To remind people the practical aspects of airborne FDEM.
So far we have described the airborne FDEM survey on book. There can be other important items that worth considering in practice.
An airborne EM data set may contain signals from non-geologic object, for example, power lines, pipes, etc. Some of them can act as active EM sources transmitting interfering signals, while others may be surficial good conductors. Measures have been taken by the system operator to minimize those negative effects, but sometimes they can still be significant in the data. In a packaged airborne data set, there is usually a channel called “power line monitoring” that can indicate the existence of power line in proximity. Advanced modeling may be necessary to take them into account for interpretation if they are difficult to suppress.
Recovery of true flight height
The flight height is usually derived from radar and/or laser altimeters. Sometimes, non-geologic objects at surface like trees can lead to miscalculation. If the EM system is operating in the inductive limit, there is hope to recover the true flight height. When a loop-loop system is saturated, the real part of the airborne FEM data asymptotes, and the imaginary part vanishes. So the data is no longer sensitive to the conductivity change caused by geologic variation. The inductive-limit data may only change if the conductivity varies within many orders of magnitude, like the contrast between the earth and the air. Effectively, such data are direct measurement of the distance from the coils to the surface.