Borehole seismic studies of a volcanic succession from the Lopra-1/1A borehole in the Faroe Islands, northern North

• Main Authors: Philip Christie, Ian Gollifer, David Cowper
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Faroe Islands; Lopra-1/1A borehole; basalt; vertical seismic profile; seismic attenuation; Lopra; Suðuroy
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.560.8602; http://www.geus.dk/publications/bull/nr9/nr9_p23-40.pdf

Extruded basalt flows overlying sedimentary sequences present a challenge to hydrocarbon explora-tion 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.