A preconditioned technique for SH- and Love-wave full-waveforminversion in time domain and crosstalk analysis

For the problem of low horizontal resolution in the method of surface wave dispersion analysis, we apply the SH- and Love-wave full-waveforms to achieve 2D and 3D imaging of the subsurface. The gradient obtained by classical full-waveform inversion (FWI) is not scaled with increasing depth. The limited source frequency band, the non-uniform coverage between shot and geophone positions and double scattering are the main reasons for the phenomenon. The Hessian operator of the misfit function can clearly predict the inherent defocusing phenomenon and the artefacts generated by the double scattering in the gradient vector, and the inverse Hessian operator is used as a deconvolution operator to realise gradient preconditioning. We present an expression of the quasi-Hessian operator in SH- and Love-wave FWI based on inverse scattering theory in prestack depth migration, and apply a new preconditioned technique to the fault and large contrast (LC) model reconstruction test. The inverted results show that the new preconditioned technique can greatly improve the imaging accuracy compared with the gradient-based methods of FWI. From single-parameter tests, we can conclude that the quasi-Hessian operator plays roles in illumination compensation and parameter complement estimation in FWI. From the final reconstructed results of a dual-parameter model, we find that the inverted result of the density is slightly closer to the true model than S-wave velocity under the condition of the same comparative distance between the initial and true model, but parametric crosstalk of density is more serious.