CHECKLES, STEVEN, FUHAO QIN, and ROBIN OWENS, Amerada Hess Corp.
Abstract: Depth Migration of Long Offsets for Sub-Basalt Imaging at the Faroe Islands
Standard seismic
data with far offsets of up to 6000 meters have shown poor data quality
below the basalt associated with the Faroe Islands. Poor penetration of
energy through the basalt and high amplitude multiples
combine to present
a very difficult imaging problem. In an attempt to evaluate the sub-basalt
exploration potential of the area three F.L.A.R.E. (Faroes Large Aperture
Research Experiment) lines were acquired.Two seismic vessels were deployed
with alternating shooting giving effective offsets of up to 30000 meters.
Synthetic seismic shots created by full elastic
modeling
with a best estimate
of the rock properties indicated the possibility that significant energy
from base of basalt and subbasalt layers may be recorded from wide angle
reflections. Depth migration is the only method that properly migrates
these events because of the very large offsets (giving non-hyperbolic moveout)
and the large velocity contrast associated with the basalt. The velocity
model input into prestack depth migration is the key factor in producing
the best image. Prestack depth migrations with standard (0 to 6000 meters)
offset ranges were carried out to derive the initial velocity model used
to migrate the far offset ranges.The far offset images were then analyzed
to update the velocities of the basalt and sub-basalt section. Good images
were obtained with events interpreted as base basalt and sub-basalt reflections.
The velocity models derived from analysis of the data indicate a velocity
inversion, suggestive of a potentially prospective sedimentary section
below the basalt.
AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England