Blackman Filter

Use the Moving Platform Gravity > 1D Filters > Blackman menu option, (geogxnet.dll(Geosoft.GX.Gravity.BlackmanFilter;Run)*), to filter noise in observed gravity data.

Blackman Filter dialog options

Input channel

Select the noise gravity channel to filter.

Script Parameter: BLACKMAN_FILTER.INPUT

Output channel

Select the output filtered channel name.

Script Parameter: BLACKMAN_FILTER.OUPUT

Filter

Select a Blackman filter to use for filtering the noise in the gravity data. A few variations of the Blackman filter are available to choose from:

  • Blackman (Exact)
  • Blackman-Harris (-70 dB, 3 terms)
  • Blackman-Harris (-62 dB, 3 terms)
  • Blackman-Harris (-92 dB, 4 terms)
  • Blackman-Harris (-74 dB, 4 terms)

The Default is "Blackman (Exact)". See Application Notes below for more information on Blackman filters.

Script Parameter: BLACKMAN_FILTER.FILTER

Time units

Select Fiducials if the time units are supplied as number of fiducials. Select the Time channel option if the units are in seconds.

Script Parameter: BLACKMAN_FILTER .USE_TIME_CHANNEL [0 – Fiducials (default), 1 – Time channel]

Filter width (fids)

or

Filter width (secs)

Specify the number of fiducials or seconds for the filter width. This length will be translated into the number of points along profile for the convolution filter. The default is 300.

Script Parameter: BLACKMAN_FILTER .WIDTH

Time channel

If you have selected the Time channel option above, you will be prompted to select a channel from the current database. Note that as the filter width is in seconds, conversion will be needed if the time channel is in hours.

Script Parameter: BLACKMAN_FILTER.TIME

Save filter file

The calculated convolution filter coefficients can be saved for future use. Specify a file name if you would like to save the coefficients to a file; the filter file can be later used with the Convolution Filter tool.

Script Parameter: BLACKMAN_FILTER.FILTER_FILE

[More]

Alpha coefficients

The alpha (α) coefficients of the selected Blackman filter are displayed when you click the [More] button. See Application Notes below for more information on the Blackman filter coefficients.

Script Parameter: BLACKMAN_FILTER.FREEFILTER_FILEAIR

Application Notes

The Blackman filter is a low pass convolution filter that helps eliminate the high frequency noise in the raw gravity data. This tool calculates and applies the convolution filter coefficients. Once calculated, you can save the coefficients in an ASCII file for future use. The saved filter file can be loaded and used with the convolution filter in Oasis montaj.

The filter length in ground units is converted into the number of coefficients in the convolution filter. It is assumed that consecutive gravity readings are relatively equidistant.

The Blackman polynomial is defined as:

Where:

N

Number of coefficients

n

(0, …, N-1)

[αn]

Alpha coefficients

The variations of the Blackman convolution filter then consist of using 3 or 4 terms along with alternate α parameters. The default is to use the 3-term Blackman (Exact) filter. The other variations of the filter produce very similar results and are provided as alternatives should you observe residual noise in the filtered data that may be removed by the use of one of the Blackman variations.

The α parameters of the Blackman filter coefficients are:

Blackman (Exact)

Blackman-Harris -70dB

Blackman-Harris -62dB

Blackman-Harris -92dB

Blackman-Harris -74dB

*The GX.NET tools are embedded in the geogxnet.dll file 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" directory, then supply the GX.NET tool in the specified format. See the topic Run GX for more details on running a GX.NET interactively.

See Also: