Abstract
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/m2. A heat flow of this magnitude is consistent with the Faroe Islands being underlain by continental crust.
Highlights
From our analysis we may conclude that the thermal regime and our reported temperatures, temperature gradients and heat-flow value from below a depth of 1200– 1400 m represent conductive equilibrium conditions without significant disturbances from the effect of drilling, ground-water flow or palaeoclimatic surface temperature variations
With respect to heat flow, an assumption of similar lithology may not be necessary
Summary
Deep boreholes generally provide the most reliable and undisturbed direct information on temperature, temperature gradients, thermal conductivity and heat flow. The Lopra-1/1A borehole is situated in the southern island (Suðuroy) of the Faroe Islands (Fig. 1) (at 61°26′ 36′′N, 6°46′30′′E) It was drilled in 1981 as a research borehole to a depth of 2175 m below ground level (Berthelsen et al 1984). The purpose of the present paper is to integrate, analyse and discuss all available thermal information from the whole depth range of the Lopra-1/1A borehole. It includes new high-precision continuous temperature logging results from the original hole measured a long time after drilling, temperature measurements from the deepened part acquired as part of the commercial logging runs during and shortly after drilling. A new terrestrial heat-flow value for the Lopra-1/1A site is presented
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have