Obiettivo
Involving high-resolution, digital reflection seismic acquisition and processing, and shear wave measurements, an alternative form of offshore site investigation is searched for, which should be available at a fraction of the cost of conventional offshore drilling and geotechnical programs.
The aim is the construction of a three-dimensional analytical model of the investigated offshore site, illustrating areas of potential hazard for any foundation and providing the basic data frame for subsequent simulation studies. The development of an offshore shear wave refraction capability is envisaged, making it possible to measure remotely from the surface, data relating to the top few tens of meters of seafloor material and to gather information on physical properties prior to drilling.
The first task included the development of the data acquisition methodology.
AUGER have successfully developed a system capable of making in situ shear wave refraction measurements on the sea floor. The sea bed towed sledge with P and S wave sources has been designed in a modular format so that a range of components can be housed onto a basic sledge frame. The design of the electromechanical shear waves sources is fundamentally new, operating on a low voltage/high current basis, with all driving electronics and associated power packs housed on the sledge itself.
RCMG developed a new sparker source (the "CENTIPEDE) and a very high resolution true 3D acquisition system, the "SEISCAT", consisting of a catamaran mounted source and 12 dual-channel ministreamers. With this system, a true 3D survey has been scaled down to what is in the present state of art the uppermost edge of accuracy : high resolution data have been sorted and processed in 1x1m2 bins, yielding coherent time slices every 0.25 ms.
Field testing of the shear wave system and various variants of 3D data acquisition, ranging from economical pseudo-3D approaches to true 3D acquisition, have been tested on various sites and scales of structural deformations in open marine and estuarine environments. The results of the 3D structural information has been processed with a new interactive 3D structural modelling package, "GEOFOX", producing with the help of new algorithms an accurate representation of faulted, multi-layered systems on an entry-level colour workstation.
In situ seismic measurements have been implemented with specialized geotechnical laboratory tests, e.g. resonant column tests defining the variation of shear modulus as a function of stream amplitude. The findings of these tests have been integrated into a 3D geo-acoustical model, defining the variation of the following information with depth; shear and compressional wave velocity, small strain shear modulus and Young modulus, porosity, density. Such a model, integrating structural information and geotechnical properties over a depth of several tens of metres, is a basic requirement for the evaluation of prospective offshore sites.
The first phase includes the development of the data acquisition methodology. A marine shear wave source, detectors and a signal processing system will be designed and constructed by AUGER.
Contemporarily a specific high resolution reflection methodology is optimized for geothechnical applications by RCMG.
Meanwhile a preliminary literature survey and conceptual preparative works for the subsequent 3-D acquisition, for P-wave velocity measurements and for the processing and modelling phases is executed.
During the second phase initial sea trials are carried out of the shear wave refraction system and the reflection system. Possible redevelopment efforts are to follow these trials in the subsequent months.
Phase 3 is a data processing and data analysis phase. The in situ measurements are supplemented with laboratory mechanical and geophysical tests. The reflection data processing and analysis aims to enhance the signal-to-noise ratio of reflection profiles, to develop a 3-D data processing package and to analyse the velocity information. These processing routines should be available on the emerging generation of relatively low cost microcomputers suitable for field use in geotechnical contracting work.
A joint sea trial of the integrated-system will be carried out during the fourth project phase.
Phase 5 deals with mathematical analysis and modelling. The three-dimensional reflection survey yield the structural framework of the investigated site. Sediment characteristics and geotechnical quantities are translated in geomechanical attributes on the basis of analytical or empirical relationships. A mathematical model of the soil properties under varying stress conditions is produced. Ultimately a final reporting phase (phase 6) is foreseen.
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DEM - Demonstration contractsCoordinatore
9000 Gent
Belgio