Magnetic Structural Inversion

The Magnetic Structural Inversion function modifies the elevation of the selected layer in order to minimize the magnetic misfit. Inversion updates the calculated response and error to reflect the structural changes.

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.

To Perform a Magnetic Structural Inversion

On the Calculate menu, select Magnetic Structural Inversion. The Magnetic Structural Inversion dialog appears.
- This dialog can also be accessed by right-clicking the model layer on the Layers tab of the Model Explorer and selecting Magnetic Structural Inversion.
From the dropdown list, select the Inversion Surface. This layer will be altered.
Enter a Maximum Number of Inversion Iterations. Inversion process will stop after this many iterations, even if the convergence target has not been reached.
Enter the Maximum Number of Taylor Series terms. This should be odd number less than 12. Higher values put more emphasis on short wavelengths. More than 5 terms are seldom useful.
Enter the Convergence Limit. 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.
Enter the Lower High-Cut Limit. A cosine roll-off high cut filter is applied to the error before inverting. This is the beginning of the cosine taper. The default value is equivalent to 0.5 times the Nyquist.
Enter the Upper High-Cut Limit, which is the short-wavelength end of the taper. 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.
Enter the Z0. This is the 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.
Enter the DC. This is the 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.
Enter the Regional Offset. This constant will be subtracted from the observed magnetic data before inverting.
Browse to the Constraints Grid. This controls where the inversion surface may change. Values of 0 indicate no change; 1 indicates maximum change.
Click Yes to continue.
If prompted, click Yes to recalculate. The calculation is processed.