Imaging Fractures Applying Time Reversal Nonlinear Elastic Wave Spectroscopy

T. J. Ulrich 1, Paul A. Johnson 1, and Robert A. Guyer 2,1
1 - EES Division, Geophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
2 - Department of Physics, University of Massachusetts, Amherst, Massachusetts 07030, USA

We currently are working on developing crack ("damage") imaging methods that combine Nonlinear Non-Destructive Evaluation (NDE) [also termed Nonlinear Elastic Wave Spectroscopy, NEWS] and time reverse acoustics.

The general method is termed Time Reversal Nonlinear Elastic Wave Spectroscopy (TR NEWS). In short, the basis of the method is to excite two waves in a solid. If a crack is present, nonlinear mixing results at the crack, and sidebands (sum and difference frequency waves) are formed. Other scatterers such as layer boundaries, voids etc. produce only linear scattering. The full signal train is detected, and one filters out all but the sidebands.

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Following this, one time-reverses the filtered signal, re-emits it from the detector, and it focuses at the nonlinear source, i.e., the crack. An example of a TR NEWS imaging experiment is shown in the figure above. The crack is shown as a thin white line.

The hot spots are associated with the crack opening and crack tip. A secondary hot spot is believed to be due to geometrical scattering of the sideband wave.

In the case of internal cracks in a 3-D solid one cannot detect at the location of a fracture. Thus a modified method is used whereby the detected signal is filtered and time reversed as in the laboratory, but the wave is back-propagated through a velocity model. In this manner, the crack can be located if the velocity model is good.

Efforts are being made to apply very similar methods for earthquake source localization and to study source complexity.

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