Ocean acoustics turbulence study: Acoustic scattering from a buoyant axisymmetric plume

Abstract

Limited in situ measurements from high-frequency underwater acoustic echosounders have suggested that there may be circumstances in which acoustic scattering from ocean temperature variability is sufficiently intense to be observable over volume reverberation due to biologics. A new laboratory program, the ocean acoustics turbulence study (OATS), has been undertaken to quantify such scattering. The laboratory employs a very precise computer-controlled positioning system and allows a scattering geometry from a near forward-scattering angle, 5°, to a near backscattering angle, 160°. This article reports on results of an initial set of experiments utilizing a 1-MHz, 1-cycle transmitted pulse scattering from a temperature anomaly field produced by a laminar buoyant plume. The objective of this experiment is to compare observations of acoustic scattering from the temperature anomaly with that predicted by the Bragg scattering condition. A parameter regime is chosen such that the laminar plume has an approximately axisymmetric and Gaussian temperature profile. For the two-dimensional axisymmetric case, the Bragg scattering condition allows prediction of an acoustically derived two-dimensional Fourier transform of the temperature field. The acoustically derived two-dimensional Fourier transform and the one derived directly from in situ temperature measurements are in good agreement, except at the higher frequency range of the bandwidth of the scattered signal. Discussion of the possibility of inverting such measurements for a direct calculation of the temperature field is presented.