Gravity Inversions

Use the Calculate > Gravity Inversions option to optimize either the top relief surface (i.e., "structure") or the lateral density distribution of the selected layer, based on the Gz or Gzz component of the gravity field.

Gravity Inversions dialog options

Layer to optimize	Select the layer you wish to optimize using the dropdown list of the model’s layers.
Optimize	Select whether to optimize the layer Top surface relief or layer Density distribution.
Field component to fit	Select whether to use Gz or Gzz component in the inversion. When Gzz is selected, it will be integrated to produce Gz for "Stage 1" iterations. Depending on whether you select "Gz" or "Gzz" as the field component, you will have different options to set below.
Gz
Max number of iterations	The inversion will stop after this many iterations, even if the convergence target has not been reached.
Convergence limit (mGal)	Inversion will stop when either the mean error or the standard deviation of the error is less than this limit. Note that under certain conditions, the calculations may not converge at all. The algorithm attempts to detect this and terminate accordingly.
Gzz
Max Stage 1 iteration	The Gz-based inversion process will stop after this many iterations, even if the convergence target has not been reached.
Max Stage 2 iterations	The Gzz-based inversion process will stop after this many iterations, even if the convergence target has not been reached.
Convergence Limit (mGal)	Inversion will stop when either the mean error or the standard deviation of the error is less than this limit. Note that under certain conditions, the calculations may not converge at all. The algorithm attempts to detect this and terminate accordingly.
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Lower High-Cut limit	A cosine tapered high cut filter is applied to the misfit before inverting. This is the beginning point of the cosine taper. If these numbers are <= 1.0, they are interpreted as a fraction of the Nyquist - 0.5 and 0.7 are the defaults. If the limits are > 1.0, they are treated as ground unit wavelengths.
Upper High-Cut limit	This is the end of the cosine taper.
Regional Offset	This constant will be subtracted from the observed gravity before inverting.
Constraints Grid	Grid to control where the inversion surface may change. Values of 0 indicate no change; 1 indicates maximum change.
Invert for Relief
Maximum elevation	Specify an absolute elevation, above which the inverted layer will not be allowed to go. This value is combined with the non-pierceable layers above the inverted layer to determine the maximum elevation allowed at any X,Y location.
Minimum elevation	Specify an absolute elevation, below which the inverted layer will not be allowed to go. This value is combined with the non-pierceable layers below the inverted layer to determine the minimum elevation allowed at any X,Y location.
Max elevation change	Limit the amount of elevation change allowed in each inversion iteration.
Invert for Density
Maximum density (g/cm³)	Specify the maximum density allowed for the layer.
Minimum density (g/cm³)	Specify the minimum density allowed for the layer.

Application Notes

Both Gz and Gzz algorithms perform "Stage 1" iterations using Gz (in the Gzz case, by integrating to produce Gz). Gzz inversion then performs "Stage 2" iterations using Gzz. Stage 1 converges quickly on the longer-wavelength components and is more robust, but is not as effective at fitting the short wavelength components. Stage 2 focuses on the short wavelength components and converges more quickly on them. However, it is less robust than Stage 1 and can diverge. You may use any combination of the two stages but the GX always runs Stage 1 iterations first. Stage 1 always improves the fit but usually more slowly than Stage 2.

The “Regional Offset” (RO) does impact the gradient structural inversion process. It subtracts the RO from the error grid that drives the inversion calculations. (This is equivalent to subtracting the RO from the observed grid because ERR=OBS – CALC – RO.) This is temporary and is not used when computing the final error grid or statistics at the completion of the inversion run. However, the RO has a significant impact on the inversion process since the sign of the error determines the direction of movement of the inversion surface.

Limitations

Structural inversion is not allowed on the top layer of the model
A density inversion is not allowed if the layer is not defined by a lateral density distribution (grid). This option will be disabled, in the dialog, if that is not the case.
The model must contain observed data for a component in order for that component to be enabled in the dialog.
Inversion is not supported if the density distribution of the layer to be inverted, or of the layer above, is a 3D distribution defined by a voxel.