Two-dimensional elastic full-waveform inversion of Love waves in shallow vertically transversely isotropic media: synthetic reconstruction tests
In most shallow-seismic applications of full-waveform inversion, the subsurface is assumed to be isotropic, although near-surface materials may exhibit strong seismic anisotropy. Ignoring anisotropy will lead to inexact solutions when simulating wave propagation or imaging the subsurface using full-waveform inversion. For shallow structures, vertically transversely isotropic media provide a suitable approximation due to the fine horizontal layering of the sediments. We investigate the effects of anisotropy on surface waves and on shallow-seismic full-waveform inversion in vertically transversely isotropic media. The comparisons of seismograms calculated in isotropic and vertically transversely isotropic models show that the sensitivity of full-waveform inversion towards anisotropy is significantly higher for Love waves than for Rayleigh waves. This observation indicates that it is more promising to perform full-waveform inversion on Love waves rather than on Rayleigh waves to identify anisotropy of near-surface materials. Therefore, we performed synthetic twodimensional reconstruction tests of anisotropic full-waveform inversion using only shallow-seismic Love waves. These tests revealed that the parameters describing vertical transverse isotropy can be accurately reconstructed by full-waveform inversion. Although the inversion for density is still problematic, this does not affect the results for the seismic velocities in a significant way. The tests on synthetic data thus prove the general applicability and the benefits of an anisotropic inversion of shallow-seismic Love waves, which can provide a more comprehensive subsurface characterization in shallow anisotropic media.