Abstract
As one of the few deep-earth imaging techniques, magnetotellurics provides information on both the structure and physical state of the crust and upper mantle. Magnetotellurics is sensitive to electrical conductivity, which varies within the earth by many orders of magnitude and is modified by a range of earth processes. As with all geophysical techniques, magnetotellurics has a non-unique inverse problem and has limitations in resolution and sensitivity. As such, an integrated approach, either via the joint interpretation of independent geophysical models, or through the simultaneous inversion of independent data sets is valuable, and at times essential to an accurate interpretation. Magnetotelluric data and models are increasingly integrated with geological, geophysical and geochemical information. This review considers recent studies that illustrate the ways in which such information is combined, from qualitative comparisons to statistical correlation studies to multi-property inversions. Also emphasized are the range of problems addressed by these integrated approaches, and their value in elucidating earth structure, physical state, and processes.
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Notes
Wave-propagation techniques, such as ground-penetrating radar, operate at higher frequencies (100 MHz–1 GHz), have better spatial resolution than magnetotellurics, but are very limited in depth penetration (≤50 m).
The converse is not necessarily true. For example, along a symmetry axis of a three-dimensional conductivity distribution, the electromagnetic field, and hence the impedance tensor, may exhibit a lower dimensionality.
The forward problem involves calculating the surface electric and magnetic fields expected for a given subsurface conductivity distribution.
An automated, random structure modification algorithm has been suggested by Muñoz and Rath (2006).
Regularization is itself a constraint on the inversion, though it is global rather than localized in its extent.
The calculated pre-eruptive water content of silicic magmas is on the order of 2.5–6.5 wt%, suggesting they originate at depth in water-undersaturated melt conditions (Burnham 1997).
Conductivity anomalies within subduction zones and sutures are sometimes attributed to serpentinite, a product of low-temperature mafic metamorphism. Though waters released during dehydration may be conductive, the conductivity of serpentinite itself is moderate, rising to only 0.01 S/m at dehydration temperatures of 560°C (P = 600 MPa, Bruhn et al. 2004). Higher conductivities have been reported (Stesky and Brace 1973), but may be attributed to unusually high porosity or grain-boundary magnetite within the measured samples.
Pseudosections are plots of magnetotelluric response parameters, ρ a and ϕ, versus frequency and distance along a profile. As frequency is a non-linear proxy for depth, pseudosections give a crude sense of how conductivity varies within the subsurface.
While seismic velocities are determined during the migration of reflection data, they are not as robust as those obtained via refraction tomography.
A thermal explanation would imply that the boundary deepens with increasing plate age as the plate cools.
This is sometimes referred to as cooperative inversion (Lines et al. 1988).
Anomalous heat flow due to tectonic activity typically remains for ∼10 Ma following the cessation of activity.
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Acknowledgments
I wish to thank the organizing committee for the 18th EM Induction Workshop for the opportunity to prepare this review. I would like to also thank those within the EM community who have drawn my attention to a range of studies. Special thanks to Kavita Jeerage for helpful discussions of mechanisms of subsurface conductivity. This review has benefited greatly from reviews by Phil Wannamaker, Mike Friedel, Stephen Box, Seth Haines, and an anonymous reviewer. Guest editors John Weaver and Pilar Queralt are thanked for guiding this manuscript to publication in a timely fashion.
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Bedrosian, P.A. MT+, Integrating Magnetotellurics to Determine Earth Structure, Physical State, and Processes. Surv Geophys 28, 121–167 (2007). https://doi.org/10.1007/s10712-007-9019-6
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DOI: https://doi.org/10.1007/s10712-007-9019-6