Given well known environmental conditions, matched field processing has been shown
to be a promising signal processing technique for the localization of acoustic sources.
However, when environmental data are incomplete or inaccurate, a 'mismatch' occurs
between the measured field and model field which can lead to a severe degradation of
the localization estimator. We investigate the possible mismatch effects of surface and
internal waves on matched field processing in a shallow water waveguide. We utilize a
modified ray theory, based on the work of Tindle, to calculate the acoustic pressure field.
This allows us to simply incorporate range dependent environmental conditions as well
as to generalize our work to deeper waveguides. In general, the conventional (Bartlett)
matched field beamformer does not provide sufficient resolution to unambiguously locate
a source, even in a perfectly matched environment. The maximum likelihood method
(MLM) matched field beamformer has much better resolution but is extremely susceptible
to mismatch. The mismatch due to surface roughness can result in a large reduction
of the estimator peak. Part, but not all, of the peak can be regained by 1)using a
model which includes incomplete reflection at the surface based on actual sea surface
statistics and 2) short time averaging of the measured signal, with times on the order
of the period of the surface waves. Mismatch due to internal waves can also result in a
large degradation of the estimator. Averaging over the same time period as surface waves
provides little improvement and leads one to surmise that internal waves may be a limiting
constraint on matched field processing. Finally, we combine the surface and internal
wave fields with a slowly moving source. This example highlights the necessity for the
development of a beamformer which has a broader mainlobe while maintaining adequate
sidelobe suppression, and we address this issue by looking at two such beamformers.