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Seismic Reflection Surveys

Seismic reflection method. Vibroseis sound source with geophone spread.

LITHOPROBE's scientific work is spearheaded by the seismic reflection method because this is the geophysical technique which produces the best images of boundaries between rock units and of structures in the subsurface. It is the principal method by which the petroleum industry explores for hydrocarbon-trapping structures, that is oil and gas traps, in sedimentary basins. We don't have the time to explain oil and gas exploration here, but it, too, is a fascinating subject to get involved in.
Extension of the seismic reflection method to deep crustal studies began in the 1960s and since the late 1970s reflection technology has become the principal procedure for detailed studies of the deep crust.

In its simplest form, the method is an echo technique based on the same principles as sonar, i.e. bouncing sound waves off the boundaries between different types of material: water and bottom sediments for sonar, rock layers for crustal studies. The form of presentation of the data is similar to that of a geological cross-section and needs to be interpreted in terms of geology. Thus, geoscientists must work together to provide the most complete interpretation; this is the procedure followed in LITHOPROBE.

In its application, the seismic reflection method is highly complex, involving a skilled acquisition crew with millions of dollars in equipment, whether on land or at sea. As in the petroleum industry, LITHOPROBE contracts the data acquisition to qualified industrial contractors.

On land, LITHOPROBE has been using large truck-mounted mechanical vibrators as sources (the "Vibroseis" method) because they are logistically appropriate and environmentally safe for the hard-rock areas in which we have worked; in one transect dynamite has been used for the purpose of sub-crustal imaging and may be used again, just as it sometimes is in industry. In all cases, federal and provincial environmental and permitting regulations are strictly followed.

In a typical regional survey at the present time, 480 groups of 9 to 12 geophone sensors per group are spaced at 25-m intervals over a 12-km length to record ground vibrations caused by the Vibroseis units. Sensor outputs are recorded on a truck-mounted computer system.

In addition, high-resolution surveys have been undertaken with the participation and collaboration of industry and provincial government agencies. These very detailed surveys aim at geological targets (typically, the geological setting of ore bodies) in the uppermost crust, i.e. the practical depth range for mining. To achieve the desired greater detail, the 480 geophone groups are spaced at smaller intervals (5 to 10 m) over only 2 to 5 km.

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