S. V. Egerev
A. N. Ivakin
O. B. Ovchinnikov
A. E. Pashin
N. N. Andreev Acoust. Inst., 4 Shvernik St., 117036 Moscow, Russia
The remotely driven (by means of a CO[sub 2]-laser pulse) underwater near-surface optoacoustic source was used for the first time for remote sea floor sensing. The laser pulse of 5- to 10-(mu)s width irradiates a spot at the water surface. Optical energy is absorbed by a thin (0.001 cm) surface layer. The burst of surface pressure generates a hydroacoustic pulse that is somewhat like a marine mammal's echo-location pulse. The pressure level at a distance of 1 m from the surface is about 0.1 MPa, the total acoustic energy transmitted to the ocean medium can amount to 1--10 J or even more. Other advantageous features of an explosive laser-induced underwater acoustic source (LIUAS) are most significant for remote sea-bed sensing: (1) the signal is of ultra wide band character (1--100 kHz), hence the single shot record produces a set of narrow-band reverberation responses; (2) scattering coefficient versus angle of incidence for a variety of frequencies can be extracted from these data since the LIUAS angular pattern is precisely known; (3) high spatial resolution of the LIUAS signal and the possibility of laser spot movement thus changing the sensing configuration. Full scale experiments were conducted in the shallow water of the Black Sea. An appropriate sea-bed scattering model was developed taking into account sound scattering by smooth large-scale (Kirchhoff) roughness of bottom relief and by anisotropic volume inhomogeneities of the subsurface sediment layer. A comparison of experimental and calculated data allows one to conclude that the sea-bed reverberation in the region of the experiment was of a volume character rather than of a surface one. Also, it was possible to estimate the scattering coefficient by sediment inhomogeneities and their anisotropy factor (i.e., the ratio between vertical and horizontal inhomogeneity scales) as well.