Borehole Seismic Analysis: up and downward continuation, migration velocity updating, and a Bakken shale case history
Du, Yue 1987-
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The Vertical Seismic Profile (VSP) with receivers or sources in the boreholes has been widely used to provide in-situ rock properties and a high-resolution image around the borehole. In this thesis, we investigate and develop three algorithms for use in borehole seismic processing, and specifically for a Bakken shale case history. First, a walk-away VSP is transformed to simulate a set of surface seismic shot records by convolving and summing together the appropriate sets of VSP records. Some artifacts can be reduced by appropriate weighting before stacking. The quality of the result depends upon the borehole receiver and VSP shot coverage. Second, downward continuing surface shots to each level of the borehole receivers is outlined. After obtaining each of the redatumed common receiver gathers, a joint Normal Moveout (NMO) velocity analysis is performed. This method gives a quick mearsurement of interval velocites from walk-away or 3D VSP data. It can be used to help build an initial velocity model below the borehole receivers. Third, we develop a method by assessing residual moveout after migration for VSP data to derive and update the velocity model below the borehole receivers. We derive a quantitative relationship which provides the residual moveout of events after migration. Using this relationship, we can invert velocities from unstacked migrated data. This method uses full prestack depth migration to handle the overburden velocity and then inverts for “local” velocities around borehole efficiently. Finally, a study in the Bakken shale, North Dakota using VSP data is undertaken. A zero-offset VSP and a walk-away line VSP are processed to image the Bakken formation. The corridor stack of the zero-offset VSP data ties the well logs convincingly and assists in building a velocity model. A North-South walk-away line VSP provides more detailed information below the receivers. Prestack Kirchhoff migration image indicates the Bakken formation is at a depth of 9950 ft (3033 m) with about 150 ft (46 m) thickness. Initial migration velocities flattened common image gathers that can be nominally improved using RMO analysis. The techniques in this thesis assist in several aspects for VSP processing and analysis.