Magnetotelluric Research Overview

Magnetotellurics (MT for short) is a remote sensing technique that can image the earth's electrical resistivity structure from depths of a few 100 metres to several 100 kilometres. The basic physics is illustrated above:

Natural electromagnetic waves are generated in the earths atmosphere by a range of physical mechanisms. As these travel into the Earth's interior they decay at a rate dependent upon their wavelengths.

• High frequency signals originate in lightning activity
• Intermediate frequency signals come from ionospheric resonances
• Low frequency signals are generated by sun-spots

The low frequency signals travel further into the earth than high frequencies. By measuring electric (E) and magnetic (H) fields at the earth's surface, and then going through some tortuous mathematics, we can generate electrical resistivity models of the earth. The electrical resistivity can then be interpreted, guided by resistivity measurements made in the laboratory and other fields observations, such as surficial geology.

In general, rocks containing fluids such as water rocks at high temperature will have a low resistivity. Similarly, dry and cold rocks will have high resistivities.

Magnetotelluric Research Overview / 6 October 1996 / unsworth@geophys.washington.edu