This book was written for students and researchers in geophysics, geology, and other Earth sciences, who wish to apply or understand the magnetotelluric (MT) method. It is intended to be an introduction to the subject, rather than an exhaustive treatise. At the same time, we do not shirk raising controversial issues, or questions for which there are no easy answers, as we do not wish to give the impression that all of the interesting problems have been solved. MT is very much a dynamic, evolving science.

We acknowledge a bias towards long-period MT studies of the deep crust and mantle. Just as one cannot drink the water from the bottom of a glass until one has drunk the water from the top (unless one has a drinking straw), electromagnetic waves cannot penetrate the deep crust or mantle without being influenced by overlying crustal structures. Hence, longer-period electromagnetic waves, which penetrate deeper into the Earth than shorter-period waves, will necessarily image a higher level of complexity than shorter-period waves. The student who has understood long-period MT sounding should, therefore, have no problem applying their knowledge to audiomagnetotellurics (AMT) and shallow crustal studies.

We have organised the chapters according to the sequence of steps most likely to be encountered by a student embarking on an MT project: from theory to field campaign, to data processing and modelling, through to tectonic and geodynamic interpretation. Some mathematical tools and derivatives are included in the Appendices.