Title page for ETD etd-12042000-222017


Type of Document Master's Thesis
Author Carney, Brooke J.
Author's Email Address bcarney@vt.edu
URN etd-12042000-222017
Title Building Velocity Models for Steep-Dip Prestack Depth Migration through First Arrival Traveltime Tomography
Degree Master of Science
Department Geological Sciences
Advisory Committee
Advisor Name Title
Hole, John A. Committee Chair
Coruh, Cahit Committee Member
Imhof, Matthias G. Committee Member
Keywords
  • Steep-Dip Imaging
  • Prestack Depth Migration
  • Velocity Model Building
  • Refraction Tomography
Date of Defense 2000-11-20
Availability unrestricted
Abstract
Although the petroleum industry has imaged reflections from the sides of salt domes,

steeply dipping structures have not been imaged as reflectors outside of sedimentary

basins; to do so requires appropriate data acquisition, prestack depth migration, and an

excellent seismic velocity model. Poststack time migrated seismic images, normal

moveout velocity analysis, well logs, and other geologic information are used to build the

velocity model. In regions of interest outside of sedimentary basins, such as major strike-slip

faults, seismic reflectivity is often sparse and little is known of detailed subsurface

geology. Alternate methods of velocity model construction must be used. First arrival

(refraction and turning ray) traveltime tomography is proposed to construct the

preliminary velocity model for steep-dip prestack depth migration in settings with little a

priori subsurface information. A densely spaced synthetic seismic data set with long-offset

recording, modeled after a real survey across the San Andreas Fault, was

constructed using a finite-difference algorithm. First arrival traveltimes were picked

from the data and a velocity model was constructed using tomography. The velocity

model was used to perform a Kirchhoff prestack depth migration of the synthetic shot

gathers. The subsurface structure was sufficiently reconstructed that the velocity model

could be refined through migration velocity analysis. A series of tomography tests was

used to determine the spatial resolution limits of the velocity model. Isolated erroneous

anomalies with sizes near the resolution limits were added to the velocity model derived

from tomography and used as input for migration. This pessimistic test provided an

adequate image and identifiable arrivals in migrated common image gathers, allowing the

velocity model to be improved through migration moveout analysis. Data acquisition requirements for tomography include long recording offsets and times, larger sources, and dense spacings, very similar to the requirements for steep-dip reflection imaging.

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