Modelling Acoustic Backscatter from Near-Normal Incidence Echosounders - Sensitivity Analysis of the Jackson Model
Abstract
For a number of years systems, such as RoxAnn and QTC-View, have been attached to ships' echosounders in order to estimate sediment bottom types from the acoustic returns. In this report the physics of acoustic backscatter is examined, at near-normal incidence grazing angles, to determine if these systems can be made less empirical. The Jackson model is used to examine backscatter versus grazing angle, for angles between 65 and 90 degrees, at a frequency of 50 kHz. This is a typical frequency for a standard ship's echosounder. Backscatter predicted by the Jackson model depends on six input parameters and a range of different physical mechanisms. In order to simplify interpretation of results, it seems worthwhile to examine the sensitivity of the model results to different input parameters as a function of sediment type. It is likely that not all six parameters will always be needed, and that some scattering mechanisms can be ignored in some cases. Backscatter versus time for a pulsed system will be examined in a subsequent report. From the model the level of the backscatter and the shape of the backscatter versus grazing angle curve depend principally on the size of surface roughness, the ratio of sediment acoustic impedance to that of water, and a volume scattering coefficient. Other model parameters may be replaced by constants. The Kirchhoff approximation describes well the surface backscatter component, for all but the very roughest surfaces. It is concluded that it should be possible to separately estimate the surface roughness height, the volume backscatter and the sediment's acoustic impedance from acoustic returns. Sediment impedance is well correlated with other sediment properties which can then be estimated.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2000
- Accession Number
- ADA388451
Entities
People
- P. J. Mulhearn
Organizations
- Defence Science and Technology Group