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par DANIEL Romuald - 12 octobre 2011

4 récupérations & largages d’OBS et 1 récupération & largage de BBOBS

OBS Experiment During BATHYLUCK09 (31 Aug. – 28 Sept. 2009)

R. Daniel, A. Rai, I. Veludo, A. Blin, C.Courrier Contact :

Version au 8/10/2009 à 17:00

Introduction : The 5 Ocean Bottom Seismometers (OBSs) deployed during BBMOMAR-I (18-27 July, 2006) and BBMOMAR-II (8-17 Aug. 2008) have provided important information about the distribution of seismicity at the Lucky-strike segment of the Mid-Atlantic ridge. The region is well known for numerous active hydrothermal vents. To study the relation between seismicity and hydrothermal activity in the region, it was decided to continue the OBS experiment at nearly the same sites by re-deployment of 5 new OBSs for another year. OBS experiment contributes significant earthquake data to the multi-disciplinary BATHYLUCK09 experiment conducted between 1st Sept. and 28th Sept. 2009. During the first two days of the BATHYLUCK09 experiment (01/09/09 and 02/09/09), five multi-channel (one pressure, two horizontal and one vertical component) ocean-bottom seismometers (OBSs), deployed during BBMOMAR-II experiment were recovered after one-year of successful operation at Lucky strike segment. Five new OBSs were deployed at nearly same locations on 02/09/2009. The four short-period and one broad-band ocean-bottom seismometer deployed during this experiment will record the seismicity in the region for the next year.

(1) Instruments deployed during BBMOMAR-II, recovered during BATHYLUCK09 Name Lat.(°) Long.(°) Notes

B1 37°19.3000 -32°16.6625 array N B2 37°17.9097 -32°19.4248 array W B3 37°17.5066 -32°17.2777 array central B4 37°16.9622 -32°14.4609 array E B7 37°15.6354 -32°17.8473 array south

(2) Instruments deployed during BATHYLUCK09 to be recovered in summer 2010 Name Lat.(°) Long.(°) Reloc. Lat. Reloc. Long. Notes

C1 37°19.3000 -32°16.639 37°19.2148 -32°16.6964 array N (SP) C2 37°17.918 -32°19.426 37°17.8499 -32°19.4913 array W (SP) C3 37°17.565 -32°17.288 37°17.4767 -32°17.3593 array central (SP) C4 37°16.967 -32°14.476 37°16.8423 -32°14.5015 array E (SP) C7 37°15.6354 -32°17.8473 37°15.6146 -32°17.8856 array south (BB)

*SP short period, BB : Broad Band.

Description of work done during BATHYLUCK09 :

Instruments were tested before deployment on 31st Aug. and 1st Sept for acoustic communication between the command unit and the OBSs, both on-board and hanging the acoustic unit at a depth of 1000 m,. 2009. On 2nd Sept., we recovered all the five OBSs deployed during BBMOMAR-II cruise and deployed five new instruments at almost the same locations (14 Hours / 5 persons). To recover the OBSs from ocean-bottom, an acoustic signal corresponding to the particular OBS was sent using EdgeTech equipment (model 8011XS). The acoustic signal activates the unlock mechanism between the OBS and its heavy metallic platform, and OBS moves upward because of buoyancy of the floaters. Once on the ocean-surface, the OBS is brought back to the ship, and is washed with fresh water, after investigating the signs of corrosion, etc. The data-logger was then extracted from the unit, it was de-vacuumed and clock-drift was measured using Zyfer GPS antenna. We used OBSDUMP software to copy the raw data from the data-logger to the external hard-disk. After correction de Raw-data with fix software (Lcheapo bug), the Fix-data is then converted to SAC format using ANYPLOT software module, to be able to be read by analysis software. The same procedure is adopted for every other OBS recovered during the cruise.

Corrosion :

After more than one year under the ocean, the instruments came back with some important corrosion. Critical parts, such as pull pins, screw washer & nuts, floating structure are very attacked by corrosion and must be replaced. Most of damaged screw are in stainless steel A2 that is not recommended for oceanic application under water, the corrosion started to attack the stainless steel structure because the 2 metal have a different composition : from now only stainless steel 316L must be use for metallic parts.