Apparent Susceptibility Calculation (SUSC)

Use the Apparent Susceptibility Calculation option to apply the apparent susceptibility calculation filter.

Apparent Susceptibility Calculation dialog options

Date (YYYY/MM/DD)	Enter the date when the survey was conducted to automatically calculate and populate the inclination and declination of the IGRF field. A Calculate button appears to the right of the date entry once a valid date is entered. This field is optional.
Field strength	Total geomagnetic field strength in nT (F).
Inclination	Geomagnetic inclination in degrees from the horizon (I).
Declination	Geomagnetic declination in degrees azimuth (D).
Depth to find susceptibility	Vertical distance (h) in ground units, relative to the observation level, at which the susceptibility should be calculated (between the observation datum and the ground). If the flying height is defined in the MAGMAP Control file, it will be used.
Pole reduction amplitude inclination	From an inclination of zero up to this inclination (I a), only the phase component is used in the reduction operator. Beyond this inclination, the operator equation includes both the phase and the amplitude. The default is ±20°.

Application Notes

Apparent Susceptibility Calculation

A susceptibility filter is a compound filter that performs:

• reduction to the pole

• downward continuation to the source depth

• correction for the geometric effect

• division by the total magnetic field

to yield susceptibility.

Apparent susceptibility mapping assumes a simple geometric model. The response is assumed to be caused by a collection of vertical, square-ended prisms of infinite depth extent, whose horizontal dimensions are equal to the input grid‑cell size. This is an idealized approximation—hence the term apparent.

Where:

I	Geomagnetic inclination in °
D	Geomagnetic declination in ° azimuth
h	Depth in ground units, relative to the observation level
Ia	Inclination for amplitude correction (never less than I) Default is ±20°. If |Ia| is specified to be less than |I|, it is set to I.
a	½ cell size

Wavenumber Domain Variable Definitions

The following variables are used in the wavenumber domain:
k	Wavenumber increment, used to depict a radially symmetrical variable.	Where: np: number of points cs: cell size
μ	X-component of the wavenumber.
v	Y-component of the wavenumber.
r	Radial component of the wavenumber.
θ	Angular (polar) component of the wavenumber.

Susceptibility Filter Assumptions

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

• The IGRF has been subtracted from the input magnetic field prior to filtering.

• There is no remanent magnetization.

• All magnetic response is caused by a collection of vertical, square-ended prisms of infinite depth extent.

• The resulting susceptibility data is dimensionless (cgs units).

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

To calculate the IGRF parameters, the date and geographic location are required.

If this information is not already supplied, you will be prompted to define it.

The IGRF field strength is required only for apparent susceptibility calculation, but it is recalculated—along with inclination and declination—in the other controls to ensure values remain synchronized.