Imaging with ambient noise

© 2010 American Institute of Physics, S-0031-9228-1009-030-7 Recent developments in seismology, ultrasonics, and underwater acoustics have led to a radical change in the way scientists think about ambient noise—the diffuse waves generated by pressure fluctuations in the atmosphere, the scattering of water waves in the ocean, and any number of other sources that pervade our world. Because diffuse waves consist of the superposition of waves propagating in all directions, they appear to be chaotic and random. That appearance notwithstanding, diffuse waves carry information about the medium through which they propagate. During the past decade, experimental and theoretical work has shown that such waves can produce an elastic response to a point source in the medium. That response is valuable because it can be used to determine the properties of the medium—for instance, using waves reflected from the medium’s discontinuities to provide the location and nature of those discontinuities or waves transmitted through the medium to infer their acoustic or seismic velocity. Perhaps surprisingly, the elastic response can be determined from recorded diffuse waves through a simple processing step: cross-correlation, a statistical measure of the waveforms’ similarities at different points in space as a function of the time lag applied to one of them. More specifically, if one were to measure a diffuse wave field that propagates through two arbitrary points in space, the cross-correlation of the two noise registrations would give the same response of the medium that would be measured if there were a source at one of the two points and a receiver at the other. Thus, by just listening passively to ambient noise and applying a simple dataprocessing operation, Imaging with ambient noise