Time-Lapse Ambient-Seismic-Noise Tomography at Valhall

We show that ambient seismic noise can be used to detect long-term velocity changes in Scholte-wave velocities at microseism frequencies. Two approaches to detect a time-lapse change in Scholte-wave velocities are tested: a data-domain and an image-domain approach. We rely on straight-ray tomography of perturbed traveltimes for imaging. The first approach is based on differentiating phase traveltime changes in the data domain. The second approach is based on differentiating group velocities in the image domain. Both methods work very well and compare well to a time-lapse image computed from controlled-source data. The time-lapse response is dominated by near-surface geomechanical effects of productioninduced reservoir compaction.