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 has been conducted to automatically calculate and populate the inclination and declination of the IGRF field. The calculate button appears to the right of the date entry once a valid date is entered. This is an optional field.
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 to calculate the susceptibility (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 the Inclination of zero to this inclination I a only the phase component is used in the reduction operator. Beyond this inclination the operator equation comprises both the phase and the amplitude. The default is 20.

Application Notes

Apparent Susceptibility Calculation

A susceptibility filter is a compound filter that performs a reduction to the pole, downward continuation to the source depth, correction for the geometric effect and 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, the horizontal dimensions of which are equal to the input grid cell size. This is an idealized approximation hence the descriptor 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 degrees. If |Ia| is specified to be less than |I|, it is set to I.
a	½ Cell size

Wavenumber domain variable definition:

k	Wavenumber domain increment, used to depict a radially symmetrical variable.	where: np is the number of points cs is the cell size
u	X component in the wavenumber domain.	k = 2π ( i μ+j ν )
v	Y component in the wavenumber domain.
r	Radial component in the wavenumber domain.	also 2πk
θ	Polar component in the wavenumber domain.

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 is naturally subject to how well the actual observed field conforms to these assumptions.

To calculate the IGRF parameters, the date and geographic location are needed. If this information is not already supplied, you will be prompted to define it. The IGRF field strength is only required for apparent susceptibility calculation, but is re-calculated along with the inclination and declination in the other controls, so the values are always synchronized.