HR: 0800h
AN: MR11A-0904    [Abstracts]
TI: Linear and Nonlinear Time Reverse Acoustics in Geomaterials
AU: Sutin, A
EM: asutin@stevens-tech.edu
AF: Davidson Laboratory Stevens Institute of Technology Davidson Laboratory Stevens Institute of Technology Davidson Laboratory Stevens Institute of Technology Davidson Laboratory, Stevens Institute of Technology, 711 Hudson Str., Hoboken, NJ 07030 United States AU: * Johnson, P A
EM: paj@lanl.gov
AF: Geophysics, EES-11, Los Alamos National Laboratory of the University iof California, Mail Stop D443, Los Alamos, NM 87545 United States
AU: TenCate, J
EM: tencate@lanl.gov
AF: Geophysics, EES-11, Los Alamos National Laboratory of the University iof California, Mail Stop D443, Los Alamos, NM 87545 United States
AB: Linear and Nonlinear Time Reverse Acoustics in Geomaterials P. A. Johnson, A.Sutin and J. TenCate Time Reversal Acoustics (TRA) is one of the most interesting topics to have emerged in modern acoustics in the last 40 years. Much of the seminal research in this area has been carried out by the group at the Laboratoire Ondes et Acoustique at the University of Paris 7, who have demonstrated the ability and robustness of TRA (using Time Reversal Mirrors) to provide spatial control and focusing of an ultrasonic beam (e.g. Fink, 1999). The ability to obtain highly focused signals with TRA has numerous applications, including lithotripsy, ultrasonic brain surgery, nondestructive evaluation and underwater acoustic communication. Notably, the study of time reversal in solids and in the earth is still relatively new. The problem is fundamentally different from the purely acoustic one due to the excitation and propagation of both compressional (bulk) and shear waves as well as the scattering and potentially high dissipation of the medium. We conducted series of TRA experiments in different solids using direct-coupled transducers on solids in tandem with a large bandwidth laser vibrometer detector. A typical time reversal experiment was carried out using the following steps (Sutin et al. 2004a). Laboratory experiments were conducted in different geomaterials of different shapes and sizes, including Carrera marble, granite and Berea sandstone. We observed that, in spite of potentially huge numbers of wave conversions (e.g., compressional to shear, shear to compressional, compressional/shear to surface waves, etc.) for each reflection at each free surface, time reversal still provides significant spatial and temporal focusing in these different geophysical materials. The typical size of the focal area is approximately equivalent to the shear wavelength and the focal area, but becomes larger with increasing wave attenuation (Sutin et al. 2004a; Delsanto et al., 2003)). The TR-induced focusing of wave energy at a point in space and time is ideal from the perspective of enhancing elastic wave, nonlinear response (for example, higher harmonic generation or wave modulation effects). We call this technique Nonlinear Time Reverse Acoustics (NLTRA) (Sutin et a. 2004b). We investigated the harmonic generation in TRA signals focused above a small crack (2mm) in a glass cube. Large second harmonic amplitudes were observed above the crack. Scanning of the surface by applying the laser vibrometer simultaneous with TRA focusing of the signal to an array of corresponding scanning points provided nonlinear imaging of the surface, showing all cracks in the scanned region. References: Delsanto, P. P., P. A. Johnson, M. Scalerandi, J. A. TenCate, LISA simulations of time-reversed acoustic and elastic wave experiments, J. of Physics D: Applied Physics 35, 3145-3152, (2003). M. Fink, Time Reversed Acoustics, Scientific American, 91-97 (1999). Sutin, A., J. TenCate and P. A. Johnson, Single-channel time reversal in elastic solids, J. Acoust. Soc. Am., in press (2004a). Sutin, A., P. Johnson, and J. TenCate, Development of nonlinear time reverse acoustics (NLTRA) method for crack detection in solids, Proceedings of the World Congress on Acoustics (Paris) [http://www.sfa.asso.fr/wcu2003/] 121-124 (2003b).
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 5102 Acoustic properties
DE: 5194 Instruments and techniques
DE: 3220 Nonlinear dynamics
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting