Scattering of Sound by Internal Wave Currents: The Relation to Vertical Momentum Flux

Internal waves scatter sound by two related perturbations: I) those associated with vertical particle displacements '(x, y, z, t) in the presence of a vertical gradient of (potential) sound speed(&: = ,o,cp); and 2) those associated with horizontal particle velocities u(x, y, z, t). The combined fractional perturbation in propagation velocity is 8c/c + u/c. The second term, generally neglected, introduces a nonreciprocity w.hen source and receiver are interchanged. Nonreciprocity is expected to be relatively small except for transmission along a deep downward loop. The principal internal wave contribution to nonreciprocity is from inertial frequencies. The sum and difference of reciprocal travel times are a measure of ' and u, respectively, along the ray path; the quadrature spectrum of reciprocal travel times is related by an integral equation to the spectrum of the momentum flux (tu). Precise measurements ofnonreciprocity could provide an estimate of the vertical mome!Jtum flux in an internal wave field.