NEAR-FIELD SCATTERING THROUGH AND FROM A 2-DIMENSIONAL FLUID-FLUID ROUGH INTERFACE

A general analytical expression for the time-dependent mean-square inc oherent field scattered fi-om or through (penetrating) a 2-D fluid-flu id rough interface for a narrow-band incident plane-wave source is der ived and expressed in terms of the second moment of the rough interfac e T-matrix. This analytical expression is independent of the scatterin g solution technique, and for distances greater than only a few wavele ngths from the interface, is equivalently expressed in terms of the bi static scattering cross section per unit area per unit solid angle (di fferential cross section) of the rough interface. Using this rigorousl y derived result, the scattered field for a narrow-band point source i s heuristically derived. This derivation leads to the usual sonar equa tion in the limit as the narrow-band signal approaches the cw (continu ous wave) case. First-order perturbation calculations for the case of a baseband Gaussian shaped source pulse illustrate narrow-band pulse d ispersion effects of the incoherent field for forward scattering into a lossy sediment. For the case of incidence below the critical grazing angle, first-order perturbation computations also show that the incoh erent field scattered through a rough interface can be much greater th an the zeroth-order field (coherent) transmitted below the correspondi ng flat-surface depending on loss and receiver depth. These computatio ns for the first-order mean square incoherent field penetrating the ro ugh interface are compared to the results for the flat-surface case, f or both plane-wave and point sources. (C) 1998 Acoustical Society of A merica.