The importance of accurate altimetry in AEM surveys for land management

Airborne electromagnetic (AEM) systems are increasingly being used for mapping conductivity in areas susceptible to secondary salinity, with particular attention to near-surface predictions (conductivities in the top 5 or 10 metres). Because measured AEM response is strongly dependent on the height of both the transmitter loop and receiver coil above conductive material, errors in measurements of terrain clearance translate directly into significant errors in predicted near-surface conductivity.

Radar altimetry has been the standard in airborne geophysical systems for measuring terrain clearance. In areas of agricultural activity, artefacts up to five metres in magnitude can be present. One class of error, found to correlate with ploughed paddocks and hence termed the "paddock effect", results in overestimation of terrain clearance. It is hypothesised that this is due to surface roughness and soil moisture levels in these paddocks. A second class of error, related to dense vegetation and hence termed the "canopy effect", results in underestimation of terrain clearance.

A survey example where terrain clearance was measured using both a radar and a laser altimeter illustrates the consequences of the paddock and canopy effects on shallow conductivity predictions. This shows that the combination, of the dependence of AEM response on terrain clearance and systematic radar altimeter artefacts spatially coincident with areas of differing land-use, may falsely imply that land-use practices are the controlling influence on conductivity variations in the near surface.

A laser altimeter is recommended for AEM applications because this device is immune to the paddock effect. Careful processing is still required to minimise canopy effects.