Colour Sonar: Multi-Frequency Sidescan Sonar Images of the Seabed in the Inner Sound of the Pentland Firth, Scotland

Duncan Tamsett,Andrew Watts,Jason Mcilvenny

doi:10.3390/jmse4010026

Abstract

The backscatter response of a seabed to an incident sonar signal is dependent on the carrier wave frequency: i.e., the seabed is acoustically colourful. Colour is implemented in a prototype three-frequency sidescan sonar system deployed in the Pentland Firth, north Scotland. Sonar amplitude data as a function of frequency are processed to render them an unconfounded effect of the seabed normalized to the response at a reference inclination angle, for colour to be a meaningful property of the seabed. Methods for mapping data at sonar frequencies to optical primary colours for human visualisation are explored. We recommend methods that in our opinion generate colour characteristics harmonious with human vision in which: shadow is white; saturation black; colour shade darkness is proportional to backscatter strength; and shades of red, green and blue are seen in proportion to the backscatter amplitudes of the low-, mid- and high-frequency sonar data. Frequency equalisation is applied to achieve a balance in colour responses in images. The seabed in the survey area is acoustically colourful. Using the “negative BGR” colour mapping method: a weakly backscattering sand dune in the north of the survey area appears as shades of light blue and purple; a strongly backscattering halo of cobbles around the dune appears as shades of hazel brown; a strongly backscattering gravel ridge across the south of the survey area appears as shades of royal blue; and exposed rock as textures ranging in colour from light brown to light blue/green. There is evidence for colour anisotropy (a dependence of colour on the direction of ensonification). Similarities between anthropic colour sonar and the natural sonar of Microchiropteran bats are noted. Bats’ sonar satisfies the information criteria for acoustic colour, and it is hypothesized that it informs a colourfully-perceived world view.

Highlights

This article is about the colour sonar method in concept and principle and a practical implementation based on a prototype three-frequency sidescan sonar system
Before considering methods for rendering multi-frequency sonar data as colour images, we look a little more deeply at the concept of the acoustic colour of the seabed
Multi-frequency sidescan sonar trace amplitudes were corrected for non-seabed effects and the effect of seabed backscatter functions were normalised to the seabed response at a reference grazing angle (30 ̋)

Summary

Introduction

This article is about the colour sonar method in concept and principle and a practical implementation based on a prototype three-frequency sidescan sonar system. Whilst the sonar amplitude backscatter response of the seabed is a function of sonar system frequency, it is a function of several other things (e.g., Reed and Hussong [2]; Searle et al [3]; Augustin and Lurton [4]), which confound the effect of frequency These lead to a ploughed field effect on mosaicked images in which distracting “plough swaths” are seen to correspond to sonar swaths. For sidescan sonar image amplitude, and in a multi-frequency system, colour, to be a meaningful property of the seabed, the confounding effects need to be corrected. We describe: (i) a prototype multi-frequency colour sidescan sonar system and its deployment in a survey of the seafloor in the Inner Sound of the Pentland Firth, northern Scotland;. It is overlain by a ridge of gravel running across the southern half of the survey area and by a sub-aqueous dune of sand underlain and surrounded by a field of cobbles in the northern half

Sonar Data Processing

The Acoustic Colour of the Seabed

Mapping Data at Sonar Frequencies to Optical Primary Colour Frequencies

RGB Colour

Natural Sonar

Summary Remarks