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Title: Seismic properties of Faroe Island basalts
Author: Shaw, Felicia M. J.
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2006
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The Faroe Islands are part of the North Atlantic Igneous Province and comprise three subaerially erupted basalt formations: the Upper, Middle and Lower Series. Vertical VSP data, core data and wireline log data, acquired in two boreholes, were the basis for studying the seismic attenuation properties of Faroe Island basalts. Over a restricted bandwidth of 40 - 150 Hz, mean Q in Glyvursnes is estimated to be ~ 25. There is evidence that Q increases with depth, corresponding with a transition from the Upper to Middle Basalt Series. Over 20 - 500 Hz, mean Q in Vestmanna is also estimated to be ~ 25, appearing constant over the full VSP interval. 1-D scattering is likely to be significant only in the upper section of Glyvursnes, where the Upper Basalt Series occurs. In Vestmanna and the lower Glyvursnes section, 1-D scattering Q is substantially higher than effective Q derived from the VSP data. 1-D scattering in basalts is found to be a frequency-dependent phenomenon, linked to periodicity in velocity structure. Analyses of log data reveal that geological factors exert primary controls on multifractal character. Multifractality tends to diminish 1-D scattering effects by disrupting the resonance of periodic velocity structures. 1-D multifractality does not give a clear indication of 2-D / 3-D heterogeneity, and synthetic multifractals must be used with caution in basalt modelling studies. Field observations suggest that 3-D heterogeneity mainly arises from variable morphology of pahoehoe flows, which have low-velocity vesiculated crusts and high-velocity cores. However, pahoehoe-based models appear to produce less 3-D scattering with increasing structural variation. Overall, 1-D and 3-D scattering cannot fully account for the high attenuation in field data. Hence, while evidence for the necessary fluid flow is inconclusive, indications are that intrinsic attenuation may be a significant component of energy dissipation in Faroe Island basalts.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available