Seismic Attenuation of Flood Basalts in the Brugdan and William Wells and Stratigraphic Correlation on the Faroes Shelf

Abstract

The Brugdan (6104/21-1) and William (6005/13-1A) wells in the Faroe-Shetland Trough penetrated stacked basalt flows for sub-basalt exploration. We examined seismic attenuation of compressional (P-) and shear (S-) waves through the basalts using short-offset vertical seismic profile (VSP) and well-log data. Seismic quality factors (Q) were calculated from spectral ratios and the root-mean-square amplitude technique in the time-domain. We found the latter method gave more robust results especially when signals were analysed from two-receiver pairs within the stacked flows. For the Brugdan basalts, we obtained an effective Q of 19‒24 and 12‒18 for P- and S-waves, respectively, while for the William prospect we estimated an effective Q of 22‒28 for P-waves. Elastic modelling was used to quantify the scattering loss using well-log-derived synthetic seismograms. We found that while P-wave attenuation could be explained by scattering mechanisms, intrinsic attenuation appeared necessary to explain the measured S-wave attenuation. Finally, a well-tie was constructed for the Brugdan well to compare migrated surface seismic data with the VSP and synthetic seismograms generated using the well-log data. While top basalt, some thick inter-basalt flows and the dolerite intrusions below base basalt showed strong reflections, enabling a confident tie, the hyaloclastite sequence shows weak reflections.