Determine Coherence Anomaly Threshold

Use the Determine Coherence Anomaly Threshold option (Geosoft.uxo.gxnet.dll(Geosoft.GX.UXO.UxaDetermineCoherenceAnomalyThreshold;Run)*) to help you decide the cut off to use when picking targets from the model coherence anomaly grid.

ClosedExpand to see the locations (menus) where this option is available.

UX-Analyze extension:

  • AGC Dynamic Data > Coherence Anomaly
  • AGC Expert User > Dynamic Data Prep and QC

Determine Coherence Anomaly Threshold dialog options

Database prefix

Select the sensor (data) database prefix.

Script Parameter: UXANALYZE.DYN_DBPREFIX

Lines to process from located database

Select which lines to process: Selected lines or All lines. They will be read from the Located database.

Default: Selected lines

Script Parameter: UXANALYZE.LINES_TO_PROCESS

Sensor channel suffix - leveled

Specify the sensor channel suffix for the levelled/background removed data.

Default: Norm_Lev

Script Parameter: UXANALYZE.SENCHAN_SUFFIX

Gates

Select the low and high time gates from the list, which limit the range of data used when modelling the data.

Default, low gate: gate value based on UXA_GateFirstValidTime channel

Default, high gate: the maximum gate number

Script Parameters:

  • UXANALYZE.DYN_LOGATE
  • UXANALYZE.DYN_HIGATE

Pitch channel

Specify the pitch channel.

Default: UXA_Pitch_Filt

Script Parameter: UXANALYZE.PITCH

Roll channel

Specify the roll channel.

Default: UXA_Roll_Filt

Script Parameter: UXANALYZE.ROLL

Heading channel

Specify the heading channel.

Default: UXA_COG (Course Over Ground)

Script Parameter: UXANALYZE.YAW

Amplitude channel

Specify the amplitude channel.

Script Parameter: UXANALYZE.AMPLITUDE

Anomaly grid cell size (m)

Enter the cell size for the amplitude anomaly grid.

It should be chosen to properly define the anomalies; the typical range is usually 0.1 to 0.5 metres. The grid will be named:  [PREFIX]_[DATATYPE]_AmpAnomaly.grd.

Default: 0.1 metre.

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_GRID_CELL_SIZE

[Adv. Grid Parameters]

When selected, the Grid Parameters dialog is displayed; that enables you to set the additional gridding parameters.

Library database

Select the signature library database.

Script Parameter: UXANALYZE.LIBRARYGDB

Library group

Select the library group.

Script Parameter: UXANALYZE.LIBRARYGROUP

Description channel

Select the channel that contains the name or description of the items. Typically, this is the Library ID channel.

Script Parameter: UXANALYZE.LIBRARYID

TOI

Pick the selected TOI. The list is populated from the values in Description channel.

Script Parameter: UXANALYZE.FWDMODEL_TOI

Project depth (m)

Enter your project TOI depth.

Default: none

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_PROJECT_DEPTH

Background amplitude threshold

Non-editable parameter. The background amplitude threshold value is calculated automatically based on the TOI and Project depth parameters.

Default: none

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_AMP_THRESHOLD

Minimum depth (m)

Enter the minimum TOI depth.

Default: 0 metres.

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_MIN_DEPTH

Maximum depth (m)

Enter the maximum TOI depth.

Default: 2 metres

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_MAX_DEPTH

Probable source depth (m)

Enter the probable source depth for your project and TOI.

Default: 0.2 metres

Script Parameter: UXANALYZE.PROBABLE_SOURCE_DEPTH

Plot file name suffix

Enter the file name suffix, which will be added to the default names for coherence anomaly database and output plot:

Coherence anomaly database: [PREFIX]_[DATATYPE]_CohAnomaly_[SUFFIX].gdb

Coherence anomaly plot: CohAnomaly_[SUFFIX].png

Default: Blank

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_THRESHOLD_SUFFIX

[More]

Data chip cross-line dimension (m)

Enter the data chip cross-line dimension.

This dimension should extend across at least 3 survey lines.

Default: 1.6

Script Parameter: UXANALYZE.DATACHIP_CROSS_LINE

Data chip along-line dimension (m)

Enter the data chip along-line dimension.

This dimension should extend along the line approximately 1.5 times the sensor array length.

Default: 1.2

Script Parameter: UXANALYZE.DATACHIP_ALONG_LINE

Number of samples

Enter the number of samples.

Default: 2000

Script Parameter: UXANALYZE.COHERENCE_ANOMALY_NO_SAMPLES

Application Notes

Based on the project depth, the Determine Coherence Anomaly Threshold plots will help you decide on the cut off to use when picking targets (Pick Target Locations) using the grid created by Determine Model Coherence Anomalies. The threshold plot can be found in the \UXANALYZE_wrk\Coherence Threshold folder.

The processing steps are:

  • Grid and threshold the amplitude data, by masking out data above a background amplitude threshold value.
    • Calculate the percentage of the site below the background threshold.
    • Warn the user if less than 20%, with the option to stop. This value is stored in uxa.config as <modelCoherence/gridPointsThreshold>.
  • From the amplitude data below the background threshold, locations are picked at random, up to the Number of Samples. While you can use a lower number of samples in your initial testing, we recommend using 2000, as this makes it easiest to reliably determine the coherence anomaly threshold.
  • For each location, extract a data chip (window). The data chip (window) is checked to ensure it contains at least 15 (the minimum number of points in the data chip) of valid data measurements, and all the data points are below the background threshold. Otherwise, pick another location. The minimum number of points in the data chip is saved in uxa.config as <modelCoherence/minNoOfPointsInDataChip>.
  • Calculate a synthetic (forward) model of the selected TOI and add the synthetic response to a copy of the data chip.
    • The model uses the item at a horizontal orientation, but with a random azimuth and random depth.
      • Azimuth can range between 0 to 359.
      • Depth can range from Depth Min to Depth Max.
  • Invert the data using single dipole model for both the data chips; with the added synthetic TOI response and without (blank).
    • Record in a GDB:
      • the X and Y of the window center
      • the RMS Noise of the blank (without) chip
      • the Fit_Coh from the inversion of blank (without) chip
      • the Fit_Coh from the inversion with synthetic TOI chip
      • the depth (used for the synthetic model)
      • the azimuth (used for the synthetic model)
  • Create a plot of Depth vs Fit_Coh.
    • A graph of depth vs maximum amplitude (monostatic z). With a horizontal line at 5 times the average RMS, of the maximum amplitude (monostatic z).
    • A graph of depth vs coherence.
    • Both graphs show the result from the inversion with the synthetic TOI response in black and without (blank) in red, along with a vertical line for the Project depth.
    • Also included are small Polarization Plots of the TOI. Below, the following parameters are listed:
      • Database prefix name
      • TOI
      • Background Amplitudes Threshold
      • Percentage of the site below the Amplitude Threshold
      • Number of samples used
      • Date and time the plot was created

*The GX tool will search in the "gx" folder. The GX.Net tools, however, are embedded in the Geosoft.uxo.gxnet.dll located in the 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.