Browsing by Author "Smith, Richard"

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Forward modeling and 3D inversion of electromagnetic data collected over the McArthur River uranium deposit in the Athabasca Basin, Canada
(2022-03-01) Mir, Reza; Fullagar, Peter; Darijani, Mehrdad; Smith, Richard; Scott, Shawn; Ross, Martin; Shamsipour, Pejman; Chouteau, Michel; Ansdell, Kevin; Gouiza, Mohamed
Detection and assessment of the deeply buried high-grade uranium deposits in the Athabasca Basin rely on geophysical methods to map conductive rocks. Variable Quaternary surface cover can mask the anomalous signals from depth and affect the interpretation of inverted conductivity models. We present the analysis of several electromagnetic (EM) modeling studies and two field data sets to demonstrate the effects of varying Quaternary cover resistivity and thickness, on the ability to resolve the parameters of underlying sandstone, alteration, and basement conductors. Synthetic data, assuming a typical shallow EM sounding system and realistic resistivities found in the Athabasca Basin, indicate that resistivity and thickness parameters of the Quaternary cover can be separately recovered in cases in which this cover is more conductive than the underlying sandstone, but not when the cover is significantly more resistive. A 3D modeling study indicates that by using airborne EM data, it is possible to detect a basement conductor of 20 S at a depth of at least 600 m below the surface, even in the presence of Quaternary cover thickness variations of the up to 20% (40–60 m). Furthermore, although Quaternary cover variations and deeper sandstone alteration can produce comparable anomalous signal amplitudes in a time-domain EM response, their effects are most visible in distinctly separate time windows. Ground-penetrating radar and other data to characterize the Quaternary cover in the McArthur River area indicate that this cover consists mostly of sandy tills ranging in thickness from 0 to 117 m. Constrained 3D inversion of an airborne EM data set from the same area indicates basement conductors consistent with the depth and location of a known fault. Elevated conductivity in the sandstone by up to a factor of two over the background values could indicate possible alteration.
On the Time Decay constant of AEM systems: A Semi- Heuristic Algorithm to validate calculations
(2020-01-01) Martinez, José Manuel; Smith, Richard; Vazquez, Danilo Diaz
The time decay constant or “tau” of airborne EM systems (AEM) is commonly used to indicate the presence and the relative conductivity or conductance of conductors in the survey area. In fact, it is not a constant since it depends on: the system; the survey design and the method of calculation. The system dependence is a consequence of parameters relating to the acquisition and pre– and post– processing of the signal. Here we propose a method for calculating tau, which is simply the time at which the transient voltage decays to 37%, or V37, of some initial value. The model utilises a semi-heuristic algorithm that estimates V37 for each transient in the database and then it calculates the delay time that voltage is measures, which is the estimates tau value. No calculation is involved with the data, instead tau is given by a weighted average of the delay times associated with the windows either side of the V37 value. We illustrate how this algorithm works using data collected by MEGATEM II in the Reid-Mahaffy test site. Results shown a good agreement between tau-grids reported in previous studies and those calculated with our V37 – method. To account for all effects coming from the acquisition and processing of EM data, the algorithm allows the emphasis to be shifted from early to late time parts of the transient. It is envisage that since this method does not apply any mathematical operation to the data it may serve as a robust means of for validating other methods.
Potential-field modelling of the prospective Chibougamau area (northeastern Abitibi subprovince, Quebec, Canada) using geological, geophysical, and petrophysical constraints
(2021-01-01) Maleki, Amir; Smith, Richard; Eshaghi, Esmaeil; Mathieu, Lucie; Snyder, David; Naghizadeh, Mostafa
This paper focusses on obtaining a better understanding of the subsurface geology of the Chibougamau area, in the northeast of the Abitibi greenstone belt (Superior craton), using geophysical data collected along a 128 km long traverse with a rough southwest–northeast orientation. We have constructed two-dimensional (2D) models of the study area that are consistent with newly collected gravity data and high-resolution magnetic data sets. The initial models were constrained at depth by an interpretation of a new seismic section and at surface by the bedrock geology and known geometry of lithological units. The attributes of the model were constrained using petrophysical measurements so that the final model is compatible with all available geological and geophysical data. The potential-field data modelling resolved the geometry of plutons and magnetic bodies that are transparent on seismic sections. The new model is consistent with the known structural geology, such as open folding, and provides an improvement in estimating the size, shape, and depth of the Barlow and Chibougamau plutons. The Chibougamau pluton is known to be associated with Cu–Au magmatic-hydrothermal mineralisation and, as the volume and geometry of intrusive bodies is paramount to the exploration of such mineralisation, the modelling presented here provides a scientific foundation to exploration models focused on such mineralisation.