SUBMARINE CABLE OBS USING A RETIRED SUBMARINE TELECOMMUNICATION CABLE- GEO-TOC PROGRAM

In order to study the Earth's structure and subduction zone tectonics, seismic data from the oceanic region are extremely important. The pre sent seismograph distribution in the oceanic region, however, provides a very poor coverage. To improve this poor seismic coverage, a cable OBS system using a retired submarine telecommunication cable is propos ed. The GeO-TOC cable runs from Ninomiya, Japan, to Guam through the I zu-Bonin forearc and the Marina Trough. The total length of the cable is 2659 km. An OBS, IZU, using the GeO-TOC cable, was successfully ins talled at the landward slope of the Izu-Bonin Trench in January 1997. The IZU OBS is located approximately 400 km south of Tokyo. The instal lation method is similar to repair work on submarine cables. The IZU O BS is equipped with three accelerometers, a hydrophone, a quartz press ure gauge, and a quartz precision thermometer with a few temperature s ensors to monitor overheating of the internal electronics. After insta llation, the voltage increase is 90 V when the current is maintained a t a constant 370 mA. Data from accelerometers are digitized by 24-bit A/D converters and sent to Ninomiya at 9600 bps for each component. Hy drophone data are sent to Ninomiya as analog signals using the AM (Amp litude Modulation) method for safety reasons. Hydrophone data are digi tized at the shore station. Other slow-rate data are multiplexed and s ent to the shore at 9600 bps. The instrument can be controlled by a sh ore computer. All data will be transmitted from Ninomiya to Tokyo and combined with other existing seismic data. (C) 1998 Elsevier Science B .V. All rights reserved.