It is demonstrated that seismic interface waves on the surface of a natural beach can be used to identify the position of a buried object. For this experiment, the waves were created with a sediment-coupling transducer and received on a three-element horizontal line array of triaxial geophones. The source and its coupling to the medium provided a high degree of signal repeatability, which was useful in improving signal-to-noise ratio. Reception of all three directions of particle velocity made it possible to augment conventional beamforming techniques with polarization filters to enhance interface-wave components. Reverberation in the beach was found to be large, though, and coherent background subtraction was required to isolate the component of the sound field reflected by the target. Propagation loss measurements provided comparisons of reflected signal power with predictions made previously, and the two were found to agree closely.

This paper describes the use of an onionskin composite for the backing material of piezoceramic elements in a high-frequency transducer array. It is the purpose of this paper to cite certain advantages of using this particular composite over a more rigid type material. Measurements on experimental test arrays consisted of single-stave directivity patterns and receiving sensitivity versus hydrostatic pressure. In order to show that a sonar array performance can be affected by the piezoceramic backing material, calculated array pattern data, based upon measured and calculated single-stave patterns, are presented in tabular form. The fabrication technique of putting the onionskin on a curved-face housing is presented. In the process, the composite is precompressed such that after being potted in polyurethane, the array will withstand high hydrostatic pressures without an extreme degradation in the sound isolating, tuning, and reflecting properties of the array.

A curved face hydrophone array has been designed, constructed, and tested for 20 000-ft-depth operations. A basic description of the hydrophone design, in which the hydrophone consists of a beamforming array of staves mounted on the circumference of a 20-in-diam. cylindrical housing, is presented. Since conventional pressure-release materials cannot be used in the construction, solutions to problems encountered, such as improper backing impedances for the individual elements and flexural waves genernated in the housing wall, will be briefly discussed. A presentation of the hydrophone frequency response and beam patterns shows that the transducer properties are insensitive to hydrostatic pressures as great as 1000 psi.

Acoustic measurements of insertion loss and echo reduction at normal incidence were made for several materials for evaluation of their use as underwater sound transducer windows in the frequency range of 50–500 kHz. Materials tested included the commercial polyurethane products such as PR-1527, CPC-19, Scotchcast 221, Scotchcast 8, and some of these materials with talc added during curing to vary the density. Values of sound velocity are derived from the echo reduction data and allow determination of ρc values for each material. A computer model of the echo variation with frequency accurately matches the measured echo reduction plots.