Reduce to Magnetic Pole Filter

Use the Reduce to Magnetic Pole option to apply a Fast-Fourier Transform Reduction to the Magnetic Pole filter to a channel.

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

• Geophysics: 1D FFT > 1D Filtering
• Gravity and Terrain Correction: Moving Platform Gravity  > Apply Single FFT Filter

Reduce to Magnetic Pole filter options

Geomagnetic inclination	Geomagnetic inclination.(degrees). Default is 0. Script Parameter: FFTRPOLE.INC
Geomagnetic declination	Geomagnetic declination.(degrees). Default is 0. Script Parameter: FFTRPOLE.DEC
Amplitude correction inclination	Amplitude correction inclination.(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. When reducing to the pole from equatorial latitudes, North-South features can blow-up due to the strong amplitude correction that is applied when the declination + wavenumber direction is ð/2 (i.e. a magnetic East-West wavenumber). By applying a higher latitude for the amplitude correction alone, this problem can be reduced or eliminated at the expense of under-correcting the amplitudes of North-South features.

An amplitude inclination of 90 causes only the phase component to be applied to the data (no amplitude correction), and a value of 0 (zero) causes phase and amplitude components to be applied over the entire range. The default amplitude inclination value is 20 degrees in the Northern Hemisphere. In the Southern Hemisphere, the amplitude inclination value is negative and the default is –20 degrees.

The pole reduction filter needs to know the azimuthal orientation of the sensor θ in order to perform the reduction to the pole. This tool assumes that lines are relatively rectilinear, and calculates the orientation of each line using the first and last point of the line.

Reduction to Magnetic Pole Filter

Where:

I	Geomagnetic inclination
Ia	Inclination for amplitude correction (never less than I)
D	Geomagnetic declination

Parameters

Ia	Inclination to use for the amplitude correction. Default is 20. (Ia = 20, if I >0; Ia = (-20), if I <0 ). If |Ia | is specified to be less then |I |, it is set to I.