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 both in Fourier and Space domains. While you interactively modify your filter parameters, the results can be immediately viewed in the spectrum and profile windows.

1D Filtering dialog options

Spectrum Pane

At the onset, this pane is empty. As soon as you select a Channel to filter and define a Filter and its parameters, the interactive spectrum graph displays the radially averaged power spectrum of the input channel (current line) as a function of wavenumbers. This spectrum is plotted in black. Once you define a filter, the filter response is displayed in blue. The filtered spectrum is displayed in red. If a combination of filters is applied, their filter and output spectra acquire other distinct colours. A legend along the bottom of the graph helps identify them.

The range of the abscissa spans from 0 to 1 / (2 x distance increment) in the wavenumber domain. For convenience and because you provide the parameters in units of wavelengths rather than wavenumbers, you can toggle between wave numbers and wavelength (1/wavenumber). However, this does not affect the graph.

To allow for a finer control in the longer wavelengths (or lower wavenumbers), the minimum wavelength can be adjusted to up to 10 times the minimum wavelength.

The ordinate is annotated with the logarithm of the power spectrum on the left and the range of the filter coefficients on the right. You can change the range of the power spectrum by editing the end values of the axis.

A reset button () will allow you to return to the initial data range.

The Spectrum pane is resizable in both directions. To horizontally resize the pane, grab the horizontal grip by holding the left mouse button down on the grip.

Profile Pane

At the onset, this pane is empty. After you select a Channel to filter and define a Filter and its parameters, the Graph displays the profile of the filter applied to the current line of the current database. The input channel profile is displayed in red. The filtered (output) response is displayed in green. If a combination of filters is applied, the output channel profile will be updated to reflect the effect of the filters on the input channel.

If the filter is one of the following types:

Reduce to magnetic pole (RTP), Continuation, low pass filter (Butterwortth, Band-pass, Cosine roll-off, Gaussian) — the output (filtered) channel profile is displayed in the same vertical scale as the input channel profile (the minimum and maximum values are determined from both profiles).
Derivative, high pass filter (Butterwortth, Band-pass, Cosine roll-off, Gaussian) — the output channel profile is displayed using the same vertical scaling factor as the input channel profile, but each profile has its own minimum and maximum range and the profiles are centred individually on their own mid-data values.
For all the other filters — the output channel profile is scaled separately (with its own minimum and maximum axis scale values).

The Profile pane is resizable in both directions. To horizontally resize the pane, grab the horizontal grip by holding the left mouse button down on the grip.

Channel to filter

The input channel to filter.

Script Parameter: FFT1MULT.IN

Output channel

The output channel (if it does not exist, it will be created).

Script Parameter: FFT1MULT.OUT

FFT Sampling Parameters

Distance increment

Optionally, specify the distance increment in ground units. If not specified, the nominal data spacing will be calculated.

Script Parameter: FFT1MULT.SAMPINCR

Interpolation method

Select the Interpolation method to be used:

Minimum Curvature
Linear
Akima

If the default Minimum Curvature interpolation introduces artifacts in the filtered output, changing the interpolation method to Linear or specifying a distance increment that is appropriate to your data would likely prevent/reduce artifacts.

Script Parameter: FFT1MULT.METHOD

Filter Definition

This is where you define the filter(s) and their parameters. You can choose from a set of 15 filters. Add filters by clicking on the button located at the bottom right of the pane. Each filter has its own specific parameters. The button beside each filter provides the detailed description of that filter.

After specifying a filter and clicking <Enter>, the spectrum and profile windows are updated to reflect the outcome of the new filter combination. Click the Clear button to clear the filter(s) and their parameters from the dialog. The spectrum graphs and profiles will reset as well.

Filter #/name

Select a filter. Up to six filters can be selected from the drop-down list.

The following filters are available:

- Apparent Density Calculation

- Apparent Susceptibility Calculation

- Band-pass

- Butterworth

- Continuation

- Cosine Roll-off

- Gaussian

- Horizontal Derivative

- Horizontal Integration

- High-pass

- Low-pass

- Reduce to Magnetic Pole

- Resistance Capacitance

- Vertical Derivative

- Vertical Integration

The Fourier domain filters are commutative and the order in which they are supplied is not important.

Script Parameters: FFT1MULT.FILTER1 [FILTER2, FILTER3, ... FILTER6)

Filter Parameters

You will be prompted for additional parameters according to each filter’s requirements. Parameters for each filter can be adjusted by entering the values directly in the parameter fields. After entering a parameter, press the <Enter> key, and the spectra and profiles will automatically update.

Application Notes

To allow you to directly maximize the preview panes, the 1D Filtering interface has a Maximize button in the title bar. The Minimize and Maximize buttons, however, cannot co-exist with the Help button in the title bar of Windows Forms. To offer both capabilities in 1D Filtering, the Help button was added outside the title bar, and to follow Oasis montaj convention, it has been added alongside the OK and Cancel buttons at the bottom of the dialog.

This tool applies multiple filters to a channel in one go. The process consists of the following steps:

Extend data to allow for ensuring continuity across the two edges.
Interpolate the data to remove all dummies – by default, the Maximum Entropy method is used to fill the data, but you could choose an alternate method by running the Set Expansion Method.
Fourier Transform (FT) the preprocessed data to the wavenumber domain.
Apply the filter(s) using a simple commutative multiplication process.
Inverse Fourier Transform the filtered data back to the space domain.

Filters that emphasize the high frequency content such as the Vertical Derivative & Downward Continuation, High-pass type filters, tend to add numerical noise to the data. If the noise is observed in the output data, you could attenuate it with an accompanying smoothing filter.

To get appropriate results, the data must be regularly sampled. If the data has wide gaps or multiple lines are merged and they change direction, irregularities/ jumps may be observed in the output data. If you notice such issues, closely inspect the path and take the corrective measures.

Finding Help for a Specific Filter

Once you have selected your filter of interest, the filter information is available by clicking the button located beside the Filter drop-down or by using one of the links below.

To allow you to better visualize the preview panes, maximize and minimize buttons are located on the dialog's title bar, while the main Help button has been placed alongside the OK and Cancel buttons at the bottom of the dialog.

*The GX tool will search in the "...\Geosoft\Desktop Applications \gx" folder. The GX.Net tools, however, are embedded in the geogxnet.dll located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" folder, then supply the GX.Net tool in the specified format.