Small-aperture acoustic imaging using model-based array signal processing

Acoustic imaging systems are often used underwater where light is either unavailable or severely attenuated. Acoustic images are typically constructed by transmitting a pulse of acoustic energy into the environment and then striving to resolve the three dimensional spatial origin of received echoes or backscatter. This thesis introduces a new concept for spatially localizing acoustic backscatter called Computed Angle-of-Arrival Transient Imaging (CAATI) that constructs an image by explicitly computing plane wave arrival angles and amplitudes as a function of range from the sonar (i.e. two way travel time after transmit). In contrast to lens or beamformer based imaging methods, the resolution of the new approach is not limited by the physical size of the receive transducer but instead by model uncertainties, and therefore high resolution may be obtained using a small aperture.; Plane wave decomposition based on one degree-of-freedom (i.e. a single plane wave is assumed with a time varying amplitude and angle-of-arrival) has for some time been employed by interferometric sidescan sonars to perform 3D imaging of the seafloor using two closely spaced array elements. However, the inability of interferometry to separate multiple concurrent plane wave components has relegated these systems to use in predominantly flat, slowly varying seafloor geometries under conditions of no multipath. CAATI on the other hand extends the number of resolveable concurrent plane wave arrivals to N/2 under coherent signal conditions and N-1 for uncorrelated signal conditions using an N element linear array.; The principles underlying the new approach are presented and the factors affecting the validity of the approach under both ideal and non-ideal signal conditions are analyzed and discussed. Alternative array signal processing methods for computing plane wave arrival angles are reviewed and considered in the context of acoustic backscatter imaging. Simulation results are presented in which acoustic backscatter is synthesized for an image phantom (corner reflector) chosen to demonstrate both the viability of the method under non-ideal signal conditions and its advantages over interferometric and beamformed imaging methods. A prototype design for a new seafloor imaging sonar concept called Small Aperture Range vs Angle sonar (SARA) is then described and results are presented for 3D imaging experiments conducted in both a test tank and a lake.; The success of CAATI and SARA demonstrates the practical utility of small aperture acoustic imaging using model based array signal processing and opens the door to a new field of imaging research.