Discussion

Dr Denness: Paper 3

A study of representative sampling intervals for geotechnical mapping was recently undertaken in the Forties field where seabed lithologies were found to vary from silty sand in varying degrees of com paction to plastic clay ranging from very soft to hard, according to its depositional history. In this survey it was decided to examine in more detail the lateral and vertical variation of geotechnical, geo physical and geological parameters. To that end an area of some 700 km2 was sampled on approxi mately a ten kilometre grid with vibrocores. Within this broad grid, seabed lithologies and sequences were examined over the region. Then a smaller scale grid of sampling points with three kilometre spacing was introduced covering one of these seabed types in a neighbouring area of 300 km2. A sampling interval within this smaller grid was defined on approximately a geometric scale. That is to say reducing along a line at intervals of approximately 1 km, 300 m, 100 m and 30 m. The testing pro gramme includes acoustic scanning of all the cores to produce acoustic logs (Fig. Bl). Undrained strength profiles from penetrometer testing, based on the encouraging correlation already derived between strengths from penetrometer and vane testing (Fig. B2), are also being prepared. The results of this programme should put us in a better position to judge the appropriate horizontal and vertical sampling and testing interval for some of these seabed materials. Further investigations are planned for other parts of the continental shelf of engineering significance relevant to the extractive industries.