Magnetotelluric interpretation

Abstract

The magnetotelluric (MT) method is based on the physical theory of electromagnetic wave propagation in electrically heterogeneous materials. Using the “skin-depth” principle that, in a given conductor, long period signals penetrate to greater depth than short period signals, the method typically employs the rich spectrum of natural electromagnetic energy from the ionosphere, magnetosphere, and atmosphere (distant lightning storms) to “probe” the electrical structure of the earth from a few hundred meters depth to tens even hundreds of kilometers. For near-surface applications, active transmitters are used throughout the audio band (100 Hz to 20 kHz) to above a megahertz. The last decades have seen significant developments in the type of low-noise, large dynamic range instrumentation needed for such studies, along with robust real-time data processing, 3D numerical modeling, and geophysical inverse theory. The MT method involves recording and analyzing the spectral character of the complex amplitudes and phases of the electric and magnetic field components over a wide range of periods from an array of closely spaced sites on the earth’s surface. From the spatial and spectral character of such data, one can, in a sense, deconvolve the signatures of lateral and vertical variations in the bulk properties of the subsurface to develop insight on fundamental processes in the earth.