Abstract:
In full-waveform inversion of seismic body waves, often
the free surface is ignored on grounds of computational efficiency.
Asynthetic study was performed to investigate the effects
of this simplification. In terms of size and frequency, the
test model and data conform to a real long-offset survey of the
upper crust across the San Andreas fault. Random fractal
variations are superimposed on a background model with
strong lateral and vertical velocity variations ranging from
1200 to 6800 m/s. Synthetic data were computed and inverted
for this model and different topographies. A fully viscoelastic
time-domain code was used to synthesize the seismograms,
and a viscoacoustic frequency-domain code was utilized to
invert them. The inversion was focused on early arrivals,
which are dominated by P-waves but also contain strong
P-Rayleigh wave conversions from the near-field of the receiver.
Resulting waveform models show artifacts and a loss
of resolution from neglecting the free surface in the inversion,
but the inversions are stable, and they still improve the
resolution of kinematic models. The extent of deterioration
depends more on the subsurface than on the surface structure.
Inversion results were improved at no additional expense by
introducing a weak contrast along a staircase function above
shots and receivers.
