How To: Perform "depth slicing" (in the Fourier domain) on your Total Magnetic Intensity (TMI) data

Symptom/Problem:
I want to depth slice my magnetic (TMI) gridded data. How can I do this using Geosoft software?

Environment:
Applies to the MAGMAP extension for Oasis Montaj. This particular resolution applies to Oasis Montaj v7.0, but can be applied to earlier versions as well.

Resolution:
You can use the Magmap extension and the interactive spectrum filtering tool to "slice" the anomaly (of a magnetic grid) based on wavelength. Apply a suitable Butterworth filter (or Bandpass filter) to select wavelength cut-offs to define a window focusing on different depths. In more detail:

Follow the preparation of the input spatial grid, using the Magmap > Interactive filtering options, to create an FFT transform grid, i.e.

a) Prepare grid for FFT (expansion) using MAGMAP > Interactive Filtering > Prepare Grid.

b) Perform FFT using MAGMAP > Interactive Filtering > Forward FFT.

c) Create radially averaged power spectrum using MAGMAP > Interactive Filtering > Radial Average Spectrum.

d) Use the spectrum file in interactive filtering (MAGMAP > Interactive Filtering > Interactive Spectrum Filters) to define a suitable control file to apply to the input magnetic grid to output the "depth slice".

e) Apply Magmap control file to the magnetic grid to create output filtered depth sliced grid (using MAGMAP > Interactive Filtering > Apply Filters).

Note: Magmap applies filters in the wavenumber domain. To convert wavelength to wavenumber, simply calculate the inverse of wavelength (in project's ground units) to output wavenumber.

e.g. Project units = metres.
Wavelength of interest = 500m, wavenumber = 1/500 = 0.002 cycles per metre