Application of the time reversal technique to the focusing of long-range ultrasound in pipelines

Abstract

The novel time reversal method of focusing has been applied to long-range ultrasonic propagation in pipelines. Some preliminary results on a 170 mm-diameter pipe of 7.1 mm wall thickness with a 9% CSA simulated crack defect are presented which show that significant increases in the signal-to-noise ratio of defect echoes can be obtained with this approach. The results equated to a signal-to-noise ratio of 2 (6 dB), which gives a 95% POD (probability of detection) according to the normal probability integral, for a 1% CSA crack. Using the same instrumentation without focusing, the crack size must be increased to 2% CSA to achieve the same POD. The results were also slightly better than those obtained using phased array focusing with a four-element array encircling the pipe. These preliminary results display promise of further improvement. The time reversal principle is described in simple terms both by a particle mechanics analogy and using geometrical optics, ie ray theory.