Extracting coherent information from cross correlations of random wave fields

Abstract

The random nature of noise and scattered fields tends to suggest limited utility. Indeed, seismic or acoustic fields from random sources or scatterers are often considered to be incoherent. Nevertheless, it has been shown that it is possible to extract information from random noise fields of a type that would typically be associated with a coherent process. An estimate of the Green’s function (or impulse response) between two points can be obtained from the cross correlation of ambient noise recorded at these two points. Recent theoretical and experimental studies in ultrasonics, underwater acoustics, and seismology have investigated this technique in various environments and frequency ranges. These results provide a means for passive imaging using only the ambient noise field, without the use of active sources. The coherent wave-fronts emerge from a correlation process that accumulates contributions over time from noise sources whose propagation paths pass through both receivers. Results will be presented from experiments using ambient noise cross correlations for the following applications: (1) localization and synchronization of unconnected acoustic receivers; (2) passive tomography of the seismoacoustic environment; and (3) studies of marine and airborne structures embedded in turbulent flow.