Airborne electromagnetics

BGR helicopter with bird in Banda Aceh (Indonesia) at the local airport Source: BGR

Sketch of the BGR airborne geophysical system Source: BGR

Method:

Airborne electromagnetics (AEM) comprises those methods, that allow a fast investigation of the Earth’s subsurface from the air ranging from some metres up to several hundreds of metres. BGR uses a helicopter for carrying the EM transmitters and receivers that are housed by a tube, the bird. This bird is towed by a 45 m long cable about 30-40 m above ground.

The transmitting dipole signals (primary fields) are generated as continuous sinus oscillations at six discrete frequencies ranging from 380 Hz to 130 kHz. At each frequency, two components of the secondary magnetic field induced in the subsurface are recorded that are converted into resistivities (reciprocal value of the electric conductivity) using simple models. The results are presented in the form of vertical sections or horizontal slices (maps). Alternatively to this airborne electromagnetic frequency-domain method, there exist time-domain methods which generate the primary fields by turning on and off the transmitter currents. Geophysicists use both fixed-wing aircraft and helicopters for applying both methods.

Measuring technique:

The details of the measuring technique are offered as a document for download (Description of the aerogeophysical measuring system of the BGR (PDF, 111 KB)Description of the aerogeophysical measuring system of the BGR (PDF, 111 KB)).

Application:

The fields of application for BGR’s helicopter-borne electromagnetic (HEM) measuring system are manifold – the following comprehensive list is not exhaustive:

• determination of salt-/freshwater boundaries
• groundwater exploration
• geological mapping
• mapping of fault zones
• exploration of contaminated sites
• propagation of conductive drainage water
• archaeological surveys
• engineering-geological surveys

• exploration of deposits

References:

• Sengpiel, K.-P. & Siemon, B., 2000. Advanced inversion methods for airborne electromagnetic exploration. Geophysics, 65, 1983-1992.
• Siemon, B., 2009. Electromagnetic methods – frequency domain: Airborne techniques. In: Kirsch, R. (ed.), Groundwater Geophysics – A Tool for Hydrogeology, 2nd ed., Springer-Verlag, Berlin, Heidelberg, 155-170.

• Siemon, B., Christiansen, A.V. & Auken, E., 2009. A review of helicopter-borne electromagnetic methods for groundwater exploration. Near Surface Geophysics, 7, 629-646.

Projects:

• HELP ACEH
• BurVal
• D-AERO
• CLIWAT