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Magnetic transects (T62)

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INTREPID's Magnetic Transects tool provides depth estimations of unparalleled accuracy and vertical resolution, through analysing the signal due to the heterogeneity inherent in all magnetic bodies

In this manual:

Theory

References

Clifton, R., 2015, Magnetic depths to basalts: extension of spectral depths method: Exploration Geophysics 46, p 284-296, http://dx.doi.org/10.1071/EG13096

Clifton, Roger and FitzGerald, Des, 2018, Extending magnetic depths past 1000 m. ASEG Extended Abstracts 2018, 1-4

Equivalent layer of random dipoles

In a recent, major advance beyond other spectral methods based on magnetic grids, INTREPID's Magnetic Transects tool provides depth estimations of unparalleled accuracy and vertical resolution. The innovation consists of analysing the signal due to the heterogeneity inherent in all magnetic bodies. (Clifton, 2015)

Consider that the power spectrum of a heterogeneous magnetic body can be approximated by the power spectrum of a layer of randomly located elementary dipoles at or near the same depth as its top. The total magnetic intensity (TMI) of the equivalent layer is then a convolution of the field of a single dipole with its distribution in the layer. Remarkably, the power spectra of the single dipole and of the distribution separate, with the elementary dipole appearing at the low frequencies in the combined spectrum. The power spectrum, due to its distribution in the equivalent layer, is almost white, spreading across the width of the combined spectrum with minimal interference with the dipole spectrum. The geometry of real bodies is more jagged and its contribution to the power spectrum leaves the dipole signal only intermittently clean.

Inversion for depth

The power spectrum of a single dipole is special in that the slope along the spectrum is entirely determined by its depth. With that formula known (ibid), the slope can be inverted to provide an estimate of the depth. Because there are many points along the spectrum, a spectrum of slopes can be calculated and using the inversion, a ‘depth spectrum’. With recent improvements (Clifton and FitzGerald, 2018), INTREPID is now able to provide credible depths to 1500 m and beyond.

Simultaneous multiple depths

Frequently, a magnetic body occupies several depths in the same area. They often appear as separate groups of depth estimates in the same depth spectrum. Consequently, the method is able to assign depths to two or even three bodies appearing in the same survey.

Depth transects

INTREPID rearranges each depth spectrum vertically, in a ‘depth profile’, with the estimated depths arranged like a seismic trace. The sample window, typically 20 x 20 km, is moved across the grid, typically 5 km at a time. Each time INTREPID adds a depth profile for the new location. With the depth profiles gathered as in a seismic section, this provides the user with a ‘depth transect’. At the same time, INTREPID stores the results in a 3-D array for integrating with other data and models.

Noise

Where they share similar depths, neighbouring bodies can interfere and confuse the depth spectrum. Similarly, the geometry of the bodies contributes to the signal and often scatters the depth spectrum. Shallow signals, from depths similar to the flight line spacing, are severely limited. These last two effects make the method less effective in the shallowest depths, limiting its minimum depth estimates approximately to flight line spacing. By adjusting parameters in INTREPID, however, once we have identified the depth signal of a body of interest, we can track it further into the shallow noise.

Magnetic Transects provides the inverse transform for the TMI anomaly above a heterogeneous body. Forward modelling to match the same TMI anomaly with a heterogeneous body requires INTREPID's Fourier modelling tool. See Random dipole modelling (T64).

Using Magnetic Transects

Follow these steps to use the Magnetic Transects tool interactively. For batch mode, see Running the tool in batch mode.

To use Magnetic Transects interactively
  1. In Project Manager, from the Modelling menu, choose Magnetic Transects or use the command magnetic_transects.exe. INTREPID displays the Magnetic Transects windows.
  2. If you have previously prepared file specifications and parameter settings for Magnetic Transects, load the corresponding task specification file using File > Open task file. (See Specifying input and output files for detailed instructions.) If all of the specifications are correct in this file, go to step --. If you wish to modify any settings, carry out the following steps as required.
  3. An example task file exists in {install_path}/sample_data/examples/Tasks/Modelling/Magnetic_Transects/mag_transects_auto_basalt.task

    mtrmain.pngmtrmain_map.png
  4. Specify the inputs to be processed and the outputs required. Use the corresponding options from the window. See Specifying input and output files for detailed instructions.
  5. Set the options and parameters for the task. See Magnetic Transects options.
  6. If you want to record your settings so that you can re-use them, save a .task file. See Saving settings for later use for detailed instructions.
  7. Run the process. See Running the Magnetic Transects task interactively.
  8. transects.png

    While the process is running, you will be able to view the power spectrum, depth spectrum, and the depth plot by clicking on points that are green within the visualisation window.

    As the process runs, points will turn from yellow (not yet processed) to green (processed and ready to view).

  9. If you want to repeat the process, repeat steps 2–6, varying the parameters and data files as required.
  10. To exit from Magnetic Transects, select Exit from the File menu, or by pressing the exit button in the upper right.

After using this tool, you can inspect the output using 3D Explore visualisation tool (See 3D Explore (T61).

You can also execute Magnetic Transects as a batch task using a task specification (.task) file that you have previously prepared. See Displaying options and using task specification files for details.

Specifying input and output files

To use Magnetic Transects, you need to specify the grid datasets to be processed and the voxet dataset for saving the results.

In this section:

Specifying input, settings and output using a .task file

A sample .task file can be found at {install_path}/sample_data/examples/Tasks/Modelling/Magnetic_Transects/mag_transects_auto_basalt.task

Choosing to Open with task on this file will populate the fields of the Magnetic Transects window.

Specifying input manually

Specify input dataset in the Input controls. You can input both a TMI and DTM grid.

Type in or paste the path and filenames or use the Browse button (ico-java-folder.png). If you use a Browse button, INTREPID displays an Open or Save As dialog box for you locate or specify the file you require. (See “Specifying input and output files” in Introduction to INTREPID (R02) for information about specifying files).

mtrinput.png

Input TMI grid Use this option to specify the TMI grid to be processed.

INTREPID opens and displays the grid region in the Magnetic Transects Map view window.

Specifying output manually

Specify the output in the Output options area.

mtroutop.png

Output voxet— Specify the voxet that you are creating with this process.

Type in or paste the path and filenames or use the Browse button (ico-java-folder.png). If you use a Browse button, INTREPID displays an Open or Save As dialog box for you locate or specify the file you require. (See “Specifying input and output files” in Introduction to INTREPID (R02) for information about specifying files).

Using the Map View window

Intrepid will launch the Map View window at launch of the Magnetic Transects tool, once an input TMI grid has been selected it will appear in the view.

This Map View will update as you change the parameters of the tool, it is useful for displaying the lines which the tool will iterate over.

mtrmain_map.png

After you have selected Run, two new Visualisation windows will appear. The map window can be panned over by left-click and drag, or zoomed with the scroll-wheel. Both windows can be exited by using the windows exit button, or by pressing e.

transects.png

Magnetic Transects options

Noise filter dipole criterion Noise filter criterion is used by the program to determine what is noise and what is noise, the default value of 1.0 is less strict that a value of 0.9. A value of 0 will disable the noise filter.

High frequency cutoff Maximum useful frequency of signal (cycles/km).

Maximum depth Maximum estimated depth below the surface in the output

Minimum depth Minimum estimated depth below the surface in the output

Survey attitude Flying altitude of the aircraft, this is used to trim high frequency data and adjust depth estimates.

Vertical Resolution Distance between bins in this depth profile (m)

Transect Angle (°) Angle of transect relative to north (azimuth)

Transect origin latitude/longitude Specify the lat/long of the transect origin

Number of Transects Specify the number of transects

Lengths of Transects (Windows) Specify the length of the transects as the number of desired windows.

Window step size (°) Size of the window step in degrees, at the equator a step size of 0.05 is approximately 5km.

Advanced options

Map Viewer Options

Enable visualisation during processing

This will create a visualisation window which will allow you to see and investigate windows that have finished processing, windows marked red and erroneous/skipped, yellow is to be process and green is completed.

View Lines

By default this setting is on, this will display lines on the Map View window

Use Window Centres

By default this setting is turned off, it will display the centroids of the windows onto the Map View window.

Opacity of Grid %

Change the opacity of the grid in the Map View

FFT Power Spectrum Options

Number of Power Spectrum Bins

Set the desired number of Power Spectrum Bins, if the value is set to 0, the program will determine and set the maximum number of allowed bins.

Trim Early frequencies

Used to trim the first ‘n’ number of points from the power spectrum. If a value of 2 is selected, the first two points in a power spectrum will be displayed as grey. If a value of 0 is set then now points will be trimmed.

Example Magnetic Transects workflow

In this example we shall detect and locate magnetic bodies in a survey of the Tiwi Islands near Darwin, Australia. It follows the settings in the .task file sample_data/examples/Tasks/Modelling/magnetic_transects/mag_transects_auto_basalt.task .

Step 1—launch the tool

To launch the Magnetic Transects tool, choose Magnetic Transects from the Modelling menu in Project Manager, or use the command magnetic_transects.exe. INTREPID displays the Magnetic Transects window.

mtrmain.pngmtrmain_map.png

Step 2—Load the source data

Magnetic Transects requires a TMI dataset, A DTM dataset can optionally be used.

In the input area of the tool window, load the two datasets.

mtrinput.png

To locate the datasets, browse to the directory
{install_path}/sample_data/cookbooks/magnetics/DepthMethod/DM_2_GRIDS/:

Control

Filename

Input TMI grid

Tiwi_TMI_gda94_geod.ers

Elevation grid

Tiwi_bathy_elev.ers

The dataset paths appear in the Magnetic Transects window and the data region appears in the Map View window.

mtrmain_map.pngmtrloadd.png

On the map, you can zoom, pan an select regions to view more closely. See Using the Map View window

Step 3—Set Magnetic Transects options

In the Magnetic Transects window:

  • Set the Max. Depth to 800
  • The Min. Depth to -500
  • The Transect Angle to 90 degrees
  • The Origin Lat. to -11.6
  • The Origin Long. to 130.3
  • The Length of Transect to 16

Step 4—Set Advanced options

Set the Number of Power Spectrum Bins to 128.

Step 5—Specify Output

mtroutop.png

Output voxet— Specify the voxet that you are creating with this process. Type in or paste the path and filenames or use the Browse button (ico-java-folder.png). If you use a Browse button, INTREPID displays an Open or Save As dialog box for you locate or specify the file you require. (See “Specifying input and output files” in Introduction to INTREPID (R02) for information about specifying files).

Step 6—Run Magnetic Transects and view the results

Choose Run magnetic transect.

View the progress display

Open the output voxet in 3D Explore

Running the Magnetic Transects task interactively

When you are satisfied with your settings, run the operation that you have prepared and view the results.

To run the task:

  • From the File menu choose Run OR
  • From the bottom right corner of the window, choose Run. OR
  • Press ctrl+r
jmenu-file-run.png

See also:

Running the tool in batch mode

Displaying options and using task specification files

This section has information about .task file operations with this tool:

Displaying current tool settings

You can, of course, examine the settings in the Magnetic Transects window.

Alternatively, for a complete, convenient list of all settings, save a .task file. You can then examine the current tool settings in the .task file using a text editor. See Saving settings for later use.

Saving settings for later use

You can save the current settings to a .task file. This enables you to repeat the operation later with the same settings. You can also change the settings by editing the file. For more information about .task files, see INTREPID task, HISTORY, report and log files (R06). To save the .task file, use options in the File menu:

jmenu-savetask.png

Save Task File— If you want to save the current file specifications and parameter settings in the current task specification (.task) file, use this option. If there is no current file then INTREPID prompts you to specify a name for the new file. Shortcut keystroke: ctrl+s

Save Task File As— If you want to save the current file specifications and parameter settings as a new task specification (.task) file or to replace an existing one, use this option. INTREPID prompts you to specify a name for the new file. Shortcut keystroke: ctrl+shift+s

Running the tool in batch mode

You can run this tool in batch mode.

To use a task specification file for a batch mode Magnetic Transects task

Type the command magnetic_transects.exe with the switch -batch followed by the name of the task specification file.

For example, if you had a task specification file called surv329.task in the current directory, you would use the command:

magnetic_transects.exe –batch surv329.task

Task specification file example

Here is an example of a Magnetic Transects task specification file.

# Example task file - magnetic_transects # # Compute a subset FFT AND use the option to create the radial power spectrum, derived from the projected TMI grid # pass these to a visualizer for auto detect flat lying random dipole bodies - eg basalt flows # optional 3D viewer to monitor prcess # Inverse and forward modelling using random dipoles 窶 case study # author Roger Clifton, ASEG2016 # A recently published method of automatically finding magnetic depths to magnetic layers is demonstrated, # finding depths to significant details in the magnetic basement under Melville Island, Northern Territory. # By using a method that simulates a magnetic basement as a series of layers of random dipoles, # the depths to basement are satisfactorily obtained. Multiple layers appear in the results. # However the inversion method used has coarse horizontal resolution, and the layers may be separated horizontally within the sample. # To resolve the ambiguity a forward model, also composed of layers of dipoles, is built on the information obtained from the inversions. # Forward modelling requires Fourier convolution for speed. # The cycle of analysis is logically completed by comparing the synthetic depth profile # with the depth profile obtained by inverting the survey data. Just one trasect line in this case. # Usage: fmanager -batch mag_transects_auto_basalt.task # MagneticTask { Magnetic_Transects { gridName {# used for all FFT operations grid: "${cookbook}/Interpretation/minerals/DepthMethod/DM_2_GRIDS/Tiwi_TMI_gda94_mga52.ers"; type: Magnetism; Band: 1; mean_elevation: 80; } elevationGridName { grid: "${cookbook}/Interpretation/minerals/DepthMethod/DM_2_GRIDS/Tiwi_bathy_elev.ers"; type: Elevation; } outputVoxelName: "CalculatedDepths.vo"; RadarAltimeter : 80.;# flying height of aeroplane, used to trim high freqencies, adjust depth estimates MaximumDepth: 800.;# depth below surface, for controlling the output maximum depths displayed/estimated MinimumDepth: -500; DipoleCriteria: 1.0;# the criterion for what is dipole and what is noise,0.9 is more severe MaximumUsefulFrequency: 6.25;# predicted maximum useful frequency in cyl/km. if your cellsize in km for x and y is dx and dy, errors are large beyond ((0.5/dx)^2 + (0.5/dy)^2)^0.5 Trim_Early_Frequencies: 1;# trim off early frequency pts, for deepest depth estimates, first couple of freq are not well constrained for slope estimates powerSpectrumBins: 128;#Zero defaults to the max for the data given. Try not to go above 1/2 the RMS of the columns*rows of the amp grid or noise takes over Gaussian_StandardDeviation: 10;# smoothing the down probe depth estimate hits to 2*sd wide, so the pattern emerges, bigger gives more broad step: 0.05;# step out in decimal deg for lat and lon, about 5 km, along each transect # method to zero in on the lines of interest originLng: 130.3; originLat: -11.6; angleOfTransects: 90.0; numberOfTransects: 1; lengthOfTransects: 16; showViewer: true; ReportFile: "Tiwi_Magnetic_Transects.rpt"; }}