oa Surface Wave Analysis - Challenges for Application on an Ultra-shallow Structure Characterisation

We discuss several difficulties related to the application of Full Waveform Inversion using surface waves in the context of seismic data acquired on an ultra-shallow ground model (0 — 10 m depth). For instance, the choice of an accurate initial velocity model in the presence of very high velocity contrasts is complicated. The presence of surface waves increases local minima problems in the objective function, preventing from convergence towards the global minimum. For improving convergence, we propose to use an alternative functional, the windowed-Amplitude Waveform Inversion, which is similar to dispersion curve analysis but without any picking step. The source wavelet is iteratively estimated during the velocity model inversion. If previous examples have shown that this methodology is applicable in synthetic data sets, it is less obvious in real data sets as discussed here.