Seismic Survey Methods, Not Just for Hydrocarbon Exploration
Using Seismic to Explore for Sandstone/Unconformity Hosted Uranium Deposits in the Athabasca Basin
GOPH 559 Bradley Parkes
Seismic survey methods have long been accepted as a tool to enhance exploration models and reduce risk in the exploration for hydrocarbon resources (Lines and Newrick 2004). In the mining exploration industry seismic survey methods are used on a much less frequent basis due to the properties associated with metamorphic and igneous rock ore deposits, however, in 2000, the Geologic Survey of Canada and Areva Uranium Corp (COGEMA) decided to apply seismic survey methods to their unconformity/sandstone hosted uranium deposit at MacArthur River in the Athabasca Basin (White et al 2002). The success of the 2D program encouraged the acquisition and interpretation of a 3D model in 2007 by Hathor Exploration Ltd. (Hajnal et al 2007).
EXTECH-IV 2000 Study
The EXTECH-IV 2000 Study was performed by the Geologic Survey of Canada and Areva Uranium Corporation’s North American subsidiary, COGEMA, in conjunction with the University of Saskatchewan. The survey area was conducted in the south east portion of the Athabasca Basin across the P2 fault at the MacArthur River uranium deposit in a region of low magnetic susceptibility. The survey consisted of two subparallel regional seismic profiles across the P2 fault which contains the P2 orebody. The objectives of the survey was 1) to define the basement structure 2) map the stratigraphic architecture of the basin 3) image the sandstone/basement unconformity 4) locate faults that control the uranium deposit 5) establish the relationship between sedimentation and deformation and 6) establish a seismic signature that identifies uranium deposits (White et al 2003).
White et al 2002
Athabasca Basin Overview The Athabasca Basin is a Mesoproterozoic craton with up to 1500m of fluvial continental sandstone as basin fill. The sandstone deposition was the result of braided river and sheet flood deposits making the sand very quartz rich and laterally discontinuous. The basin fill sandstone uncomfortably lies on medium to high grade metamorphic rock of the Mudjatik and Wollaston Formations basement rock. These basement rocks were ductilely deformed during the Paleoproterozoic Trans-Hudson Orogeny. This period of deformation and orogeny created a number of fault systems that control the distribution uranium deposits (Hajnal et al 2007).
Sandstone/Unconformity Hosted Uranium Deposits
Uranium is mined in the form of U3O8, otherwise known informally as yellowcake. However, in the subsurface uranium can exist in an isotopic form of hexavalent uranyle ions (UO22+). U2O22+ exists as a highly mobile fluid under oxidizing conditions (Nair 2010). Unconformities in the subsurface act as conduits under which the isotopic mobile form of uranium travel until it is reduced at fault zones (Nair 2010). This feature of sandstone/unconformity type deposits led to the belief that if the fault zones and unconformity could be mapped and identified, areas of reduced uranium isotopes could be identified through the use of seismic surveys, as the acoustic impedance difference between uranium and sandstone would identify barren from mineralized faults (White et al 2002).
The EXTECH-IV 2000 survey consisted of two lines across the P2 fault zone. Line A was approximately 10km in length and Line B was approximately 30km in length. Geophone spacing was set at 5m with a 20m vibration interval. Three 22,000kg VibroSeis units were used to produce 65,000kg peak force. 960 receivers were set up to record the seismic data. Frequency ranged between 10-84 H with ten sweeps at each site. The sweep length was 28 seconds and a fold equal to 120% was derived. A 2D Finite Difference Elastic Wave Algorithm was used to interpret the traces (White et al 2002).
Elastic model used for seismic simulation. Note the density and velocity differences between sandstone and uranium. This large acoustic impedance difference helps determine barren from mineralized faults.
Geologic Difficulties of EXTECH-IV 2000
There were numerous difficulties related to topography and geology of the Athabasca Basin. The irregular nature of the topography created difficulties in placing receivers in the optimal pattern. In addition the survey had to be shot in the winter leading to high levels of noise associated with permafrost and ice responses to the seismic source. The basin fill of the survey area consists of terrestrial sandstone deposits and glacial till. Glacial till clastics have a characteristic low velocity seismic response and the variable thickness of these deposits lead to travel time delays and shallow reverberations that can overprint the seismic observations. The terrestrial nature of the sandstone deposition creates wavy and discontinuous seismic reflections. This creates a difficulty in correlating the seismic response over long distances. In addition, the lower sandstones underwent intense silicification. Silicification has the effect of increasing P-wave velocity. Velocities in the silicified region averaged 5400-5600m/s versus the 4400-4600m/s in the upper sandstones creating a pull up effect. Also creating challenges, the individual faults of the Athabasca basin have limited vertical offsets and very high dip angles making the interpretation of these faults difficult on seismic data (White et al 2003).
In addition to the geologic and topographic difficulties the study suffered from a number of geophysical interpretation challenges. In order to avoid spatial aliasing the choice of bin size was of utmost importance, as was maximum and minimum frequency. Eventually a bin size of 10*10m was determined sufficient to eliminate aliasing. Due to the irregular topography and the effects from permafrost and ice, the receivers recorded a large amount of noise. This required attempts to increase signal to noise ratio. This noise had to be filtered out before the data could be interpreted and increase the signal to noise ratio. In addition, the high dip nature of the faults created a dip moveout problem that required an adjustment to the time distance equation due to the extra unknown dip angle of the faults. Assumptions about average velocities had to be adjusted due to the pull up effect of the high velocity silicified sandstone zones (White et al 2003).
EXTECH-IV 2000 Survey Results
The EXTECH-IV 2000 Survey was the first 2D seismic survey to be shot over the Athabasca basin and was successful enough to meet most of the original objectives. The survey resulted in the following conclusions:
1) The sandstone/basement interface results in large amplitude and a laterally continuous reflection on the horizontal component, however it was generally invisible on vertical component. This emphasized the importance of S waves over the P wave reflections.
2) The orebody and fault offset in the sandstone/basement interface results in relatively weak diffracted energy that emanates from the vicinity of orebody and fault.
3) An asymmetry in amplitudes of diffractions was observed due to the dip of the orebody that results in higher amplitude down dip; however amplitude of fault zone was symmetrical.
4) The P wave to S wave converted diffraction has higher amplitude than diffracted P wave amplitude.
5) Down dip P wave diffraction and P to S converted wave is more prevalent in the horizontal component.
6) Distinct high velocities indicate mineralized zones (depicted as halo zones above the orebodies).
7) Tomographic inversion of First Break seismic reflections provided insight to acoustic properties of the basin fill.
8) A very bright horizontal zone at 2.3s TWT was believed to indicate a young intrusive, suitable as a heat source that could aid in uranium fluid mobility.
9) The unconformities associated with heavy weathering create significant changes in impedance that can be recognized on seismic.
10) The seismic diffraction response that characterizes the orebodies was similar to diffractions resulting from hosting fault zones associated with uranium deposits.
11) Symmetry in amplitudes of the orebodies might help distinguish barren faults from mineralized faults.
12) A larger response was observed on the horizontal component than vertical component suggesting that horizontal component geophones enhance the detection of steeply dipping faults. Also emphasizing the importance of S waves.
13) A characteristic diffraction response was observed on unmigrated data, suggesting the best interpretation is pre-migrated data (White 2002 and Hajnal et al 2002).
The image is an example of the halo created by the high density of uranium deposition and identifying a mineralized fault.
White et al 2002
Seismic acquisition, interpretation and processing can help model sandstone/unconformity hosted uranium deposits by identifying barren faults from the mineralized faults due to density and velocity differences leading to large variations in acoustic impedance and the reflection coefficient. The identification of mineralized faults from non-mineralized faults can lower exploration risk by allowing the exploration geologist and geophysicist to select higher probability exploration targets. This survey also indicated that uranium mineralization has a distinctive seismic signature that can be identified by high velocity haloes in zones above the orebodies. The survey was also successful in imaging the basement structure and unconformity that acts as a conduit for highly mobile uranium isotopes. The success of the 2D EXTECH-IV 2000 study confirms the usefulness of seismic surveys in the exploration for sandstone/unconformity hosted uranium deposits and led to the a 3D survey shot in 2007 and this study lead to discovery of the Midway uranium deposit by Hathor Exploration Ltd. which was subsequently purchased by Rio Tinto, further confirming this as successful technique for uranium exploration.
Hajnal, Z., Pandit, B., Reilkoff, B., Takacs, E., Annesley, I., Wallster, D. (2007): Recent Development in 2D and 3D Seismic Imaging of High-Grade Uranium Ore Deposit Related Environments, in the Eastern Athabasca Basin, Canada. In "Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Exploration" edited by B. Milkereit, 2007, p. 1131-1135
Hajnal, Z., Takacs, E., White, D., Reilkoff, B., Powell, B., and Koch, R. (2002): Regional Seismic Images Beneath the McArthur River Ore Bodies, Saskatchewan; in Summary of Investigations 2002, Vol 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.2, CD-ROM, Paper D-4, 5p.
Lines, L., Newrick, R. (2004): Fundamentals of Geophysical Exploration. Society of Exploration Geophysicists.
Nair – Geology 323 Lecture. Personal Communication (2010).
White, D.J., Hajnal, Z., Gyorfi, I., Takacs, E., Roberts, B., Mueller, C., Reilkoff, B., Koch, R., Powell, B., Annesley, I., Bernier, S., Jefferson, C. (2003): Interim Results of the EXTECH-IV Seismic Reflection Program in the Athabasca Basin, Northern Saskatchewan. Geologic Survey of Canada.
White, D.J., Roberts, B., Mueller, C., Hajnal, Z., Gyorfi, I., Reilkoff, B., Koch, R., and Powell, B. (2002): Seismic Reflection Profiling: An Effective Exploration Tool in the Athabasca Basin? An Interim Assessment; in Summary of Investigations 2002,Vol 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.2, CD-ROM, Paper D-2, 7p.