Reduce to Magnetic Pole Filter

Use the Reduce to Magnetic Pole option to apply a Fast Fourier Transform (FFT) reduction to magnetic pole 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

Reduce to Magnetic Pole filter options

Geomagnetic inclination	Geomagnetic inclination in degrees. Default: 0 Script Parameter: FFTRPOLE.INC
Geomagnetic declination	Geomagnetic declination in degrees. Default: 0 Script Parameter: FFTRPOLE.DEC
Amplitude correction inclination	Amplitude correction inclination in degrees. Default: 20 (Northern Hemisphere) or –20 (Southern Hemisphere). Script Parameter: FFTRPOLE.ACINC

Application Notes

• Reduction to the pole has an amplitude component and a phase component.

• At equatorial latitudes, North–South features can become exaggerated due to strong amplitude corrections applied when the declination plus wavenumber direction equals π/2 (magnetic East–West wavenumber).

• To reduce or eliminate this effect, apply a higher latitude for amplitude correction only. This under-corrects North–South amplitudes but prevents blow-up artifacts.

Key Behaviors:

• An amplitude inclination of 90° applies only the phase component (no amplitude correction).

• An amplitude inclination of 0° applies both phase and amplitude corrections across the full range.

• Default amplitude inclination: 20° in the Northern Hemisphere; –20° in the Southern Hemisphere.

The filter also requires the azimuthal orientation of the sensor (θ) to perform the reduction. The tool assumes survey lines are relatively straight and calculates orientation using the first and last point of each line.

Reduction to Magnetic Pole Filter

Where:

I	Geomagnetic inclination
Ia	Inclination for amplitude correction (never less than I) Ia = 20 if I >0 Ia = -20 if I <0 If |Ia| is specified as less then |I|, it is set to I.
D	Geomagnetic declination