An examination is made of the scattering cross section per unit volume per unit solid angle, &sgr;, for the case of acoustic scattering from ocean microstructure. Both monostatic and bistatic source/receiver geometries are considered. Diffraction is explicitly included in the formulation, resulting in an expression for &sgr; which accounts for finite scattering volume effects associated with vertically anisotropic microstructure. Vertically anisotropic microstructure results in &sgr; a maximum at reflection with respect to the horizontal plane, where &sgr;=&sgr;r, with an angular bandwidth of order the maximum of β0, α, where β0 is the beamwidth angle of the scattering volume, and α is the anisotropy slope angle. For the stratified part of the ocean column, neglecting doubly diffusive effects, under typical open ocean conditions, temperature induced microstructure scattering dominates that of fluid velocity scattering by several orders of magnitude. Two models of temperature microstructure, termed the classical turbulence model and the empirical microstructure model, the latter based on field observations, are used as input into the acoustic formulation.

Specific functional forms are obtained for &sgr;r in terms of environmental microstructure parameters. For frequencies of the order of 100 kHz and greater, scattering is principally the result of microstructure in the wave number regimes corresponding to the classical inertial or Batchelor subranges. Typical open ocean microstructure modeled as isotropic classical turbulence results in volume backscattering strengths SV=10 log (&sgr;r)≈-110 dB. However, for very narrow beamwidth sonar systems, SV scales as -20 log α, which for anisotropy slope angles of order that of internal waves would result in a 37-dB increase in scattering strength. Intense turbulence associated with high vertical shear zones such as that present in Stellwagen bank in Massachusetts Bay or at Knight Inlet. British Columbia, can result in volume backscattering strengths of the order of -60 dB. Bistatic scattering at the reflection angle, particularly at near-forward directions, results in general, in greatly enhanced scattering strength over that associated with backscatterin