Page 13 - Winter 2008

source waveform(s). Defects can be detected by comparing
the TR reconstruction of the source in the test sample with
the TR reconstruction obtained from an intact sample. TR of
Surface Acoustic Waves (SAWs) has also been demonstrated
in a very wide frequency range from infrasound and high
seismic frequencies, to the high ultrasound55 and phononic
range. TR using SAWs include applications in development
for characterizing thin films and plates with microscale het-
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are also in development, from the field scale to the global scale. For instance, promising approaches for landmine local- ization employing both linear and nonlinear elastic methods
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erogeneity.
Applications of TR to a variety of geophysical problems
The methods generally rely on exciting the landmine from an array located at short dis- tances from the mine, by scanning a detection laser in a raster-like manner, and conducting reciprocal TR at each scan point. The linear methods tend to rely on location by exciting resonances of the mine that give amplified signals compared to the background (the soil), or by acoustic imped- ance differences. At enhanced wave amplitudes, landmines exhibit a nonlinear response due to the mine’s structure, or the interface between the mines and the soil. In the nonlinear methods, the scanning is done to excite the nonlinear response of the mine which leads to localization upon filter-
using TR are being tested.
ing for harmonics for instance. tions will be described below.
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Other geophysical applica-
We turn now to some applications that are in develop- ment at Los Alamos, in collaboration with a number of other institutions. These include applications to NDE as well as studies of the earthquake source (first begun at the Institute of Physics of the Globe [Paris], in collaboration with LOA).
Recently, tests of the feasibility and robustness of apply- ing TRM transducers directly to the surface of a solid speci-
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These studies, illustrate that TR works very well without submerg- ing the test specimen, and in the presence of the fully elastic wavefield. They also confirm the efficiency of TRMs com- posed of a small number of elements. These studies consist of a solid sample with reflecting boundaries, including the case of a simple geometry, which does not lead to chaotic/ergodic ray path dynamics [in an ergodic cavity, a wave originating at any point reaches all other points]. Ergodic ray path dynam- ics have been shown to be the ideal case when only using a single channel TRM because the virtual aperture on a TRM
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men (a typical configuration in NDE) were reported.
Meaning, ergodic cavities are of great value when using a single transducer and recording for a long time because of the large number of reflections one
obtains.
Crack detection and imaging exploiting nonlinear elas-
ticity is a topic of significant interest, and much work in NDE of solid materials has been conducted over the last 10 years or so. The general approach is known as Nonlinear Elastic Wave Spectroscopy (NEWS). NEWS encompasses all nonlinear
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is increased dramatically.
As noted, cracks can be the source of significant elastic nonlinearity, generat- ing wave distortion in the form of harmonics, sum and dif- ference frequencies (intermodulation distortion) in the pres-
methods that employ spectral analysis.
ence of relatively large amplitude elastic waves. describe two methods below (Note much of this work was developed in collaboration with A. Sutin [Stevens Institute of Technology, Davidson Laboratory], R. Guyer [University Nevada Reno and LANL], the group of P. P. Delsanto and M. Scalerandi [Turin Polytechnic Institute], and with K. Van Den Abeele [Catholic University Leuven, Belgium (Kortrijk campus)]).
The first method we describe is called the Time Reversal Elastic Nonlinearity Diagnostic, known as TREND,65,66 and uses TR and NEWS to image surficial or near-surface, non- linear scatterers (normally cracks or near-surface disbond- ing). This is accomplished by conducting the TR process repeatedly in a raster-type scan of a region of interest. At each scan position, harmonics and/or sidebands are extracted from the focal signal by Fourier analysis. In this manner, a map of the nonlinear response is created. The method pro- vides the means for isolating mechanical damage features by scanning, and directly probing crack complexity at very high resolution. Typically TREND is performed using a laser vibrometer or other non-contact detector to facilitate the ease and speed of measuring many points on the surface of an object and applying reciprocal TR. The limitation of this method is that one can only measure at locations where a detector can be placed. TREND is also time intensive as the entire forward-propagation/time-reversal/back-propagation procedure must be performed at each scan point. The result- ing signals require little processing to be analyzed. Figure 9a
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We
Time Reversal 11