Scaling of Optical and Low-Megahertz Acoustic Properties of Turbid-Water Systems in the Context of Image Quality

摘要

In the context of underwater imaging of minelike objects, it is desired to compare the quality of acoustic images with that of optical images at various turbidity levels. For this purpose, there has been interest in scaling experiments to a smaller size. First it is shown that, when modelling a target at range si (e.g 5 metres), by experimenting at a shortened range I (e.g. 1 metre), one must (at l) use a different concentration of added matter for the acoustic than for the optical measurements. This effect arises because the acoustic attenuation that occurs in clear water is a constant and is not to be scaled. The report derives, first, the appropriate modified scaling laws, and second, the laws for maintaining constant image quality, on which the former laws depend. Because the image quality depends on the signal-to-noise ratio in the image, the latter laws depend on the properties of the acoustic noise. The laws of constant quality are derived for three noise environments: instrumentation noise, clutter and the combination of the two. Methods of carrying out the scaling experiment are described. Unfortunately the main conclusion is negative: the real noise is such that scaling experiments in the acoustic mine imaging context are not possible in practice. Here the problem for the first noise environment is that two assumptions are made that do not hold in the normal experimental arrangement; for the second and third environments, an acoustic array of reduced size would have td be built at prohibitive cost. Formulae for the visibility range are given.