Apparent Susceptibility Calculation Filter

Use the Apparent Susceptibility Calculation option to apply a Fast Fourier Transform (FFT) Apparent Susceptibility filter to a channel.

The filter option is available with the following extensions and menus:

1D FFT > 1D Filtering
Moving Platform Gravity > Filters > Apply Single FFT Filter

Apparent Susceptibility Calculation filter options

Geomagnetic inclination	Geomagnetic inclination. Default = 0 Script Parameter: FFTSUSC.INC
Geomagnetic declination	Geomagnetic declination. Default = 0 Script Parameter: FFTSUSC.DEC
Amplitude correction inclination	Amplitude correction inclination. Default = 20 Script Parameter: FFTSUSC.ACINC
Depth to sources (in ground units)	Depth to sources in ground units. Default = 0 Script Parameter: FFTSUSC.DEPTH
Low-pass cutoff (in ground units)	Low-pass Butterworth cutoff. Default = 0 Script Parameter: FFTSUSC.LP
Total geomagnetic field strength (nT)	Total geomagnetic field strength in nT. Default = 55000 Script Parameter: FFTSUSC.STRENGTH

Application Notes

A susceptibility filter is a compound filter that performs:

Reduction to the pole
Downward continuation to the source depth
Correction for the geometric effect of a vertical square-ended prism
Division by the total magnetic field to yield susceptibility

An optional Butterworth low-pass filter may also be applied to remove noise from the data.

The susceptibility filter calculates the apparent magnetic susceptibility of the magnetic sources using the following assumptions:

The magnetic field has had the IGRF removed.
There is no remnant magnetization.
All magnetic response is caused by a collection of vertical prisms of infinite depth extent.
The result is in SI units.

The validity of the results depends on how well the observed field conforms to these assumptions.

Apparent Susceptibility Filter Components

$image\ebx_34577.jpg$

Where:

H(ω): Downward continuation to depth h

G(θ): Reduction to the pole

K(ω): Geometric factor of a vertical prism (dimensions: 2a×¥×¥)

α: Half cell size

ω: Wavenumber (radians/ground unit)

θ: Wavenumber direction (azimuth)

Parameters:

h: Depth in ground units, relative to observation level at which to calculate the susceptibility.

I_a: Pole reduction amplitude inclination. This is the inclination used to apply only the phase component in the reduction to the pole. The default is $image\ebx_-259103764.jpg$ 20. If |I_a | is specified to be less then |I |, it is set to I.

I: Geomagnetic inclination

D: Geomagnetic declination

F: Total geomagnetic field strength (nT)