Reduce to Magnetic Pole (REDP)

Use the Reduce to Magnetic Pole option to apply the reduction‑to‑pole filter.

Reduce to Magnetic Pole dialog options

IGRF
Survey date (YYYY/MM/DD)	Enter the date of the survey. By default, this field is populated with the grid creation date. If you modify the date, you must click the Calculate button to the right of the entry to recalculate the IGRF field and refresh the Inclination and Declination fields. When you click OK, only the inclination and declination values are saved to the control file. The date itself is not saved. During later uses of this filter, the most recently saved inclination and declination values are loaded from the control file, and the displayed date reverts to the grid creation date. This should not cause issues because, once inclination and declination are defined, the date becomes irrelevant.
Inclination	Geomagnetic inclination I, in degrees measured from the horizon. This value is automatically calculated but can be overridden.
Declination	Geomagnetic declination D, in degrees azimuth. This value is automatically calculated but can be overridden.
Amplitude correction inclination	Amplitude correction inclination Ia, in degrees (see Application Notes). This value is automatically calculated as a function of the inclination. If you change the Inclination, you must click Calculate to recalculate the Amplitude correction inclination.

Application Notes

The IGRF parameters of an input magnetic grid depend on its geolocation and survey date.

If the grid does not have a coordinate system defined, you will be prompted to assign one.

Once calculated, the IGRF parameters are added to the workspace and used by several MAGMAP filters:

• The IGRF field strength is required for calculating apparent susceptibility.

• The IGRF field direction (inclination and declination) is used in the reduction to the pole.

Field strength, inclination, and declination are calculated together to ensure they remain synchronized.

Stability considerations at low magnetic latitudes

Reduction to the Pole includes:

• Amplitude component: sin(I)

• Phase component: i cos(I) cos(D-θ)

At near‑equatorial latitudes, the RTP filter becomes unstable near the declination direction, producing dominant artificial trends that obscure real geologic features.

To mitigate this, an amplitude‑modulating inclination I_a is introduced.

This reduces instability but results in under‑correction of amplitudes for features aligned with the declination direction.

To minimize amplitude distortion in low latitudes, I_a should be chosen relative to I—large enough to counteract instability but small enough to minimize amplitude distortion.

Reduce to Magnetic Pole Equation

if | Ia | < | I |, Ia = I

Where:

I	Geomagnetic inclination (°).
D	Geomagnetic declination (° azimuth from north).
Ia	Inclination for amplitude correction (never less than I) ->

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.