Preliminary gravity inversion model of Frenchman Flat basin, Nevada Test Site, Nevada

The accuracy of the calculation of depth to pre-Cenozoic rock is dependent on many factors, including: the accuracy of the gravity measurements, the relative spacing of the measurements on the ground, the accuracy of the constraints and assumptions built into the reduction process (e.g. assuming an average crustal density of 2670 kg/m3 for complete Bouguer and isostatic corrections), the assumption of uniformly dense, horizontal layers, the density constraints used in the inversion, and the interpolation algorithm used between the data points.
The primary source of uncertainty is the density-depth function used to calculate the thickness of the basin fill. To investigate the variation in the final model a sensitivity analysis (Hill, 1998) was performed. The model was perturbed systematically, and 1% scaled sensitivities were calculated for each model parameter (layer). The sensitivities represent the approximate change in depth resulting from a 1% change in the parameter (density) value. We can use them to gain insight into how the parameters quantitatively contribute to the model. However, the sensitivities do not scale; in other words, multiplying one result by 10 does not yield the exact amount the depth would change given a 10 percent change in density. This is because the model is non-linear, and so the values are valid only within a short range of the solution. Figures 8-11 show the 1% scaled sensitivities for the final model in Frenchman Flat. The largest range in sensitivity, indicating the largest overall influence on the model, occurs in the second layer, while the fourth layer has the greatest influence over the deepest part of the basin. Laterally the shallower layers of the model have a greater affect than the deeper layers. The deeper layers influence the deep part of the basin only; this follows from the simple fact that a layer cannot affect the model if the predicted depth is not deep enough to include the layer.
The general range of the sensitivity values is from a few meters up to 30 meters. This implies the model is fairly sensitive to density changes, since fluctuations of a few percent imply tens of meters fluctuation in the depth estimates.