Seismic Interferometry Using Walkaway DAS VSP Data: CO2CRC Otway Project Feasibility Study

Coming Stage 3 of the CO2CRC Otway project will focus on well-based CO2 monitoring techniques. Although time-lapse active source seismic is probably the most reliable geophysical method for monitoring of the injected CO2 it has some potential disadvantages: cost and invasiveness. Use of Distributed Acoustic Sensing (DAS) for permanent monitoring and utilization of the non-primary wavefield, such as free-surface multiples, can help to alleviate these issues. Permanently installed boreholes DAS receiver arrays can provide a better optimisation of active source effort, meanwhile, interferometric imaging of VSP multiples increases the illumination of the subsurface. We performed a feasibility test of interferometric imaging on the Otway synthetic walkaway VSP dataset and this test demonstrated that the proposed approach is viable for use on DAS data. Subsequently, we performed a test on the field walkaway VSP-DAS data. Both tests have shown that utilization of non-primary wavefield can significantly increase the efficiency of walkaway VSP providing more detailed subsurface image. Experiment on the field data has proved that DAS can be sensitive enough to deal with the secondary wavefield. Finally, the combination of interferometric and standard imaging for walkaway VSP could provide a subsurface image comparable to surface seismic.