Magnetotellurics: Principles and practice

This paper reviews the magnetotelluric (MT) method with emphasis on recent improvements and the emerging applications. Recent improvements are in recognition of statics effects and their treatment, and in analysis of impedance tensors in 3D structural conditions. In spite of substantial progress in the analysis of the tensor, there are still unresolved questions regarding its information content, in terms of degrees of freedom. There has also been slow but steady progress in inversion, with the development of methods of downward continuation in 1D, 2D and 3D, in time, depth and frequency domains. An improved understanding of the limitations of MT is leading to more joint use with other methods. At shallow depths MT has become an accepted weapon in the arsenal used to explore for petroleum, geothermal resources and epithermal gold and deep base metals. Improved understanding of the relations between conductivity and rock composition and fabric will make MT more useful for engineering purposes in future. Progress in understanding the cause of conduction in the deep crust and upper mantle has been less rapid, and the many anomalies remain enigmatic. Whereas MT is now a useful technique with some unique applications, there are obviously important and difficult problems remaining to be solved.