The magnetotelluric (MT) method consists of measuring the temporal fluctuations of the Earth's natural electromagnetic fields (electrical storms, ionospheric currents) on the surface and determining the distribution of electrical resistivity as a function of frequency (period), that is, as a function of depth (from a few hundred meters to a few hundred kilometers).

Methodology

The external energy coming mainly from solar activity is the primary electromagnetic field (Hp) that reaches the Earth's surface. Part of this energy is reflected and another part is transmitted into its interior. The Earth acts as a conducting medium inducing an electric field (telluric currents) that gives rise to a secondary magnetic field (Hs):

Simple diagram of how electromagnetic induction works: 

• Hp, primary magnetic field.
• Hs, secondary magnetic field.

The simultaneous records of these electric and magnetic time series are the data that are measured in this technique through two electric dipoles (unpolarizable electrodes) placed in the shape of a cross, and three induction magnetometers aligned in orthogonal directions to measure the temporal variations of the components of the magnetic field:

Diagram of the arrangement of measuring instruments with the AMT technique.

Subsequently, the time series are transformed to the frequency domain and, based on the linear relationships between the components of the electric and magnetic field, the responses of the ground are obtained at all stations of the profile, which will give rise to a model of subsoil electrical resistivities:

Electrical resistivity model resulting from the processing of AMT data.

Applications

• Definition of contacts between subsurface materials.
• Detection of rock substrate.
• Detection of water table.
• Identification of fractured areas and discontinuities.
• Location of areas contaminated with leachates.
• Definition of areas affected by salt intrusion.
• Geothermal studies.
• Location and monitoring of geological reservoirs for storing CO₂.