Gzz Structural or Density Inversion

Use this function to perform structural or density inversion on a GM-SYS 3D model using Gzz.

3D Gravity Inversion dialog options

Inversion surface	Drop down list of the model’s surface grids. The chosen surface defines the top of the inversion layer. With structural inversion, this grid will be altered. With density inversion, the layer's corresponding lateral density distribution grid will be altered.
Max Stage 1 Inversion Iterations	Gzz inversion uses a two-stage approach (see Application Notes). Inversion process will stop after this many iterations, even if the convergence target has not been reached.
Max Stage 2 inversion iterations	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.
Lower high-cut limit	A cosine tapered high cut filter is applied to the error before inverting. This is the beginning point of the cosine taper.
Upper high-cut limit	… and this is it’s end. These numbers are expected to be given as a fraction of the nyquist - 0.5 and 0.7 are the defaults
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.
[Limits]	Set limits to the amount of change allowed in each inversion iteration.

Application Notes

Gzz inversion uses a two-stage approach:

• Stage 1 converges quickly on the longer-wavelength components, and it is more robust.

• Stage 2 focuses on the short wavelength components and converges more quickly on the short-wavelength components. However, it is less robust than Stage 1, and it can diverge.

You may use any combination of the two stages, but the dialog 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 is significant on the inversion process since the sign of the error determines the direction of movement of the inversion surface.

Before inverting, the model is checked to assure that the calculated model response is up to date with respect to the other model files.