A Study Of Imaging Technique And Application Based On Seismic Interferometry Method

Seismic interferometry method and its application in the field data, greatly enrichedthe understanding and study of seismic wave propagation, it is considered to be a majoradvance in the development of geophysics. The scope of application of seismicinterferometry method is from the one-dimensinal layered acoustic medium to thethree-dimensional elastic medium, arbitrary inhomogeneous media or anisotropic media,and even can be extended to the moving media and attenuation media. The basic idea ofseismic interferometry method is retrieval of useful signal from the chaotic disorderseismic signal, new seismic signal is generated by the interference of recorded seismicsignal, the new seismic signal not only include the properties of original seismic signal, butalso retrieve some important features that the original signal does not have. Seismicinterferometry method can be used in both passive seismic measurements and controlledsource seismic measurements, the source can be certain source or noise source. Activesource seismic interferometry provide a new approach for seismic exploration, it can beused for a variety of data types. Take application of vertical seismic profile (VSP) data asrepresentative, VSP data can be transferred into the other data type by seismicinterferometry method, such as single seismic profile (SWP) data, surface seismic profile(SSP). Passive source seismic interferometry method applied to retrieval of useful signalfrom ambient seismic noise to obtain geological properties of medium, and then infer thecharacteristics of subsurface structure. In this paper, we respectively study and discussionthe application of seismic interferometry method for active source seismic data and passivesource seismic data.First of all, describe the basic principle of seismic interferometry methods, based onreciprocity theorems, underlying assumption of inhomogeneous, lossless medium, acousticand elastic Green's function representations can be derived. The reconstructed Green'sfunction not only include the direct wave between two receivers, but also include primaryreflection and multiple scattering. Retrieved Green's function by seismic interferometry isbased on the assumption that receivers are surrounded by a closed source surface. Undernormao circumstances, the source distribution conditions required by seismicinterferometry can not be met. So in this paper, we investigate that when the circumstancethat theory condition can not be met, the sources distributed in the subsurface, how the properties of source distribution effect on the retrieval of reflected waves. In the practicalapplication of seismic exploration, it always hard to meet the theoretically required for acontinuois source distribution, but during the calculation, in order to avoid aliasing, thesource interval should be set intensively. When the source only distributed in the localsubsurface area, only part of reflected event can be reconstructed in the retrieved gathers,the signal to noise ration is decreased seriously. In general, retrieved reflection informationfrom subsurface transmission response by seismic interferometry method is very effective.Process the retrieved reflection information and then used to migration process, theresulted image can be improve the incomplete reflected waves in the retrieved commonsource gathers and obtain satisfactory results.In the paper, we study on application of seismic interferometry method for activesource seismic data, here mainly target at VSP data. When the conventional seismic dataprocessing methods failed to acquire the satisfactory imaging results of seismic datainclude overburden complexity, we could apply the virtual source method to convert thetraditional vertical seismic profile gathers into virtual single well profile gathers. After that,redatumed geometry of source-receiver arrays is closer to the structure target and belowthe complex overburden so that we can get the better image resolution of the target afterconventional process and avoid the effected by unknown velocities of complex overburden,provide an efficient way to handle this complex seismic data. In the paper, the virtualsource method applied to detect steep structure such as faults present in the subsurface.Though the synthetic data found that the steeper structure the better image results, andautomatically correct the statics. The virtual source method is still work well for elasticdata sets. Moreover, modeling experiment show that virtual source method is still effectivefor the sparse geometry, we could get the good imaging results. Although the signal tonoise ratio decreased as source interval and receiver interval increased, the image result iscorrect.Although the imaging accuracy of the standard VSP data is high, the imaging range isvery limited that is defined by a small triangle, with the triangle apex at the shallowestreceiver. In order expand the imaging range of subsurface structure, one can transformVSP data into virtual SSP data by correlation algorithm of seismic interferometry, and thenthe imaging process used to virtual SSP data. The imaging range of subsurface structurefrom the results has same area as that of the original VSP source distribution along thesurface. In addition, this transform eliminates well statics and the need to know the sourceor receiver position in the well and excitation time of source in the well. In this paper,VSP-SSP correlation transformation effectively transform synthetic VSP data into virtual SSP data, and then migrated by a standard surface seismic data processing to give ainterferometric image of subsurface structure. From the results, compared with standardVSP image, VSP-SSP correlation transformation is a huge increase in the subsurfaceillumination. Test on the effect of geometry on the VSP-SSP correlation transformation bynumerical simulation. When the receivers in the well distributed on local area,reconstructed SSP gathers by VSP-SSP correlation transformation with various degrees ofeffect, part of reflected event can not be reconstructed correctly, the signal to noise rationof gathers decreased. When the VSP-SSP correlation transformation applied to sparsegeometry, the reconstructed SSP gathers still has good quality. When integrate VSP andSSP data that recorded over the same area, one can transform VSP data into virtual SSPdata by VSP-SSP convolution transformation. Resulted virtual SSP data can be comparedwith actual SSP data, and used to determine the lithology of subsurface reflector body.During the transformation process, it is no need to know the location information ofreceivers in the well. We transform VSP data into virtual SSP data by VSP-SSPconvolution transformation, and then the conventional seismic processing used to virtualsurface gathers to obtain the image of subsurface structure. Compared with virtual SSPdata generated by correlation transformation, the signal to noise ratio of virtual SSP datagenerated by convolution transformation is higher, the resolution of deeper reflector isbetter, but the imaging range is limited. Test on the effect of sparse geometry on theVSP-SSP convolution transformation by numerical simulation. When the receivers in thewell distributed on local area, reconstructed SSP gathers by VSP-SSP convolutiontransformation with various degrees of effect, reflected event of shallow reflector can notbe reconstructed correctly, but the signal to noise ration of gathers is still high. When theVSP-SSP correlation transformation applied to sparse geometry, the reconstructed SSPgathers still has good quality. Combine VSP-SSP correlation transformation and VSP-SSPconvolution transformation, the advantages from two transformations combined together.Process the virtual SSP gathers by conventional image processing, the resulted imagingrange is same with it from correlation transformation and result in a huge increase in thesubsurface illumination, at the same time, the imaging accuracy is same with it fromconvolution transformation and deeper subsurface structure can be detected.Passive source seismic data collected from the underground microseismic, generally,recorded ambient noise signal has relatively weak energy and low signal to noise ratio. It ishard to directly process the passive seismic data and use it to study of geological structurein the reservoir. Retrieved new seismic data set from ambient noise by seismicinterferometry, the noise into useful signal. The retrieved surface waves and reflected waves can be used to infer geological properties in the shallow subsurface structure anddeeper subsurface sreucture. This paper study application of seismic interferometrymethod for passive source seismic data by synthetic data and ambient seismic noise datasets. First of all, design forward modeling scheme suit for passive source seismic data,and then generate synthetic passive seismic data. Then we apply the seismic interferometrytechnique to approximately25hours of recordings of ambient seismic noise at the Ketzinexperimental CO2sequestration site, Germany. Common source gathers were generatedfrom the ambient noise for all available receivers along two seismic lines bycross-correlation of noise records. The retrieved response includes surface waves, refractedwaves and reflected waves. Comparison with surface waves and reflected waves fromactive source seismic survey, confirming the validity of the passive data processing.Stacking of common offset gathers increases the signal to noise ratio of retrieved commonsource gathers. A surface wave dispersion curve could be estimated from the retrievedsource gathers. Inversion of the dispersion curve results in an average1D and2D S-waveprofile below the site that is consistent with velocity models from traveltime tomographyof active source data from the area, but has higher resolution for the shallow subsurface. Inaddition, compared with velocity profile from other previous active source data, velocityprofile from surface wave dispersion curves also show similarities. Data processing of theretrieved common source gathers results in a stacked section that is in reasonableagreement with that of an active source section from same survey lines. The stackedsection from passive seismic survey have the potential to provide information on thedeeper subsurface structure, provide a effective technical support for the practicalapplication of passive source seismic exploration.