TWO-DIMENSIONAL VERSUS THREE-DIMENSIONAL GEOLOGICAL RESOURCE MODELING – URANIUM AND GROUNDWATER RESOURCE EVALUATION CASE STUDIES

From the late 1950's through the 1980's, uranium resources were estimated by picking the tops and bottoms or ore zones from wireline logs. The thicknesses of these zones were then vertically totaled and contoured as “grade thickness maps”. A planimeter was then used to compute the area of the individual contours. The resultant area calculations were then multiplied by the thicknesses represented by the planimitered contours and combined to determine the total volume of ore. Finally, the volumetric total was multiplied by the rock density to determine the mass of U3O8. These tasks were typically performed manually, without the benefit of a computer.

The aforementioned methodology is no longer suitable thanks to in-situ mining methods that are based on three-dimensional models of the site geology. Instead, the data from the wireline logs (e.g. gamma values) must be digitally stored during the probing process (or digitized from paper logs), geometrically transformed into discrete Cartesian xyzg points, interpolated into block models, filtered by economic parameters, and converted into models that can be used as the basis for in-situ leach-field planning. All of these steps must be performed with computer software.

The same transition in data processing has occurred within groundwater evaluations. In the past, hydrogeologists would pick tops and bottoms for aquifers based on downhole gamma, resistivity, and gamma logs. The thicknesses represented by these intervals would then be used to create isopach maps which were processed in a fashion similar to the previously described uranium evaluation. The volumetric results were then multiplied by transmissivity (permeability) and storativity (porosity) constants en-masse to determine the total amount of available water. Given the practical constraints (time and money), this approach was perfectly reasonable.

Today, aquifers no longer have “tops” and “bottoms”. Instead, aquifer contacts are considered to be gradational boundaries that can only be modeled with computer software. Lateral and vertical spatial variations in transmissivity and storativity are readily handled by three-dimensional modeling algorithms. The net result is more geologically-reasonable resource assessments.