Seismic Interferometry without Equations
Monitoring oilfield reservoirs by downhole seismic arrays typically consists of picking travel-times of direct arrivals and locating the hypocenter of the fracturing rock. The 3-component array typically has no more than a handful of geophones located in one or more wells. To broaden the use of this downhole recording array I propose that it can, in principle, also be used to record 3D VSP data from controlled surface sources. These VSP data with free surface multiples can then be interferometrically transformed into virtual surface seismic reflection data, so that the subsurface reflection coverage is almost the same as a surface seismic experiment. To support this proposal, I present interferometric imaging results from 2D and 3D VSP data. Images from free surface related multiples show that the subsurface reflection illumination is almost as wide as that of a surface seismic experiment and is much larger than that from a conventional VSP image.