An adaptive technique using measured Green´s functions for extending spatial coherence in aberrating materials

Many common engineering materials can aberrate high frequency acoustic waves. The source of the aberrations is spatial velocity variations resulting from the material microstructure. Aberrations can degrade acoustic measurements by distorting the acoustic wavefront and causing acoustic speckle. A new technique is demonstrated which optimises the spatial distribution of the generation of the ultrasound to compensate for acoustic aberrations. The technique uses a Green´s function for the material which is experimentally determined. In aberrating materials the Green´s function is a function of source and observation position, G(R,S). The spatial coherence is optimised in the required measurement region to suppress acoustic speckle. The technique is used to improve the accuracy of velocity measurements in steels with high frequency SAWs. The technique was performed using the adaptive optical scanning acoustic microscope (AOSAM) at the University of Nottingham, UK.