Ocean tomography

The Buried Object Scanning Sonar (BOSS) is being developed at Florida Atlantic University to image targets buried under the seabed. Tomographic images are constructed using a sequence of sonar transmissions while the vehicle is moving. This motion causes image distortion and should be measured and removed by mapping the echoes received to an absolute coordinate system. The aim of this thesis is to develop and simulate a technique for generating BOSS images that provide an accurate representation of target shape and size, by removing vehicle motion while mapping the image pixels. Synthetic acoustic data sets are generated by convolving the auto-correlated FM transmission pulse with the impulse response of an elastic sphere. Synthetic outputs of a Doppler velocity log and a 3-axis inertial measurement unit are generated to simulate vehicle motion. Noise is added to the sensor data to show the effects of motion sensor errors on image quality.

Experimental measurements have been conducted to investigate the effects of a three dimensional bathymetry on ocean acoustic propagation and our abilities to use array processing for localizing sources. This work is unique because it uses laboratory scale measurements to isolate the effects of the bottom bathymetry. Previous investigations using laboratory scale measurements have only used simplistic bottom profiles. In addition, experiments which have investigated the effects of the bottom bathymetry at sea have encountered difficulties isolating these effects due to range dependent sound speed profiles and the uncertainties of ocean acoustic experiments. The first part of this dissertation investigates the tracking of an acoustic source in a three dimensional shallow water environment. This work is comprised of two studies. The first study uses matched field processing for identifying the trajectory of a source. The second investigation uses experimental measurements and theoretical predictions to evaluate the beating angle bias caused by the use of plane-wave beamforming in the presence of bathymetric refraction. The second part of this dissertation uses laboratory scale measurements to analyze two and three dimensional propagation over a realistic bottom bathymetry. This series of investigations uses an inverse approach based on normal mode theory. The inversion algorithm is used to extract the normal mode amplitudes for the purpose of analyzing the measurements for two dimensional mode coupling and bathymetric refraction. The results of this investigation show that the bathymetry has a strong influence on the three dimensional acoustic field. Analysis of the experimental measurements identify that mode coupling and bathymetric refraction are important for propagation over the laboratory scale model and these effects adversely influence our abilities to localize sources in three dimensional shallow water environments. It is also shown that by incorporating three dimensional propagation models into the signal replica used by the array processor a significant improvement in performance can be achieved.

High-resolution sonar systems are primarily used for ocean floor surveys and port security operations but produce images of limited resolution. In turn, a sonar-specific methodology is required to detect and classify underwater unexploded ordnance (UXO) using the low-resolution sonar data. After researching and reviewing numerous approaches the Multiple Aspect-Fixed Range Template Matching (MAFR-TM) algorithm was developed. The MAFR-TM algorithm is specifically designed to detect and classify a target of high characteristic impedance in an environment that contains similar shaped objects of low characteristic impedance. MAFR-TM is tested against a tank and field data set collected by the Sound Metrics Corp. DIDSON US300. This thesis document proves the MAFR-TM can detect, classify, orient, and locate a target in the sector-scan sonar images. This paper focuses on the MAFR-TM algorithm and its results.