Technical Notes - Helicopter Electromagnetics

What is that Noise on a DIGHEM/RESOLVE System?

What is noise, indeed? The internal electronic noise of a well-adjusted helicopter electromagnetic system is well below the numbers shown on the chart below - usually below 1 ppm. The noise levels shown here are "average" under field conditions, and may include sferics, power lines, vibrations, etc.

The second dimension to noise is the frequency content. These values are a rough measure of the shorter period (higher frequency) noise, from a few 10's of seconds to higher. This is the noise that is critical in the anomaly detection band, which is around 2 to 10 seconds. Any noise shorter than about 1 second is easily filtered because it doesn't affect any signal in which we are interested.

The critical noise for resistivity mapping is the very long period - normally called drift. The drift quoted in this table is the uncorrected drift between high altitude base level checks. This drift is greatly reduced by a linear interpolation between the values measured at the high altitude tests. The only remaining drift in the data given to the interpreter is the non-linear component of this drift. This last component is removed to less than 1ppm in the leveling and processing.

DIGHEM/RESOLVE STANDARD VALUES RMS

Greg Hodges, Chief Geophysicist, 2001