MAGMAP Filter Design

Use the interactive filter design dialog to define the Fourier domain filters and to interactively preview the original grid and the filtered grid(s). If the filter file provided in the MAGMAP Filtering dialog is a pre-existing filter control file (*.con) , the filter values stored in that file will be loaded by default in this dialog. Filter parameters can then be interactively modified to obtain the best results for your data.

The preview pane windows enable you to view both your original grid and your resultant filtered grid(s). As you interactively modify your filter parameters, the results can be immediately viewed in the Filtered Grid window.

MAGMAP Filtering | Create/Edit Filter dialog options

Spectrum Pane

The interactive spectrum graph (See Figure 1) displays the radially averaged power spectrum of the input grid as a function of wavenumbers. This spectrum is plotted in black. Once you define a filter, if it has a radial representation, the filter graph will be displayed in blue. The icon flags filters that do not have a radial representation. 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 identifies them.

The range of the abscissa spans from 0 to ½ cell size. For convenience and because you provide the parameters in 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 up to a factor of 10. You can change the range by editing the value at the right end of the abscissa.

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 ordinate.

When you change the graph range, the reset button (), located at the bottom left of the graph, becomes active and allows you to return to the initial data range.

The Spectrum pane is resizable in both directions. To resize the pane, grab the horizontal or vertical grip by holding the left mouse button down on the grip (Figure 2).

Filter Definition

This is where you define the filter(s) and their parameters. You can choose from a set of 29 filters. MAGMAP will accept up to four concurrent filters. Add filters by clicking on the Add Filter () button located at the bottom right of the pane. Each filter has its own specific parameters. The button located beside each filter provides the detailed description of that filter.

After specifying a filter and clicking <Enter>, all the other panes are updated to reflect the outcome of the new filter combination. As a further convenience, low/high/bandpass filters offer live sliders that allow you to interactively see the effect of their settings.

Filter #/name

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

The following filters are available:

- Apparent Density Calculation

- Apparent Susceptibility Calculation

- Bandpass

- Butterworth

- Butterworth Bandpass

- Butterworth Notch

- Conversion between Field Components

- Cosine Roll-off

- Cosine Roll-off Bandpass

- Derivative

- Differential Reduction to Magnetic Pole

- Directional Cosine

- Directional Pass/Reject

- Downward Continuation

- Gaussian Regional/Residual

- General Radially Symmetric

- Gravity Earth Filter

- High-pass

- Horizontal Integration - X

- Horizontal Integration - Y

- Low-pass

- Notch

- Pseudo-ogravity

- Reduce to Magnetic Equator

- Reduce to Magnetic Pole

- Transform from Magnetic Pole

- Upward Continuation

- Vertical Integration

- Wiener Optimum

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

Filter Parameters

You will be prompted for additional parameters according to each filter’s requirements. Parameters for each filter can be adjusted either by moving the slider controls where available (), or by entering the values directly in the parameter fields (). After entering a parameter, press the <Enter> key, and the spectra and preview images will automatically update.

Preview Pane

The preview pane offers an interactive view of both the original grid and the filtered grid(s). The images in the two preview windows are linked and the cursor location is tracked. The most recent colour table assigned to the input grid is used to display the images in the preview pane.

The pane is resizable; grab the grip at the bottom right of the pane to resize the pane.

Original grid

This is an image view of your original grid.

Filtered grid(s)

This is an interactive image view of your filtered grid(s).

At the onset, this pane is empty. As soon as you define filters, the outcome is reflected in this pane. If you defined multiple filters, the Preview filter result list becomes active and you could view their effect individually or combined.

[Zoom]

To zoom into a location on both grids, click the Select and zoom to an area of interest ( ) button. Note that your cursor changes to a select mode when it crosses either the Original grid or the Filtered grid(s) windows.

Using your cursor, draw a box around the desired area, then click inside the boxed area to zoom to the selected location. Click the Show entire data extent () button to return your zoomed images to the original full size.

Application Notes

Figure 1: MAGMAP - Interactive filter layout

Figure 2: MAGMAP - Interactive definition of filters

Grid Size Consideration

If the input grid is very large, the interactive refresh of the graph/images in the MAGMAP filter window could be slow. To circumvent the potential delay, when the input grid is larger than 2048 cells in either dimension, the grid is decimated to dimensions less than 2048. The decimated grid is saved in the working folder and is reused in subsequent invocations of the MAGMAP interactive filter. If you delete the preview grid, next time you run MAGMAP it will be recreated. The decimated grid inherits the base name of the input grid appended by "_preview". Its transform inherits the base name of the input grid appended by "_preview_trn". The decimated grid and its transform are only used to produce the graph/images in the Magmap filter window. The final grid is produced using the original grid as input.

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. Additional information can be found in the 2D Filtering Learning Path.

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.