Technical Notes - Helicopter Electromagnetics

Coil Separation and Conductor Detection Limits

Coil separation between the transmitter and receiver has a strong influence on HEM system noise, and an apparent effect on the response to a conductor. Much of the noise in a frequency-domain HEM system comes from the changes in coupling between the transmitter and receiver, causing the receiver to detect some of the transmitted primary. Because the primary field decreases with distance by a factor of the cube of the increase in the distance, increasing the coil separation greatly reduces the strength of the primary field at the receiver. For an increase in coil separation from 6.5 to 8m, the primary field is reduced by a factor of (6.5/8)3 = 0.54, with a similar decrease in the primary field noise.

The second effect of decreasing coil separation is an apparent reduction in the secondary field. This effect is caused by the normalisation of the secondary field response to the strength of the primary measured at the receiver - double the primary, and the normalised secondary field has half the value. This effect is called "apparent", because in actual fact there is no reduction in the true strength of the secondary field. For an HEM system, the true strength of the secondary field is governed by the strength of the primary field, the geology, and the system altitude. The same conductor will give the same secondary magnetic field, but will be defined as 2ppm for an 8m coil separation, and 1ppm for a 6.5m coil separation system.

To compare system responses, one needs to compare the amplitude of the anomaly measured by the system to the detection limit for the system. The commonly accepted detection limit for discrete anomalies is a peak response more than three times the system noise limit.

This graph compares the HEM anomalies from three bedrock conductors, at depths of 25m, 50m, and 75m. For the models shown, the noise limits (as published by the manufacturers) are 1ppm for the 8m-long HEM system, and 2 ppm for the 6.5m system. This gives anomaly detection limits of 3 ppm and 6 ppm respectively. The anomaly for the conductor modeled at 25m depth-to-top exceeds the detection limit for both systems. For depths of 50m and 75m, the anomaly exceeds the detection limit of the 8m coil separation system (but not the 6.5m system) due to the significantly lower noise limit. The shorter coil separation system will likely miss both of the deeper conductors.

Greg Hodges, Chief Geophysicist, September 2003