On Approximate Treatment of Reflection of Ultrasound from a Group of small reflecting Bodies in the Sea and some Experiments connected herewith

Abstract

Approximate equations denoting the treatment of the reflection of ultrasound from such small objects as plankton, air bubbles, mud grains etc in the sea are described with reference to the already published papers, and target strength or reflection losses of the above objects are calculated and graphed according to the equations. Measurements of reflection loss of the group of plankton (calanus plumchrus, 2〜3mm in length), air bubbles (1〜3mm in diameter) at 50kc and 200kc ultrasound are carried out in the experimental water tank and the measured values are compared with those calculated. A few experiments in the sea given too. Main results obtained are as follows: (1)Target strength T_s or reflection loss β_1 of the small spherical object is expressd approximately in the following form. β_1=20log_&lt10&gt[a/(2*10^2)*2ka^2|(1-gh^2)/(3gh^2)+(1-g)/(1+2g)|]^&lt-1&gt where, a:radius of sphere k:2π/(λ) λ:wave length of ultrasound in the surrounding medium g:(ρ')/ρ relative density h:(c')/c relative sound velocity (2)Experiments show that the reflection loss of the group of plankton (calanus plumchrus, 2〜3mm in length) at 50kc ultrasound decreases in nearly inverse proportion to √&ltN&gt (N is the total number of plankton) up to the average distribution density of plankton about (0. 3)/(cm^3). (3)Experiments show that the reflection loss of the group of air bubbles (about 2mm in diameter) at 50kc ultrasound is smaller than that at 200kc ultrasound since it is expected that such air bubbles acting as massive reactance elements in these frequency range vibrate less strongly as the frequency increases. Measured values of these reflection losses are about 5〜10dB smaller than that of the calculated ones.