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Title: Geophysical investigations of the deep structure of the Bristol Channel and South Wales
Author: Llewellyn, D. J.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1982
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During August 1975, a series of seven seismic profiles, up to 63.8 km in length, were established in the Bristol Channel area. Marine shots of Aquaflex line charge ware fired from the R.R.S. Edward Forbes into land-based seismometer arrays. Analogue seismic recordings were digitised to permit digital filtering and automatic computer plotting of the seismograms. Interpretation of seismic P-wave arrivals was performed in terms of planar-layer models. The Carboniferous Limestone facies is characterised by a P-wave velocity of 5.1-5.3 km s-1; Old Red Sandstone strata exhibit velocities of 4.6-4.8 km s-1 and often constitute a seismic hidden layer beneath the Limestone. Variscan folding of Carboniferous Limestone and O.R.S. strata complicates the seismic top-layer. A refractor of velocity 5.4-5.7 km s is correlated with a Lower Palaeozoic horizon, and a regional 6.0-6.3 km s-1 basal refractor is correlated with Precambrian crystalline basement, except beneath north Devon. The basal refractor in general deepens southwards across the area. A single-ended quarry-blast profile between Cornelly and Penderyn quarries, sited on opposite rims of the South Wales coal basin, suggests a northward shallowing of the basal refractor beneath the coalfield. Aeromagnetic and Bouguer gravity anomaly data for the area were collated and digitised. These data were variously filtered and transformed, using wavenumber-domain techniques, to facilititate a qualitative interpretation. Several anomalies were interpreted quantitatively using methods of spectral analysis and direct anomaly inversion. Interpretation of the potential field data supports a southerly increase in basement depth beneath the region. Intra-basement susceptibility contrasts are largely responsible for the magnetic anomalies over South Wales, where seismic and cagnetic basements are approximately coincident. Southwards, they apparently diverge towards north Devon, where a shallow high velocity refractor, of Lower Palaeozoic or Late Proterozoic age, is possibly structurally emplaced above a deep magnetic basement.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available