Acoustic backscattering from salp and target strength estimation

Abstract

Although zooplanktonic organisms nearly have the acoustical impedance of water, they backscatter the sound. Strong acoustical echoes from salps were recorded in the Weddell sea at 150 kHz (1992), in the Indian ocean at 12 kHz (1994) and in the Mediterranean at 120 and 38 kHz (1996). Sphere and cylinder models were applied to estimate the salp target strength. The salp is modelled by a cylindrical gelatinous body (length L) and a spherical nucleus (radius a). Backscattering from each part was estimated individually. With a detection threshold of –100 dB, the minimum body length and the minimum radius of nucleus to be theoretically detected were estimated for each of the following frequencies: 12, 38, 120 and 150 kHz. A small salp ( L = 14 mm and a = 1 mm) would be only detected at 120 and 150 kHz whereas a big salp ( L = 100 mm and a = 7.5 mm) would be also detected at 12 and 38 kHz. The nucleus of the salp can reveal by itself the salp's presence in the medium by concentrating a maximum weight into a minimum volume. The effects of variations in the input parameters on the estimated acoustical backscattering are discussed.