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Long-Term Stability of Acoustic Benchmarks
Deployed on Thick Sediment for GPS/Acoustic
Seafloor Positioning
Hiromi Fujimoto, Motoyuki Kido, Yukihito Osada, Keiichi Tadokoro,
Takashi Okuda, Yoshihiro Matsumoto, and Kozue Kurihara
Abstract
GPS/Acoustic seafloor positioning has become an indispensable geodetic
observation for the monitoring of crustal activities in subduction zones. There
remain, however, some key problems to be settled. Among them is long-term sta-
bility of acoustic benchmarks deployed on the seafloor: long-term attitude stability,
especially against ground motions of earthquakes, and their electrical/mechanical
durability for long-term geodetic observation. M7-class earthquakes occurred in
2004 off Kii Peninsula, Central Japan, and then coseismic seafloor crustal move-
ments were detected by using the GPS/Acoustic (GPS/A) observations. This event
gave us a unique opportunity to test the stability. We carried out diving surveys in
2006 and visually inspected the benchmarks that detected the crustal movements.
All of them stood stably on the flat sediment. No effects of the earthquakes were
observed. In case that the slight tilts of the instruments were caused by the earth-
quakes, the effect on the observed crustal movements is estimated to be within 1 cm.
One of old benchmarks deployed 6 years before was recovered and inspected. There
was no problem on the battery, and no damages on the outer frame of the instrument.
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