The wave equation is solved to determine the transmission and reflection coefficients for plane waves at oblique incidence on a system of n layers of plane parallel plates. Both shear- and longitudinal-wave properties of the materials are included in the derivation. Equations are presented in a form which will facilitate adaptation to a digital computer. Data measured in the frequency range 100 –700 kHz for one, two, and three solid layers immersed in water compare favorable with computer results.
The temporal phase stability of the medium was measured at a site used for the collection of synthetic-aperature sonar data. These measurements were conducted over a two-year time span, and were made at several different heights-above-bottom. Results obtained show that for small heights-above-bottom, the standard deviation of the phase is as small as 0.04 radians over a 20-min measurement interval. At larger heights-above-bottom, the standard deviation of the phase is much larger, getting as large as 0.31 radians over a 2-min interval for one measurement.
The liquid-filled target sphere is a high target strength reflector which, since its first use in 1971, has proven to be of great utility in sonar research. The spherical metal shell filled with a low-velocity liquid is being used as a calibrated, passive, aspect-independent target. To date, the characteristics of this target have only been described in a narrow ka range by simple ray theory models. In this paper, a comprehensive treatment for 0<ka<100 for various shell materials and filling fluid refractive indices is presented, using results from wave theory analyses. Comparisons are made with air-filled and water-filled spheres and with solid spheres. Form function versus ka plots, echo time history, and target strength versus frequency are presented. Results show good agreement between computed and measured target strengths and permit an accurate prediction of target strength over a wide range of frequency and fluid velocities.
A specially designed sonar rail facility was used to collect synthetic-aperture sonar data in the frequency range from 20 to 100 kHz. A target field laid out at the rail site containing simulated point objects was used to measure the beamforming performance of synthetic-aperture sonar at these frequencies. The synthetic-aperture beam patterns obtained in these tests show excellent compression in azimuth and have side-lobe levels of about-17 dB. The mechanical and electronic features of the sonar rail facility are described, and the experimental procedures outlined. A passive auto-focus technique used to correct rail nonlinearities is described, as well as measurements made to determine the temporal coherence of the medium at the rail site.
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