Jacques Meunier

Characterization of Soft Deepwater West Africa Clays: SHANSEP Testing is Not Recommended for Sensitive Structured Clays

With the development of deepwater fields offshore West Africa, in water depths currently ranging between about 600 and 1,500 metres, geotechnical characterization of the sediments has become an issue since the properties of the soft Gulf of Guinea clays differ from those of the equivalent Gulf of Mexico or North Sea clays. In particular, the highly plastic (plasticity index Ip over 80 %) West Africa deepwater clays have a very low unit weight, are highly compressible, and have a relatively strong natural structure. As part of a joint research effort carried out within the CLAROM framework in France, a high quality triaxial testing programme was carried out on clay specimens from an oilfield located in 1,300 metres of water. The paper describes the laboratory test procedure and the results obtained, comparing the results of SHANSEP testing with the results of other triaxial tests at low confining stresses representative of the in situ conditions. With the SHANSEP testing procedure, the micro-structure of the soft West Africa clay is altered by the reconsolidation stresses in the over-consolidated range through large consolidation deformations. The paper shows that the SHANSEP procedure provides undrained shear strength values that are similar to those of a disturbed or reconstituted sample. High quality triaxial testing under confining stresses representative of the in situ stress level appears as the only way to provide geotechnical parameters that are representative of the in situ clay strength. Therefore, the SHANSEP triaxial testing procedure is not recommended for the soft sensitive structured clays from deepwater West Africa sites.

PASISAR: processing of very high-resolution near-bottom seismic data

The PASISAR very high resolution seismic data are recorded through a single channel streamer attached to a deep towed side-scan sonar, using conventional seismic sources located at the sea surface. This device provides an outstanding improvement of both horizontal and vertical resolutions of seismic profiles in deep waters. However, the motions of the towed fish and the dissymmetry of the acquisition device dramatically alter the quality of data. Therefore a dedicated processing is mandatory in order to provide the interpreters with legible seismic sections.<<ETX>>

Computation of Shear Waves by Integral Equations Methods in Stratified Media

This paper presents the SISMOS code developed by IFREMER. The principle of SISMOS code lies on integral equations applied to the Kennett’s formalism. Sediments are caracterized by homogeneous horizontal stratified layers. Several examples of synthetic seismograms on which shear waves appear are presented in homogeneous and stratified media.

Tests In Offshore Monaco Of New Techniques For A Better Integration Of Geotechnical And Seismic Data

It is recognised by most offshore foundation engineers that a poor level of correlation still exists between standard seismic records and geotechnical investigation data. The GEOSIS project aimed to define and develop new investigation procedures enabling a better integration of seismic and geotechnical data. Research has focused on multi-channel very high resolution seismic surveying and seismic measurements in geotechnical boreholes. Towards the end of 1992, tests were carried out in 50 m of water depth offshore Monoco. The tests included multi-channel seismic surveys, continuous coring and sampling down to 80 m, sonic and nuclear logging and, for the first time, P and S-wave vertical seismic profiling. The paper describes the offshore field tests and the data processing procedures. Results demonstrate that both seismic cone and standard VSP techniques are able to provide data usable for the calibration and the interpretation of very high resolution 24-channel seismic surveying carried out on the same site.

Multichannel processing of very high resolution seismic

The installation of offshore structures requires a fine survey of the near subsurface to detect the nature of the ground and eventual obstacles. Seismic reflection methods are the most commonly used. But up to now at least, these methods were limited to single channel acquisition, sometimes digital (in the best cases) due to technology limitations. A fire cycle of 0.5 or 1 sec and a sampling rate of 5kHz could not allow multichannel recording. The advent of new technologies allows the use of the multichannel concept for superficial subsurface exploration and brings: (1) a significant improvement of signal to noise ratio, (2) real depth information of the interfaces using velocity analysis, (3) the possibility to acquire data in rougher sea conditions because of its multiplicity.

Integration of geotechnical and geophysical data: GEOSIS project

The GEOSIS project has improved the geotechnical and geophysical methods of seafloor survey for the installation of offshore structures. Specifications for Vertical Seismic Profiling and Very High Resolution multichannel seismic were established and processing methods were fitted. The integration of these data increases the validation of the interpretation, and should improve the extrapolation of lithological parameters between distant boreholes.<<ETX>>

SHRIMP: an investigation tool for pipeline and cable burial

Pipelines and cables are more and more usually buried into the seabed. Several techniques are available (ploughing, trenching, jetting or dredging), but the choice of the appropriate tool and the estimation of the work advancement speed need precise information concerning the seafloor characteristics. Conventional methods including soundings and reflection seismics are not appropriate for survey before burial. A new tool based on refraction seismic on the seabed has been developed. The real time processing gives the P-waves velocities and the thickness of the different layers inside the sediment. The depth of investigation into the seafloor is between 3 and 10 metres. The accuracy in layer thickness is about 25 centimetres.<<ETX>>