Abstract
A prototype three-frequency (114, 256, and 410 kHz) colour sidescan sonar system, built by Kongsberg Underwater Mapping Ltd. (Great Yarmouth, UK), was previously described, and preliminary results presented, in Tamsett, McIlvenny, and Watts. The prototype system has subsequently been modified, and in 2017, new data were acquired in a resurvey of the Inner Sound of the Pentland Firth, North Scotland. An image texture characterisation and image classification exercise demonstrates considerably greater discrimination between different seabed classes in a three-frequency colour sonar image of the seabed, than in a multi-frequency colour image reduced to greyscale display, or in a single-frequency greyscale image, with readily twice the number of classes of seabed discriminated between, in the colour image. The information advantage of colour acoustic imagery over greyscale acoustic imagery is analogous to the information advantage of colour television images over black-and-white television images. A three-frequency colour sonar image contains a theoretical maximum of a factor of 3 times the information in a corresponding greyscale image, for independent seabed responses at the three frequencies. Estimates of the average information per pixel (information entropy) in the colour image, and in corresponding greyscale images, reveal an actual information advantage of colour sonar imagery over greyscale, to be in practice approximately a factor of 2.5, empirically confirming the greater information based utility of three-frequency colour sonar over greyscale sonar. Reference: Tamsett, D.; McIlvenny, J.; Watts, A. J. Mar. Sci. Eng. 2016, 4(26).
Highlights
Multi-frequency acoustic colour imagery is not a new idea, and along with multi-frequency radar colour imagery, was patented a surprisingly long time ago [1]
Three-frequency multi-beam acoustic images are shown in Hughes Clarke and Muggah [3] and Hughes Clarke [4], and three-frequency sidescan sonar acoustic images were generated by Tamsett and McIlvenny [5], Tamsett, McIlvenny, and Watts [6], and McIlvenny et al [7]
It has been shown by demonstration that three-frequency colour sonar imagery is more informative than analogous greyscale imagery, and achieves considerably greater discrimination between different types of seabed than is achievable, either by single-frequency data imagery, or by three-frequency data reduced to greyscale imagery
Summary
Multi-frequency acoustic colour imagery is not a new idea, and along with multi-frequency radar colour imagery, was patented a surprisingly long time ago [1]. A new and considerably improved attitude sensor card provided by Kongsberg Underwater Mapping (Great Yarmouth, UK) replaced the previous one, which had inadequately measured sonar fish heading. This replacement proved invaluable in the surveys in 2017 providing essential data supplementary to the heading data from the differential antenna in the ERI’s (Environmental Research Institute) GPS system. This requires that the carrier wave frequencies be sufficiently separated for independent seabed responses. The swaths are shown in this paper overlain from west to east to form montages for which the direction of ensonification is to the west, except along the eastern half of the easternmost swath
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