P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
3. Mathematical description of electromagnetic relations 3.6. The influence of magnetic permeability In MT, magnetic permeability is usually assumed equal to the permeability of free space µ0 = 4π10 −7 Vs/Am, rather than the product of µ0 and the permeability of a specific medium (i.e. the relative permeability) µr. Neglecting the effect of the relative permeability during the calculation of the electric resistivity results in a frequency independent deviation from the correct value of factor µr (cf. Eqs. 3.29 and 3.30). The approximation that µ = µ0 is valid for most MT cases, since µr is almost always close to 1, except for some uncommon minerals like Magnetite (µr = 5) and Phyrrhotite (µr = 2.55) [Telford et al., 1990]. 48
Distortion of magnetotelluric data 4 Over time, various definitions for distortion of magnetotelluric (MT) data have been given, wherein in the very most cases the used definition is tailored to the current topic. Distortion is commonly attributed to an effect that impedes or prevents an interpretation of the obtained MT data. These effects include, amongst others, deviation of the source field from the plane wave assumption (Sec. 2.3), and generation of secondary electromagnetic (EM) fields due to regions of different electric conductivity in the area under investigation. Since modern MT processing is capable of dealing with most cases of 1D and 2D subsurface structures, the latter usually refers to situations where responses from regional structures are disturbed by small-scale or 3D features [e.g. Berdichevsky et al., 1973]. In here, a more general definition is chosen, attributing distortion to effects that cause a deviation of the electric currents from a straight-lined behaviour within the region of interest, i.e. J = Jp + Ja, (4.1) with Jp: undistorted component of the electric current from primary EM fields and Ja: anomalous current due to distortion. In this Section different types of distortion are presented, their frequency dependence and the effects of dimensionality of the distorting body are investigated, mathematical formulations of the distortion effects are shown, and an overview about approaches used to remove the effects of distortion are given. Alterations of electric current behaviour due to deviation of MT source fields from the uniform case are examined separately in Section 2. Comprehensive overviews about distortion of MT data are given for example in the review papers by Jiracek [1990] and Ledo [2005], as well as in the publications by Berdichevsky et al. [1989], Vozoff [1987], Groom and Bahr [1992], and Simpson and Bahr [2005]. In addition, various papers have been published regarding specific aspects of distortion and attempts for their removal; these will be referred to in the following sections when the respective topic is addressed. 49
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3. Mathematical description of electromagnetic relations<br />
3.6. The influence of magnetic permeability<br />
In MT, magnetic permeability is usually assumed equal to the permeability of free space<br />
µ0 = 4π10 −7 Vs/Am, rather than the product of µ0 and the permeability of a specific<br />
medium (i.e. the relative permeability) µr. Neglecting the effect of the relative permeability<br />
during the calculation of the electric resistivity results in a frequency independent<br />
deviation from the correct value of factor µr (cf. Eqs. 3.29 and 3.30). The approximation<br />
that µ = µ0 is valid for most MT cases, since µr is almost always close to 1, except for<br />
some uncommon minerals like Magnetite (µr = 5) and Phyrrhotite (µr = 2.55) [Telford<br />
et al., 1990].<br />
48
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