A. V. Bunchuk
A. N. Ivakin
N. N. Andreev Acoustics Institute, 4 Shvernik St., 117036 Moscow, Russia
Experimental and theoretical investigations of acoustic properties of the deep-sea bottom covered with manganese nodules have been conducted. The objectives were (1) to study the possibility of remote acoustic detection and outlining of nodule deposits and (2) to find out the possibility of remote acoustic monitoring and estimation of the nodules abundance, mean size, and other parameters. Field experiments were conducted in four regions of the Pacific and Indian Oceans at the frequency range of 2--20 kHz, and a number of energy and correlation characteristics of bottom echo signals were studied. The field experiments have shown that availability of manganese nodules is displayed by an increase of both the sound reflectivity and scattering strength and by the appearance of their unusual, comparing nodules-free bottom, frequency-angular dependences. A theoretical model of sound scattering and reflection by a set of nodules laid on the bottom has been developed [A. V. Bunchuk and A. N. Ivakin, Sov. Phys. Acoust. 35(1), 5--11 (1989)] in which the explicit form of connection between characteristics of bottom reradiated acoustic signal and nodules parameters have been established. As a result, it became possible to explain all the obtained experimental data and to solve some inverse problems. The spatial distribution of amplitudes of signals reflected and scattered from the bottom (and of some other signal parameters) can be mapped and these maps appear to be the one of the nodules' abundance over the ocean bottom. Finally, real-time algorithms for the inverse problem solution were developed and their validity for adequacy was performed.