Magnetotellurics

Magnetotellurics (MT) is a passive geophysical method which uses natural time variations of the Earth's magnetic and electric fields to measure the electrical resistivity of the sub-surface.

Electrical resistivity of rocks and minerals is an important physical property to measure as part of attempts to understand geological structure and processes. It varies by many orders of magnitude, from very resistive crystalline igneous rocks, to very conductive saline-filled sedimentary rocks. As measured by the magnetotelluric method, the resistivity obtained is a bulk property of a volume of Earth material and is associated with factors such as rock composition, porosity and permeability as well as rock fluid composition and temperature.

The Earth's magnetic field varies continuously in both time and space. By measuring at ground level sites time variations of the magnetic field and the electric field, the ratio of the electric and magnetic variations provides a measure of the electrical resistivity. Depth information is obtained by measuring the time variations over a range of frequencies. High frequencies penetrate the Earth to shallow depths only, while low frequencies penetrate deeper. Information is obtained from a few hundred metres depth to hundreds of kilometres depth.

Geoscience Australia has commenced a program of magnetotelluric data acquisition along onshore seismic acquisition lines. The electrical information obtained is complementary to seismic information, and is most valuable when combined with seismic and other geophysical measurements such as magnetism and gravity in multi-disciplinary interpretations.