Acoustic Imaging in a Turbid Underwater Environment

Abstract

Theoretical and experimental studies are presented concerning underwater acoustic imaging in general with particular emphasis on acoustic visualization in highly turbid water. A derivation is made of the relationship between image-plane and object plane pressure distributions in an acoustic imaging system embodying a single spherical lens, and the theory is applied to both diffusely and specularly reflecting objects. The attenuation and scattering coefficients of sea water containing various ocean-bottom sediment suspensions are determined; it is found that for an acoustic frequency of 2.5 MHz (employed in the experimental work), several hundred parts per million suspended sediment concentration effects an acoustic attenuation twice that of sea water alone. A method for underwater acoustic image conversion is described which involves a sampled matrix of piezoelectric elements. Two novel switching techniques are discussed for sequentially connecting with high selectivity the various sensor elements to the output terminal. Typical shadow, focused shadow, and backscatter acoustic images are displayed that were obtained with a 100-element collinear version of the image-conversion system. The collinear array has a sensitivity threshold of better than 10−10 W/cm2 and a resolution on the order of 1.5 mm. Some of the images which are presented were taken at a range of approximately 4 m in water containing several hundred parts per million of suspended diatomaceous earth of micron particle size.