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

• Main Authors: Niels Balling, Niels Breiner, Regin Waagstein
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Faroe Islands; Lopra; Sion
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.535.7052; http://www.geus.dk/publications/bull/nr9/nr9_p91-107.pdf

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 tempe-rature logs measured a short time after drilling to a depth of 3430 m (Lopra-1/1A). The high-preci-sion temperature log determines accurately levels of inflow of groundwater to the borehole and signif-icant 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 varia-tions are interpreted to be due to thermal conductivity variations and to reflect varying secondary mineralisation and mineral alterations.