P-wave velocity distribution in basalt flows of the Enni Formation in the Faroe Islands from refraction seismic analysis

The main objective of this work is to establish the applicability of shallow surface-seismic traveltime tomography in basalt-covered areas. A densely sampled ~1300-m long surface seismic profile, acquired as part of the SeiFaBa project in 2003 (Japsen 2006) at Glyvursnes in the Faroe Islands, served as the basis to evaluate the performance of the tomographic method in basalt-covered areas. The profile is centred at a ~700-m deep well. V _P, V _S and density logs, a zero-offset VSP, downhole-geophone recordings and geological mapping in the area provided good means of control. The inversion was performed with facilities of the Wide Angle Reflection/Refraction Profiling program package (Ditmar 1999). We tested many inversion sequences while varying the inversion parameters. Modelled traveltimes were verified by full-waveform modelling. Typically an inversion sequence consists in several iterations that proceed until a satisfactory solution is reached. However, in the present case with high velocity contrasts in the subsurface we obtained the best result with two iterations: first obtaining a smooth starting model with small traveltime residuals by inverting with a high smoothing constraint and then inverting with the lowest possible smoothing constraint to allow the inversion to have the full benefit of the traveltime residuals. The tomogram gives usable velocity information for the near-surface geology in the area but fails to reproduce the expected velocity distribution of the layered basalt flows. Based on the analysis of the tomogram and geological mapping in the area, a model was defined that correctly models first arrivals from both surface seismic data and downhole-geophone data.