Seismic Velocity Model Building in from Area of Complex Geology, Southern Alberta, Canada

Isaac, Helen ¹; Lawton, Donald ^1
1 Geoscience, University of Calgary, Calgary, AB, Canada.

We reprocessed in time and depth two seismic lines from the Turtle Mountain area of Southern Alberta, Canada. Turtle Mountain is the site of the Frank Slide, where a large portion of the mountain became detached and caused a debris slide in 1903. One of the contributing factors to the slide is the geology. Steeply dipping, and sometimes overturned, folded and faulted carbonates overlie weaker clastic rocks to create an unstable situation.

Our objective was to improve the seismic imaging of the complex structures that form the mountain. Processing in the time domain was designed to attenuate noise and enhance signal in the data. The poststack and prestack time migrated sections are not adequately focussed and do not image correctly the structure deep in the section, which is expected to be relatively undeformed. Prestack depth migration provided better images. Our most effective velocity models for prestack depth processing were obtained by integrating all sources of geological and velocity information into the interpretation of seismic depth sections which were created through iterative updates of the velocity models. Our choice of velocities for the depth migrations was guided by constant velocity migrated sections, near-surface velocity models from refractions statics analysis, averaged sonic interval velocities and limited migration velocity analysis in regions of coherent reflectivity.

The prestack depth migrated sections are better focussed, making the sections easier to interpret than the time sections, and represent the expected structure of deep, relatively undeformed reflectors more reliably than the sections migrated in the time domain. Although the reprocessing improved the quality of the processed seismic data, we were not able to image the details of the shallow, steeply dipping strata of Turtle Mountain. In particular, the overturned beds are not imaged by the seismic data. The detailed interpretation of Turtle Mountain relies strongly upon the geological model since the seismic data do not image adequately the structural complexities.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009