Acoustic waveform inversions are an increasingly popular tool for
extracting subsurface information from seismic data. They are computationally
much more efficient than elastic inversions. Naturally, an inherent
disadvantage is that any elastic effects present in the recorded
data are ignored in acoustic inversions. We investigate the extent
to which elastic effects influence seismic crosshole data. Our numerical
modeling studies reveal that in the presence of high contrast interfaces,
at which P-to-S conversions occur, elastic effects can dominate the
seismic sections, even for experiments involving pressure sources
and pressure receivers. Comparisons of waveform inversion results
using a purely acoustic algorithm on synthetic data that is either
acoustic or elastic, show that subsurface models comprising small
low-to-medium contrast (?30%) structures can be successfully resolved
in the acoustic approximation. However, in the presence of extended
high-contrast anomalous bodies, P-to-S-conversions may substantially
degrade the quality of the tomographic images. In particular, extended
low-velocity zones are difficult to image. Likewise, relatively small
low-velocity features are unresolved, even when advanced a priori
information is included. One option for mitigating elastic effects
is data windowing, which suppresses later arriving seismic arrivals,
such as shear waves. Our tests of this approach found it to be inappropriate
because elastic effects are also included in earlier arriving wavetrains.
Furthermore, data windowing removes later arriving P-wave phases
that may provide critical constraints on the tomograms. Finally,
we investigated the extent to which acoustic inversions of elastic
data are useful for time-lapse analyses of high contrast engineered
structures, for which accurate reconstruction of the subsurface structure
is not as critical as imaging differential changes between sequential
experiments. Based on a realistic scenario for monitoring a radioactive
waste repository, we demonstrated that acoustic inversions of elastic
data yield substantial distortions of the tomograms and also unreliable
information on trends in the velocity changes.