Seismic velocities in the siting area of the Norwegian Seismic Array (Norsar) have been monitored over a time period of 1 week by using a hydroelectric power plant as a continuous wave generator. Propagational phase angle differences have been measured over travel distances ranging from 4.7 to 13.7 km, and group velocities of the order of 3.5 km/s are derived. This is close to the expected (phase) velocity for S waves, and the particle motions derived at a distance of 4.7 km also correspond well with those for S waves. The obtained precisions are 10-3 for a time period of about 2 hours and 10-4 when 1 week of data is used. The phase difference data exhibit a clear semidiurnal variation which probably corresponds to variations in the seismic velocities, with peak to trough amplitudes around 10-3. There is some possibility that the observed variations could be caused by tidal effects. Observed spatial variations show that the influence of the local geology is significant.