HR: 1340h
AN: G13B-1234    [Abstracts]
TI: Monitoring the Lateral Gradient of Sound Speed in Ocean Toward Fast GPS/Acoustic Seafloor Positioning for the Cabled System
AU: * Kido, M
EM: kido@aob.geophys.tohoku.ac.jp
AF: RCPEV, Tohoku Univ., 6-6 Aoba, Aramaki, Aoba-ku, Sendai, 980-8578, Japan
AU: Osada, Y
EM: osada@aob.geophys.tohoku.ac.jp
AF: RCPEV, Tohoku Univ., 6-6 Aoba, Aramaki, Aoba-ku, Sendai, 980-8578, Japan
AU: Fujimoto, H
EM: fujimoto@aob.geophys.tohoku.ac.jp
AF: RCPEV, Tohoku Univ., 6-6 Aoba, Aramaki, Aoba-ku, Sendai, 980-8578, Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: DONET, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan
AB: The GPS/acoustic technique is now in practical use for seafloor positioning to monitor crustal deformation beneath the ocean, where land-based GPS networks are not available. To achieve semi-realtime monitoring of the strain accumulation and possible precursor for the expected Nankai earthquake in Japan, JAMSTEC and others have started so called DONET project (Development of Dense Ocean-floor Network System for Earthquakes and Tsunamis), funded by MEXT Japan, where numerous seismometers, pressure gauges, and acoustic ranging instruments are going to be equipped through the planing seafloor cables at Kumano-nada. A GPS/acoustic system will be combined in part of the cable system. The present GPS/acoustic survey, which acoustically measures slant ranges between a surface transducer and three seafloor transponders, has a fault to get position in semi-realtime. The problem setting supposes a laterally stratified sound speed structure. Violation of this condition with lateral gradient in sound speed results in the deviation of apparent position of the transponders. At present, ~5~cm of accuracy is achieved after taking time-average more than 1~day to cancel-out the time-varying direction of the gradient. In addition, if a long-lived gradient appeared, we have no way to distinguish seafloor displacement from the gradient. To overcome the present status, we propose a new survey style which actively estimates the sound speed gradient and makes its correction on apparent positioning by using five transponders. This rather complicated survey style requires severe layout of the transponders and observing position to stably resolve five unknowns: the horizontal displacement vector, stratified sound speed, and its gradient vector. We numerically investigated the best arrangement by evaluating the condition number of the observation equations. For further application, we also diagnosed the case of the reduced number of the unknowns and transponders for lower-cost construction of a seafloor station, such that direction of the displacement is known or expected in advance based on other geophysical or geological informations.
UR: http://www.aob.geophys.tohoku.ac.jp/dmg/gpsa/
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1222 Ocean monitoring with geodetic techniques (1225, 1641, 3010, 4532, 4556, 4560, 6959)
DE: 3050 Ocean observatories and experiments
DE: 3094 Instruments and techniques
DE: 3260 Inverse theory
SC: Geodesy [G]
MN: 2007 Fall Meeting