Effects of second-order scattering on high resolution sonars

Abstract

To achieve simultaneous high volume coverage and high spatial resolution, sonars employing multiple narrow steered receiver beams in conjunction with a single wide transmitter beam are quite often used. The effects of second-order scattering on multibeam sonars have been calculated. The analysis is in terms of the backscattered energy (or ‘‘integrated echo’’) due to clouds of point scatterers. The work applies to scattering such as from clouds of bubbles or dense schools or ‘‘swarms’’ of swimbladder-bearing fish. Under certain conditions second-order scattering will increase the echo energy an appreciable amount, especially as the ratio of transmitter beamwidth to receiver beamwidth increases. The major conditions are (1) dense clouds of nondirectional scatterers are insonified, (2) the sonar signal suffers measurable attenuation due to the extinction cross section of the scatterers, and (3) the absorption cross section of the scatterers is small. The echo energy due to both first- and second-order scattering are calculated for several beamwidth ratios. It is shown that the backscattered energy is increased by up to 100% due to second-order scattering.