P. Schmoldt, PhD - MTNet - DIAS

Using magnetotellurics to gain information about
the Earth
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In order to investigate the Earth’s subsurface with magnetotellurics (MT), time-series data
of the natural electromagnetic (EM) variations, described in Section 2, are recorded at the
surface above the area of interest. Advanced instrumentation and installation procedures
are used in order to enhance quality of recorded data, which in turn determine the grade
of all subsequent steps.
Processing of the recorded electric and magnetic field data involves transformation
from the time domain into the frequency domain and deduction of electric impedance.
During the impedance deduction, different processing schemes, such as remote reference
and robust processing, are used to enhance impedance estimate quality.
In MT investigations, impedance estimates (or the related apparent resistivity and phase)
allow for derivation of a subsurface structure distribution, using forward modelling and
inversion algorithms. Application of forward modelling and inversion algorithms is computationally
very expensive and various schemes have been proposed to lower computational
load and to speed up the process. Commonly used schemes hold advantages and
disadvantages, and the choice of a scheme usually depends on computational facilities,
the characteristics of the problem, and the personal preference of the investigator.
6.1. Recording of magnetotelluric signals
6.1.1. Instrumentation
Since the beginning of MT investigation in the early 1950’s [Tikhonov, 1950; Cagniard,
1953] multiple advances have been made to the initial systems and today a variety of
instruments are available. During the PICASSO Phase I fieldwork campaign in Spain,
two different types of instruments were used: The Phoenix broadband MTU-5 recording
system [Phoenix Geophysics, 2005] with 1.41 m long MTC-50 induction coils and the
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