Can Full-Waveform Inversion Compensate for the Lack of Illumination in Crosswell Tomography?

Seismic illumination can be defined as the maximum angle between rays that pass through a point. Only interfaces completely contained in the angular apertures can be correctly imaged with first arrival traveltime tomography (FATTT). We investigate if 2D acoustic full-waveform inversion (FWI) can compensate for the lack of illumination of a crosshole tomography experiment. We use synthetic data generated with Ricker wavelets with peak frequencies at 100 or 500 Hz, resulting in small overlapping in the frequency bandwidths, allowing the use of a multiscale FWI approach. We investigate two FWI cases: in the first case (FWI T), just the waves recorded at the opposite borehole are used whilst, in the second case (FWI T+R), the waves recorded at the two boreholes are used. For a single interface, the shape of the transmitted waveform only varies significantly when it is contained in the angular apertures. Accordingly, shot gathers for layered models with interfaces outside the angular apertures can be roughly reproduced with equivalent homogeneous media. Thus, in comparison with FATTT, both FWI cases give mild improvements for models with interfaces inside the angular coverage, and cannot compensate for the lack of illumination of the experiment. However, in the mixed condition where layers with interfaces inside the angular coverage are cut by a fault, FWI offers substantial improvements over FATTT, even if the fault plane is outside the angular coverage. In this mixed situation, resolution increases when FWI T+R is used. However, for the studied cases, no significant improvements were obtained in the FWI stage where the 500 Hz peak frequency was used.