Applications of Acoustical Holography to Underwater Viewing
Authors: Greene D.C. , Hildebrand B.P. 
Published: October 2008

ABSTRACT
Optical viewing systems are severely hampered in muddy water. Under these circumstances, increased viewing distance (but less resolution) can be expected from "viewing" systems using longer wavelength acoustical energy. Acoustic images approaching optical quality can be obtained at short ranges using high frequency ultrasound. Images may be formed either by focusing acoustic energy or by reconstruction of acoustical wave-fronts (holography). Systems based on both principles are examined and recent results obtained using holography are presented.

INTRODUCTION

Scope
In attempting to survey all work in underwater acoustical imaging, we quickly realized that there had been far too much done to include in a thirty-minute presentation. For this reason we have selected as material to be discussed only that which seems applicable to underwater viewing at sea. This presentation consists of a very brief analysis of problems associated with underwater viewing, a survey of others' work toward solving these problems and a presentation of that part of the Battelle-Northwest work in acoustical holography (sponsored by the Holotron Corporation that we feel is applicable to underwater viewing. In discussing the work of others, we have been selective, as mentioned earlier. Much work of significance has been done that we cannot discuss because of military classification. We may, through pure ignorance, neglect to mention significant work that is in the open literature.

The Problem
Muddy water is difficult to see through. This everyday phenomenon can delay or even prevent the performance of work on silty bottoms or in permanently turbid waters such as in rivers, bays, or their outflows. Even in normally clear water, any operation involving crawling along the bottom, or using hydrodynamic thrusters near the bottom, will stir up clouds of sediment. Depending on the current over the bottom, vision may be impaired for as long as half an hour. At close range, the free diver can substitute his sense of touch for vision, but the operator of a submersible is not intimately coupled to the work object by his manipulator. In such cases, one would like to be able to "see" through the murky water.
Figure 1 shows the relationship of frequency to wavelength for underwater acoustic and light energy. The problem, here, is caused by particulate matter in the water. The individual particles may be invisible to the naked eye, yet large compared to the wavelengths of visible light. As a consequence, the particles reflect the light preventing the object from receiving full illumination and providing a background of backscattered illumination that masks the reflection of the light that does penetrate to (and is reflected by) the objects being viewed. Figure 2(a) shows the dependence of light absorption on wavelength.
Water-suspended particles having size equal to and larger than the wavelength of visible light cause the water to be murky by scattering incident light. Figure 2(b) shows the distribution in size of suspended particles in a sample of turbid water. The smaller particles are the more numerous. The larger particles do not remain as long in suspension as do the smaller particles