Noise Threshold

Use the Noise Threshold menu option (UCENOISE GX) to flag the sections of the survey lines that exceed the specified first or fourth difference noise tolerance or where data is negative.

Noise Threshold dialog options

Line selection

Choose all or selected lines for processing

Script Parameter: UCENOISE.LINESEL

Channel

Data channel.

Script Parameter: UCENOISE.CHAN

Noise test

Choose from 1st difference, 4th difference, Savitzky-Golay filter or Negative values.

Script Parameter: UCENOISE.TEST

New map

Select New map or Current map to plot.

Script Parameter: UCENOISE.MAP

Noise Threshold (Savitzky-Golay) dialog options

Savitzky-Golay tolerance

Choose the Savitzky-Golay tolerance to use to flag sections. See the Statistics (UCENOISEST GX) topic to determine standard deviations for the data.

 

Script Parameter: UCENOISE.TOL

First or Fourth Difference dialog options

1st-difference tolerance

Choose tolerance to use to flag sections. See the Statistics (UCENOISEST GX) topic to determine standard deviations for the data.

 

Script Parameter: UCENOISE.TOL

4th-difference tolerance

Application Notes

This quality control option creates a channel called FLAG_NOISE. First or fourth differences can be calculated for the input channel. In the case of differences, the values outside the range defined by +/- the specified tolerance will have the value of 1 placed in the FLAG_NOISE channel. Values within the tolerance range (or where data is positive for the negative values test) will be flagged with a 0.

The negative values test is useful for identifying sections of the survey where instrument failures are indicated by negative data values.

The first difference filter calculates the numeric horizontal derivative of the data along the transect. If indicates the point-to-point variation in the data.

The fourth difference is the fourth horizontal derivative of the data. The purpose of the fourth difference is to enhance general disturbances or noise which will aid in the identifying of a true signal and the quality of data. The application was originally intended for magnetometer data, although it is now widely used on EM data. While the first difference still contains the inherent characteristics of the data, the fourth difference is strictly a noise indicator.

The responses of targets may be detected only if they are greater than the background noise level. Geophysical noise (not sensor sensitivity) is therefore the limiting factor in determining thresholds and detection depths. The noise encountered in geophysical surveys is generally of four types:

  • Instrument Noise

  • Ambient (Disturbance Field) Noise

  • Motional or Dynamic Noise (mechanical vibration, etc.)

  • Terrain Noise (site-specific, repeatable response of rocks, soils, and metal clutter

The difference filter is calculated by the subtraction of the successive data values of the channel:

D1(V(i)) = V(i+1) - V(i)

Higher differences are formed by successive operation, e.g.

D2(V(i)) = V(i+1) - 2V(i) + V(i-1)

D3(V(i)) = V(i+2) - 3V(i+1) + 3V(i) -V(i-1)

D4(V(i)) = V(i+2) - 4V(i+1) + 6V(i) - 4V(i-1) + V(i-2)

The Savitzky-Golay noise filter used is a symmetrical filter of a power of 4.  This filter can be found in the Oasis montaj 'etc' directory.

Acknowledgments

The following document has been used in the compilation of this Help file, and further information can be obtained directly from it:

  • ORDNANCE AND EXPLOSIVES DIGITAL GEOPHYSICAL MAPPING GUIDANCE - OPERATIONAL PROCEDURES AND QUALITY CONTROL MANUAL (DGM QC Guidance), U.S. Army Corps Engineering and Support Center, Huntsville, Prepared by NAEVA Geophysics, Inc. December 2003.