Quantitative modeling of bottom-interacting ocean acoustic waves is
complicated by the long propagation ranges and by the complexity
of the scattering targets, We employ a two-dimensional (2-D) hybrid
technique combining Gaussian beam, finite difference, and Kirchhoff
integral solutions of the wave equation to simulate ocean acoustic
experiments within half of a convergence zone in the SOFAR channel,
The 2-D modeling approach is reasonable due to the one-dimensional
(1-D) velocity distribution in the water column and the strong lineation
of the seafloor morphology parallel to the mid-ocean ridges. Full-waveform
modeling of ocean acoustic data requires that the topography and
the material properties of the seafloor are available at scales that
are several orders of magnitude smaller than typical bathymetric
sampling rates. We have therefore investigated the effects on the
ocean acoustic response of a stochastic interpolation scheme used
to generate seafloor models, For typical grazing angles of the incident
wave field (approximately 5 degrees-20 degrees), we found that different
stochastic realizations of the same seafloor segment (sampled at
200 m) yield an intrinsic uncertainty of the order of 3-8 dB in amplitude
and 0.1-0.3 s in time for individual prominent events in the reverberant
acoustic field, Hybrid simulations are compared to beam-formed ocean
acoustic data collected during the Acoustic Reverberation Special
Research Program (ARSRP) cruises, Side lobe noise in the observed
acoustic data is simulated by adding band-limited white noise at
-30 dB relative to the maximum intensity in the synthetic data. Numerical
simulations can be limited to the response of only one of the mirror
azimuth beams provided that the experimental geometry is suitably
chosen, For the 2-D approximation to be valid, the cross-range resolution
of the observed data must be smaller than the characteristic scale
of seafloor lineations, and the beams of interest must be approximately
perpendicular to the dominant structural grain. Under these premises,
the arrival time and maximum intensity of the observed backscattered
acoustic events can be modeled within 0.3 s and 5-10 dB, respectively,
The mismatch in arrival time is interpreted to be due predominantly
to intrinsic uncertainties in the stochastic interpolation of the
seafloor profiles, whereas the fact that the intensity of the backscattered
events is systematically overestimated is attributed to 3-D effects
within the ''footprint'' of the beam and/or to underestimated noise
levels.