Practical aspects of wavefield decomposition for permanent seismic rotatory sources

Continuous reservoir surveillance requires permanent seismic sources. One such source is a seismic orbital vibrator (SOV). An SOV excites a circularly polarised seismic signal by the rotation of an eccentric mass alternately clockwise and counter-clockwise. By summing or subtracting the two rotation records, circular polarisation could be decomposed into vertical and horizontal forces, which enhances P and S wavefields respectively. However, asymmetry of the signal for each rotation pattern creates decomposition artefacts that require some compensation. We use several SOVs for time-lapse seismic monitoring at the Otway research site in combination with DAS downhole system. Analysis of the VSP shot gathers acquired continuously for many months reveals that the S wavefield changes quite drastically over a period of three months and thus causes significant non-repeatability. As the S and converted SP waves mask the target primary P wavefield reflections, we propose to attenuate S and SP wavefield components and improve repeatability for the target interval using wavefield decomposition. The method compensates for the asymmetry of the signal for different rotations by using the estimated SOV signature from the direct arrivals observed on the VSP data. The presented method shows significant improvement of the wavefield repeatability. The SOV signature can be split into vertical and horizontal forces allowing for separate processing of P and S wavefields. However, the method requires the source signature measurement and thus may be limited to VSP applications only.