Abstract
This thesis develops a technique and its hardware realization, to generate three dimensional images of targets located in the near field By merging a planar phased array of omnidirectional hydrophones, with a beamformer and a back projection technique, it was possible to generate a volumetric representation of the targets. The concept was first simulated by placing an oscillating point source in the near field of a virtual 3D space and a virtual planar array centered at the coordinate origin in a direct ping configuration, and successfully visualizing the point source at different azimuth and elevation angles. To implement the technique and prove the feasibility of the concept, an acoustic source was mounted with a rectangular array of hydrophones and signal conditioning electronics in an underwater projector-receiver apparatus linked to a custom designed data acquisition and processing electronics via an umbilical cable. To maintain the signal coherence an array of sampling and hold circuits were used in conjunction with a fast multiplexing scheme and a high speed AID converter. The stored data was later processed using a beam forming and a back projection scheme to create a volumetric representation of the acoustic targets. Two sets of experiments were carried out at the RSMAS dock. A direct ping experiment in which an acoustic source in the near field was visualized, and a backscattering experiment in which the dock pilings located in the near field were visualized in 3D.