4D Surface Wave Tomography Using Ambient Seismic Noise

In 4D land and especially for Permanent Reservoir Monitoring (PRM), changes of the subsurface induce unwanted signal variations that interfere with the 4D signal recorded from the reservoir. A three-month PRM pilot was carried out for Shell on the Peace River heavy oil field in Alberta, Canada in 2009. During this period, reservoir production was monitored using active buried sources and buried receivers. We took advantage of this continuous seismic recording to extract surface waves from recorded ambient noise using cross-correlation techniques. Surface wave tomography is then applied to produce daily time-lapse surface wave velocity maps that monitor velocity variations within the shallow subsurface. We provide an image of the shallow subsurface velocities showing generally higher values in the southern part of the area. This pattern correlates fairly well with the known presence of swamp (muskeg) in the area and the wells pad location. Calendar observation of velocity maps shows stronger variation at low frequencies with good spatial coherence. In the case of PRM and continuous seismic monitoring, these findings could help to discriminate, at least qualitatively, contributions due to shallow subsurface variations from actual reservoir 4D variations.