Underwater measurements of acoustic backscattering coefficients from gravel surfaces at oblique incidence were made in a laboratory tank. A side scan geometry was adopted in these measurements using a conventional, 300kHz, side scan transducer to insonify an area which is beam width controlled in one direction and a pulse length controlled in the other. As the pulse travels along the rough surface the variation of the backscattered signal with grazing angle was studied. In the second set of measurements a normal incidence geometry was adopted to investigate the dependence of the backscattering coefficient upon range from the rough surface insonified. Two different frequencies (1MHz and 250kHz) and two different rough surfaces (a pressure release and a gravel surface) were used in this experiment. The backscattered signal was measured by a small LC5-2 hydrophone which was inserted on the acoustic axis of the transmitter. The experimental results were compared with the theoretical values developed using the Helmholtz-Kirchhoff integral. In the oblique incidence case the Fraunhofer phase approximation was used to evaluate the scattering integral. The predicted values compared well with the experimental results measured for the gravel surfaces. The backscattering coefficient was found to vary rapidly with angle at low grazing angles while this variation was not so obvious at grazing angles between 30 to 60. In the normal incidence mode the scattering integral derived using the second order, Fresnel, approximation was used to predict the theoretical backscattering coefficient values. A good agreement with the experimental results was obtained even at ranges close to the rough surface. The normal incidence backscattering coefficient of a rough surface is shown to be dependent upon the surface properties alone, only in the farfield region of the scattering area. In the nearfield of the surface, the backscatterintg coefficient is shown to be depending on the surface reflection coefficient only. In the region between these two ranges the backscattering coefficient is shown to be depending upon both the surface statistics and the measurements geometry.
|Date of Award||1984|