| ABSTRACT: | ENHANCEMENT OF SYNTHETIC APERTURE FOCUSING TECHNIQUE (SAFT) BY
ADVANCED SIGNAL PROCESSING
M. Jastrzebski1, T. Dusatko1, J. Fortin1, F. Farzbod1, A.N. Sinclair1; M.D.C. Moles2 and F. Honarvar3
1University of Toronto, Toronto, Canada;2 R-D Tech, Mississauga, Canada;3KNT University of Technology,
Tehran, Iran.
The synthetic aperture focusing technique (SAFT) is a well-established method for improving the resolution
of an ultrasonic image. Two major shortcomings are the inherent assumptions that the ultrasonic transducer
acts as a point source, and that the pulse reflected from a flaw has a spectral content that is independent of
the flaw’s location relative to the transducer.
In this project, we address the first issue by application of Huygen’s principle and ray theory to calculate the
field from a wideband source of finite diameter. This enables the SAFT processing scheme to achieve a
lateral resolution that is considerably tighter than the diameter of the transducer Ð a result that would not be
possible with a single A-scan captured with an unfocused transducer.
The second shortcoming of the classical SAFT algorithm is addressed by filtering the flaw echo signals to
compensate for the relatively wide divergence angle of the lower frequency components; compensation is
also included for the spectral shifting due to frequency-dependent attenuation. This allows the image of the
flaw to be reconstructed with less distortion. The signal processing schemes are applied to synthetic flaw
signals, and then to real defects of the type found in girth welds of gas pipelines. When combined with
Wiener filtering, the result is a marked sharpening of both lateral and depth resolution, such that time-of-
flight calculations can be used to obtain accurate measures of defect height.
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