Transmitter-receiver coil separation is one of many factors affecting the depth of penetration and background noise levels for Helicopter ElectroMagnetic (HEM) surveys. Some of the others are transmitter power, bird stability, electronic noise levels and real-time filter quality.

Frequency domain HEM systems transmit a primary field, and measure the secondary response from the earth in the presence of the primary. As a consequence, the response is generally measured normalised to the strength of the primary field, in parts per million. The minimum measurable signal is defined by the smallness of signal that can be resolved in the presence of the very large primary field and is usually defined in ppms. Therefore, the ppms of one system will differ from another.

The depth of detection and the fineness of resistivity resolution, depend on the ability to resolve weak signals and subtle changes in signal in the presence of the noise. "Noise" includes both high-frequency variations in the range of anomaly wavelengths, and drift, which interferes with the system's ability to map resistivity accurately over a large area.

The smaller the actual ppm value is, for the same ppm noise level, the finer the system resolution. This creates an apparent contradiction: the more powerful the transmitted field, the better, but the smaller the primary field interfering with the measurement at the receiver, also the better. This is best accomplished by having the receiver as far as practical from a powerful transmitter. The farther away the receiver is from the transmitter, the lower the primary field at the receiver will be, allowing for the detection of smaller secondary fields. (When the system altitude is significantly higher than the transmitter-receiver separation, the response from the earth depends only on the transmitter power and frequency.)

The decrease in primary field is a function of the coil separation cubed. Thus for an increase in coil separation from 6m to 8m, the decrease in received primary field would be (8/6)3=2.4 times. The anomaly responses, measured in ppms, would be 140% bigger.

Most systems measure their noise level in ppm, and about 2 ppm is common. To compare systems, the noise level of each must be divided by the coil separation cubed. To compare two systems with claimed noise levels of 2 ppm, one with 5 m coil separation and one with 8 m coil separation, the relative lower noise limits would be 2/53=0.016 to 2/83=0.004 for 5 m and 8 m respectively. This means that the 8 m bird ppms have a resolution 4 times better than the 5 m bird.

The closest analogy to this effect which is familiar to most geologists is ground HLEM , such as the Max-Min. The Max-Min uses longer coil spacing (up to 250m) to see deeper, because the receiver is farther from the transmitter, so the secondary field is much larger relative to the primary. (The absolute secondary field is actually weaker, since the transmitter and receiver are farther from the conductor.) Unlike Max-Min, because the coils are much farther above the ground than they are from each other, the coil separation of a HEM system does not effect spatial resolution.

Greg Hodges, Chief Geophysicist, 2001