Geoacoustic inversion performed from two source‐receive arrays in shallow‐water waveguide.

Abstract

Raylike propagation of acoustic waves in a shallow‐water waveguide between two vertical line arrays is investigated by applying a double beamforming algorithm, which performs time‐delay beamforming on both emitting and receiving arrays and allows identification of eigenrays by their emission and reception angles and arrival times. From the intensity of each eigenray, it is possible to determine reflection coefficient from the bottom of the waveguide as a function of an angle of incidence. The procedure was initially tested in a small‐scale tank experiment for an acoustic waveguide with either steel or Plexiglas bottom. By fitting an experimentally found reflection coefficient with a corresponding theoretical expression, an estimate for the speed of shear waves in the bottom material was obtained. Similar analysis was subsequently applied to the data obtained during an at‐sea experiment, which was performed between two vertical transducer arrays in shallow‐coastal waters of the Mediterranean. An angle‐dependent bottom reflection coefficient was extracted and geoacoustic inversion was performed by fitting the data with theoretical calculations, in which bottom sediments were modeled as a multilayered system. Good agreement between experiment and theory was observed. Our results indicate possible application of eigenray intensity analysis based on double beamforming algorithm for geoacoustic inversion problems.