Multispectral Acoustic Backscatter from Multibeam, Improved Classification Potential

Most multibeams today provide a measure of the received seabed backscatter intensity. With proper radiometric and geometric reduction, the seabed backscatter strength and its angular variation may be derived, which can then be used to attempt seabed classification. Most multibeams, however, are monochromatic in the sense that they transmit using a single center frequency (even if with some pulse bandwidth). The seabed scattering is thus specific to that wavelength. As a result ambiguities in classification can result when differing scattering mechanisms (e.g. surface and volume) result in a similar intensity at that wavelength. If, in contrast, the seabed can be imaged using a triplet of discrete center frequencies, each spaced about an octave apart, the frequency dependence may be used as an additional classifier. Additional complications exist, however, with the added requirement to perform those same corrections, but now for three different systems. A major concern is the proper accounting for path length attenuation for the highest frequency involved, as that limits the maximum depth over which the combined systems can be used. Multispectral imaging has been implementing twice now using co-located 70-100 kHz and 200-400 kHz multibeams. Examples of improved seabed discrimination are presented. Clear variations in the shape of the angular response curve as well as the relative scattering between frequencies are demonstrated.