The interpretation of gravity and aeromagnetic data for the geological investigation of the Sellafield area, west Cumbria

Abstract

SUMMARY Potential field modelling has been used to test structural models of the Permo-Triassic cover sequence in the Sellafield area. An analysis of rock density data indicated that the main density interfaces occur at the base of the Calder Sandstone and the base of the St Bees Sandstone, and these horizons were employed in 3D gravity stripping. In addition, 2.5D gravity and magnetic modelling was conducted along selected profiles, with structures constrained by boreholes and good quality seismic reflection data, but subject to modification elsewhere. These studies highlighted shortcomings in earlier models for the margins of the Ravenglass Sub-basin and have led to new versions which agree better with available data. Subsequent drilling has confirmed the validity of the re-interpretation. Gravity data provide strong evidence of a north-west-trending fault zone which bounds the north-eastern corner of the Ravenglass Sub-basin. The sharp change in the strike of the basin margin represented by this fault zone probably arose because of the influence of pre-existing basement structures (granites and lines of weakness) on basin evolution. Relatively magnetic rocks within the Borrowdale Volcanic Group can be traced beneath Permo-Triassic cover using aeromagnetic data; the magnetic signature relating to the displacement of these rocks at the margins of the Ravenglass Sub-basin provides corroboration of models based on gravity data.