Bottom interaction of low-frequency acoustic signals at small grazing angles in the deep ocean

Abstract

The results of a deep-ocean bottom interaction experiment are presented in which the effects of both bottom refraction and subbottom reflection were observed. Data were obtained in the Hatteras Abyssal Plain using a deep towed 220-Hz pulsed cw source and two receivers anchored near the bottom. For ranges between 1 and 6 km, corresponding to bottom grazing angles less than 13 °, the quadrature components of the received signals were recorded digitally. The observed amplitude shows a strong spatial interference pattern which is composed of the direct and bottom interacting arrivals. It is shown that for small source–receiver separations, the bottom return is dominated by a strong subbottom reflection. With increasing separation, this arrival evolves into a refracted arrival due to the presence of a positive sound-speed gradient in the sediment overlying the subbottom. Because of the gradient, a caustic is formed, and corresponding high intensity regions are observed in the data at the expected ranges. Values of sediment layer thickness, sound-speed gradient, and sound-speed drop at the water–bottom interface are obtained from best fits to the data using ray theory, normal mode theory, and the parabolic equation method. These values are consistent with those obtained in nearby locations by other workers. The success of the parabolic equation method indicates that at small grazing angles, the bottom interaction process may be modeled as a propagation process combined with the effect of a perfect, soft subbottom reflector. A value of sediment attenuation, 0.0015 dB/m at 220 Hz, is also inferred from the data and is among the lowest values reported to date in the literature.