Scientific results from the deepened Lopra-1 borehole, Faroe Islands: Magnetic logs from the Lopra-1/1A and Vestmanna-1 wells, Faroe Islands

• Main Authors: Waagstein, Regin, Abrahamsen, Niels
• Format: Article in Journal/Newspaper
• Language: English
• Published: Geological Survey of Denmark and Greenland 2006
• Subjects: Magnetic logging; rock magnetism; susceptibility; NRM; magnetic reversals; Faroe Islands; Lopra; Vestmanna; North Atlantic; Geology; QE1-996.5; Geophysics. Cosmic physics; QC801-809; Geological Survey of Denmark and Greenland Bulletin
• Online Access: https://doaj.org/article/d24bd2c0d3a74d91b7d3506886a46b51

Susceptibility measurements from cores (representing basalt, lapilli-tuffs and tuffs) and magnetic logs from the Lopra-1/1A well are presented. The basalts fall into high- and low-susceptibility groups with no overlap. The high-susceptibility basalts (seven cores) have susceptibilities between 4 and 88 ×10–3 SI and consist of basalt with < 1% vesicles from thick massive units. The low-susceptibility basalts are intergranular, intersertal or hypocrystalline and contain no or very little (< 1%) visible magnetite, are generally more altered than the high-susceptibility basalts and have susceptibilities in the range from 0.6 to 1.4 × 10–3 SI (seven cores). The susceptibility of ten volcaniclastites of lapilli-tuff or tuff varies from 0.4 to 3.8 × 10–3 SI. The cores from the Lopra-1/1A well reveal a bimodal distribution of magnetic susceptibility. Low susceptibilities ranging from 0.4 to 4 are characteristic of altered basalts poor in magnetite, lapilli-tuffs and tuffs. Thus single measurements of susceptibility are of little use indiscriminating between these three types of rock. Susceptibility logs from the Lopra-1/1A well show that the variation below 3315 m distinguishes clearly between volcaniclastics (hyaloclastites) with low and fairly constant susceptibility and basalt beds of between 5 and 10 m thickness (with high susceptibility). The volcaniclastics comprise some 60–70% of the sequence between 3315 and 3515 m with the maximum continuous sediment layer being 80 m thick. A 1½ m core of solid basalt at 2381 m and sidewall cores of basalt from the Lopra-1/1A well have a mean susceptibility of 22.1 ± 3.5 × 10–3 SI (standard deviation (σ) = 23.6, number of samples (N) = 46), while samples of hyaloclastite (lapilli-tuff and tuff ) have a mean susceptibility of0.85 × 10–3 SI (σ = 0.39, N = 17).The mean values of the rock magnetic parameters for 303 basalt plugs from the Vestmanna-1 well are: Qave = 13.3 ± 0.6 (σ = 11), Save = 11.8 ± 0.6 × 10–3 SI (σ = 11) and Jave = 4.64 ± 0.25 A/m (σ = 4.4).The reversely ...