Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands

Information on temperature, temperature gradients, thermal conductivity and heat flow from the c. 3.5 km deep Lopra-1/1A borehole in the Faroe Islands is presented and analysed. The upper 2450 m of the drilled sequence consists of thick tholeiitic basalt flows and the deeper parts of hyaloclastites and thin beds of basalt. Temperature data originate from high precision temperature logging a long time after drilling to a depth of 2175 m (the original Lopra-1 borehole) and from commercial temperature logs measured a short time after drilling to a depth of 3430 m (Lopra-1/1A). The high-precision temperature log determines accurately levels of inflow of groundwater to the borehole and significant thermal disturbances to a depth of c. 1250 m. Below 1300 m, no significant disturbances are seen and interval temperature gradients for large depth intervals show only small variations between 28 and 33°C/km. The mean least-squares gradient for the depth interval of 1400-3430 m is 31.4°C/km. In clear contrast to these overall very homogeneous, large-interval, mean temperature gradients, great local variability, between gradients of 20-25°C/km and 45°C/km, was observed between about 1300 and 2175 m (maximum depth of the high-resolution temperature log). These gradient variations are interpreted to be due to thermal conductivity variations and to reflect varying secondary mineralisation and mineral alterations. A preliminary analysis of the Lopra-1/1A temperature-depth function in terms of long-term palaeoclimatic signals indicates subsurface temperatures below about 1300 m to be in equilibrium with mean surface temperatures significantly below zero during the last glacial period. A subsequent temperature increase of 12-16°C occurred at around the termination of the last glaciation. The measured temperatures (some after correction) and the thermal regime below 1300 m seem to represent conductive equilibrium conditions without significant disturbances from the effect of drilling, groundwater flow or long-term palaeoclimatic surface temperature variations. Thermal conductivity measured on samples of basalt taken from drill cores and surface outcrops in the area of the borehole shows values within a rather narrow range and a well-defined mean value for low porosity basalts of about 1.8 W/m°C, while a few samples of lapilli-tuff/tuff from the borehole gave values around 1.9 W/m°C. Lapilli-tuff and tuff seem to have higher matrix (grain) conductivity than basalt. Heat flow is estimated at 60 ± 5 mW/m ². A heat flow of this magnitude is consistent with the Faroe Islands being underlain by continental crust.