CROSS-CUTS Plumbing the depths

Did subduction of one of the lithospheric plates off Vancouver Island end a million years ago? Or does this still continue today? For the answer, you may have to wait a few years. But an answer there will be, thanks to a project conducted by various Canadian government agencies and several universities. As a result, we may be in the midst of solving this and numerous other mysteries concerning the Canadian lithosphere. This first phase of the project known as Lithoprobe should also bring a better understanding of the origins of Vancouver Island. After the west coast study, Lithoprobe will move to five or six other regions across Canada. Eventually, this abundance of geological knowledge will form the basis of 3-dimensional models of the areas studied.

The Lithoprobe project uses 20-tonne trucks equipped with hydraulic jacks. A special plate is lowered from the truck onto the road at sample sites. When vibrated by a compressed air system, the plate produces shock waves in the earth that bounce off discontinuities below the surface and return to geophones placed above ground in a line a few kilometres long. The truck’s equipment and the sensing devices communicate with a central computer, which ensures that the shock waves are produced simultaneously by the plates. Since the speed at which the waves travel depends on the density and elasticity of the rocks through which they pulse, the data produced by seismic surveys can be translated into a 3-dimensional model of the depths of the earth.

The seismic reflection technique is the most common method used in geo physics for shallow subsur face studies. Lithoprobe, a joint effort of Energy, Mines and Resources Canada, the Natural Sciences and Engineering Research Council of Canada, and several universities, will produce a 3-D model of an area 40 km beneath the surface. With a commitment of $25 million over a 5-year period, this is the largest project ever funded by nserc. The work is expected to produce economic spinoffs.