Abstract: Pitfalls in Velocity
Analysis
Using
Common-Offset Time Migration
Y. C. Kim, J. R. Krebs
Integral based common-offset time migration is a robust and cost-effective tool for migration velocity
analysis
. In practice this method is not applied iteratively, and travel times are often computed without properly accounting for ray bending in a layered earth. For dipping reflectors these practices can lead to erroneous
velocity
estimates.
The velocity
of an event measured by common-offset time migration
velocity
analysis
is fairly insensitive to the initial migration
velocity
even for dipping reflectors. However, the time at which that event occurs is very sensitive to the initial migration
velocity
. This is because errors in the initial
velocity
result in under or over migration of the event. For example, if the initial
velocity
is too slow, the data will be under migrated, and the
velocity
event from a dipping reflector would appear at approximately correct
velocity
but at a time that is too early. When velocities increase with depth, this error would cause us to pick a
velocity
function that is faster than the correct function. However, this new
velocity
function will generally be closer to the correct
velocity
than
the initial migration
velocity
. Convergence to the correct
velocity
usually occurs after a few iterations.
It is important to account for ray bending when computing travel times, even for time migration. Coincident reflections having different dips focuses at different velocities if ray bending is ignored. Thus, we must choose between the velocity
that optimizes one dip or the other but not both. By properly accounting for ray bending, all dips are focused at the same
velocity
, producing significant improvements in the final image.
AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994