The object of this project is to compare validity of these different solutions in order to determine optimal device when considering both technical results and costs. Tentatively improved modelling programs and theoretical interpretation of results are considered.

In order to collect four components seismic data at sea, Ocean Bottom Seismometer   (OBS) is the basic tool currently used in the industry. Whatever water depth may be, deployment of a spread of OBS generates several operational difficulties, and sensors’ coupling is one of the major problems to be solved for insuring high quality records.
 
Universities, contractors, manufacturers propose different methods to obtain as good a coupling as possible. The object of this project is to compare validity of these different solutions in order to determine optimal device when considering both technical results and costs. Tentatively improved modelling programs and theoretical interpretation of results are considered.
 
Data acquisition campaign. 
In cooperation with Ifremer and Istambul Technical University, a campaign is presently taking place as one component of a scientific campaign in Marmara sea, where natural seismic activities are quasi-permanently high. Six different OBS are layed on sea bottom, (water depth 1300 m), each OBS being equipped with different coupling devices. Laying operations were done with Ifremer manned submarine Nautile, and each OBS was filmed.
 
            Two of these OBS are Ifremer OBS 
 
Microbs: Geophones are integrated in a glass sphere fixed on a heavy concrete plate to insure efficient coupling. An acoustic device allows retrieving equipment and data, concrete plate remains on sea bottom. (Fig. 1) 
 
 
Fig 1 : Microbs

Lotobs: Geophones are integrated in a small lenticular titanium container as per fig 2. This container is dropped only when the lotobs reaches sea bottom. 

 
  
 Fig 2: Lotobs
 
Four OBS are CGGVeritas property:
Trilobit: geophones are integrated in the container. The weight of the device insures the coupling ( fig 3) 

 
 
Fig 3 : Trilobit

MSS: geophones are integrated in the device shown fig 4. Upon arrival on sea floor, this device vibrates (Sercel Australia patent) in order to be partially buried. 
  
 
 
Fig 4 : ARMSS

Span: The device shown Fig 5 contains a titanium container with geophones. The results of the experiment will be analysed before making the decision to equip the device with a jetting apparatus, which showed during different experiences ability to bury the sensors. (CGG patent) 
  

 
Fig 5 : Span
 
 
Subtech: Geophones are integrated in the needle shown Fig 6. This needle is Installed following  geotechnical coring method ( Kullenberg coring apparatus),  insuring deep burying of sensors ( CGG patent) 
  
 
Fig 6 : Subtech
 
Scope of the work.
 
Upon completion of data acquisition, data will be synchronised and formatted, and characteristic sequences collected during passive recording time will be determined in order to allow analysis and comparisons. Data recorded while shooting lines with airgun will be processed.
Relevant parameters will be collected and a model of seismic responses will be established with existing programs.

Comparison of 4c recorded data of each OBS and of models will be conducted, discrepancies analysed and theoretical explanations will tentatively be formulated.
From conclusions of the whole process, recommendations relative to optimisation of sensors coupling for OBS will be formulated and optionally, new trials for validation will be conducted.

Task  
Task 1: Supplying or manufacturing OBS, Shipping and installation of the spread: CGGVeritas and Ifremer, achieved May 15 th.
Task 2: Retrieval of equipment and of data:

CGGVeritas and Ifremer, Scheduled June 15 th

Task 3: Synchronization of records, reformatting of data, choice of sequences, processing of seismic
Proposed: CGGVeritas, CMR, (in cooperation with Ifremer),
Task 4: Modelling of seismic responses:
Proposed: CMR
Task 5: Analysis of results

Proposed; CGGVeritas and CMR

Task 6: Theoretical works and recommendations:
Proposed : CGGVeritas and CMR
 


 
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