Upper Mantle Structure in Western Canada

• Author / Creator

  Hemmings, Charles David

• Refraction arrivals from the upper mantle were recorded on a 3500 km. profile stretching from Lake Superior to Alaska. Recordings from the middle section of the profile (1000 - 2000 km.) were digitized, and various frequency and velocity filters were used to enhance this data. One version of the velocity filter, a non-linear operator, obtained the highest degree of data enhancement, but at the expense of signal distortion. The analysis presented in this paper rests primarily on these enhanced records and secondarily on the analog records. The velocity structure was obtained by matching a theoretical travel time curve calculated from proposed structures to the observed travel time curve. The most successful model thus produced was model U of A #3. This model contained four striking features; two velocity reversals, one at a depth of 42 to 70 km. and the other at 125 to 170 km., both of which were followed by rapid velocity increases, and two rapid velocity increases at depths of 455 and 660 km. These velocity reversals were necessary to reproduce the shadow zone at a distance of 900 km. and its accompanying cusp at 300 km., and the shadow zone at 1500 km. with its cusp at 800 km. The rapid velocity increases were used to produce two cusp pairs, the first at 3200 and 1700 km., and the second at 3300 and 2200 km. The additional velocity reversals at depths of 198 to 230 km. and 250 to 325 km. with a small velocity increase between them were needed to produce the shadow zones at 2000 and 2400 km. respectively. These reversals were too small to produce any detectable cusping. To complete the details of this structure, and to obtain the velocity behaviour in the transition zones, the theoretical amplitudes caldulated from the model must be made to match the observed amplitudes. This amplitude study is not presented in this paper. However, the computer program listed in the appendix calculates both amplitudes and travel times, and is presently being used to complete this study.

• Subjects / Keywords

  • Upper mantle
  • Refraction arrivals
  • Velocity
  • Velocity reversals

• Graduation date

  Fall 1969

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-tt4n-tg70

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.