Far‐field considerations in surface and bottom scattering

Abstract

It is sometimes argued that the concept of scattering cross section is inapplicable for most surface and bottom scattering measurements, because the acoustic receiver is invariably in the near‐field zone of the ensonified patch and scattering cross section is defined in the far‐field limit. Winebrenner and Ishimaru [IEEE Trans. Antennas Propag.AP34, 847–849 (1986)] have shown that the correlation length of the surface field defines the relevant far‐field region for stochastic scattering. This sets a less stringent constraint than the far‐field criterion based on the ensonified patch size and greatly extends the domain of applicability of the scattering cross‐section concept. Using a standard formalism that includes near‐field phase terms, it is shown how the scattering cross section can be used even in the near field of the ensonified patch. The result is a proof of the method commonly used in reverberation calculations: One integrates over the ensonified patch with the appropriate propagation loss factors and bistatic scattering cross section as integration variables. This assumes that the far‐field criterion based on the correlation length is satisfied. [Work supported by ONR.]