Deep geomagnetic sounding - used by geophysicists to study the phase and the thermodynamic states of the earth's crust and upper mantle - has yielded many valuable results. This book presents some new approaches that can extend the potential of deep geomagnetic sounding. The authors present an analysis of the fundamental models of electromagnetic induction in the earth, thus paving the way to solving direct and inverse geoelectric problems. Numerous theoretical innovations are included, aimed at ensuring a more comprehensive interpretation of geoelectric data, many of them supported by practical examples. The book is primarily written for scientists and postgraduate students in geomagnetism and geoelectrics, but may also be useful to geophysicists dealing with mathematical theory of interpretation and, in certain sections, to specialists in electrodynamics.