Seismic interferometry
[1] Seismic interferometry (SI) utilizes the crosscorrelation of signal pairs to reconstruct the impulse response of a given media.
Papers by Keiiti Aki (1957),[2] Géza Kunetz, and Jon Claerbout (1968)[3] helped develop the technique for seismic applications and provided the framework upon which modern theory is based.
The crosscorrelation of passive noise measured at a free surface reproduces the subsurface response as if it was induced by an impulsive point source, which is, by definition, equal to Green's function.
Seismic interferometry uses this previously–ignored background wavefield to provide new information that can be used to construct models of the subsurface as an inverse problem.
[1][6] The long term average of random ultrasound waves can reconstruct the impulse response between two points on an aluminum block.
In a similar case, it was shown that the expressions for uncorrelated noise sources reduce to a single crosscorrelation of observations at two receivers.
The interferometric impulse response of the subsurface can be reconstructed using only an extended record of background noise, initially only for the surface and direct wave arrivals.
[9] Seismic interferometry can produce a result similar to traditional methods without limitations on the diffusivity of the wavefield or ambient sources.
[1][5] Recent work[11] has mathematically demonstrated applications of crosscorrelation for reconstructing Green's function using wave field reciprocity theorem in a lossless, 3D heterogeneous medium.
Traces are most often extended records of passive background noise, but it is also possible to utilize active sources depending on the objective.
Seismic interferometry essentially exploits the phase difference between adjacent receiver locations to image the subsurface.
[13] Seismic interferometry is fundamentally similar to the optical interferogram produced by the interference of a direct and reflected wave passing through a glass lens where intensity is primarily dependent upon the phase component.
[9] In the simplest case, consider a rotating drill bit at depth radiating energy that is recorded by geophones on the surface.
In one example, passive listening and the crosscorrelation of long noise traces was used to approximate the impulse response for shallow subsurface velocity analysis in Southern California.
[18][19] Seismic interferometry can virtually move a source into a downhole location to better illuminate and capture steeply dipping sediments on the flank of a salt dome.
Seismic interferometry can locate the position of an unknown source and is often utilized in hydrofrac applications to map the extent of induced fractures.
[9] It is possible that interferometric techniques can be applied to timelapse seismic monitoring of subtle changes in reservoir properties in the subsurface.
One of the biggest remaining challenges is extending the theory to account for real world media and noise distributions in the subsurface.
