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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ftjgeusbullet:oai:geusjournals.org:article/4856 2023-05-15T16:11:02+02:00 Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic Christie, Philip Gollifer, Ian Cowper, David 2006-05-31 application/pdf https://geusbulletin.org/index.php/geusb/article/view/4856 https://doi.org/10.34194/geusb.v9.4856 eng eng Geological Survey of Denmark and Greenland (GEUS) https://geusbulletin.org/index.php/geusb/article/view/4856/10490 https://geusbulletin.org/index.php/geusb/article/view/4856 doi:10.34194/geusb.v9.4856 GEUS Bulletin; Vol. 9 (2006): Scientific results from the deepened Lopra-1 borehole, Faroe Islands; 23-40 2597-2154 2597-2162 info:eu-repo/semantics/article info:eu-repo/semantics/publishedVersion Peer-reviewed Article. 2006 ftjgeusbullet https://doi.org/10.34194/geusb.v9.4856 2022-03-15T17:22:19Z 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 remained within 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 reflected wavefield 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 seismic velocities, 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 layered medium 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 of the 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 GEUS Bulletin (Geological Survey of Denmark and Greenland) Faroe Islands Lopra ENVELOPE(-6.771,-6.771,61.444,61.444) Suðuroy ENVELOPE(-6.830,-6.830,61.518,61.518) Geological Survey of Denmark and Greenland (GEUS) Bulletin 9 23 40 |
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GEUS Bulletin (Geological Survey of Denmark and Greenland) |
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English |
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 remained within 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 reflected wavefield 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 seismic velocities, 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 layered medium 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 of the 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 |
Christie, Philip Gollifer, Ian Cowper, David |
spellingShingle |
Christie, Philip Gollifer, Ian Cowper, David Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
author_facet |
Christie, Philip Gollifer, Ian Cowper, David |
author_sort |
Christie, Philip |
title |
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_short |
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_full |
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_fullStr |
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_full_unstemmed |
Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North Atlantic |
title_sort |
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 (GEUS) |
publishDate |
2006 |
url |
https://geusbulletin.org/index.php/geusb/article/view/4856 https://doi.org/10.34194/geusb.v9.4856 |
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 |
genre_facet |
Faroe Islands North Atlantic Suðuroy Suðuroy |
op_source |
GEUS Bulletin; Vol. 9 (2006): Scientific results from the deepened Lopra-1 borehole, Faroe Islands; 23-40 2597-2154 2597-2162 |
op_relation |
https://geusbulletin.org/index.php/geusb/article/view/4856/10490 https://geusbulletin.org/index.php/geusb/article/view/4856 doi:10.34194/geusb.v9.4856 |
op_doi |
https://doi.org/10.34194/geusb.v9.4856 |
container_title |
Geological Survey of Denmark and Greenland (GEUS) Bulletin |
container_volume |
9 |
container_start_page |
23 |
op_container_end_page |
40 |
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1765996147186335744 |