Abstract

Some of the conditions necessary for the use of seismic reflection techniques for subsurface mapping in Precambrian Shield terranes have been determined from field studies carried out near Flin Flon, Manitoba.In areas of unconsolidated overburden, geoflex-type surface energy sources provide sufficient energy. However, in outcrop regions, boreholes have to be drilled to a minimum depth of 1.5 m, preferably in patterns of 2–6 holes. Explosives with a higher detonation velocity than those presently available would be useful.A near-surface, low velocity layer was discovered on top of all examined rock types, and appears to be the result of open fractures in the rock. The thickness of this layer varies from 20–44 m in the rocks studied.A velocity contrast of 783.3 m/s exists between the Amisk volcanic rocks and Missi sedimentary rocks, making reflection mapping possible. Seismic events which were interpreted as reflections were identified, near contacts between these formations in the subsurface. A fault contact between Amisk and Missi rocks has been mapped to a depth of about 1.6 km, and a normal stratigraphic contact between the Amisk and Missi Groups has been mapped to a depth of about 0.25 km.Because of the complex structure in most Precambrian Shield terranes, it is necessary to locate seismic lines carefully with respect to the geological features being studied. In particular, it is best to keep the line within a rock unit that has constant velocity throughout, and to use short lines, so that a limited number of structures are intersected.

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