HR: 0800h
AN: S31A-0287    [Abstracts]
TI: Investigating unknowns in seismic interferometry with noncontacting ultrasonics
AU: * van Wijk, K
EM: kasper@acoustics.mines.edu
AF: Physical Acoustics Laboratory and Department of Geophysics, 1500 Illinois st, Golden, CO 80401 United States
AB: Cross-correlating an arbitrary input of a linear system with the resulting output is the basic idea behind the matched filter. As far back as 1950 it was known that using random noise as an input, the cross-correlation would synthesize the impulse response of a linear system. Matched filtering later found its way into geophysics, most prominently in compressing vibroseis sweeps. Now, derivations showing the emergence of the Green function between receivers have more recently (re)surfaced. This result can intuitively be related to matched filtering, considering that each receiver is a secondary Huygens source. Arguments based on stationary phase, time-reversal, and normal modes have led to the same conclusion that the impulse response between two receivers can be obtained by the derivative of the cross-correlation of the recorded wave fields in the presence of equipartition of energy. Equipartition can be achieved in two ways. First, an impulsive source can scatter in a heterogeneous medium enough times so that an equal amount of energy is propagating in all directions. Alternatively, one can imagine a scenario where a (random) distribution of sources directly creates equipartitioned energy. The late-time equipartitioned signal in the scattering case and a distribution of random sources is conceptually the same. However, it is not clear that equipartition is likely in the Earth in general: scattering in the Earth can be strong, but so can absorption, which attenuates the (higher frequency part of the ) signal. On the other hand, passive sources (ocean waves, microseismic activity) might be limit in space and time as well. Nevertheless, recent imaging results in global and exploration scale seismic interferometry point out that equipartition is not necessary to retrieve vital subsets of the Green function between receivers. Especially, results in the literature for retrieving the surface-wave response between receivers are spectacular, while body-wave reflections between receivers have not been observed. Here we present computer-controlled and noncontacting laser ultrasonics that address outstanding issues in seismic interferometry regarding the location, amount, correlation and spacing of sources.
DE: 0935 Seismic methods (3025, 7294)
DE: 7205 Continental crust (1219)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005