Crosswell nonlinear seismic waveform inversion without downhole sources and its application to time-lapse monitoring

Crosswell seismic measurements enable obtaining high resolution, high accuracy images of the subsurface between boreholes. They are, however, generally expensive considering the need of deployment of special downhole sources. In this study, we develop a novel nonlinear waveform inversion to estimate velocity structures between two vertical boreholes using VSP data without downhole sources. Contrary to the conventional full waveform inversion (FWI), the effect of wave propagation between surface sources and one of the vertical boreholes is appropriately cancelled using representation theory. Furthermore, it enables us to calculate partial derivatives of the cost function without explicitly resolving the Green’s functions in seismic interferometry. We test numerically this new approach of time-lapse monitoring of a deeper target layer, considering also the effect of changes in the complex, shallow vadose zone. We assume that the temporal changes in velocity in the vadose zone are larger than those at the deeper target layer. Our results show that in contrast to conventional FWI, the newly developed approach has the advantages of expensive crosswell seismics involving downhole sources. The estimated velocity is robust against spatiotemporal changes in the near-surface. The approach will be very useful when accurate time-lapse seismic measurements are needed in a cost-effective manner.