean bottom seismograph was performed by a remotely operated vehicle (ROV) "Hyper-Dolphin" of JAMSTEC. Using a hydraulic pump, the ROV pumped out sediment from a PVC caisson on the seafloor to submerge it in the seafloor. The broadband OBS was then installed inside the caisson. The caisson buried in the seafloor was finally filled with glass beads to eliminate space between the broadband sensor and the caisson. Improvement of the background noise level by the burial operation of the broadband OBS was apparent in two frequency bands; below 0.1 Hz and above 4Hz. Below 0.1 Hz, the background noise became about 20dB or more quieter after burial for both vertical and horizontal components. Above 4Hz, a persistent noise peak around 5 Hz disappeared after burial. Thus, we consider this peak as an artificial noise due to the seismograph package. After burial, we could clearly notice several noise peaks above 2Hz which was only just visible before burial. These peaks probably show seismic resonance in the sediment layer.
DE: 7294 Instruments and techniques
DE: 7209 Earthquake dynamics and mechanics
DE: 3094 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting