Complex seismic signatures of the Athabasca basin subsurface
|Location||International Geological Congress,oslo 2008|
|Author||Pandit, Bhaskar۱; Hajnal, Zoltan۱; Annesley, Irvine۲; White, Donald۳; Reilkoff, Brian۱; McCready, Alistair۴; Wallster, Dale۴|
|Holding Date||07 October 2008|
Modern multifold reflection techniques were introduced to the Athabasca Basin in the winter of 2001. Since then, several very successful 2D and 3D surveys were carried out, mainly in the eastern segment of the uranium exploration and mining district. Within this region of the basin, predominantly horizontal sandstones and conglomerates of the Manitou Falls Formation constitute the sedimentary fill. The crystalline basement lithologies comprise highly folded and interleaved Paleoproterozoic meta-sediments and Archean orthogneisses. Despite this apparently simple geology, complex seismic signatures are generated within the sedimentary fill and close to the sandstone-basement unconformity, as well as in the underlying basement rocks. Results from in-situ geophysical and geological logs, and rock-property measurements reveal that the reflectivity within the basin fill strata is dominantly controlled by porosity variations; and also, in part, by post-depositional hydrothermal alteration zones and grain-size lithological changes.
Internal, lithology-based reflectivity properties and their variations within the sandstone column, although weak, are still identifiable in many cases. The seismic waveforms related to the unconformity are highly variable. Contact(s) between fresh sandstone and comparable basement rocks generate strong and simple seismic signals. Interfaces between altered sandstone and variably-altered basement segments (regolith) all reduce the strength of the reflectivity; and in combination with zones of fracture density variations, lead to complex multi-cycle wave patterns. Core sample analyses reveal that certain, moderately dipping lithological variations in the basement can be recognized seismically. Numerous examples from different regions of the basin illustrate that the seismic method is not only capable of imaging the sub-surface structural environments, but also can map lithological changes within the Athabasca Basin. Seismic attribute analyses of 2D sections and 3D data volumes have great potential in providing additional constraints in mapping and identifying the structurally complex mineralizing systems of the Athabasca unconformity-type uranium deposits.