IMPROVED MIGRATIONS IN LATERALLY VARYING MEDIA, WIND RIVER, WYOMING
, by Heloise Bloxsom Lynn, Laura Gagnon, and Don Seeburger
A COCORP dataset from the south end of the Wind River Mountains, Wyoming, showing the Wind River thrust
juxtaposing Precambrian crystalline rocks and Mesozoic sediments, was the input of a series of migrations,
wherein the velocity model was a function of depth and lateral distance. A well 1.6 km from the line provided
constraints from the V(x,z) model input to the 45 degree w-finite difference program. Too high velocities in
the near surface and in the Mesozoic-Precambrian transition zone produced "smiles" in the migrated sections
(classic overmigration). The "smiles" observed in undermigrated sections were attributed to pullup in z caused
by unusually slow near surface and fault zone velocities. The two best velocity models were used to migrate a
larger, composite dataset of 470 traces (31.4 kim) - 6 seconds. The results were interpreted geologically as
follows: (1) The thickness of the Wind River thrust zone varies with depth (0-~3 km depth: 0.25-1.0 km thick;
3-7 km: ~1.5 km thick; 7-10 km: imbricate thrusting is the dominant feature; +10 km: ~2 km thick). (2) At
least three subsidiary (imbricate) thrustus are seen between 4 and 9 km depth in the sedimentary sequence.
(3) The dip angle of the thrust fault changes from 30 degrees to 0 degrees where it passes upward from
Precambrian crystalline rock to sedimentary rock. The difference in rock properties (strength, anisotropy)
between a layered sedimentary sequence and Precambrian crystalline basement may explain the change in thrust
angle.
STANFORD