The primary objective of the high-resolution survey was to offer a detailed subsurface image of the complex P2 fault zone, hosting the world's largest high-grade uranium deposits. Raw seismic data have low signal-to-noise ratio; nevertheless the implemented processing procedure considerably improved its quality. Interpretation of the two lines integrates all available background information such as regional geology; structural, stratigraphic, and diagenetic features associated with the borehole information including stratigraphic and petrophysical properties; and seismic analogies for the basement from other high-grade metamorphic terranes. The resulting interpretations generally agree with previous geological knowledge, but add new exploration insights, such as: 1) the complex 3-D geometry of the P2 fault zone, 2) the existence of additional dekameter-scale, fault-controlled structures associated with the P2 zone, 3) syntectonic deposition of the upper parts of the Manitou Falls Formation in this area; and 4) the internal architecture of the metamorphic basement.

Seismic-reflection data and a vertical seismic profile were acquired in the vicinity of the McArthur River mining camp. These data are interpreted with the aid of in situ geophysical and geological logs and rock-property measurements, which indicate that reflectivity within the basin-fill strata is controlled largely by porosity variations (Phi = 0 - 11%) that are attributed primarily to zones of silicification (postdepositional hydrothermal horizons), but also to grain-size lithological variations. The reflection data clearly image the unconformity zone and associated fault offsets including the P2 mineralized fault zone. A prominent shallow- to moderately dipping zone of reflections that extends downward from the surface location of the P2 fault is interpreted as a major crustal shear zone that partially controlled the locus of high-grade uranium ore deposition. The seismic techniques have demonstrated their utility in defining some of the key geological variations that are relevant to identification of prospective ores zones.

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Interim results from the EXTECH-IV 2000 seismic-reflection program at the McArthur River mine site include interpretations of the two-dimensional high-resolution and regional profiles, simulated seismic responses for the P2 orebody, and preliminary images from the pseudo three-dimensional survey. Interpreted seismic-reflection data 1) accurately image the sandstone-basement unconformity at a depth of approximately 400 to 600 m; 2) map its offset by the P2 reverse fault; and 3) trace the extension of the 2.5km thick P2 fault system upward through the overlying Athabasca Group sandstone and downward more than 4 km as a listric structure that curves past the southeastern limit of the survey. Lateral variations in sandstone porosity, silicification, or facies changes are inferred from variable reflection response and velocity variations, and angular discordances in stratigraphy suggest onlaps or unconformities that may have resulted from tectonic control on sedimentation. A reflection response from the vicinity of the orebody is apparent. The entire survey and a LITHOPROBE survey to the east are underlain by a bright reflector interpreted as an extensive thick sill, at a depth of approximately 6 km, that may be associated with the Mackenzie Igneous Event (1.265 Ga).

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EXTECH-IV is a multidisciplinary study designed to improve the geoscience framework and develop exploration technology for unconformity-type uranium deposits of the Athabasca Basin. A multi-element seismic reflection program was conducted in the vicinity of the McArthur River uranium mining camp to test this technology for imaging the subsurface geometry of the ore deposits and the geology that hosts them. The seismic program consisted of 2-D reflection profiling (39 km of regional and 8 km of high resolution), a limited 3-D high-resolution survey, and vertical seismic profiling. Results from preliminary processing of the high-resolution 2-D seismic and vertical seismic profiling data show: 1) laterally continuous reflectivity regionally associated with the basement unconformity beneath the basin-fill sediments; 2) local reflectivity within individual units of the Manitou Falls Formation that is generally comparable to reflectivity of the boundaries between the formation members; and 3) local strong reflectivity associated with an abrupt increase in density that occurs within the Manitou Falls b member.

Two subparallel, regional seismic reflection profiles of the multidisciplinary and multi-institutional EXTECH-IV project outline fundamental relationship between the local structures of the McArthur River mine of the Athabasca Basin and the underlying deep crustal tectonic framework. This deep reflection sounding investigation crosses over the P2 ore body, which is located in the region of a magnetic low. Tomographic inversion of the first-break (FB) arrival times indicated lateral changes in P-wave velocities of the sandstone along the survey lines. Distinct high velocity zones mark alterations of the sandstone lithology over known mineralized locations of the P2 ore body.The migrated time sections are characterized by robust reflection signals from near surface to past-Moho depths. The sandstonebasement unconformity is clearly imaged, however, its correlation is more difficult along the central and southern part of the profiles due to influences of major structural disturbances. The differing patterns of reflections observed along the profiles will potentially allow distinction of the Wollaston/Mudjatik Transition Zone from the Wollaston Domain. South dipping reflections, traceable to 4 km depth in the crust, are correlated with the P2 shear/fault that is well recognized below the main ore body of the McArthur mine site. This structure is over 2.5 km thick and its seismic signature suggests multi-phase deformation. Strong, arcuate, and in places subparallel, zones of reflectivity dominate the middle and lower crust. The origin of these prominent seismic events appears to be located at the southern margin of the seismic investigation.Highly prominent, bright and sub-horizontal reflections, at 2.3 s two-way time (TWT), are the most distinct seismic signatures on the sections. These events are directly comparable to reflections observed along the northern half of the 1994 LITHOPROBE regional line S2B. The origin of this enigmatic zone of reflectivity is still under considerable discussion; Mandler and Clowes (1997) associate the 1994 data with the post-Hudsonian Mackenzie magmatic activity (1.265 Ga). The Moho is well defined at about 10.5 s TWT and is characterized by a number of laterally recognizable gently south dipping reflections. Below the Moho, a zone of strong reflectivity delineates the upper mantle, revealing considerable tectonic involvement of the lithospheric mantle within the depth range of 35-45 km. A gentle southeastern dip in the tectonic development of the region symbolizes the overall structural attitude of this highly reflective zone.