Magnetotellurics is a broad-band electromagnetic geophysical technique, which uses natural time-dependant variations in the earth’s magnetic field as the source, and electric fields induced in the earth as output. These data are used to determine (usually via 2- or 3-D inversion) the resistivity distribution within the earth to depths of many kilometers. This is then interpreted in terms of lithology.

DATA ACQUISITION
At a single MT measurement point, we measure the input fields (two horizontal magnetic components, Hx and Hy) and the output from the earth (two horizontal electric fields, Ex and Ey, and a vertical magnetic field, Hz). The magnetic fields are measured by induction coils , and the electric fields by grounded dipoles. Using robust FFT techniques, the recorded data (time series) are transformed to electromagnetic impedances as a function of frequency.

A 5-channel MT set-up at 5200 m asl in Chile	Example of time series, duration 1024 seconds. From top to bottom, channels are Ex, Ey, Hx, Hy, Hz
For the purpose of display and 2D interpretation, the impedance tensor is reduced (“decomposed”) to two orthogonal apparent resistivities and phases, xy and yx, typically in the strike and dip directions. In the 3D inversion, the components xx, yy are also used.
2 AND 3D INVERSION To transform the impedances to the resistivity distribution within the earth, we use 2- or 3-D inversion. Geosystem is the world leader in these techniques, and has developed code which is widely used both industrially and academically. The image to the right shows the top of a carbonate target recovered by 3D inversion, below a zone of complex overthrust structures.

 

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