3D Magnetic Structural Inversion

Use the Calculate > Magnetic Structural Inversion option to perform magnetic structural inversion on a layer in the GM-SYS 3D model.

Magnetic Structural 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 number of inversion iterations	Inversion process will stop after this many iterations, even if the convergence target has not been reached.
Max number of Taylor Series terms	Should be odd number less than 12. Higher values put more emphasis on short wavelengths. More than 5 iterations are seldom useful.
Convergence limit (in ground units)	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 roll-off high-cut filter is applied to the error before inverting. Enter the beginning of the cosine taper. The default value is equivalent to 0.5 times the Nyquist.
Upper high-cut limit	Enter the short-wavelength end of the taper. The default is the wavelength equivalent to 0.7 times the Nyquist. If you enter numbers that are <= 1.0, they are interpreted as fractions of Nyquist. If you enter numbers that are > 1.0, they will be interpreted as wavelengths (distances) in ground units.
Z0 (nominal top of surface)	Downward continuation depth relative to sea level. This level should be above the highest elevation you expect in the inverted structure. If Z0 is too deep, the inversion will fail to converge. The default value is the magnetic survey elevation.
DC (nominal mean of surface)	Mean depth of the resulting inverted surface relative to sea level, which must be deeper than Z0. The default value is the mean of the selected layer's relief surface. If the inversion is triggered from the layer context menu, the default is calculated from the selected layer's relief surface and displayed; otherwise "*" is displayed to indicate that it will be calculated during the inversion.
Regional offset	This constant will be subtracted from the observed magnetics before inverting.
Constraints grid	Designated Constraints Grid for restriction of calculated values.

Application Notes

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

Because the magnetic structural inversion involves a downward continuation of the magnetic anomaly data to the "Z0" depth, a high-cut filter is necessary to prevent the input anomaly from "blowing up" when the highest frequencies are amplified by the downward continuation filter. If the input anomaly grid has high frequency noise from survey resolution limits or high frequency signal from shallow magnetic sources (e.g., volcanics or culture), the inversion algorithm cannot produce a basement structural relief at significant depth to account for the high frequency portion of the input residual anomaly. The inversion algorithm will attempt to oscillate rapidly up and down in its attempt to fit the high frequency signal. This is undesirable and geologically improper. The user-specified high-cut filter can help stabilize the inversion and prevent the inversion process from oscillating. However, you should also avoid over-filtering, as the resulting inversion surface may be smoother than desired. You can iterate by repeatedly running inversion, filtering heavily on the first attempt and then gradually lessening the filter.

Magnetic structural inversion can be improved by an understanding of the local, relative vertical relief of the output results in relation to the selected grid interval. In cases where the chosen grid interval is small, (less than one tenth of the local vertical relief), the Taylor Series order number needs to be increased to a larger odd number (say 7, 9 or 11) in order to allow enough high frequency variation into the inversion grid to fit the rapidly varying relief. Unfortunately there are limits to how much the Taylor Series order can be increased to fit sharp structures at significant depth because the inversion can oscillate and "blow up". Even high-cut filtering of the magnetic anomaly does not completely solve this problem of the structural inversion "blowing up". The ratio of the local relief on the structure layer divided by the grid spacing is critical. If this ratio is greater than ~10:1, and the Taylor Series Order is increased to 9 or 11, the process can still "blow up" even with a moderate high-cut filter. Until the user is comfortable with the simultaneous influences of vertical relief/depth of inversion surface, noise and filter cutoffs - stick to a ratio of 10:1 or less.