Coherence of bottom‐reflected sound in the Tagus Abyssal Plain

Abstract

Coherence of bottom‐reflected sound in the Tagus Abyssal Plain was determined as a function of bearing at frequencies 50–500 Hz for a fixed grazing angle of 15°. SUS charges were dropped at 244 m depth in a circular pattern of 28 km radius around a receiver at 2500 m depth in water 5100 m deep. The bottom reflected signals were isolated and processed to remove the decorrelating effects of varying bubble pulse periods. Computed was a coherence function ρ(ω,φ) = |Smn(jω)|4/Sm(ω)Sn(ω), where S is a sample estimate of the power spectral density, j is −1, ω is the radian frequency, and m, n are the indices of adjacent shots at an average bearing φ. For a given frequency, the measured coherence function vs bearing exhibits a degree of symmetry in which features occur in pairs 180° apart. This observation suggests a directionally dependent, coherence‐degrading effect operating at the bottom, in the water column, or both, and significant over an aperture of 5 km. [Work supported by NORDA Ocean Programs Office.]