After a promising start in the 1970s and 80s, the UK rather fell behind other countries in the search for viable mid-enthalpy
geothermal resources. This situation began to turn around in 2004, when the first of three deep geothermal exploration boreholes
were drilled in northern England. What distinguished these from earlier drilling in Cornwall was the deliberate search for naturallyhigh
permeability associated with major faults, especially those that have undergone strike-slip reactivation during the Cenozoic.
Boreholes at Eastgate in the North Pennines targeted buried radiothermal granite, whereas the 1,821m-deep Science Central
Borehole in Newcastle upon Tyne targeted a postulated deep sedimentary aquifer (the Fell Sandstones), which were inferred to be
connected laterally to the granitic heat source by a major fault (the reactivation of the Iapetus geo-suture). The drilling was in both
cases rewarded with impressive heat flows, and in the case of Eastgate with what is believed to be the highest permeability yet
found in a deep granite batholith anywhere in the world. In parallel with these developments, a re-assessment was made of the preexisting
geothermal heat flow database for the UK, applying newly-standardised correction protocols for palaeoclimatic and
topographic distortions, which were found to be particularly marked in Scotland (where only shallow boreholes had been used to
establish geothermal gradients in the original 1980s analysis), Similar prospects in northern England (similar to that drilled at
Science Central) are now the focus of commercial exploration efforts. Appraisal of fault dispositions relative to the present-day
maximum compressive stress azimuth are being used to identify the most promising areas for intersecting fault-related permeability
at depth. New geophysical tools – most notably atomic dielectric resonance scanning – are also being appraised for their ability to
directly detect features (such as hot brines) which are indicative of localised convection in target fault zones and aquifers