Quantitative acoustic color displays for classification with broadband sonars

Abstract

Many sonars now use broadband signals. Although such sonars can produce impressive high resolution, high contrast images of the underwater acoustic scene, there is usually substantial ambiguity about what is actually being viewed. Some means of further determining what makes up the acoustic scene is highly desirable. Conventional sonar imaging technology depicts, in some way, variations in the intensity of the acoustic return over the image area. With broadband sonars, there is additional information about the nature of acoustic contacts, which is lost in such a display. In particular, the spectrum of the acoustic return may contain significant information about the object that caused the echo. A technique for displaying this additional spectral information in a quantitative manner using a mapping from the echo spectrum to color is presented. The resulting color image is roughly the acoustic analogue of color vision, where the color provides additional information about the object in the image, and is not merely a colorful alternate way to display acoustic intensity. Basic definitions of color terminology and equations describing the mapping from acoustic return spectra to color are provided. Techniques for calibrating the spectral levels in order to obtain true acoustic color are discussed. The use of broadband scattering models to provide a quantitative basis for the coloring scheme is described. Finally, our implementation of acoustic color and an example of an acoustic color image for a broadband sonar operating in a complex environment are shown and interpreted.