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

Abstract

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.

Seismic attenuation in the continental crust SW of England

“…9b), a 10 km thick layer of granite is buried beneath 200m of basement. This depth is consistent with gravity and thermal models (Edwards 1984a;Sams & Thomas-Betts 1988). Basement, with the same physical properties as in the non-granite model, constitutes the rest of the crust.…”

“…1). This adamellitic granite batholith is thought to be of crustal origin (Shackleton, Ries & Coward 1982), and to have a depth extent from 10 to 15 km and an average width of about 30km (Edwards 1984a;Sams & Thomas-Betts 1988). A prominent, 20-30mgal low in gravity coincides with the outcrop of granite on land, and the anomaly's southwestward extension more than 200 km out to sea indicates that the batholith's buried length is at least of that order.…”

Deep geoelectric survey of the Carnmenellis granite

“…The regional geometry of the batholith has been examined by gravity modelling (WillisRichards 1986;Sams & Thomas-Betts 1988). These studies, combined with a few conclusions that can be drawn from seismic experiments, support the idea of the granite being homogeneous down to a fiat base.…”

“…The Cornubian batholith is characterized by the highest measured heat flow values in the UK. The heat flow field and subsurface temperatures for the Carnmenellis granite are discussed by Wheildon & Rollin (1986), Lee (1986) and by Sams & Thomas-Betts (1988). The predicted sub-surface temperature profile for the Carnmenellis granite provides a temperature of 200°C at about 5.4km and indicates a temperature of 370 deg C at a depth of 10 km.…”

The Cornubian geothermal province: heat production and flow in SW England: estimates from boreholes and airborne gamma-ray measurements

“…It was suggested that heat flow refraction played a significant role due to the complex 3D form of the batholith. This suggestion was later assessed by Sams and Thomas-Betts (1988) who performed 3D numerical modelling of the heat flow field in the SW. The study confirmed that the surface heat flow pattern is dominated by the 3D form of the batholith with heat refraction due to thermal conductivity contrasts enhancing heat flow in the granite and lateral flow of heat caused by heat production contrasts diminishing it.…”