Seismic investigation of the Faeroe basalts and their substratum

Abstract

Abstract Various seismic techniques have been used to investigate the subsurface in the area around the 2.2 km deep Lopra-1 well located on the Faeroe Islands. The techniques include vertical seismic profiling, walk-away vertical seismic profiling and refraction seismic profiling. Integration of these three different techniques, using a combined profiling method, makes it possible to overcome some of the problems arising from the large acoustic impedance contrasts and S-wave scattering which often exist in volcanic areas. The three seismic profiles show distinct compressional (P) and shear (S) energy arrivals. The direct S-wave is generated at basalt interfaces at or just below the sea bed and interferes in part with groundroll in the refraction recordings. The low noise activity before first arrival indicates the very high quality of the VSP data. The VSP and the walk-away VSP recordings show clear evidence of reflected P-waves, while reflected P-waves are more difficult to recognize on the surface recordings. The velocity through the basalt flows is shown to be dependent on the thickness of the basalt beds. Thick beds yield high velocities, while thin beds generate lower velocities. Velocity anisotropy is observed in the upper 700 m, probably resulting from cracks in the columnar basalt flows or from master joints. The seismic examination of the three profiles shows excellent interbasaltic reflections. A very strong reflector with a negative reflection coefficient is observed at a depth of c. 2.35 km and is interpreted to be the base of the basalt flows. Deeper reflections are interpreted as sills in the substratum of the basalt flows.