WAVELET-BASED PERMEABILITY COMPENSATION TECHNIQUE FOR CHARACTERIZING MAGNETIC-FLUX LEAKAGE IMAGES

The magnetic flux leakage method, used for nondestructive evaluation o f ferromagnetic objects, generates greyscale images that are represent ative of the integrity of the specimen. Defective areas typically appe ar as bright regions in the image. Unfortunately, the task of defect c haracterization becomes more challenging due to the effects of variati ons in the test parameters associated with the experiment. One such te st parameter is the permeability of the test object. Conventional inva riant pattern recognition algorithms are not capable of performing inv ariance transformations to compensate for such variations. This pa per describes a novel technique that uses wavelet basis functions to prov ide selective invariant features and eliminate image intensity variati ons from undesirable changes in operational variables. The performance of the invariance transformation is demonstrated by applying the meth od to magnetic flux leakage images obtained using a finite element sim ulation of in-line inspection of natural gas transmission pipelines. ( C) 1997 Elsevier Science Ltd.