2020 Volume 47 Issue 4 Pages 129-138
For surveying underwater objects in the vast ocean, it is necessary to mount an ambient noise imaging system into a movable vessel such as an autonomous underwater vehicle (AUV). However, the concave lens developed in our previous studies is not suitable to mount it on the bow of AUV. Because the concave lens does not fit to the AUV’s bow shape, its water resistance is large. Our group studied convex lenses which have aspherical surfaces to mount on an AUV’s bow. These lenses were composed with solid lenses faced to sea water, and inner liquids placed in the AUV’s bow. In this study, we measured the temperature-dependences of three Syntactic foams (TG-26/3000, TG-28/4000, TG-39/11500) for solid lens, and of Fluorinert (FC-72) for inner liquid, in order to select suitable materials to the convex lens. The results of tank experiments showed that the sound velocities decrease with increasing temperature, for all materials. The temperature-dependences of the refractive indexes were estimated by the first-order approximation formulas obtained from the experimental results. Finally, the total transmittances through the solid lens from sea water to inner liquid were obtained from the viewpoint of acoustic impedance matching. The best syntactic foam for the solid lens to be used in combination with the FC-72 inner liquid was found to be the TG-28/4000.