3-D numerical modelling of the conductive heat flow of SW England

Abstract

SUMMARY The effect of 3-D geology on conductive heat flow was investigated using the finite element technique on two models from SW England, one a regional model centred on the Comubian batholith as a whole and the other a more detailed model centred on the Cammenellis pluton. Only two rock types, granite and country rock, were used to represent the geology of the region and yet good agreement between modelled and observed data was obtained provided that the granite was assumed to be highly homogeneous, with surface heat production values persisting throughout its depth, and the base of the batholith was assumed to be flat at a depth of 15 km. The results show that the surface heat flow pattern is dominated by the 3-D shape of the batholith which controls the relative importance of the two opposing effects that operate at its margins: heat refraction due to conductivity contrasts enhancing heat flow in the granite, and lateral flow of heat caused by heat production contrasts diminishing it. The highest heat flows occur at the boundaries of outcrops facing the axis of the batholith, where there is significant heat refraction but little lateral loss. A high degree of correlation between mineralized zones and areas of very high heat flow, in both granite and country rock, is also noticeable. The temperature field at any depth displays wide-ranging lateral variations over relatively short distances, the highest values occurring in areas of granite which are covered by insulating country rock and which lie towards the centre of the batholith. Regions of highest heat flow are not necessarily the regions where the highest sub-surface temperatures are likely to be found and it is shown that the hot dry rock geothermal test project at Rosemanowes is not optimally sited from temperature considerations. When the constraints on the physical parameters of the model are sufficiently reliable, as they are in SW England, the heat flow and temperature predictions from 3-D numerical models can be of enormous value in the assessment of the geothermal potential of a region.