Using Pseudoorthogonal Signals to Reduce Noise Level in Ultrasound Testing of Highly Absorptive Materials

Abstract

To increase the signal-to-noise ratio when testing materials with a high level of absorption, it has been proposed to use complex signals, formed on the basis of code sets employed in Code Division Multiple Access (CDMA) technology, as probing signals. The code sequences in the code sets have a low level of cross-correlation function and a delta-like autocorrelation function. Echo signals are recorded by the double-scanning method, in which the elements of an antenna array sequentially emit unique probing signals phase-shift keyed according to the Kasami code, with the echo signals being recorded in each radiation cycle concurrently by all the elements in the antenna array. Since the shape of the “side lobes” of the compressed echo signals is different for each shot, reconstructing the image of reflectors using digital focusing of the antenna (DFA) reduces the level of the “side lobes”. As a result, when different code sequences of length 15 are used for each antenna-array element, this level may prove to be lower than when one sequence of length 63 is used for all the elements. Provided that a unique code set is used in each position, reconstructing the DFA image when scanning with an antenna array should further reduce the noise level and the level of the “side lobes”. The effectiveness of the proposed approach was demonstrated in a model experiment on reconstructing the image of side-drilled holes in a Plexiglas™ SO-1 calibration block, when using Kasami codes of lengths 15 and 63.