Technical Notes - Helicopter Electromagnetics

Fugro's Coplanar Coil Convention

At the end of 1998, Fugro Airborne Surveys changed the way we normalise the DIGHEMV coplanar measurements. This change brought us into conformity with the convention employed by other helicopter EM systems. This change has the effect of doubling the perceived coplanar signal strength: for example what was 100 ppm (parts per million) signal will now become 200 ppm. This change has no effect at all on the quality of our data.

The most obvious effect of Fugro converting to this normalisation procedure for the coplanar coils is an apparent doubling of our noise and drift statistics. This change is exactly offset by a doubling of the signal for all responses, both background and anomalies. Hence, there is no change in the Signal to Noise ratio, and no effective reduction in the high quality of data collected by a Fugro survey.

Fugro Airborne Surveys can archive data for clients in either normalisation standard. There is no change to the coaxial data. It will be the same as always.

The Details

All of the major HEM survey systems normalise the measured signal from the earth to the transmitted primary field. But there is more than one way to do this:

• We assume that all transmitter coils (Tx) generate the same primary field P (at the transmitter). We can call the measured primary field strength from the vertical coaxial Tx kP at the vertical coaxial receiver coil (Figure 1, Right). (The plane of the coil is vertical, and perpendicular to the axis of the bird.) If so, then the primary measured at the horizontal coplanar receiver coil (Rx) from the coplanar Tx will be kP/2 (Figure 1, Left).

• Each transmitted field generates secondary response from a homogeneous halfspace - the earth. If the secondary from the earth is S from the coplanar Tx, then the response from the vertical coaxial Tx will be S/2.

Figure 1: Coil Pairs and EM Fields

Because frequency domain systems measure in the presence of the primary field, the secondary response is normalised to the primary, typically in parts per million (ppm). For the coaxial coil pair, the normalised secondary response is (S/2) / kP. We can call this 1ppm. For the coplanar pair, if the response is normalised to the primary at the receiver we get S / (kP/2)=4ppm.

However, by geophysical theory, the absolute response of a layered halfspace to a horizontal coplanar coil is only twice that of a vertical coaxial coil transmitting the same field. (The rest of the 4:1 difference is in the normalisation.) Standard DIGHEM systems applied a ½ factor to the coplanar data to define the halfspace signal as 2ppm, to keep the ratio of coplanar to coaxial measured responses from the halfspace matched to geophysical theory.

Neither method of normalisation has an inherent advantage. However, many of Fugro Airborne Surveys's clients who were modelling DighemV data for layered earth responses requested that Fugro use the method of normalisation to the primary field at the receiver, which is a more common standard for electronic and geophysical modelling applications.

Greg Hodges, Chief Geophysicist, 2001