Thomas N. Lawrence
Nancy R. Bedford
Appl. Res. Lab., Univ. of Texas at Austin, P. O. Box 8029, Austin, TX 78713
Synthetic aperture processing is a method of extracting horizontal wave numbers from recordings on one hydrophone. Such an approach has been previously suggested; most recently by Collins et al. [J. Acoust. Soc. Am. 92, 2366(A) (1992)], who proposed single hydrophone matched-field processing. A similar method was used by Frisk et al. [J. Acoust. Soc. Am. 86, 1928 (1989)] to derive bottom properties from data. The experimental method requires that a source (or receiver) move at a constant depth and a constant velocity with respect to a fixed receiver (or source). The moving source thus sweeps out a synthetic aperture with range, and array element recordings are processed by means of a Fourier transform of complex pressure with range, yielding the wave-number spectrum (or modal eigenvalues) over the chosen aperture. A series of overlapping apertures over the source track shows the evolution of the wave-number spectrum with the change in source--receiver range. Using a vertical array, estimations can be made for mode depth functions over the aperture of the array, mode attenuation, and mode eigenvalues with source range, i.e., those parameters needed for matched-field processing. Comparisons will be made to low-frequency acoustic data from a range-dependent environment. Experimental results will be compared with parameters obtained from an adiabatic normal mode model.