Coda wave interferometry (CWI) is a sensitive ultrasound method for the detection of weak and local changes in complex inhomogeneous media. In a nonlinear modification of the method discussed here, a high-frequency probe coda is compared to its replica obtained in the presence of low-frequency pumping. If, after the filtering-out of low frequencies, the coda signals are different, this is attributed to nonlinear pump-probe interaction induced by contact acoustical nonlinearity in the damaged zone. Actually, the CWI methods are used for global inspection of complex media, such as for example, concrete structures. In this work, a step forward is made; it consists in combining the CWI with the time-reversal (TR) technique in order to allow one to focus the pump wave on a selected area in the structure and to detect and localize a flaw. Time-reverse pump is possible only in pulsed mode due to the spatio-temporal wave compression. By this reason, the particularities of coda wave mixing in conventionally used continuous and pulsed pump mode are considered. It has been experimentally observed that an aftereffect of a pulsed pump provides a nonlinear interaction between pump and probe waves of a sufficient overall level for defect detection with TR. Finally, it was shown that a TR focusing even with the minimal available quality i.e., with only one transducer produces a sufficient contrast allowing to distinguish intact and damaged zones with nonlinear CWI.

中文翻译：

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结合时间反演的非线性尾波干涉法局部损伤检测

尾波干涉测量法 (CWI) 是一种灵敏的超声方法，用于检测复杂非均匀介质中的微弱和局部变化。在此处讨论的方法的非线性修改中，将高频探测尾音与其在低频泵浦存在下获得的复制品进行比较。如果滤除低频后尾声信号不同，这归因于损坏区域中接触声学非线性引起的非线性泵浦-探针相互作用。实际上，CWI 方法用于复杂介质的全局检查，例如混凝土结构。在这项工作中，向前迈进了一步；它包括将 CWI 与时间反转 (TR) 技术相结合，以便将泵浦波聚焦在结构中的选定区域并检测和定位缺陷。由于时空波压缩，时间反向泵浦仅在脉冲模式下是可能的。由于这个原因，考虑了传统使用的连续和脉冲泵模式中尾波混合的特殊性。已经通过实验观察到，脉冲泵的后效应提供了泵波和探测波之间的非线性相互作用，其整体水平足以用 TR 检测缺陷。最后，结果表明，即使在可用质量最低的情况下，即仅使用一个换能器的 TR 聚焦也能产生足够的对比度，从而可以区分具有非线性 CWI 的完整和损坏区域。已经通过实验观察到，脉冲泵的后效应提供了泵波和探测波之间的非线性相互作用，其整体水平足以用 TR 检测缺陷。最后，结果表明，即使在可用质量最低的情况下，即只有一个换能器的 TR 聚焦也能产生足够的对比度，从而可以区分具有非线性 CWI 的完整和损坏区域。已经通过实验观察到，脉冲泵的后效应提供了泵波和探测波之间的非线性相互作用，其整体水平足以用 TR 检测缺陷。最后，结果表明，即使在可用质量最低的情况下，即仅使用一个换能器的 TR 聚焦也能产生足够的对比度，从而可以区分具有非线性 CWI 的完整和损坏区域。