A case-history tutorial describing the incorporation of geophysical, petrophysical and geological constraints to generate realistic geological models of the Matheson Study Area, Ontario

The model that is used to explain potential-field data is highly dependent on the constraints applied in the modelling process. Many studies demonstrate the necessity of constraining gravity and magnetic models. However, typically they do not demonstrate the individual enhancements that come as a consequence of integrating each constraint into the geophysical model. In this paper, we show that when there are no constraints, it is possible to find an inverse model that is consistent with gravity data, but the model is unrealistic, as one sedimentary basin is too deep. Adding a depth weighting constraint can ensure the depth is correct, but all other features have the same depth, which is unrealistic. Including densities from a density compilation makes the densities at surface realistic, but the dips are all close to vertical and the thicknesses are similar, which is unrealistic. In this case, the inversion is believed to have found a local minimum close to the starting model. Reflection seismic data was used to constrain a two-dimensional (2D) modeling exercise (on multiple profiles) to determine the geometry of one sedimentary sub-basin. These 2D models were then combined to build a realistic three-dimensional (3D) starting model. An inversion from this model fixed the densities of each lithology, but allowed the thicknesses of the layers to vary. The resulting model was realistic, with the dips and thicknesses away from the seismic constraints being consistent with geological expectations. Although the fit to the data was much better than the previous model, it was poorer than hoped. If the densities were then allowed to vary within a realistic range of values, the fit could be improved so that both the fit to the data and the geologic model are realistic.