Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic
Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs a...
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Geological Survey of Denmark and Greenland
2006
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ftdoajarticles:oai:doaj.org/article:2b46f85bc6294162bf726b46d514641a 2023-05-15T16:10:38+02:00 Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic Cowper, David Gollifer, Ian Christie, Philip 2006-07-01T00:00:00Z https://doaj.org/article/2b46f85bc6294162bf726b46d514641a EN eng Geological Survey of Denmark and Greenland http://www.geus.dk/publications/bull/nr9/nr9_p23-40.pdf https://doaj.org/toc/1604-8156 1604-8156 https://doaj.org/article/2b46f85bc6294162bf726b46d514641a Geological Survey of Denmark and Greenland Bulletin, Vol 09, Pp 23-40 (2006) Faroe Islands Palaeogene basalts Lopra-1/1A borehole vertical seismic profile seismic attenuation Geology QE1-996.5 Geophysics. Cosmic physics QC801-809 article 2006 ftdoajarticles 2022-12-31T09:42:17Z Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here.The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remainedwithin the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflectedwavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismicvelocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well.The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported.Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layeredmedium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth ofthe well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence. Article in Journal/Newspaper Faroe Islands North Atlantic Suðuroy Suðuroy Geological Survey of Denmark and Greenland Bulletin Directory of Open Access Journals: DOAJ Articles Faroe Islands Lopra ENVELOPE(-6.771,-6.771,61.444,61.444) Suðuroy ENVELOPE(-6.830,-6.830,61.518,61.518) |
institution |
Open Polar |
collection |
Directory of Open Access Journals: DOAJ Articles |
op_collection_id |
ftdoajarticles |
language |
English |
topic |
Faroe Islands Palaeogene basalts Lopra-1/1A borehole vertical seismic profile seismic attenuation Geology QE1-996.5 Geophysics. Cosmic physics QC801-809 |
spellingShingle |
Faroe Islands Palaeogene basalts Lopra-1/1A borehole vertical seismic profile seismic attenuation Geology QE1-996.5 Geophysics. Cosmic physics QC801-809 Cowper, David Gollifer, Ian Christie, Philip Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
topic_facet |
Faroe Islands Palaeogene basalts Lopra-1/1A borehole vertical seismic profile seismic attenuation Geology QE1-996.5 Geophysics. Cosmic physics QC801-809 |
description |
Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon exploration using reflection seismic techniques. The Lopra-1/1A re-entry well on the Faroese island of Suðuroy allowed us to study the seismic characteristics of a thick sequence of basalt flows from well logs and borehole seismic recordings. Data acquired during the deepening operation in 1996 are presented here.The re-entry well found that the seismic event at 2340 m, prognosed from the pre-drill Vertical Seismic Profile (VSP) as a decrease in impedance, was not base basalt and the deepened well remainedwithin the lower series basalts. Nonetheless, compressional and shear sonic logs and a density log were recorded over the full open hole interval. These allowed a firm tie to be made with the reflectedwavefield from a new VSP. The sonic logs show a compressional to shear wavespeed ratio of 1.84 which is almost constant with depth. Sonic compressional wavespeeds are 3% higher than seismicvelocities, suggesting dispersion in the basalt flows. Azimuthal anisotropy was weakly indicated by the shear sonic log but its orientation is consistent with the directions of mapped master joints in the vicinity of the well.The VSP downgoing compressional wavelet shows good persistence, retaining a dominant period of 28 ms at 3510 m depth. Average vertical velocity is 5248 m/s, higher than previously reported.Attenuation can largely be modelled by geometrical spreading and scattering loss, consistent with other studies. Within the piled flows, the effective Q from scattering is about 35. Elastic layeredmedium modelling shows some hope that a mode-converted shear wave may be observed at moderate offsets. Like its predecessor, the 1996 VSP indicates a decrease in impedance below the final depth ofthe well. However, it is unlikely to be basement or sediment and is probably an event within the volcanic sequence. |
format |
Article in Journal/Newspaper |
author |
Cowper, David Gollifer, Ian Christie, Philip |
author_facet |
Cowper, David Gollifer, Ian Christie, Philip |
author_sort |
Cowper, David |
title |
Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_short |
Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_full |
Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_fullStr |
Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_full_unstemmed |
Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_sort |
scientific results from the deepened lopra-1 borehole, faroe islands: borehole seismic studies of a volcanic succession from the lopra-1/1a borehole in the faroe islands, northern north atlantic |
publisher |
Geological Survey of Denmark and Greenland |
publishDate |
2006 |
url |
https://doaj.org/article/2b46f85bc6294162bf726b46d514641a |
long_lat |
ENVELOPE(-6.771,-6.771,61.444,61.444) ENVELOPE(-6.830,-6.830,61.518,61.518) |
geographic |
Faroe Islands Lopra Suðuroy |
geographic_facet |
Faroe Islands Lopra Suðuroy |
genre |
Faroe Islands North Atlantic Suðuroy Suðuroy Geological Survey of Denmark and Greenland Bulletin |
genre_facet |
Faroe Islands North Atlantic Suðuroy Suðuroy Geological Survey of Denmark and Greenland Bulletin |
op_source |
Geological Survey of Denmark and Greenland Bulletin, Vol 09, Pp 23-40 (2006) |
op_relation |
http://www.geus.dk/publications/bull/nr9/nr9_p23-40.pdf https://doaj.org/toc/1604-8156 1604-8156 https://doaj.org/article/2b46f85bc6294162bf726b46d514641a |
_version_ |
1765995802647330816 |