Background Removal-Demedian Filter

Use the Demedian Filter option (Geosoft.uxo.gxnet.dll(Geosoft.GX.UXO.UxaFilterDynamicData;Run)*) to level or remove the background from dynamic survey data.

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

UX-Analyze extension:

  • AGC Dynamic Data > Drift and Background Removal
  • AGC Expert User > Dynamic Data Prep and QC

Dynamic Data Filtering dialog options

Database prefix

Select the databases prefix for the sensor (Data) database.

Script Parameter: UXANALYZE.DYN_DBPREFIX

Input channel suffix

Select the input channel suffix from the list of available suffixes.

The pre-requisite to this GX is Dynamic Data Processing GX. In order to populate the suffix list, the GX looks for the data channel names with the word NORM in them, indicating the data have been normalized for the current, and considers what comes after NORM as the suffix.

Script Parameter: UXANALYZE.INPUT_CHANNEL_SUFFIX

Output channel suffix

Enter a new output channel suffix, or select it from the list.

The output channel name would be the input channel including the suffix with the output suffix appended. For example: UXA_RX2Y_TXZ_NORM_[OUTPUT_SUFFIX].

Script Parameter: UXANALYZE.OUTPUT_CHANNEL_SUFFIX

Lines to process

Select the lines you want to process.

The available options are: Selected lines, Current line, and All lines.

The default is Selected lines.

Script Parameter:UXANALYZE.LINES_TO_PROCESS

Number of passes

Select the number of times the filter is applied.

Default value is 5.

Script Parameter: UXANALYZE.FILTER_NPASSES

Amplitude threshold

Enter the background amplitude threshold used in the filtering.

Default value is 10.

Script Parameter: UXANALYZE.AMP_THRESHOLD

Filter width

Enter the filter width. If this value is even, it is increased by 1 to give a window that is symmetric about each data point. Width should be 10 to 15 sec wide and can also depend on the size of the anomaly from your TOI.

Default value is 101.

Script Parameter: UXANALYZE.FILTER_WIDTH

Shrink filter at ends

Shrink filter at ends to eliminate edge effects. If yes, the filter shrinks near the ends so that it remains of equal size on both sides of the individual data points. This can eliminate edge effects.

Default value is Unchecked.

Script Parameter: UXANALYZE.FILTER_SHRINK_ENDS

Application Notes

The dynamic survey data can be leveled by filtering the data. Each Tx-Rx pair are filtered by:

  • Removing the median from the data.
  • Selecting the data less than the background amplitude threshold (that is, removing outliers).
  • Over a moving window defined by the Filter width, calculating a running median and subtracting the median from the data point at the center of the window.

Subsequent passes do the same thing except the amplitude threshold is divided by 2 for each pass.

While this method of levelling dynamic survey data works well in many situations, caution should be exercised when working with datasets that have a mixture of both dense and sparse anomaly areas. You may need to consider alternate methods of leveling the data in these situations.

Other output channels

In addition to calculating levelling for each Tx-Rx pair, two other channels are calculated:

  • UXA_SumMonostatZ – the sum of the monostatic data for Z component Tx's and Rx's.
  • UXA_SumCoxialZ – the sum of the coaxial data for Z component Tx's and Rx's.

Where:

  • Coaxial: the Tx and Rx are oriented in the same direction – in this case vertical (normal vector 0,0,1).
  • Monostatic: the Tx and Rx pairs are coincident (same center) and coaxial (same direction).

Similarly named channels generated by the Dynamic Data Processing are overwritten with the level/background removed data.

*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.