Recently the use of ultrasonic arrays for imaging defects in metal components has
become economically attractive in Non-Destructive Testing. Given a certain array,
the image quality strongly depends on how the measurements are process into an
image. The current state-of-the-art imaging algorithm in actual use is delay-and-sum
beamforming, which has a resolution capability that is fundamentally limited by the
physical approximation used to describe how waves interact with matter.
This thesis explores the practical use of alternative non-linear “super-resolution”
imaging algorithms that use more accurate physical models, and can theoretically
achieve unlimited resolution. This is made possible by utilising additional sources of
information contained within the measurements, in particular the small amplitude
multiply scattered signals.
The distribution of information contained in the measurements, and utilised by the
imaging algorithms is studied in the context of information capacity of signals. We
discover some insights into the limits of imaging which depend on the signal-to-noise
ratio.
The accuracy of non-linear imaging algorithms can be strongly dependent on the
accuracy of the measurements. Therefore several experiments are performed to
assess their performance in practice. The experimental implementation of these
methods poses a number of challenges, including removal of the incident field, and
compensating for array element directivity.
Super-resolution capability is demonstrated in a highly attenuative medium for the
first time. To further improve the image quality we explore the possibility of using
mirror reflections. This gives an increase in the effective aperture. We perform
simulated and experimental reconstructions of a complex scatterer and find that the
completeness of the image is improved.
The mirror interface also allows quantitative speed-of-sound imaging of penetrable
scatterers using the HARBUT algorithm. This is tested experimentally for the first time.