Ultrasonic Propagation Measurements in Sea Water up to 400 kc

Abstract

The propagation characteristics of ultrasound in sea-water and its ultrasonic reflection loss on the sea-bottom or on fish-schools must be studied for both the design and the practical operation of echo-sounders, fish-finders, and SONARs. This paper shows a practical method for the above-mentioned study, together with the results of measured propagation characteristics in both vertical and horizontal directions, as well as the ultrasonic reflection loss on the sea-bottom and on fish-schools, using 10, 14.5, 28, 50, 200, 300, and 400 kc. The propagation attenuation in a direction vertical to the sea-surface is due mainly to spherical divergence, but the attenuation in a horizontal direction is partly caused by absorption as well. This absorption becomes larger at higher frequencies: in fact, the results of experiments show that the absorption is 10–20 db/km at 28 kc, 31–50 db/km at 200 kc, and about 120 db/km at 400 kc. The reflection loss of the ultrasound at a fish-school decreases as frequency increases. On the contrary, the reflection loss of the sea-bottom increases as frequency increases. At 200 kc, therefore, the echoes from ground-fishes sometimes become stronger than that from the sea-bottom.