P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS P. Schmoldt, PhD - MTNet - DIAS
5. Earth’s properties observable with magnetotellurics Depth (km) 0 −500 −1000 −1500 −2000 −2500 −3000 −3500 −4000 −4500 −5000 −5500 −6000 Velocity (km/s), Density (g/m 3 ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 50 100 150 200 250 300 350 400 Pressure (GPa) 0 −500 −1000 −1500 −2000 −2500 −3000 −3500 −4000 −4500 −5000 −5500 −6000 Depth (km) Legend Vp Vs Density Pressure Fig. 5.1.: The Preliminary reference Earth model (PREM); after Dziewonski and Anderson [1981]. Note that the same scale is used for seismic velocity and density (with different units). distribution for the Earth. Fortunately, the Earth’s core (Sec. 5.2.3), and with some exceptions the mid and lower mantle (Sec. 5.2.2), can be assumed to exhibit an approximately radial-symmetric conductivity–depth profile. This assumption is based on observed similarity of structures at various depths across a range of areas of the Earth [e.g. Ichiki et al., 2001; Schultz et al., 1993; Kuvshinov et al., 2005; Neal et al., 2000; Olsen, 1998; Tarits et al., 2004]. Therefore, these deeper regions of the Earth are commonly described by 1D conductivity models, however, it is important to point out that these finding may be biased due to the decreasing sensitivity of EM methods with depth (Sec. 3.3). Significant lateral conductivity variations at certain depth regions are likely considering the present conditions, e.g. local hot-spot generations at the D”-layer, but are not observable on the surface. Over time, various 1D conductivity–depth profiles have been presented, starting with the fundamental work of Lahiri and Price [1939], stratifying the Earth into radial shells with different conductivity properties (Sec. 5.2), where particular sections within the Earth are governed to a certain extent by the different types of electric charge transport. In this Chapter, electric conductivity of the different region within the Earth will be discussed, but first electric charge transport mechanisms are examined. 82
Anode (positively charged terminal) + Cation (positively charged ion) + 5.1. Electric charge transport in rocks and minerals electrolytic fluid - - Fig. 5.2.: Illustration of electrolytic conduction. + - Anion (negatively charged ion) Cathode (negatively charged terminal) 5.1. Electric charge transport in rocks and minerals An excellent summary of electrical properties of crustal and mantle rocks and the related conductivity mechanisms are given in the review papers by Nover [2005] and Yoshino [2010]. The three main contributors to charge transport in rocks and minerals of the Earth are electrolytic conduction, electronic conduction, and semiconduction; however, the effect of the conductive component in a multi-phase medium on MT measurements is highly dependent on interconnection of the conductive component in the host medium. Influence of different charge transport types for a region within the Earth is dependent on type and condition (e.g. temperature, fluid saturation, melt percentage) of the local mineral composition (cf. Sec. 5.3). 5.1.1. Electrolytic conduction Electrolytic or ionic conduction usually refers to charge transport by ion mobilisation in an electrolytic fluid; other ionic conduction, such as hydrogen diffusion, is usually considered separately (cf. Sec.5.3). The electrolytic process can be illustrated using a laboratory setup in which two devices with different electric charge are placed in an ionic fluid (Fig. 5.2). Chemical processes at the positively charged terminal (anode) generate positively charged ions (cations) while the negatively charged terminal (cathode) generates negatively charged ions (anions) through removal or addition of electrons to the neutral ions in the fluid, respectively. The cations then migrate from the anode to the cathode and the anions from the cathode to the anode in order to equalise charge. Responses, recorded during in situ EM experiments, which originate from such charge difference within the ionic fluid can therefore result from different causes, such as natural or artificial EM wave signals, or local charge imbalances (e.g. poorly isolated power devices); cf. Sections 2 and 4. Electrolytic conduction is a very important parameter regarding electric conductivity in the Earth’s subsurface, in particular at crustal depth where electrolytic conduction of fluids in porous media is often dominant. For fluid saturated porous rocks an empirical 83
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Anode<br />
(positively charged<br />
terminal)<br />
+<br />
Cation<br />
(positively charged ion)<br />
+<br />
5.1. Electric charge transport in rocks and minerals<br />
electrolytic fluid<br />
- -<br />
Fig. 5.2.: Illustration of electrolytic conduction.<br />
+<br />
-<br />
Anion<br />
(negatively charged ion)<br />
Cathode<br />
(negatively charged<br />
terminal)<br />
5.1. Electric charge transport in rocks and minerals<br />
An excellent summary of electrical properties of crustal and mantle rocks and the related<br />
conductivity mechanisms are given in the review papers by Nover [2005] and Yoshino<br />
[2010]. The three main contributors to charge transport in rocks and minerals of the<br />
Earth are electrolytic conduction, electronic conduction, and semiconduction; however,<br />
the effect of the conductive component in a multi-phase medium on MT measurements<br />
is highly dependent on interconnection of the conductive component in the host medium.<br />
Influence of different charge transport types for a region within the Earth is dependent<br />
on type and condition (e.g. temperature, fluid saturation, melt percentage) of the local<br />
mineral composition (cf. Sec. 5.3).<br />
5.1.1. Electrolytic conduction<br />
Electrolytic or ionic conduction usually refers to charge transport by ion mobilisation in<br />
an electrolytic fluid; other ionic conduction, such as hydrogen diffusion, is usually considered<br />
separately (cf. Sec.5.3). The electrolytic process can be illustrated using a laboratory<br />
setup in which two devices with different electric charge are placed in an ionic fluid (Fig.<br />
5.2). Chemical processes at the positively charged terminal (anode) generate positively<br />
charged ions (cations) while the negatively charged terminal (cathode) generates negatively<br />
charged ions (anions) through removal or addition of electrons to the neutral ions<br />
in the fluid, respectively. The cations then migrate from the anode to the cathode and the<br />
anions from the cathode to the anode in order to equalise charge. Responses, recorded<br />
during in situ EM experiments, which originate from such charge difference within the<br />
ionic fluid can therefore result from different causes, such as natural or artificial EM wave<br />
signals, or local charge imbalances (e.g. poorly isolated power devices); cf. Sections 2<br />
and 4.<br />
Electrolytic conduction is a very important parameter regarding electric conductivity<br />
in the Earth’s subsurface, in particular at crustal depth where electrolytic conduction of<br />
fluids in porous media is often dominant. For fluid saturated porous rocks an empirical<br />
83
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