Seismic properties of Faroe basalts from borehole and surface data

The seismic velocities, anisotropy and reflectivity of layered basalts in the Faroe Islands were studied using borehole data and vertical seismic profiles from the Vestmanna borehole, combined with reflection and wide-angle seismic data recorded by arrays of both borehole and land multicomponent receivers. The correlation of in-situ downhole ultrasonic-scale velocity and density measurements, borehole logs and core samples with the seismic-scale velocities and reflection data allows us to build a model of the seismic properties across a range of scales. The match of observed travel-times of wide-angle P-wave data into borehole and surface seismometers with those predicted from the borehole sonic measurements suggests that the basalts are homogenous over horizontal distances on the kilometre scale. We find that any P-wave transverse anisotropy in the basalts is less than 5%, and probably lower than half that. A pronounced intra-basalt reflector visible on the surface seismometer data is likely to be caused by thick flows near the top of the Beinisvørð Formation, but is not precisely at the geological boundary between the Malinstindur Formation and the Beinisvørð Formation. Using full-waveform synthetic seismogram models, we demonstrate that strong intra-basalt reflections are likely to be caused by interference of the seismic signal with the layered sequence rather than necessarily representing specific stratigraphic or lithological boundaries.