HR: 0830h
AN: G21D-0296    [PDF]
TI: On-Line Operating 3-D Seafloor Positioning System (1)
AU: * Eguchi, T
EM: eguchi@nda.ac.jp
AF: Earth and Ocean Sciences, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, 239-8686 Japan
AB: We propose a new observation system of on-line 3-D positioning which will be deployed on the sea-bottom of convergent type plate boundaries where large inter-plate seismic events occurred historically. The system has observation sites at assigned intervals along optical fiber cables. Using the several cables, crossing each other, we can construct a real-time operating network of triangular base lines. Each observing site on the cable will be equipped with two-kind high gain instruments i.e., the laser ranging and pressure gauge sensors, as well as additional apparatuses to remove the influence of temperature and salinity etc. on the data. Attenuation rate of visible rays in seawater is relatively smaller at bands of blue-color (wave length; $\sim$ 450nm) to yellowish green-color ($\sim$ 550nm). The attenuation rate of optical signals of blue to yellow-green color in highly transparent seawater is 0.1 $\sim$ 0.5 dB/m. If we can utilize the high power optical laser output of the blue to yellow-green band for the positioning, the signals can reach the target receiver station with highly sensitive detector located at the distance of 10**2 m or larger. Using additional data of thermal and salinity fields etc. for compensating refractive index of laser signal ray path in clean seawater, we may attain the resolution of laser ranging at an order of 1 mm for each triangular base line with the total length of 1 $\sim$ 2 km. The base line consists of several secondary positioning stations with the spacing of $\sim$ 10**2 m. To improve the data resolution, we apply signal processing such as low-pass filtering etc. As is important, we cannot decompose the change of the base line distance data into 3-D individual components. We need another kind data, such as pure vertical coordinate of the positioning sites to resolve the 3-D components. To measure the vertical coordinate of the seafloor stations, we utilize data from the high gain pressure sensor. In the case of crystallized quartz-based pressure sensor which are commonly used in several on-line sea-bottom seismic observation systems in Japan, the resolution of water depth change is $\sim$ 1 mm being sufficient to detect the arrival of larger tsunamis or similar phenomena. As a result of our previous observation based on the crystallized quartz-based pressure sensor, the maximum amplitude of pressure change due to tsunamis on the flat seafloor with the depth of 2 $\sim$ 3 km is approximately ten-times smaller than that at the tide gauge station at neighboring open shore. Generally, most of flat sea-bottom at plate boundary regions are covered with less consolidated sedimentary layer with variable thickness. During large shallow seismic events at regional distances, freely settled sea-bottom stations from the sea surface may suffer some additional or differential movement(s), especially in the horizontal directions, from deeper basal rock layer. To get observation data with high reliability even for long periods including some intermittent activity of shallow large earthquakes with the epicentral distances of several hundred km or less, we must carefully monitor the position not of instruments landed freely on sea-bottom surface but of those tightly fixed with the deeper basal rock. Thus, we have to install and fix observation sites of the network on deeply rooted boreholes or on tightly coupled tool with the basal rock layer, as many as possible. To raise the ratio of the number of observation items vs. the total system expense, we should add some other geophysical instruments such as gravity, broad-band seismic, or electro-magnetic sensors at selected stations.
DE: 1200 GEODESY AND GRAVITY
DE: 1206 Crustal movements--interplate (8155)
DE: 3094 Instruments and techniques
DE: 4564 Tsunamis and storm surges
DE: 7200 SEISMOLOGY
SC: Geodesy [G]
MN: 2003 Fall Meeting