Responsible: Satish Singh, Laboratoire de Géosciences Marines, IPG Paris
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.
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.
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:
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.
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.
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.
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
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.
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.
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.