The Study Of Virtual Source Imaging Method Based On Seismic Intefrerometry

Seismic interferometry technique is a new and popular seismic data processingmethod in recent years, which is based on the reciprocity equations of convolutionand correlation types. Its basic principle is to get response between two receivers bycorrelating the wavefields recorded by them and stacking over all physical sources, asif there is a real source buried at the receiver location. It improves signal to noise ratioand resolution of seismic records by reconstructing new seismic records with theoriginal seismic records. The virtual source method is an important application ofseismic interferometry.In conventional seismic exploration, complex overburden makes us incapable ofimaging the deeper underground structure effectively. Large and rapid velocityvariations in all spatial directions lead to strong scattering that severely distorts anddisperses wave-fronts propagating both downward to the reflectors and returningup-ward. In these circumstances, conventional imaging techniques based ongeometrical ray theory or other approximations might not adequately describe thewave propagation and thus might produce poor results. Wave-equation imaging couldgive better results, pro-vided that a detailed velocity model of the overburden hasbeen constructed. In fact, obtaining an accurate velocity model remains a problem forall imaging approaches. The virtual source method provides us with a promisingsolution, which can image below a complex overburden without the knowledge ofoverburden velocities and near-surface changes.When people found it impossible to achieve a major breakthrough from existingmature traditional surface seismic imaging, some people placed detectors overlyingbelow the most complex part. Shooting on the surface, then use time reversaltechnique to eliminate all the near-surface propagation effects without knowing thevelocity model information. Thus each detector is converted into a virtual source.Underground collection system allows us to completely eliminate the establishment ofnear-surface velocity model, which is a weak part in traditional surface imaging. Thevirtual source method helps us redatum the real sources into the horizontal well andbrings about a new seismic record, thus subtly avoid the distortion effects of the complex overburden, which is the essence of the virtual source method. What’s more,sources become closer to target body after using the virtual source method, whichattributes to better imaging resolution.In this paper, we introduce the principle of seismic interferometry techniques indetail. To start with, we describe the relationship between traveltime shift andcross-correlation, and prove their consistency using mathematical methods. Second,we derive the reciprocal equation of correlation type using the Green’s function,which is the mathematical basis of seismic interferometry techniques. Finally, wesimplify the reciprocal equation of correlation type using the far-field approximationand clarify the physical meaning of this equation using stationary phase method.We study a special case of the seismic interferometry, the virtual source method,based on the reciprocal equation of correlation type. We introduce the theory of thevirtual source method and derive the reciprocal equation of correlation type for thevirtual source method, based on which we simulate the forward process undercomplex overburden with sources on the surface and receivers in the horizontal well.Then we reset the real sources into the horizontal well using the virtual source methodand get a new seismic record as if there are real sources buried in the horizontal well.Finally we image the underground structure using Kirchhoff migration and verify thefeasibility and advantages of the virtual source method.