Seismic-reflection techniques have been applied in several studies over the last 20 years as a uranium-exploration tool within the Athabasca Basin and have been utilized to provide the larger structural context for known uranium deposits within the basin. At the crustal scale, deposits within the eastern Athabasca Basin are shown to be associated with deep-seated shear zones that originated during Trans-Hudson orogeny and have subsequently been reactivated during and subsequent to deposition of the basin-fill sandstones. Seismic properties of the Athabasca sandstones and underlying basement have been determined through in situ borehole measurements. Reflectivity within the sandstones is generally weak. Seismically recognizable signatures are primarily associated with variations in fracture density, porosity, and degree of silicification. The basement unconformity and regolith, a prime target of exploration, is widely imaged as it is characterized by variable but generally distinct reflectivity. Results from the McArthur River mine site suggest that the spatial coincidence of seismically imaged high-velocity zones and deep-seated faults that offset the unconformity may be a more broadly applicable exploration targeting tool. Three-dimensional (3-D) seismic imaging near existing ore zones can define the local structural controls on the mineralization and point the way to new targets, thus leading to more efficient exploration drilling programs. Furthermore, seismically generated structural maps of the unconformity and rock competence properties may play a significant role at the outset of mine planning.

The Millennium uranium deposit is located within the Athabasca Basin, in northern Saskatchewan, Canada. The deposit is hosted within moderately dipping Paleoproterozoic gneisses that are unconformably overlain by more than 500 m of flat lying, porous Paleoproterozoic to late Mesoproterozoic Athabasca Group sandstones. The deposit is associated with the sandstone-basement unconformity, post-Athabasca structure, and hydrothermal alteration. These features combine to create a complex 3D hydrogeologic setting that presents challenges with respect to mine development, production, and safety. In 2007, as part of a prefeasibility study for potential mine development, a seismic program consisting of a 3D surface survey, vertical seismic profiling, moving source profiling, and side-scan surveys was undertaken to map the complex geology. The geometry and resolution of these different seismic surveys allowed for direct imaging of the geologic targets of interest, regardless of orientation and size. After integration with drill-defined geology, the program successfully imaged the location and character of the unconformity, the post-Athabasca structural setting at camp and deposit scales, and the alteration around the deposit. This information increased the understanding of geotechnical aspects of the geology hosting the deposit, and is currently being used to help minimize risk and costs associated with mine development. Seismic surveys are now viewed as an integral part of risk reduction associated with mining in the Athabasca Basin.

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the ∼6.5 km 2 survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wideangle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial.The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

The Millennium uranium deposit is located within the Athabasca Basin of northern Saskatchewan. The basement rocks, comprised primarily of paleo‐Proterozoic gneisses, are electrically resistive. However, the deposit is associated with highly conductive graphitic metasediments that are intercalated with the gneisses. An unconformity separates the basement rocks from the overlying, horizontally stratified, Proterozoic sandstones of the Athabasca Group (which are also highly resistive). The strike extents of the graphitic metasedimenary packages are extensive and therefore electromagnetic (EM) survey techniques are successful at identifying these zones but do not identify the specific locations where they are enriched in uranium. Through drilling it has been noted that hydrothermal processes associated with mineralization has altered the rocks in the vicinity of the deposits, which should in theory result in a resistivity low. A significant resistivity low has been mapped coincident with the Millennium deposit using ground resistivity survey techniques. However, a comparison of the airborne EM and ground resistivity results reveals that the two data sets have imaged different features. The resistive‐limit (on‐time) windows of the MEGATEM data show conductive features corresponding to lakes located to the west and south of the deposit. The late‐time windows show a feature to the east of the deposit, interpreted as being associated with the east‐dipping graphitic basement conductors (similar to that observed in historical ground EM data collected in this area). The early‐time TEMPEST windows (delay times less than 0.2 ms) show a broad resistivity low located at approximately the same location as where the alteration has been identified through drilling. Modelling the data is not easy but a response that decays prior to 0.3 ms is consistent with 500 Ωm material in the sandstone, a resistivity value close to the lower limit with respect to the hydrothermally altered Athabasca group sediments in this area. The MEGATEM system does not see a conductive zone over the alteration as clearly but the high signal‐to‐noise ratio in the late‐time MEGATEM data means that the conductive material at a greater depth is more coherently imaged.

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As part of EXTECH IV, three-dimensional audio-magnetotelluric data were collected in the McArthur River uranium mining camp, northern Saskatchewan. One hundred and thirty five audiomagnetotelluric stations were acquired along 11 profiles over the P2 and P2 North mineralized zones with an average site spacing of 300 m. The new audio-magnetotelluric data extend the coverage of an earlier two-dimensional survey and were acquired to provide a three-dimensional view into the subsurface conductivity structure of the McArthur River deposit, the overlying Athabasca Group sandstone, the basement rock types and offsets, and the alteration assemblages associated with the deposit. Digital comb filters were tuned to and removed strong harmonics; then robust audio-magnetotelluric responses were calculated. The resulting induction arrows map different domains of coherent and complex electrical strike. Data qualities and distribution are ideal for the next stage-calculation of a three-dimensional audio-magnetotelluric model.

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