Broadband acoustic reflectivity and its application to the characterisation of materials

Underwater acoustic imaging and classification sonars are being progressively extended into broadband and interest is shifting to higher frequencies for use in shallow waters with particular emphasis on object classification for mine counter-measures. These shifts raise the opportunity to use the frequency-dependent scattering properties of different materials to characterise target composition. Many mechanisms may play a role in the total backscattered signal, including specular reflections, scattering from surface irregularities and multiple resonances (often modified by internal structures). These contain a great deal of unexploited information that can tell us a lot about the nature of the object. The acoustic backscattering properties of an object depend to a large extent on the materials used on the object surface and on its structure, yet little is documented about the broadband reflective properties of common marine materials such as rubber foams, Aluminium, Steel, etc., in open literature. In this paper, we explore the reflective properties of these materials over a range of frequencies and incident angles through direct measurement via ensonification by a broadband source in an acoustic tank. An experimental set up is described that can record the reflected energy over a several discrete angles simultaneously. Compact source pulses are used to permit multiple reflections from tank walls and the free surface to be separated out