Acoustical Imaging with Linear Transducer Arrays

Abstract

The extension of acoustical holography to uncontrolled media outside the laboratory requires rapid scan of a large hologram surface. We have studied and digitally simulated a scheme that will scan sizes up to a few tens of feet in times as short as 0.1 sec. The basis of the design is two crossed linear arrays of transducers, one transmitting and one receiving array. The receivers listen simultaneously while transmitters are pulsed in rapid sequence. By listening to all N transmitters with each of N receivers, the system gathers N² data which constitutes a virtual hologram.

The interference fringes in the hologram are shifted slightly from those of conventional holography. This causes two aberrations that will be significant in certain cases. One occurs in reflection holography when parts of the object field are at significantly different ranges. The image field appears rotated by twice the ratio of the scan velocity to the sound velocity. The other image aberration occurs when a wide-angle hologram is made into a transparency for optical image reconstruction. Complex corrective optics can be designed to remove it. However, for digital image reconstruction the exact wide-angle formulation is much simpler with the new geometry. We have simulated images of point reflectors and of triangular reflectors, the latter both specular and diffuse. Only minor image effects result from the unorthodox geometry.