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
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