1D Filtering
Use the 1D FFT > 1D Filtering menu option (geogxnet.dll(Geosoft.GX.FFT1D.FFT1DFiltering;Run)*) to apply up to six 1D Fast Fourier Transform filters to a channel and to interactively preview the power spectrum and resulting filter responses in both the Fourier and Space domains. As you adjust the filter parameters, the updated results are displayed immediately in the spectrum and profile windows.
1D Filtering dialog options
Spectrum Pane
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At the start, this pane is empty. Once you select a channel to filter and define a filter with its parameters, the interactive spectrum graph displays the radially averaged power spectrum of the input channel (current line) as a function of wavenumber. This spectrum is plotted in black. After you define a filter, its response curve appears in blue, and the filtered spectrum is shown in red. When multiple filters are applied, each filter and its output spectrum are shown in additional distinct colours. A legend along the bottom of the graph identifies each series.
The horizontal axis ranges from 0 to 1/(2 x distance increment) in the wavenumber domain. For convenience—because filter parameters are specified in units of wavelength rather than wavenumber—you can toggle the axis display between wavenumber and wavelength (1/wavenumber). This toggle affects only the axis units, not the underlying graph data.
To enable finer control at longer wavelengths (lower wavenumbers), you can adjust the minimum wavelength to up to ten times its standard value.
The vertical axis shows the logarithm of the power spectrum on the left and the range of filter coefficients on the right. You can modify the displayed power-spectrum range by editing the axis limits.
The Reset  button returns the display to its initial data range.
The Spectrum pane is resizable in both directions. To resize horizontally, drag the horizontal grip while holding down the left mouse button.
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Profile Pane
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At the start, this pane is empty. After you select a channel to filter and define a filter with its parameters, the graph displays the profile of the filter applied to the current line of the current database. The input channel profile is shown in red, and the filtered (output) response is shown in green. When multiple filters are applied, the output channel profile is updated to reflect the combined effect of all filters on the input channel.
If the filter is one of the following types:
- Reduce to Magnetic Pole (RTP), Continuation, or low-pass filters (Butterwortth, Band-pass, Cosine roll-off, Gaussian) — the filtered (output) channel profile is displayed using the same vertical scale as the input channel profile. The minimum and maximum axis values are determined from both profiles together.
- If the filter is a Derivative or high-pass filter (Butterwortth, Band-pass, Cosine roll-off, Gaussian) — the filtered profile uses the same vertical scaling factor as the input profile, but each profile has its own minimum and maximum axis range, and each is centered independently on its own mid‑data value.
- For all other filter types— the filtered (output) channel profile is scaled independently, with its own minimum and maximum axis values.
The Profile pane can be resized in both directions.To resize it horizontally, drag the horizontal grip while holding the left mouse button.
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Channel to filter
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The input channel to be filtered.
Script Parameter: FFT1MULT.IN
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Output channel
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The output channel. If the channel does not exist, it will be created.
Script Parameter: FFT1MULT.OUT
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FFT Sampling Parameters
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Distance increment
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Optionally specify the distance increment in ground units. If not specified, the nominal data spacing will be calculated automatically.
Script Parameter: FFT1MULT.SAMPINCR
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Interpolation method
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Select the interpolation method to use:
- Minimum Curvature
- Linear
- Akima
If the default Minimum Curvature interpolation introduces artifacts in the filtered output, switching to Linear interpolation or specifying a distance increment appropriate for your data will generally reduce or eliminate these artifacts.
Script Parameter: FFT1MULT.METHOD
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Filter Definition
This section is where you define the filter(s) and their parameters. You can choose from a set of 15 filters. Add a new filter by clicking the Add Filter  button located at the bottom right of the pane. Each filter has its own set of parameters.
The Help  button beside each filter provides a detailed description of that filter.
After selecting a filter and pressing Enter, the Spectrum and Profile windows update automatically to reflect the results of the current filter combination.
Click Clear to remove all filters and their parameters from the dialog. The spectrum graphs and profiles will reset accordingly.
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Filter #/name
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Select a filter from the drop-down list. You may define up to six filters.
The following filters are available:
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- Apparent Density Calculation
- Apparent Susceptibility Calculation
- Band-pass
- Butterworth
- Continuation
- Cosine Roll-off
- Gaussian
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- Horizontal Derivative
- Horizontal Integration
- High-pass
- Low-pass
- Reduce to Magnetic Pole
- Resistance Capacitance
- Vertical Derivative
- Vertical Integration
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Fourier domain filters are
commutative, so the order in which the filters are applied does not affect the result.
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Script Parameters: FFT1MULT.FILTER1 [FILTER2, FILTER3, ... FILTER6)
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Filter Parameters
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Each filter may require additional parameters. You will be prompted to enter these values according to the filter’s requirements. Parameters can be adjusted by typing directly into their fields. After entering a value, press Enter to update the spectra and profiles automatically.
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Application Notes
To allow you to maximize the preview panes directly, the 1D Filtering interface includes a Maximize button in the dialog’s title bar. Because Windows Forms does not permit Maximize/Minimize buttons to co‑exist with a Help button in the title bar, the Help button has been placed outside the title bar—following Oasis montaj conventions—alongside the OK and Cancel buttons at the bottom of the dialog.
Processing Workflow
This tool applies multiple filters to a channel in a single operation. The workflow consists of the following steps:
- Extend the data to ensure continuity across both edges.
- Interpolate the data to remove all dummies.
- By default, the Maximum Entropy method is used to fill the data.
- You can choose an alternate method by running Set Expansion Method.
- Compute the Fourier Transform (FT) of the preprocessed data to convert it to the wavenumber domain.
- Apply the filter(s) using a simple commutative multiplication process.
- Perform the Inverse Fourier Transform to rerun the filtered data to the space
domain.
Considerations
Filters that emphasize high-frequency content—such as Vertical Derivative, Downward Continuation, and High-pass type filters—may introduce numerical noise. If noise appears in the output data, consider adding a smoothing filter to attenuate it.
For reliable results, the data must be regularly sampled.
If the data contains large gaps, or multiple merged lines with changes in direction, irregularities or jumps may appear in the output. If such issues arise, inspect the line path closely and apply appropriate corrections.
Finding Help for a Specific Filter
After selecting a filter, detailed information is available by clicking the button located beside the Filter drop-down list. You can also use the filter‑specific links provided below (if applicable).
For improved visualization of the preview panes, Maximize and Minimize buttons are included in the dialog’s title bar. The main Help button is located at the bottom of the dialog, next to the OK and Cancel buttons.
*GX.NET tools are embedded in the geogxnet.dll file located in the \Geosoft\Desktop Applications\bin folder. To run this GX interactively (outside the menu), navigate to the bin directory and specify the GX.NET tool in the required format. See the topic for more guidance.
