HR: 0800h
AN: G21A-0141    [Abstracts]
TI: A Synthetic Test of Precision in GPS/Acoustic Measurements of Seafloor Positioning
AU: * Kido, M
EM: kido@aob.geophys.tohoku.ac.jp
AF: AOB, Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AU: Sweeney, A D
EM: aaron@jamstec.go.jp
AF: JAMSTEC, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Osada, Y
EM: osada@aob.geophys.tohoku.ac.jp
AF: AOB, Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AU: Fujimoto, H
EM: fujimoto@aob.geophys.tohoku.ac.jp
AF: AOB, Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AU: Miura, S
EM: miura@aob.geophys.tohoku.ac.jp
AF: AOB, Tohoku Univ., Aoba-ku, Sendai, 980-8578 Japan
AB: In this decade, much efforts have been made in detecting seafloor movement using GPS/Acoustic ranging method. GPS/Acoustic measurement consists of two parts, one is the positioning of a transducer at sea surface using K-GPS and the other is measuring travel-time or slant-range between the transducer and transponders put on ocean bottom using acoustic ranging. Currently precision of the seafloor positioning reaches less than 10cm. Several possible factors arise, which still decrease the precision, such as GPS positioning itself in 1 Hz, interpolating of exact position of the transducer at an arbitrary timing of acoustic measurement, uncertainty of the sound velocity variation both in time and space as well as depth variation due mainly to ocean tide. A linear or spline interpolation can be applied to undulation of the transducer position when GPS antennas and the transducer are equipped on a ship. On the contrary when using a buoy system, one need additional physical sensors to detect flutter of the buoy in higher frequency. For the sound velocity, one can estimate it based on CTD or XCTD/XBT data. However, such data cannot cover entire period and space of observation. In general, sound velocity thought to varies smaller in lateral than in time. Accounting for this nature, Scripps group developed a strategy that set transponders like triangle and make acoustic measurement with the transducer kept laterally equidistant to the transponders. In this method most of laterally stratified component of sound velocity variation in time will be cancelled to determine the center of the triangle. Prior to this measurement, position of individual transponder must be estimated. Error in this estimation will biases the ``absolute'' final position of the center, which is not important to detect the crustal ``relative'' movement, as long as one take the same estimations in the next survey. However this error increase the misfit of travel-time, which results in the final position to be sensitive to other possible error source. Therefore position of the individual transponder should be solved simultaneously with the center position. In addition, we have to account for the randomness of the error to consider the ``mean distribution'' of the center position as the precision of survey. We will present theoretical estimate of the precision with an application to actual data set.
DE: 4294 Instruments and techniques
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3094 Instruments and techniques
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting