The objective of the ‘GEOSIS’ research project was to Improve the integration of shallow seismic and geotechnical data for offshore site investigations. In the first phase of the project, an experimental site investigation was performed offshore Monaco, including a very high resolution (VHR) digital seismic survey and vertical seismic profiling (VSP) in geotechnical borings. The VHR shallow seismic survey was carried out with the newly developed multichannel digital acquisition system 'DELPH 24'. The VSPs were performed from a geotechnical drilling vessel by using a standard seismc cone and a standard VSP probe as seismic receivers in the soil borings. Correlation between seismic compression and shear wave velocities Vp and Vs and the cone resistance qc were established In a second phase, the use of standard VSP processing and exploration oil reservoir assessment tools (1.e. stratigraphic deconvolution and inversion techniques) allowed the VHR seismic tune sections to be transformed, first into acoustic impedance versus tune, and then into cone resistance versus depth sections The promising results obtained provide evidence that it is now possible to derive and to quantitatively extrapolate consistent geotechnical data around or away from sod brings using VHR seismic data. Before completion of the GEOSIS project, this was considered as theoretically possible but not proven. Although application of the proposed methodology involves extra costs with respect to current standard practice, integration of seismic and geotechnical data should become a major improvement in future offshore site investigation practice.
Although the concept of integrated geosciences studies is considered in the forthcoming ISO Standard 13819–2 for offshore structures, in particular for deep water siteinvestigations, no significant advances have occurred in the past ten years for actually integrating very high resolution (VHR) geophysical and geotechnical data in routine offshore site investigation practice. Recent technological developments manly concerned application of single channel digital shallow seismic, and use of 3D exploration seismic data with associated advanced processing for the assessment of drilling hazards and preliminary engineering studies for offshore facilities (Kolk and Campbell, 1997)
Concerning shallow VHR seismic site surveys for geological/geotechnical investigation of the first 100 m of sea bottom sediments for foundation engineering studies, the current routine techniques and surveying practice generally yield seismic data of relatively ‘poor’ quality in terms of resolution, which makes the two distinct seismic and geotechnical site surveys difficult to cross-correlate. Thus, large uncertainties std remain in the assessment of sod strata variation and geotechnical properties when a structure location is some distance away from a soil boring since the degree of correlation between geophysical and geotechnical data depends on the quality and reliability of the data as well as on the geology of the site and the sod characteristics themselves.
With the development of digital acquisition techniques for shallow VHR seismic site surveys, it is now considered as increasingly feasible to extrapolate consistent geotechnical information from seismo-acoustic measurements (Haynes et al., 1993). The ‘GEOSIS’ project was aimed at improving the correlation between VHR seismic and geotechnical data from integrated site investigations.