Electromagnetics

Electromagnetic measurement Source: BGR

Rocks, especially clays and ores, and mineral-bearing fluids have a great influence on the electrical conductivity of the earth’s subsurface (see figure). With the help of electromagnetic methods, geoscientists can assess the electrical conductivity. Compared to potential and spectral methods, these methods have the advantage, that not only mapping is possible but also, due to the vertical resolving capabilities, sounding and thus a three-dimensional investigation.

Conductivities of rocks and fluids Source: BGR

Conductivity mechanisms in the subsurface can depend on electrons (graphite and ores), ions (salts dissolved in water) or cation exchange (clays). Besides the electrical conductivity, the magnetic permeability and the electric permittivity of the rocks and fluids have, under certain conditions, a noticeable influence on the electromagnetic measurements.

Geophysicists use natural or artificial electric or magnetic transmitting sources causing variations in the magnetic field (primary fields) and inducing electrical currents into the subsurface. The strength of these currents can vary depending on the electric conductivity. The interpretation of the magnetic fields (secondary fields) generated by the induced currents yields the electric conductivity. Variations of the transmitter-receiver-properties (shape of signal, frequency spectrum, geometry) are used by geophysicists in order to investigate different depths. Measurement and interpretation of the EM fields can be done in a time or frequency domain, where transients or discrete frequency spectra are analyzed.

BGR uses the following EM methods:

• Airborne electromagnetics (AEM)
  
• Transient electromagnetics (TEM)
  

If direct currents or low frequency alternating currents are induced into the subsurface directly by electrodes and the generated electric potential is measured by potential electrodes, the method is called direct current geoelectrics.

Reference:

Nabighian, M.N., 1987. Electromagnetic methods in applied geophysics, Vol. 1, Theory. Society of Exploration Geophysicists, Tulsa, OK.