VSP Reverse Time Migration Angle Gather Domain Imaging Method And Application

Vertical seismic profiling (VSP) is borehole seismic technology which thereceivers are arranged in the borehole and sources are located on the surface. Migrationis an important part of VSP data processing flow to derive the detail of undergroundstructure and to help analysis lithology. Reverse-time migration (RTM) is able to imagefor many kinds of wavefield, and born with amplitude preserved property for seismicwavefield. Compared with the surface seismic RTM, there are some imagingcharacteristics and noise for VSP RTM. Imaging noise of VSP RTM affects theresolution of migration section seriously. To suppress imaging noise and get bettermigration result, some studies are conducted in this thesis：1、According to the diffraction model test and its analysis, imaging noise appearon the other side of the borehole, whose position correspond to the location of theimaging point. This kind of noise weaken with the increase of underground coverage.2、The wavefield of the graben model and the multi-layer horizontal model areseparated into direct waves, up-going waves and down-going waves. The results provethat the reverse time migration imaging for direct waves and down-going wavesproduces inverse scattering noise on profiles and angle domain gather. In order to getimaging profiles of higher SNR, data of up-going waves are used for imaging.3、The low frequency noise of reverse-time migration affects imaging resolution.Local wavefield decomposition method is employed to separate wavefield, and cross-correlation imaging is applied to wavefield of different direction. According to theimaging results of several model tests and field data, low frequency noise is suppressedsuccessfully in imaging profile.4、Angle domain common imaging point gathers (ADCIGs) demonstrate thecontribution of reflection waves from all angles and contain rich local information. Inthis paper, the extension imaging condition method is used to calculate ADCIGs. Tiltedmodel is calculated to test its accuracy.5、To suppress the artifact of VSP RTM imaging, I present a method that combineangle domain imaging and reflection angle constraint. The imaging correlation for forward wavefield of sources and backward wavefield of receivers is conducted in theangle gather domain, but the reflected angle is approximated by ray tracing and isapplied to select efficient data. The resolution of imaging is improved and the imagingartifact is suppressed.Finally, two sets of wavefield data, an Offset VSP case and a Walkaway VSP case,are applied to the above method to get imaging profiles. Artifacts are weaken and theevents are displayed clearly, which shows our methods is better process for VSP RTM.