Araki, Eiichiro

Kaneda, Yoshiyuki

Kaneko, Sho

Kawaguchi, Katsuyoshi

UPCommons

Installation requeriments for seismic observation in the seafloor

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INSTRUMENTATION VIEWPOINTSessions41INSTALLATION REQUIREMENTS FOR SEISMIC OBSERVATION IN THE SEAFLOORSho KANEKO, Eiichiro ARAKI, Katsuyoshi KAWAGUCHI, Yoshiyuki KANEDAJapan Agency for Marine-Earth Science and Technology2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061 Japan+81-46-867-9354, kanekos@jamstec.go.jp1. IntroductionFrom 2006, a project was started to construct submarine cable net-work for large earthquakes and tsunami monitoring in the Nankai Trough, Japan (Development of Dense Ocean-fl oor Network System for Earthquakes and Tsunamis; DONET). 20 sets of state-of-the-art seismic sensor which cover whole frequency band and dynamic range will be deployed in the project.In seafl oor observation, installation method of seismic sensor is very important to improve the measurement performance. A precise observation cannot be achieved without the consideration on envi-ronmental eff ect. This paper introduces the past results of seafl oor seismic observation from several conditions of measurement envi-ronment and a new approach to conduct high quality seismic obser-vation in the seafl oor.2. SummaryScientifi c seismic observation in the seafl oor requires very wide dy-namic range and frequency band. The target observation range of motion of DONET project is acceleration range from the weakest mo-tion of 10-9 m/s2 to the largest motion as 20 m/s2 and frequency band from 1/years to a few 100 Hz. It is very diffi  cult to cover this broad frequency and dynamic range by single sensor. Development of a complex sensor system which consists of broadband seismom-eter, strong motion accelerometer, and geophones is necessary to achieves the requirement. This approach is as same as land seismic network in Japan (F-net, Hi-net).Reduction of background noise is very important to get the best per-formance of seismic sensor in the seafl oor. The past results of seismic observation in the seafl oor proved that the quality of data obtained from the seismic sensors is aff ected by their installation condition. The seismic sensors set up in the seafl oor were infl uenced by seafl oor current and fl uctuation of thermal condition of seawater. These fac-tors are the major causes of seismic noise encountered in the seafl oor setting. Coupling seismic sensors to ground motion in good fi delity is diffi  cult in the seafl oor, because most seafl oor is covered by soft and water saturated sediment. Burying seismic sensors into the seafl oor is a remedy for these factors. By burying seismic sensors, seafl oor cur-rent is isolated from seismic sensors, and thermal condition of seis-mic sensors can be stabilized. Methods to install seismic sensors into the seafl oor have been developed, and several trial installations have been conducted so far. The most typical example of methods is anobservation using boreholes drilled by scientifi c drill ship from 1999 to 2001. The measurement result of the borehole observation was the quality close to those in the best land observatories.It was also tried to bury seismic sensors surfacially in the sediment by using remotely operated vehicle and other tools. Long period noise from these buried seismic sensors was reduced in some cases, but there were some cases of unsatisfactory results. Experiences from these surfacially buried seismic sensors suggest that more eff orts should be made to increase coupling to ground and to isolate water around seismic sensor.To obtain the best performance of seismic observation in DONET project, it is scheduled to develop the sophisticated complex sensor system and also to evaluate the most suitable installation method for seafl oor.THE GEOAZUR/GURALP <<HIPPOCAMPE>> OBS/LTOBSYann Hello (1), Cansun Guralp(2), Ben Yates (1), Alain Anglade (1), Olivier Desprez (1) Andy Smith (2), Martin Rowe (2), Diane Holland (2), and Philippe Charvis (1)(1) Géosciences Azur, Observatoire Océanologique de Villefranche-sur-Mer, IRD, France Tel :+33(4)93763885, E-mail : yann.hello@geoazur.obs-vlfr.fr(2) Guralp System Ltd.The study of continental margins, subduction zones and oceanic basins as well as the quantitative assessment of seismic hazard near densely populated coastal areas request the deployment of a large number of Ocean Bottom Seismometers (OBS) during period of sev-eral weeks for active tomography, up to several months or years for passive experiment.Geosciences Azur (joint IRD1, CNRS2 , UPMC3  and UNSA4  labora-tory) has developed a new, easy-to-use, 4-components OBS named “Hippocampe”. The Hippocampe OBS exists in a short period version based on 3 gimbaled, 4.5 Hz, geophones self leveled installation, en-capsulated in a 150 mm diameter, glass sphere. The broadband sen-sor was designed in cooperation with Guralp System™ and on the basis of a CMG-40T seismometer gimbaled in a similar glass sphere, with a magnetometer and tiltmeter for position on the bottom (op-tional). The data logger developed at Geosciences Azur consists in a 24 bits analog/digital converter synchronized by a high accuracy Seascan clock (2 10-8). Data are buff ered in a 1 Gb fl ash memory then stored on a 80 Gb hard disk. Power consumption is ~500 mW for continuous recording of 4 channels at 200 samples per seconds allowing 6 to 12 months recording autonomy on the sea bottom. The data logger and batteries fi t in a 432-mm diameter glass sphere. A second sphere is used to increase the fl oatability during long-term deployment.For recovery an acoustic code trigger simultaneously an electro-me-chanical release system, (developed in cooperation with Guralp Sys-tem Ltd) and an electrolytic, burn-wire release. At the surface fl ash M3

Installation requeriments for seismic observation in the seafloor

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