A simple heat-flow quality function and appraisal of heat-flow measurements and heat-flow estimates from the UK Geothermal Catalogue

Abstract

A comprehensive database of temperature, heat flow, thermal conductivity and geochemistry is the basis of geothermal modelling. The latest revision (1987) of the UK Geothermal Catalogue (UKGC) contains over 2600 temperatures at over 1150 sites and over 200 observations of heat flow. About 93% of the temperature data are from depths less than 2000 m and about 50% are Bottom Hole Temperatures (BHT). Heat-flow density distribution models can be expanded to include estimates of heat flow derived from BHT and thermal conductivity data contained in the UKGC. A simple quantitative assessment of the quality of individual heat-flow measurements has been devised, based on the type and frequency of temperature and conductivity measurements and the borehole depth. The dimensionless quality function (hfQ) has been defined so that for a heat-flow borehole with two equilibrium temperature measurements made across an interval with a mean depth of 1 km or greater, with complementary thermal conductivity determinations made at intervals of 1 m across this zone, the heat-flow quality function is 1.0. Over 50% of the UK heat-flow dataset has a hfQ function less than 0.10; the 4 highest quality observations have values above 1.0. The same function can be used to assess the quality of estimates of heat flow by assigning quality functions to various types of temperature and thermal conductivity observation schemes. Appraisal of the UK heat-flow dataset indicates that some of the observational data have a similar quality rating to estimates of heat flow derived from convolution of temperature and thermal conductivity data for deep boreholes within the catalogue. In eastern England, the heat-flow density pattern is significantly modified by incorporation of estimates of heat flow from deep boreholes which have hfQ values comparable with early heat-flow measurements made in this region.