Spatial coherence and rough bottom scattering in shallow water

Abstract

Shallow-water acoustic data collected off Mallorca in March 1993 are analyzed for spatial coherence, and their relation to rough bottom scattering is investigated. Bistatic reverberation data were collected by a horizontal receiver array with equispaced hydrophones suspended 1 m above the bottom. A flextentional transducer towed behind the ship was used as a source, generating a 209-dB source level at center frequency of 400 Hz, with 5-ms signal length, and 10-s interval. The ship made a hexagonal run with the horizontal receiver array inside the track. A theoretical source model has been designed to approximate the actual source pulse, and is used to match-filter the data. The modal arrivals are clearly identified in the resulting impulse responses, allowing for mode-by-mode analysis. The total field is decomposed into a mean field (coherent field) and scattered field (incoherent field) by a stacking procedure. The spatial coherence is then calculated using a correlation and a coherence function. The magnitude square of the complex coherence function (MSC) of the total field was 0.9 while the MSC of the scattered field was 0.1, which suggested that the bottom roughness was very small, allowing for modeling by perturbation theory.