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

A detailed airborne geophysical survey has been made of part of west Dyfed at a flying height of 75 m with magnetic, electromagnetic (VLF-EM) and radiometric equipment mounted in a helicopter. The area of 670 km2 includes the Precambrian anticlines of St David’s and Hayscastle, the Fishguard, Sealyham and Trefgarne volcanic groups, and the adjacent Lower Palaeozoic sediments with associated basic intrusions. Ground geophysical surveys were carried out at 33 localities to confirm the nature and the sources of the airborne anomalies, and a geological examination was also made at selected localities. Rock samples were collected for petrographical examination and physical property determinations. A regional gravity survey was also carried out. Some details of the geophysical methods and data presentation are included together with a map at a scale of 1:50 000 summarising the main results obtained from interpretations of the airborne geophysical surveys. The aeromagnetic data clearly show the distribution of the Precambrian rocks, the numerous dolerite intrusions, and some of the pillow lavas associated with the Fishguard Volcanic Group. This distribution generally confirms the outcrop pattern based on geological mapping. The magnetic data are likely to be more reliable for mapping on a more detailed scale, due to the extensive drift cover which hinders geological mapping in many places, and they have also revealed some large scale structures, in- The airborne VLF anomalies are most obvious at lithological boundaries with large resistivity contrasts such as the Precambrian/Lower Palaeozoic sediment and dolerite/sediment contacts. The radiometric data provide little obvious additional information. Although the west Dyfed area has no significant mining history, some exploration activity has taken place there in recent years (Allen, Cooper and others 1985; Cameron and others, 1984; Brown and others, 1987). One of the main restrictions on exploration has been the lack of rock exposure in much of the inland area and the airborne geophysical survey was undertaken as partial compensation for this