Full waveform inversion of SH- and Love-wave data in near-surface prospecting

We develop a two-dimensional full waveform inversion approach for the simultaneous determination of S-wave velocity and density models from SH - and Love-wave data. We illustrate the advantages of the SH/Love full waveform inversion with a simple synthetic example and demonstrate the method's applicability to a near-surface dataset, recorded in the village achtice in Northwestern Slovakia. Goal of the survey was to map remains of historical building foundations in a highly heterogeneous subsurface. The seismic survey comprises two parallel SH-profiles with maximum offsets of 24 m and covers a frequency range from 5 Hz to 80 Hz with high signal-to-noise ratio well suited for full waveform inversion. Using the Wiechert-Herglotz method, we determined a one-dimensional gradient velocity model as a starting model for full waveform inversion. The two-dimensional waveform inversion approach uses the global correlation norm as objective function in combination with a sequential inversion of low-pass filtered field data. This mitigates the non-linearity of the multi-parameter inverse problem. Test computations show that the influence of visco-elastic effects on the waveform inversion result is rather small. Further tests using a mono-parameter shear modulus inversion reveal that the inversion of the density model has no significant impact on the final data fit. The final full waveform inversion S-wave velocity and density models show a prominent low-velocity weathering layer. Below this layer, the subsurface is highly heterogeneous. Minimum anomaly sizes correspond to approximately half of the dominant Love-wavelength. The results demonstrate the ability of two-dimensional SH waveform inversion to image shallow small-scale soil structure. However, they do not show any evidence of foundation walls.