Technical Notes - Helicopter Electromagnetics

Magnetic Compensation

Because the HEM sensors are removed from the aircraft by approximately 30m, the EM signal detected from fields induced in the helicopter are well below the level of the signal detected from even highly resistive geology. Similarly, the magnetic anomaly created as the system changes orientation relative to the earth's magnetic field is also below the threshold of measurable natural magnetic anomalies.

Greg Hodges, Chief Geophysicist, 2003