Full Microtremor H/V(z,f) Inversion for Shallow Subsurface Characterization

Abstract:

The H/V spectral ratio has emerged as a single station method within the seismic ambient noise
(SAN) analysis field by its capability to quickly estimate the resonance frequency of a site and
through inversion the average profile information. Although it is easy to compute from experimen-
tal data, its counter theoretical part is not obvious when building a forward model which can help
reconstructing the derived H/V spectrum. This has lead to the simplified assumption that the noise
wavefield is mainly composed of Rayleigh wave and furthermore to consider only the left or right
flank around the H/V peak frequency in the inversion for subsurface 1D shear wave velocity pro-
file. A new theoretical approach for the interpretation of the H/V spectral ratio has been presented
by Sánchez-Sesma et al. (2011). Here, the fundamental idea behind their theory is presented as it applies to receivers at depths. A smooth H/V(z, f ) spectral curve on a broad frequency
range is obtained by considering a fine integration parameter which is in turn time consuming. We
show that this can be considerably optimized by first smoothing the Directional Energy Density
(DED) spectrum. Further analysis shows that the obtained H/V(z, f ) spectrum computed by the
mean of the imaginary part of Green’s function method could also be recovered using the reflec-
tivity method for a medium well illuminated by seismic sources. Inversion of synthetic H/V(z, f )
spectral curve is performed for a single layer over a half space. The striking results allow to poten-
tially use the new theory as a forward computation of the H/V(z, f ) to fully invert the experimental
H/V spectral ratio at the corresponding depth for the shear velocity profile (Vs) and additionally
the longitudinal velocity profile (Vp) using receivers both at the surface and in depth. Although
the final best density profile in this synthetic case looks reasonable, the marginal distribution of
misfit shows less sensitivity to the expected density values. We use seismic ambient noise data
in the frequency range of 0.2 − 10Hz recorded at two selected sites in Germany where borehole
information is also available. The obtained 1D Vp and Vs profiles are correlated with geological
log information. Results from shallow geophysical experiment are also used for comparison.