EurOCEAN 2000 - Vlaams Instituut voor de Zee

VERY HIGH RESOLUTION MARINE 3D SEISMIC METHOD FOR
DETAILED SITE INVESTIGATION
Bruno Marsset 1 , Jean-Pierre Henriet 2 , Mark Noble 3 ,
Nigel Wardell 4 , Angela Davis 5 , Peter Andresen 6
1 IFREMER, Brest, France; 2 RCMG, Gent, Belgium; 3 ARMINES, Fontainebleau, France;
4 OGS, Trieste, Italy; 5 UWB, Gwynned, Great Britain; 6 HAL, Surrey, Great Britain
INTRODUCTION
The project is aimed at a cost-effective detailed 3D reconnaissance of seabed
sediment properties for geotechnical and geological site investigation purposes. It involves two
main objectives. The first objective is the development of a very high resolution 3D seismic
method to be used for small-scale (100 m x 100 m) as well as intermediate-scale (1 km x 1 km)
sites. The second objective is the extraction of geotechnical information from the 3D seismic
data and the creation of a 3D physical model to the subsurface.
Given the variation in scale of the target sites, the acquisition task (Task 1) was
involved in two different approaches. (1) Redimensioning and marinisation of an existing rigid
3D-field system for small-scale site surveys involving shallow water (up to 30 m). (2)
Development of a flexible 3D-field system for intermediate-scale site surveys involving deeper
water (up to 100 m).
Several attempts to achieve 3D Very High Resolution seismic surveys were carried
out during the first two years of the project. Whereas seven selected european sites have still to
be surveyed, three 3D VHR seismic data sets were successfully acquired in the framework of
Task 2 (Rhone Delta, Strait of Dover and St-Austell Bay).
Paying attention to the frequency content of the seismic data, specific 3D
processing techniques have to be developed in order to correctly position the seismic events
and to prepare the data for the extraction of geotechnical information (Task 3). In order to
obtain optimal 3D imaging, pre-stack migration was applied on the data sets previously
acquired. The final 3D image of the subsurface underlines the validity of the proposed
acquisition and processing scheme.
A subset of the acquired seismic data will be treated for seismic attribute
extraction (Task 4). Velocity analysis will be performed on additional « long offset (100 m) »
2D multichannel data. Determining the attenuation (Q-factor) will be carried out using the
spectra ratio method. The recovery of acoustic impedance will require the use of seismic
inversion techniques. Additional laboratory measurements will provide supplementary
information to help assess the validity of the inversion (Task 5). Based on the information
from the seismic attributes, and on the inter-relationships between geotechnical and acoustic
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