MT and AMT are the same type of survey. The key thing is that they are using naturally circulating electro-magnetic fields in the earth's crust as the input or source field. Because these fields are very low frequency and long wavelength they can penetrate to great depths within the earth's crust. If there is a conductive (metallic) body in the area being surveyed then the source field will generate a secondary electromagnetic field within that conductive body. Very sensitive magnetometers are used on a survey grid to take the magnetic field measurements. The induced magnetic field can be differentiated from other magnetic fields by its vector direction as it is out of phase with the input field. The strength of this induced magnetic anomaly is directly proportional to the conductivity of the target body so results are usually reported in ohm-metres. MT surveys are used extensively in North and South America. They have not caught on so quickly in Australia due to cost and competition from other methods.
