Scaled Tank Experiments of Low-Frequency Propagation in the SOFAR channel

This paper presents an original experiment reproducing a horizontally stratified marine environment in a laboratory so as to investigate the propagation of acoustic waves. For the first time, our lab has created in a tank a vertical sound speed profile in a liquid layer with a minimum as in a real ocean. This profile is obtained by a diffusion technique. We stack layers of seven different liquids whose densities increase with depth. The liquids are two aqueous alcohol mixture and four saline solutions, with a layer of pure water in between. By varying the amounts of alcohol and salt we produce mixtures of increasing densities. The mixtures can then lie one upon the other. The experiment was done on a 1 to 20000 scale with a 1 MHz source in 20 cm of water and a source-receiver separation of 4 m. This simulates a 50 Hz source in 4000 m of water, and a source-receiver separation of 80 km in the real ocean. Original synthetic materials were used to simulate the sea bottom. Besides the interest of the experimental set-up, the paper reports comparisons between results from a well-known theoretical model (the Parabolic Equation Method) and experimental results, and it describes the limits of the experimental model and its defects.