A high resolution seismic survey over an area on the central North Sea is described from it$ earliest stages up to it a consequences for the conceptual design of offshore structures.
It is shown that overlying the marine shelf deposits are near surface sediments of mainly glacial origin, Three advances and withdrawals of ice sheets are recorded in these shallowest sediments by their respective erosional surfaces and associated tunnel valleys.
All sediments except for the youngest are overconsolidated. Foundation conditions are characterised by a general lack of sandy sediments and an abundance of normally consolidated clays and silts at the seabed overlying hard clays at variable depths.
It is demonstrated that full interpretation of the reconaissance seismic survey data and the resulting geological model are of paramount importance for the selection of reconaissance geotechnical borehole locations. The resulting integrated seismic/ geotechnical data base enables the lateral prediction of geotechnical properties over the area and is beneficial for the conceptual design stage of offshore installations.
Over the period 1972-1982 Shell Exploration and Production U.K. made a number of oil and gas discoveries in a small area in the 21 quadrant of the U.K. continental shelf. All accumulations are small by North Sea standards and when development was considered in the early Eighties, it was realised that production and off take facilities would have to be shared between the fields. There was however no clear concept of the type and distribution of these facilities, and of the interconnecting network of flowlines.
In order to assess potential development plans and platform locations, it was decided to carry out a reconnaissance survey over an extensive area measuring 17 × 7.5 km, covering 4 possible oil and gas developments.
As a precursor to the survey, the upper part of conventional exploration seismic sections was studied to define the scope of the project. Although the resolution of these data is very limited, it was evident that the proposed survey area is traversed by several deep, erosional, buried channels. This directly influenced the planning of the survey in terms of preferred shooting direction, line spacing and seismic penetration.
The main objective of the survey was to accurately establish the distribution of sediments in the top hundred metres below seabed. By doing so, it was hoped that optimal soil boring locations could be selected after interpretation of the seismic data. Eventually high resolution seismic data and soil boring data would be integrated to aid in the prediction of piling and in the conceptual design of installations.
Secondly, the data should provide sufficient information to evaluate the long term loading behaviour of the sediments within a zone of some four hundred metres below seabed.
Thirdly, penetration and resolution should be sufficient to allow the detection of possible shallow gas occurrences between 300 and 450 m below sea level.
