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Parc OBS-INSU Responsible: Satish Singh, Laboratoire de Géosciences Marines, IPG Paris INSU OBS: Ocean bottom seismometers (OBS) are the basic instruments used to record seismic signals at the seafloor, either generated by earthquakes or by man-made sources such as dynamite or air-guns. They can be used to locate earthquakes and determine the velocity structure of the sub-surface. Until 2000, IRD and IFREMER, each, had about 15 short-period OBS for their own research purposes and there was a great demand by INSU community to acquire OBS for INSU. A proposal to buy/build 20 short period and 8 broadband OBS was submitted in October 2000, with IPG Paris as a lead partner. INSU allocated €864 k for the OBS project in November 2001. A CNRS marché was launched in January 2002 and a contract between CNRS and Scripps Institution of Oceanography was signed in September 2002 to build 20 short period OBS. These OBS arrived in France in October 2003 and were tested in the Mediterranean Sea in February 2004, a test cruise in which engineers/scientists from IRD, IFREMER and UBO also participated. Instruments: The short period OBS contains a hydrophone and a 2 s vertical component geophone. The recording system, batteries, and acoustic release are placed in two aluminum cases. The whole system is hosted in a plastic frame (1 m3) and weighs about 80 kilograms. It can be deployed on the seafloor for 3 months to a year. Figure: Short period INSU OBS The change in the exchange rate and increase in cost due to improvement in the designs of equipments (e.g. from Guralp 40 T to Trillium 240 s seismometers) did not permit us to order all of the 8 broadband OBS. Three broadband OBS will arrive in France in January 2006. The recording system and the design of the equipment are similar to that of short period OBS, and many parts (up to 70%) could be exchanged between the short-period and broadband instruments. The seismometer is a Trillium 240 s sensor, similar to STS 2 used on land by Geoscope. Figure: Broadband INSU OBS is being deployed at sea. Engineers: In the mean time, CNRS opened an IR position in 2001, and an engineer was recruited in November 2001. A second person, a technician, was recruited in January 2004 and a third engineer will start from January 2006. With three technical staff, the INSU OBS team is ready to provide a full service, 2-3 one-month cruises per year. Deployment of OBS: 2004 The six ONSU OBS were deployed for the first time after the 22 November 2004 Guadeloupe earthquake (magnitude 6.2) for 38 days (December 17, 2004 – January 25, 2005), which also recorded tsunami, T-phases, and earthquake signals from the 26 December 2004 Great Sumatra Earthquake. The cruise was organized at a short notice and the instruments were deployed using a small vessel. More than 2000 aftershocks were recorded during this period. 2005 The second major deployment of INSU OBS was in June-July 2005 at the Lucky- Strike segment of the Mid-Atlantic Ridge during the SISMOMAR cruise. Total of 32 deployments was made, with 100% success and excellent quality of data. One OBS was deployed in the water column at 900 m above the seafloor to record the source signal from the air guns. 2006: Year 2006 will be a very busy year for INSU OBS Parc. A month cruise is programmed on the R/V l’Atalante in January-February 2006 in the Gulf of Aden (ENCENS II), led by University of Paris 6. This will be first cruise where IPGP is not involved in science and exploitation of data from the cruise, and OBS will fulfill its true role as an INSU OBS Parc. A second major deployment (40) is planned to take place in Sumatra in July-August on board R/V Marion Dufresne, near the epicentral region of the 26th December great Sumatra earthquake. This is a major international experiment involving IFREMER, UBO and Southampton Oceanographic Centre and is a part of an international program, Sumatra-Andaman Great Earthquake Research (SAGER). This will be the first time 60 OBS will be deployed using a single vessel. INSU is also participating in an EC funded European Seismology project, NERIES, where three broadband OBS will be deployed for three years in the Mediterranean Sea and Atlantic Ocean, as a first European step towards permanent seafloor observatories. There are three major partners, Alfred Wegner Institute (Germany), INGV (Italy) and INSU-IPG Paris (France), who will be deploying and recovering their own BBOBS, one at each site. France will be responsible for the Atlantic Ocean site near the Lucky Strike, which is also a MOMAR site. The first deployment will take place in Fall 2006 for one year, with recovery and deployment each consecutive year for three years. 2007: INSU OBS are programmed to be deployed during SISMANTILLES II in January- February 2007. It will be a joint IRD and INSU operation. French OBS Pool: Over the years, it has become apparent that one may require a large number of OBS (40-60) for one single experiment (such as SUMATRA-OBS, SISMANTILLES II); therefore it would be useful to coordinate with other organizations and try to pull resources together for the French marine geosciences community. Several meetings were organized between INSU, IRD and IFREMER to create a French OBS Pool, where each partner will provide their OBS to the pool for a maximum of one month per year. A Memorandum of Understanding is in the process of being signed and should be in place for 2006. If it implemented earnestly, then anyone in France could request a maximum of 60 OBS (20 IRD + 20 INSU + 20 IFREMER), without involving scientifically any of the OBS partners. European OBS Pool: There are a total of over 150 OBS in Europe (France, Germany and UK), and it would be useful to have cooperation on European level. We are in discussion with our British colleagues, who will be participating in the SUMATRA-OBS experiment, in order to develop a long-term cooperation for the use of OBS, particularly as their OBS are the same instruments as INSU OBS. We already have collaboration with Italy and Germany on BBOBS through NERIES, which hopefully will improve in the coming year. Cost of Deployment: The cost of one short-period OBS deployment for one month is about €1500, and the cost of a subsequent deployment within one month is about €350. The cost of short- period OBS deployments for one year is about €10000, and that of a BBOBS is about €15000. One should include an insurance of 2.5% of the cost of OBS per month of deployment, which is about €950 for a month cruise, and the transport cost. Maintenance cost: Salty sea water is very corrosive and therefore, the wear and tear is very high. Keeping the OBS in ready-to-use form requires extensive maintenance: it costs about an average of €1500 for one instrument for one year, totaling to about €45,000 per year. Until now, INSU funding for the acquisition of OBS was used to cover maintenance costs, but from 2006 onwards, we will require funding of maintenance costs from INSU. Future funding/development: The next major break-through in seismology is likely to come from observation at sea using BBOBS. In January 2006, we shall have three BBOBS, and one of them will be deployed at MOMAR site in Fall 2006. Therefore, it is vital to complete the BBOBS pool of eight instruments, if France were to develop BBOBS project of its own or participate in a project with other European partners. For example, our participation would not have been possible to participate in the NERIES project if we did not have three BBOBS. We are also partner with British scientists to deploy BBOBS in Sumatra in 2007. A mi-lourds proposal of €275 k was submitted in 2005, but there has not been any input from INSU as yet. The present short period OBS have only two components but the instrument could record up to 12 channels. We would like to extend these to four component with only moderate cost of €1500 per instrument, and therefore, we require €30 k for conversion from 2-C to 4-C.
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