Survey Design


To discuss the principles and parameters that need to be chosen for an airborne FDEM survey.

Applicability of AFEM

Choosing the instrument and the survey parameters and layout is usually referred to as “survey design”. In most cases, the first step in a geophysical survey is to match the system parameters to the geologic task using rules of thumbs or empirical criteria.

AFEM is mostly suitable for large survey areas with focus from the surface to the shallow depth. It can be quickly deployed, giving a quick overview of the regional conductivity distribution. The recording system (towed bird) is usually smaller than a time-domain system, in which a large and rigid frame is used to mount the transmitter loop. So it is generally the least costly geophysical EM solution measured by dollar per line-km. Hydrological, environmental and engineering applications are the main area AFEM is used. It can also be useful for mineral or petroleum exploration at shallow depth.


In an AEM survey/system, a concept “footprint” is usually used to quantify the size of the underground volume that a system is sensitive to. Footprint can be evaluated using different formulations, but it mostly depends on the frequency and the conductivity. Higher frequency and higher conductivity can result in a smaller footprint. A lower flight height may also reduce the footprint. A first-order estimate of footprint can be from skin depth, which is a function of the frequency and the conductivity.

Survey parameters

The operating frequencies, separations and orientations of the coils in a AFEM system are usually fixed. The only variable geophysicist need to consider in the survey design is the line spacing. For a 3D survey, it is important to ensure the line spacing is at least equal or smaller than the footprint, so no “gap” is left between lines. At the locations where higher quality of data is desired, survey lines may be dense. The line spacing is usually a trade-off between the cost and resolution.

Another consideration is the sounding spacing. For a towed system, the in-line spacing is adjustable by the flight speed and sampling rates, and can easily achieve high density soundings.

Flight height can be another parameter. Generally a lower flight height is preferred as it provides better resolution for the near surface structure using a smaller footprint. But some practical conditions may limit the lowest possible flight height, for example, trees, infrastructure, severe topography, aircraft safety, etc.