  We perform helioseismic holography on realistic solar convection simulations
and compare the observed travel-time perturbations with the expected travel
times from the horizontal flows in the simulations computed from forward models
under the assumption of the Born approximation. We demonstrate reasonable
agreement between the observed and model travel times which reinforces the
validity of helioseismic holography in the detection of subsurface horizontal
flows. From the variation of the signal-to-noise ratio with depth, we conclude
that the helioseismic detection of individual flow structures with spatial
scales of supergranulation or smaller is not possible for depths below about 5
Mm below the surface over time scales less than a day. Approximately half of
the observed signal originates within the first 2 Mm below the surface. A
consequence of this is a rapid decrease (and reversal in some cases) of the
travel-time perturbations with depth due to the contribution to the
measurements of oppositely directed surface flows in neighboring convective
cells. This confirms an earlier interpretation of similar effects reported from
observations.
