Calculating Apparent Forward Loss Using APL Bottom Bistatic Scattering Cross Sections

摘要

Calculations are carried out to examine the approximation of treating forward scattering from sediments as a forward reflection. The calculations use two different bistatic scattering models, both appropriate for acoustic frequencies between 10 and 100 kHz. The models are used to determine the intensity level of the energy forward scattered into a broad-beam receiver from a transmitter while the halfwidths of the transmitter's horizontal and vertical beams are varied from 20 to 300. This intensity is compared to the level that is predicted if the sediment is assumed to be perfectly flat so that a reflection process can be used. Four different sediment types are examined: silt, sand, cobble, and rock. For a silt sediment it is found that, for single scattering and the pulse widths examined, a transmitter with equal vertical and horizontal halfwidths greater than 40 would have a peak intensity within 2 dB of that calculated for a reflection. To be within 2 dB of the reflection calculated for a rock sediment, the transmitter halfwidths would have to be greater than 110. The results for sand and cobble are within these two extremes. Smaller halfwidths lead to correspondingly larger errors when using the reflection approximation.