Enhancing resolution in ultrasonic imaging using transducer harmonic components

Ultrasonic transmitting transducers produce harmonic components that are normally lost due to the limited bandwidth of the receiving transducer. In pulse-echo mode it is natural since the same transducer is used both as transmitter and receiver. However, when using a separate receiver the higher harmonic components emitted by the transmitter can be used for improving the temporal resolution in ultrasonic imaging. This paper presents an application of this technique to the non-destructive evaluation (NDE) for improving quality of ultrasound B-scans acquired in the immersion inspection of solids. Firstly, the presence of higher components in the signal received from copper specimen is proven in a set-up consisting of two ultrasonic transducers with different center frequencies. A narrowband transmitter has lower center frequency while the receiver has a broader band and a higher center frequency. The received harmonic components, although much weaker than the echo in the fundamental frequency band, can be combined to enhance the signal to noise ratio and the resolution in B-scan images. Secondly, the broadband signals acquired in this set up are processed using algorithms enhancing the temporal resolution. It is shown that the presence of higher harmonics results in an improved temporal resolution. Practical application to NDE of electron beam weld in copper is used to illustrate the proposed technique.