Long-Term Stability of Acoustic Benchmarks Deployed on Thick Sediment for GPS/Acoustic Seafloor Positioning - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 268

Fig. 5 PXPs of JHOD. ( a ) Unit W, ( b ) Unit S, ( c ) Unit N. Location of seafloor position of each
PXP is shown in Fig. 2 . ( d ) PXP Unit W after recovery
Although the JHOD group was maintaining an observation site for GPS/A
observation some 50 km southwest of the study area (Site H in Fig. 1 ), a set of four
PXPs shown in the eastern part of Fig. 2 was selected as the target of the diving
survey. The PXPs deployed in 2000 were the first seafloor benchmarks for GPS/
Acoustic seafloor positioning around Japan. The three instruments in Fig. 5a-c
show the condition after a stay for about 6 years on the ocean bottom. Variety of
design was adopted for the first generation of the instruments. The PXP in Fig. 5b
was tilted by 6°, but there was no indication of disturbances of the sediment around
it. This suggests that it was slightly tilted at the time of landing on the seafloor after
a free-fall.
As the JHOD group planned to renew the PXP array, we recovered one of them
to check the health of the instrument. The ROV recovered the one shown in Fig. 5a
with a rope as is shown in Fig. 6a . The Fig 5d shows the instrument on the deck
after the recovery. It was impressive that the outside of the frame was almost free
from corrosion or marine lives.
Because most of benchmarks for seafloor geodetic observation in the subduction
zones are deployed on the sediment, stiffness of the surface portion of the sediment
is an important factor for the attitude stability of an instrument on the seafloor.
In each dive one or two core samples were obtained from each dive using a push
corer into the sediment nearly vertically with a manipulator of the ROV, and the
top part of the sediment was sampled. As is shown in Fig. 7 , the sediment was
composed mostly of mud. Upper 4-5 cm of the surface layer is quite soft, and is