Geoscience Reference
In-Depth Information
Fig. 6
(
a
) A snap shot showing the recovering operation of the JHOD PXP Unit W with the ROV.
(
b
) A photograph showing an anti-erosion metal made of Zn on the JHOD PXP Unit N. Owing to
the metal, the frame of the PXP was almost free from erosion by the sea water
visually different from the deeper layer. The size of the corer was 4.8 cm in outer
diameter, and 30 cm in length. The sediment except the topmost part was stiff
enough to stop the push-coring halfway.
3
Discussions
Geodetic observation by GPS on land is precise to an order of 1 mm, and a bench-
mark for the GPS positioning is founded firmly on the ground. Seafloor positioning
is now aiming at precision of 1 cm. Then the position of sensor part of a benchmark
should be stable within 1 cm relative to the seafloor. One of approaches to such
problems is to visually observe how the benchmarks stand on the sediment after a
strong earthquake occurred nearby. The 2004 M7-class earthquakes off Kii
Peninsula gave us a unique chance to investigate the problem. Visual observation
of ten acoustic benchmarks deployed on thick sediment showed that all of them
stood on the flat sediment as if there had been no earthquakes. There was no dis-
turbance on the surface of the sediment around the instruments.
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