Acoustic fluctuations due to shallow thermal microstructure

Abstract

Simultaneous measurements of waves, orthogonal water particle velocities, temperature, conductivity and acoustic phase and amplitude measurements at 20 and 40 kHz over horizontal and vertical, 2m paths were made at the NUC oceanographic tower, San Diego in April 1974. Four stochastic models for acoustic propagation including geometric, Born, Rytov and Debye approximation models were tested. The Krosil'nikov - Obhukov formulation of the Born approximation yielded reasonable results. Difficulty in establishing scales obviated good results from the Rytov and Debye models. Taylor thermal microscales from spatial correlation analyses determined vertically across the thermocline varied from 6.8 to 178 cm. The measured variance of the index of refraction ranged from 4.5x10-4 to 2.7xl0-3. Temporal stochastic analyses showed horizontal acoustic path losses were ten times vertical losses because of diffraction effects. The primary mechanisms for acoustic variability were internal and surface waves pumping the thermocline up and down.