| 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 omni-directional 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 A/D 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. |
