Search

Unconformity-type deposits supply a significant amount of the world’s uranium and consist of uranium that is generally codeposited with graphite in a fault zone. The low resistivity of the graphite produces a significant contrast in electrical resistivity, which can be located with electromagnetic (EM) methods. The Athabasca Basin in Western Canada hosts significant uranium deposits, and exploration in deeper parts of the basin has required the application of new EM methods. This paper presents an evaluation of the audiomagnetotelluric (AMT) exploration method at the McArthur River mine in the Athabasca Basin. AMT data were collected at 132 stations on a grid, and two-dimensional (2D) and three-dimensional (3D) inversions were used to generate resistivity models. These models showed two major results: (1) a significant conductor coincident with a major basement fault (P2) and the uranium deposits (this conductor begins at the unconformity at a depth of [Formula: see text] and extends to a depth of at least three km) and (2) a resistive halo which might be caused by the silicification associated with mineralization. However, synthetic inversions showed that this halo could be an artifact of smoothing function in the inversion scheme. The 2D inversions were validated by synthetic inversions, comparison with the 3D inversion models, and correlation with well-log information. 3D AMT forward modeling showed that strong 3D effects are not present in the AMT impedance data. Induction vectors showed more evidence of complexity, but the inclusion of these data in the inversion improved subsurface resolution.

The vast and growing literature on the search for stratigraphic traps for petroleum ignores gravity gradients, for which the theory has been available since the heyday of the Eötvös torsion balance decades ago. These are discussed in this paper. The horizontal and vertical gradients can be measured with available gravimeters and (to a limited extent) with the Eötvös torsion balance. A major advantage of the gradient method is that surveying to determine position and elevation of the station is not required. Both theory and practice have been reported in the geophysical literature, but the important application to stratigraphic traps has not been mentioned. We evaluate here the method for locating “pinchouts”, a term which embraces “stratigraphic” and “unconformity” traps in Halbouty’s (1972) classification. Both position and depth of the assumed pinchout are determined by the gradient anomaly. The magnitudes of anomalies of horizontal and vertical gradients are about equal. However, pending new instrumentation, only the horizontal gradient is practically useful for field surveys. The gradient method is quantitatively promising and, used in conjunction with other methods, should significantly advance the search for stratigraphic traps for petroleum.

The eroded Mississippian surface is the major unconformity in the province of Alberta. To map its erosional highs and lows is most important, because the Mississippian may be productive of hydrocarbons or may cloak the attitude of deeper sediments from which production is sought. This paper deals with the methods of presentation of reflection seismic data to that end, together with a suggested recording instrument technique. Some of the interpretive problems, and the possible significance of Mississippian porosity on the acoustic impedance of its reflection are mentioned. Maps of similar data, one geological and the other reflection seismic, are presented for comparison. The former is obtained from drilled wells and the latter from reflection shooting performed prior to drilling.