Improvement of time reversal mirror in detection of small cracks and metallurgical defects in sample with high ultrasonic speckle noise level

Ultrasonic Time Reversal Mirror (TRM) is an original solution to detect small cracks in solids of any geometrical shape. This technique is self adaptative. It achieves in real-time, without any signal processing, a natural focusing process matched to the defect shape and to the geometry of the liquid-solid interface. In highly scattering media, the detection of small defect is usually difficult due to the speckle noise. It is shown that TRM process allows a new approach of speckle noise reduction: TRM behaves differently if the signals come from an echographic speckle noise or from a defect. If the speckle noise originates from a thin microstructure whose scale is less than the wavelength used, the TRM process cannot refocus on the speckle noise sources due to the loss of information during the propagation. Using this property of TRM process we developed an original signal processing technique that allows to distinguish, during the inspection, a high level speckle noise zone from a defect zone