P. Schmoldt, PhD - MTNet - DIAS

Earth’s properties observable with magnetotellurics
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Electromagnetic (EM) methods determine the distribution of electric conductivity 1 σ in
the subsurface by measuring the relation between time-varying electric and magnetic field
components (cf. Sec. 3.2). Electric conductivity is the measure of a material’s ability to
conduct electric current, specific for a material under given conditions. Thus, EM methods
can, in principle, derive the distribution of materials in the subsurface. However, the magnetotelluric
(MT) method does not measure the electric conductivity at a certain point in
the subsurface, but rather the integrated conductivity of a volume. A volume represented
approximately by a hemisphere of radius given by the inductive distance (cf. Sec. 3.3).
Therefore, assumptions have to be made about the local conductivity distribution during
analysis and interpretation of the obtained data (cf. Secs. 3 and 6).
Electric conductivity of a material depends on a variety of different properties (cf. Sec.
5.3); accordingly, common Earth’s materials exhibit a wide range of values from 10 7 –
10 −7 S/m (i.e. resistivities in the range 10 −7 – 10 7 Ωm) (Fig. 3.5). Among the different
factors influencing the electric conductivity of materials in the Earth, temperature, water
salinity, and interconnectivity of conductors have the most significant effects. These
influencing parameters can undergo very localised changes, both in the vertical and the
horizontal directions; e.g. a massive ore body or horizontal temperature gradients in
mantle convection cells can result in conductivity changes of several orders of magnitude.
Therefore, the creation of a global 1D electric reference model possesses a number
of challenges when compared to the seismological Preliminary Reference Earth Model
(PREM) [Dziewonski and Anderson, 1981] (Fig. 5.1). The present spatial coverage with
MT measurements is not yet sufficient to take into account the large amount of regional,
let alone local conductivity anomalies, to facilitate creation of a global 3D conductivity
1 There lies a redundancy in terminology when describing the electric properties of materials; in some
cases σ is chosen, whereas in other cases electric resistivity ρ is used. As σ and ρ are simply inverses of
each other (ρ = 1/σ with [ρ] = Ωm and [σ] = S/m), the convention used in the remainder of this text is
chosen out of convenience; conductivity will be used when the majority of the elements exhibit σ > 1
(ρ < 1) and resistivity in the opposite case.
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