Frank S. Henyey
Eric I. Thorsos
K. Miguel Nathwani
Appl. Phys. Lab., Univ. of Washington, 1013 NE 40th St., Seattle, WA 98105
Reilly et al. [J. Acoust. Soc. Am. 97, 3385--3386 (A) (1995)] presented experimental results for low-frequency bottom scattering strength which appear to show pulse length dependence. It is shown that the scattering strength is independent of pulse length under the assumptions of (1) linear acoustics, and either (2a) the scatterers are not moving, as for ocean bottom scattering, or (2b) the statistics of the scatterers are time independent (a stationary process) and uncorrelated to pulse timing, as for ocean surface zone scattering. These results hold if the pulses have the same frequency spectrum. In the usual case that their spectra are slightly different, rapid frequency dependence can appear to give pulse length dependence. In case (2a), the ensemble average removes that part of rapid frequency dependence due to random interference between returns from different scatterers or different parts of the same scatterer that appear in a single return. Rapid variation of the average received power can bias the measured scattering strength for longer pulses, but this effect can be removed from the data. Statistics other than scattering strength, such as peak return, are generally pulse length dependent.