Upper mantle structure deduced from seismic records acquired during Project Edzoe in Southern Saskatchewan and Western Manitoba between distances of about 790 kilometers and 1285 kilometers

Abstract

In August 1969, the Seismology Division of the Dominion Observatory detonated a series of chemical explosions in Greenbush Lake, British Colombia; the project is known as "Project Edzoe". A total of twenty explosions were attempted in 180 feet of water. The seismic field crew from the Department of Earth Sciences, University of Manitoba, obtained eight seismic records along an east-west profile in southern Saskatchewan and Manitoba; recording distances were in the range 790 to 1285 kilometers. Signal frequencies on the records were less than 7 Hz; noise frequencies were generally above 7 Hz. Analog playbacks increased the signal to noise ratio by about 68 percent; digital filters offered no improvement over analog playbacks. An upper mantle velocity structure consisting of a linear velocity-depth gradient, below the base of the crust, accounts for first arrival times. However, uncertainty of crustal structure beneath the shot point and recording sites produces uncertainty in the velocity at the base of the crust and the velocity gradient immediately below it. A second arrival, following the first within about one second, can be explained by a rapid increase in velocity gradient occurring between depths of about 120 and 150 kilometers. Evidence is given for the existence of a very low gradient following the rapid increase.