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.