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12 Elephants Dancing In Unison

Seismic surveys are an important component of LITHOPROBE research. These experiments involve generating seismic energy by compressing and/or shearing the earth at one point, and receiving the energy at a second point. The path the waves travel through the earth is dependent upon rock properties. Sophisticated data processing techniques allow geologic structures to be imaged and rock properties to be constrained. When this information is combined with other geological, geophysical and geochemical data, interpretations of crustal structure and evolution can be made.

LITHOPROBE seismic reflection surveys use vibroseis trucks as the seismic energy source. When a truck reaches a specified source point, an on-board hydraulic system depresses a central plunger against the ground raising the 20,000 kg truck into the air. Then, the hydraulic system vibrates the mass of the truck over a precisely controlled frequency band (e.g., 10 to 56 Hz). Typically, four vibroseis trucks are operated synchronously and the resulting data permits mapping crustal structures to depths up to 50 km. The energy produced by these trucks is recorded by an array of 240 (or more) geophones. Click for more information on the seismic reflection method.

In the summer of 1995 as part of the Alberta Basement Transect, an experiment was carried out to image not only crustal structure, but structure in the upper mantle. The upper mantle is normally a very poorly reflective region, suggesting that the mantle rocks exhibit gradual contrasts in properties. However, conventional crustal reflection surveys may not generate enough energy to clearly detect subtle structures at upper mantle depths. Therefore, to explore the upper mantle using standard techniques, twelve 20,000 kg vibrators were operated simultaneously (see picture above).

Stay tuned to find out what was discovered beneath Southern Alberta.

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