Tomographic sonar image formation of difficult objects and of mines on the seabed

Abstract

Insonifying a sea mine over a complete (360°) set of look directions, while recording the acoustic returns (or echoes) from the object as a function of aspect angle, enables the two dimensional projection data (or measurement) space to be fully populated. An acoustic image is readily reconstructed by applying Fourier transform methods to the projection data. Tomographic sonar images of various (inert) sea mines are presented, where a fixed monostatic sonar insonifies the object as it rotates about its vertical axis through one complete revolution. A difficulty arises if the specular component returned from one part of the object swamps the returns from the rest of the object. The problem is solved by taking spatial derivatives of the Fourier reconstructed image, which then results in a representation that enables the object to be recognized. Another complication presents itself when structural waves, which are excited by the insonification process, contribute to the observed impulse response. Numerous examples of the effect of these structural waves on the formation of the image are presented for various objects and sonar technologies, including parametric sonar. For operational tomography, the object is fixed (a mine resting on the sea floor), which requires the sonar to circumnavigate the mine (at a safe standoff distance) while simultaneously insonifying it and compiling the multiaspect projection data space. Sample tomographic sonar images of a practice sea mine, a Mk 84 bomb, and the sea floor itself, are presented.