Wave
Equation Prestack Depth Migration
for Deep Gulf Coast Prospecting
Morgan Brown, Joe Higginbotham, and Cosmin Macesanu
Wave
Imaging Technology Inc., 11777 Katy Frwy., Ste. 156, Houston, Texas 77079
ABSTRACT
The shallow section in the Gulf Coast normally exhibits a simple, depth-variable velocity
profile, which implies that prestack
time migration (PSTM) methods generally suffice to produce a drillable image. In recent years, however, an exciting frontier has opened in the deep
section, with high geopressure preserving reservoir prospectivity. Geopressure is a double-edged sword. In addition to being a drilling hazard,
geopressured sediments often exhibit seismic velocities below the compaction trend. Complexity in the
velocity
field causes focusing effects in
the propagated
wave
fields, and challenges all the assumptions behind PSTM. Even if a PSTM image contains a seismic reflection (not a given), the
image time may no longer accurately represent the geology in depth.
For these reasons, prestack depth migration (PSDM) methods have been employed successfully for some time to image the deep
section when pressure is an issue. Provided that the velocity
complexity can be encoded into a migration
velocity
model, PSDM can provide a more
accurate picture of the geologic structure, in situations like fault shadow.
Typically, Kirchhoff PSDM algorithms have been used in the Gulf Coast. When the velocity
complexity is severe, such
as under Gulf of Mexico salt, it is generally agreed that
Wave
Equation PSDM (WEM) algorithms produce superior imaging results. For deep Gulf Coast
prospecting, the uplift from WEM over Kirchhoff might be more subtle, but the main difference is likely to be in the amplitudes. WEM algorithms can
more naturally handle the amplitude variations due to
velocity
focusing effects. Because of this, there is a good reason to use WEM for amplitude-versus-angle
(AVA) analysis, but until recently, it was not known how to efficiently decompose WEM images into angle “gathers.”
We show how a quality shot record WEM algorithm, combined with a recently developed efficient angle decomposition scheme, can produce compelling results on deep Gulf Coast imaging examples.
Brown, M., J. Higginbotham, and C. Macesanu, 2009, Wave
equation prestack depth migration for deep Gulf Coast prospecting: Gulf
Coast Association of Geological Societies Transactions, v. 59, p. 125.
AAPG Search and Discover Article #90093 © 2009 GCAGS 59th Annual Meeting, Shreveport, Louisiana