A seismic study of deep geological structure in the Bristol Channel area, SW Britain

Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray‐tracing and time‐term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s −1 . Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms −1 , the Old Red Sandstone with a velocity of 4.7–4.8 kms −1 acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s −1 refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s −1 basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra‐basement sequence. In north Devon, a 6.2 km s −1 refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.