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Abstract: Pitfalls in Velocity Analysis Using Previous HitCommonNext Hit-Previous HitOffsetNext Hit Time Previous HitMigrationNext Hit

Y. C. Kim, J. R. Krebs

Integral based Previous HitcommonNext Hit-Previous HitoffsetNext Hit time Previous HitmigrationNext Hit is a robust and cost-effective tool for Previous HitmigrationNext Hit 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 Previous HitcommonNext Hit-Previous HitoffsetNext Hit time Previous HitmigrationNext Hit velocity analysis is fairly insensitive to the initial Previous HitmigrationNext Hit velocity even for dipping reflectors. However, the time at which that event occurs is very sensitive to the initial Previous HitmigrationNext Hit velocity. This is because errors in the initial velocity result in under or over Previous HitmigrationNext Hit of the event. For example, if the initial velocity is too slow, the Previous HitdataNext Hit 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 Previous HitmigrationNext Hit 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 Previous HitmigrationTop. 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