Improved Modulated Excitation for Imaging and Parameter Identification Applications

Pulse compression techniques and array focusing are common ways to achieve better signal-to-noise (SNR) levels for reception while maintaining the resolution of a short pulse. These techniques can inadvertently create a near surface blind region due to longer transmit events and are limited by available instantaneous transmit power. Furthermore, the increased complexity at high voltage levels makes implementations more costly. An alternative to these methods is binary modulation of a predetermined signature excitation waveform. Using both computer simulation and physical experiments, this technique is shown to decouple the pulse duration versus resolution compromise even if available power is fixed. The method’s effectiveness allows for the reduction of the outgoing excitation pulse amplitude driving a conventional transducer probe that is normally of the order of hundreds of volts to voltages on the order of 1 volt peak-to-peak. These results are achieved while maintaining the scan resolution and improving SNR by amounts in excess of 30 dB. Near surface anomalies are resolved despite arbitrarily long excitations as transmission and signal recovery occur simultaneously. The recovered system response from the binary modulation technique is able to discriminate against off-path reflection sources that would otherwise appear as in-line irregularities. Reducing off-angle anomalies inherently improves imaging performance in a heterogeneous environment as energy from the off-angle heterogeneities does not effectively contribute to noise in the demodulated scan result. Demodulation into in-phase and quadrature components also provides two linearly independent acoustic parameters yielding the material makeup of anomalies. Verification is performed with conventional, nondestructive testing, ultrasonic transducers.

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