Elastic SH- and Love-wave Full-Waveform Inversion for shallow shear wave velocity with a preconditioned technique

In contrast to the Rayleigh waves, the system of SH- and Love-wave Full-Waveform Inversion (FWI) is simpler (Only S-wave velocity and density is inverted). However, the gradient is not scaled with the increasing depth in gradient-based FWI. The main reasons for the phenomenon are the limited frequency band of the wavelet, non-uniform coverage of the excitation-receiving points and the double scattering. The illumination can be enhanced by adding a preconditioned operator to the inversion. To compensate the illumination, we presented a preconditioned technique abiding by the theory of inverse scattering in SH- and Love-wave FWI. The synthetic tests demonstrate that the preconditioned technique can expedite the convergence rate and enhance the illumination intensity to the deeper medium compared with the conventional gradient-based FWI. The results also indicate that the preconditioned conjugate gradient (PCG) method has the best parameter reconstruction performance among the local optimization algorithms. Subsequently, we adopt the PCG method to inverse the field data acquired in the village of Čachtice, northwestern Slovakia. In addition, a multiscale strategy is employed and the Wiechert-Herglotz method is followed to estimate the initial S-wave velocity model in inversion. Eventually, this paper illustrates the credibility of the inverted result from the perspectives of waveform fitting, common-offset profile, signal similarity, relative error of channel energy and normalized misfit function values. This study primarily seeks to delineate the foundations of historical buildings under the survey line. According to the range of S-wave velocity, we assert that there are no foundation remains in the subsurface.