Preface to the Special Issue on “The 21st Electromagnetic Induction Workshop”
Surv Geophys
Preface to the Special Issue on ''The 21st Electromagnetic Induction Workshop''
Toivo Korja 0 1
Graham Heinson 0 1
Stephan Thiel 0 1
0 G. Heinson S. Thiel Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide , Adelaide 5005 , Australia
1 T. Korja (&) Department of Physics, University of Oulu , P.O. Box 3000, 90014 Oulu , Finland
Electromagnetic (EM) induction methods are used and continue to be developed for a wide range of applications, ranging from exploration near the Earth's surface to the investigations of the deep mantle. In this research, important scientific and societal challenges, such as to search for hydrocarbons and other Earth resources, to probe the structure and dynamics of the lithosphere, to study environmental issues and to monitor and mitigate natural hazards, are addressed. The Working Group I-2 of the International Association of Geomagnetism and Aeronomy on ''Electromagnetic Induction in the Earth'' has held biennial workshops since the Edinburgh, United Kingdom, Workshop of 1972. Here, selected topics are extensively explored by the participants, in the form of oral and poster presentations and discussion sessions. An essential and important part of the EM Induction Workshops (EMIWs) has been invited review presentations on themes selected by the program committee. These themes vary from workshop to workshop; usually, they highlight recent advances in the rapidly evolving fields of electromagnetic induction and introduce the important new directions of research as well as highlight and review results focusing on certain geological targets. The review papers presented at the workshops have traditionally been published as Special Issues of Surveys in Geophysics/Geophysical Surveys since the 1978 workshop in Murnau, Germany. This Special Issue of Surveys in Geophysics contains nine expanded articles from review papers presented at the 21st Workshop on Electromagnetic Induction in the Earth. The workshop was held between 25 and 31 July, 2012, in Darwin, Australia. It was organised by the Working Group I-2 of the International Association of Geomagnetism and
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Aeronomy and Local Organising Committee led by Graham Heinson. The scientific
programme of the Workshop was divided into nine themes, for which ten review presentations
were given in the workshop:
John Booker (University of Washington, USA) reviews the use and misuse of the
magnetotelluric phase tensor (PT), which has been widely used within the MT community
since its first postulation in 2004. His review focuses on recent examples where the PT was
used to estimate the dimensionality and strike direction. Booker revisits the theory behind
the PT and argues that correct use of the tensor including meaningful thresholds for the
third skew invariant, a 3D dimensionality indicator, as well as the importance of the error
propagation for the uncertainty estimation of the PT estimates, leads to improved estimates
to justify 2D modelling of the data. As the PT is a representation of the magnetotelluric
impedance tensor, the PT is compared with other representations including induction
arrows, the enhanced admittance phase tensor and the Mohr circle representation. Booker
describes the progress made in using the distortion-free nature to modify modelling
algorithms to purely solve for the inductive response of the impedance tensor. The paper
also investigates a PT-consistent decomposition formulation which overcomes some of the
strict 2D assumption of the traditional 2D impedance tensor decomposition methods
formulated by Bahr and Groom and Bailey. The review concludes that PTs should form a
more substantial part in 3D modelling and also in analysing the model fits of conventional
3D MT inversions. It provides a very useful resource for MT practitioners.
Anne Pommier (Arizona State University, USA) reviews the laboratory measurements
of crustal and mantle rocks which provide an important link for interpreting geophysical
measurements, in particular the electrical resistivity properties of these rocks. It is a timely
review following a review on the laboratory conductivity measurements of mantle minerals
by Yoshino (2010). Pommier begins by demonstrating how electrical measurements in the
laboratory and the field differ, followed by approaches to combine the laboratory and MT
results through joint interpretation as well as forward modelling and hypothesis testing.
The review shows the controlling factors of the laboratory measurements, highlighting the
influence of the pressure, temperature and length of experiments, scaling and oxygen
fugacity. Pommier discusses the importance of hydration of crustal and mantle materials as
a major driver to explain enhanced conductivities observed from MT. However, laboratory
measurements have to be considered carefully when interpreting magnetotelluric models.
The review concludes with recommendations to pursue multi-disciplinary approaches
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