Seismic Interferometric Migration Of VSP Data

Analogous to optical interferometry, seismic interferometry estimates the detailedproperties of the Earth by analyzing the interference patterns of seismic waves.These patternsare constructed by correlating and summing pairs of seismic traces with one another torobustly image the Earth’s elastic properties.Seismic interferometry retrieves the response of pairs of station by cross-correlation. It isbased on the Clearbout theory of one dimensional media, and expanded to three dimensionsand arbitrary medium. It has been fully derived by representation of Green theory, and it canbe applied in all medium without restrict. A major benefit of seismic interferometry is that thesource-receiver array, after interferometric redatuming, becomes closer to the imaging target.A key problem with seismic interferometry is that the reciprocity equations of the correlationtype assume a wide aperture of sources and receivers that may not be realizable in a practicalfield experimentInterferometric migration (IM) is a recently developed method that is proved to be apowerful imaging tool for VSP data. Synthetic and field data examples demonstrate that IM isrobust to velocity estimation errors, and can sometimes produce reflectivity images muchmore accurate than standard migration methods. A wave-equation interferometric migration(WEIM) method is developed for efficiently migrating multiples in VSP data. With WEIM,for each receiver of the VSP data, only one receiver of travel times and one wavefieldextrapolation need to be calculated for the migration of free-surface multiples. However, thelow fold image from the migration of a handful of receiver gathers can lead to troublesomeacquisition footprints and low S/N ratios.Wave-equation interferometric migration (WEIM) is proposed for imagingsteeplydipping saltflanks with VSP data. The major advantage of using WEIM for VSPsaltflank imaging is that only a local velocity model around the target area is needed. Thismeans that the migration is not only efficient but also accurate in practice because there is noimage distortion from erroneous velocity estimation of the overburden. However, a remainingproblem is the inability to isolate target reflections in the VSP data, which can result in many artifacts.