Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

摘要

We investigated the seismic attenuation of compressional (P-) and converted shear (S-) waves through stacked basalt flows using short-offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21-1) and William (6005/13-1A) wells in the Faroe-Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root-mean-square time-domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22-26 and 13-17 for P- and S-waves, respectively, and 25-33 for P-waves in the William well. An effective Q_S/Q_P ratio of 0.50-0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P-wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S-wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P- and S-waves in the stacked basalt flow sequence is mainly caused by one-dimensional scattering, while intrinsic absorption is small.