Imaging objects with variable offset in an evanescent wave field using circular synthetic aperture sonar and spectroscopy

Abstract

Imaging properties of objects suspended in an acoustic evanescent wave field are examined. Evanescent waves are generated using a tank containing immiscible liquids and an appropriately directed acoustic beam [C. F. Osterhoudt et al., IEEE J. Oceanic Eng. 33, 397-404 (2008)]. The source and receiver transducers are in the liquid having the higher sound velocity. Object(s) are spun about a vertical axis while scattering is measured. The object(s) offset into the wave field is then varied and the experiment repeated. In this work small spheres and other objects are used to gain insight into imaging properties as a function of the object or object(s) position in the evanescent field. Data is examined using circular synthetic aperture techniques. Additionally, a comparison is made between spectral data and a heuristic model in the case of two spheres. The spectral evolution in that case is affected by the interference from the two scatterers. Cases where the source and receiver are collocated (monostatic) and those where the source and receiver are separated (bistatic) are compared. [Work supported by ONR.]