Underwater Imaging System using Multi-frequency Ultrasonic Holography

Abstract

This paper describes an underwater imaging system using multi-frequency ultrasonic holography. This imaging system has 11X11 receiving array and reconstructs the image by holographic calculation. In conventional mono-frequency holographic system, undesirable ultrasonic refrection from non-target objects degrades the quality of image. To prevent this image degradation, the authors propose a method of multi-frequency ultrasonic holography. It prevents image degradation not by an increase in the number of receivers but by an increase in the number of holograms formed by different ultrasounds. A good quality image can be obtained from a synthesized hologram that is vector summation of the phase-corrected holograms. The reduction rate of undesirable ultrasound is estimated to be 1/√(M), where M is the number of diffent frequency ultrasounds. The upper and the lower limits of applicable frequency bandwidth are presented. Finally the prevention of image degradation is demonstrated by experiments in the water.