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 the two following techniques : multi-channel very high resolution seismic surveying and seismic measurements in geotechnical boreholes. Toward the end of 1992, tests were carried out in 50 m of water depth offshore Monaco. 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.
The installation of offshore facilities requires a precise spatial understanding of seabed stratigraphy and horizontal variations in soil characteristics. A typical site survey prior to the installation of offshore production facilities is done in two phases :
an initial geophysical survey (including bathymetry, sonar, shallow seismic down to about 100 m below seabed) over an area of one to several sq. km.
a dedicated geotechnical investigation at the location of the intended installation.
These two surveys are generally carried out at different periods of the project, by different contractors. Consequently there is quite often a lack of cross-correlation between the two data sets, Sometimes resulting in critical uncertainties for foundations engineers. Moreover, when a potential change of platform location k likely to happen, it should be particularly interesting to optimise the number of boreholes needed to obtain sufficient knowledge on soil layer distribution and characteristics over the entire production facilities layout1. The objective of the GEOSIS project is to define and develop new investigation procedures enabling a better integration of geophysical and geotechnical data and a significant improvement of methods used to extrapolate soil data around geotechnical boreholes.
In current practice, seismic reflection surveys for offshore civil engineering projects combine two main techniques :
single-channel seismic surveying, analogue for the most part, with sources and receivers of very high frequency (0.5 to 3 kHz).
multi-channel high resolution seismic surveying with sources and receivers of lower frequency(<500 Hz).
Single channel seismic numerical acquisition has been developed recently and is being increasingly used. It allows improvement of the output quality and presents the possibility of post-processing.