An airborne geophysical survey of part of west Dyfed, South Wales, and some related ground surveys

摘要

A detailed airborne geophysical survey has been made ofudpart of west Dyfed at a flying height of 75 m withudmagnetic, electromagnetic (VLF-EM) and radiometricudequipment mounted in a helicopter. The area of 670 km2udincludes the Precambrian anticlines of St David’s andudHayscastle, the Fishguard, Sealyham and Trefgarneudvolcanic groups, and the adjacent Lower Palaeozoicudsediments with associated basic intrusions.udGround geophysical surveys were carried out at 33udlocalities to confirm the nature and the sources of the airborneudanomalies, and a geological examination was alsoudmade at selected localities. Rock samples were collectedudfor petrographical examination and physical propertyuddeterminations. A regional gravity survey was also carriedudout.udSome details of the geophysical methods and dataudpresentation are included together with a map at a scale ofud1:50 000 summarising the main results obtained from interpretationsudof the airborne geophysical surveys.udThe aeromagnetic data clearly show the distribution ofudthe Precambrian rocks, the numerous dolerite intrusions,udand some of the pillow lavas associated with the FishguardudVolcanic Group. This distribution generally confirms theudoutcrop pattern based on geological mapping. Theudmagnetic data are likely to be more reliable for mappingudon a more detailed scale, due to the extensive drift coverudwhich hinders geological mapping in many places, andudthey have also revealed some large scale structures, in-udThe airborne VLF anomalies are most obvious atudlithological boundaries with large resistivity contrastsudsuch as the Precambrian/Lower Palaeozoic sediment anduddolerite/sediment contacts. The radiometric data provideudlittle obvious additional information.udAlthough the west Dyfed area has no significant miningudhistory, some exploration activity has taken place there inudrecent years (Allen, Cooper and others 1985; Cameronudand others, 1984; Brown and others, 1987). One of theudmain restrictions on exploration has been the lack of rockudexposure in much of the inland area and the airborneudgeophysical survey was undertaken as partial compensationudfor this.