Accounting for palaeoclimate and topography: a rigorous approach to correction of the British geothermal dataset

Raw heat flow measurements typically require correction for both palaeoclimate and topography if temperatures are to be reliably extrapolated to depths greater than those where temperature is measured. Such corrections are thus an essential step in quantifying geothermal energy resources. However, although both types of correction were pioneered decades ago by British workers, they have been omitted or underplayed in subsequent assessments of the UK geothermal dataset. Furthermore, as most UK heat flow measurements to date have utilised shallow boreholes, the magnitudes of the required corrections (for both effects) are exacerbated. In addition, the location of Britain, at a range of latitude with a temperate climate at present but where arctic conditions prevailed during much of the Pleistocene, means that the palaeoclimate correction, for a borehole of a given depth, is particularly large. Outside regions of high relief relative to borehole depth, palaeoclimate corrections at sites in Britain are indeed larger in magnitude than topographic corrections, and for almost all boreholes are additive. The magnitude of the palaeoclimate correction depends on assumptions about palaeotemperature anomalies and their durations, but from the available evidence it can be assessed, for a very shallow borehole in an unglaciated part of southern Britain with rocks of thermal conductivity 3 W m−1 °C−1, as 27 mW m−2. Air temperatures during Pleistocene cold stages decreased northward, but in much of northern Britain the Earth's surface was not exposed to these low temperatures for part of the Late Pleistocene due to the insulating effect of cover by ice sheets; the detailed correction for each locality thus depends on the local histories of air temperature anomalies and of ice cover, and may therefore potentially be greater or less than is typical for southern England. The past failure to recognise the magnitude of palaeoclimate corrections at sites in Britain, and to incorporate them into studies of geothermics, has led to systematic underestimation of temperatures at depth and, thus, of the overall geothermal energy resource.