Mapping of complex bedrock structure using the high-resolution reflection seismic technique
In May of 1996, four reflection seismic profiles and one VSP (vertical seismic profile) were acquired near Colonsay, Saskatchewan. This survey was conducted as part of a site-specific study of the high-resolution reflection seismic profiling, and shallow open-borehole VSP methods. The reflection seismic/VSP survey was designed with four specific objectives in mind. The sponsor wanted to determine: (1) whether high-resolution reflection seismic data could provide an essentially continuous structural image of the shallow subsurface (Quaternary alluvium and Cretaceous shale to a depth of ca 200m); (2) if the Cretaceous bedrock and overlying Quaternary strata in the study area are extensively faulted; (3) whether useful VSP data could be acquired from shallow open-boreholes in the study area; and (4) optimum high-resolution reflection seismic acquisition parameters (given the target zone and site conditions) for future production-oriented work in the area. From a technical perspective the survey was successful. The acquired high-resolution reflection seismic data effectively image the shallow subsurface, and support the thesis that bedrock and overlying Quaternary strata are extensively faulted in the study area. The VSP data establish that sonic log velocities recorded above the water table in a fluid-filled borehole are anomalously high (relative to the seismic velocity of an equivalent thickness of unsaturated sediment). On the basis of the VSP control, the sonic log data were modified such that the corresponding synthetic seismograms correlate well with the reflection seismic profiles. Additionally, on the basis of field testing, optimum site/target specific field acquisition parameters were determined. These will be of use in designing any subsequent seismic reflection and VSP surveys in the study area.
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