Velocity Inversion Using Geological Constraints
Jos Van Trier
Reflection seismics can be used to determine the geology of the subsurface. An image of the subsurface is obtained by migration of the seismic data. The migration needs a velocity model, which is normally determined from (normal moveout) NMO-velocity analysis.
However, in areas of complex geology NMO-velocity analysis fails, and migration-velocity analysis (an iterative migration process) is necessary. The result of prestack-migration velocity analysis is usually a smooth migration velocity model (i.e., the exact structure and interval velocities of the subsurface are not known).
For an accurate interpretation of the geology, it is necessary to find the structural velocity model. The structure boundaries can be determined from the migrated image of the subsurface, using geologic constraints, such as non-crossing interfaces, known dip limits, well-log information, etc.
To determine the velocities inside the structures, geological information, such as behavior of velocities inside the structures (known from well logs or common geological sense), is used to constrain the velocity inversion. Two approaches to the velocity inversion are investigated: (1) an extension of migration-velocity analysis and (2) a method that uses amplitude-vs.-offset information in the prestack migrated data.
The smooth velocity model is converted to a structural model using the boundaries in the migrated image, and the velocity model is updated with an iterative method that minimizes the curvature in the CDP-gather after migration. The method flattens events in the gather before stack, thus improving the stackpower in the stacked image. Events are defined along lines of equal instantaneous phase. So, the method incorporates traveltime and phase information in the inversion.
Amplitude information is considered in the second approach. The amplitude behavior as a function of offset of reflections in prestack migrated CDP-gathers contains information about the velocity contrast at the reflector. This information can be incorporated in the velocity inversion.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.