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...
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Geological Survey of Denmark and Greenland (GEUS)
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Online Access: | https://geusbulletin.org/index.php/geusb/article/view/4862 https://doi.org/10.34194/geusb.v9.4862 |
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ftjgeusbullet:oai:geusjournals.org:article/4862 2023-05-15T16:10:37+02:00 Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands Balling, Niels Breiner, Niels Waagstein, Regin 2006-05-31 application/pdf https://geusbulletin.org/index.php/geusb/article/view/4862 https://doi.org/10.34194/geusb.v9.4862 eng eng Geological Survey of Denmark and Greenland (GEUS) https://geusbulletin.org/index.php/geusb/article/view/4862/10495 https://geusbulletin.org/index.php/geusb/article/view/4862 doi:10.34194/geusb.v9.4862 GEUS Bulletin; Vol. 9 (2006): Scientific results from the deepened Lopra-1 borehole, Faroe Islands; 91-107 2597-2154 2597-2162 Lopra-1/1A borehole Faroe Islands temperature gradients thermal conductivity heat flow info:eu-repo/semantics/article info:eu-repo/semantics/publishedVersion Peer-reviewed Article. 2006 ftjgeusbullet https://doi.org/10.34194/geusb.v9.4862 2022-03-15T17:22:19Z 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. Article in Journal/Newspaper Faroe Islands GEUS Bulletin (Geological Survey of Denmark and Greenland) Faroe Islands Lopra ENVELOPE(-6.771,-6.771,61.444,61.444) Geological Survey of Denmark and Greenland (GEUS) Bulletin 9 91 107 |
institution |
Open Polar |
collection |
GEUS Bulletin (Geological Survey of Denmark and Greenland) |
op_collection_id |
ftjgeusbullet |
language |
English |
topic |
Lopra-1/1A borehole Faroe Islands temperature gradients thermal conductivity heat flow |
spellingShingle |
Lopra-1/1A borehole Faroe Islands temperature gradients thermal conductivity heat flow Balling, Niels Breiner, Niels Waagstein, Regin Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
topic_facet |
Lopra-1/1A borehole Faroe Islands temperature gradients thermal conductivity heat flow |
description |
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. |
format |
Article in Journal/Newspaper |
author |
Balling, Niels Breiner, Niels Waagstein, Regin |
author_facet |
Balling, Niels Breiner, Niels Waagstein, Regin |
author_sort |
Balling, Niels |
title |
Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
title_short |
Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
title_full |
Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
title_fullStr |
Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
title_full_unstemmed |
Thermal structure of the deep Lopra-1/1A borehole in the Faroe Islands |
title_sort |
thermal structure of the deep lopra-1/1a borehole in the faroe islands |
publisher |
Geological Survey of Denmark and Greenland (GEUS) |
publishDate |
2006 |
url |
https://geusbulletin.org/index.php/geusb/article/view/4862 https://doi.org/10.34194/geusb.v9.4862 |
long_lat |
ENVELOPE(-6.771,-6.771,61.444,61.444) |
geographic |
Faroe Islands Lopra |
geographic_facet |
Faroe Islands Lopra |
genre |
Faroe Islands |
genre_facet |
Faroe Islands |
op_source |
GEUS Bulletin; Vol. 9 (2006): Scientific results from the deepened Lopra-1 borehole, Faroe Islands; 91-107 2597-2154 2597-2162 |
op_relation |
https://geusbulletin.org/index.php/geusb/article/view/4862/10495 https://geusbulletin.org/index.php/geusb/article/view/4862 doi:10.34194/geusb.v9.4862 |
op_doi |
https://doi.org/10.34194/geusb.v9.4862 |
container_title |
Geological Survey of Denmark and Greenland (GEUS) Bulletin |
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9 |
container_start_page |
91 |
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107 |
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1765995790589755392 |