Any measurements that differ from those of the surrounding rocks are called anomalies, and could indicate the presence of a mineral deposit. For example, some metal ore deposits are more magnetic than the Earth's normal magnetism. Magnetometers can be used either on the ground or in an aircraft to measure magnetic anomalies. A low flying aircraft is fitted with a special transmitter and a sensitive receiver, called a 'bird', is towed behind. Rocks containing ore deposits often have different electrical conductivity than rocks without them.
Ore deposits produce different responses to electrical pulses emitted by the aircraft's transmitter.
The responses are picked up by the very sensitive receiver in the 'bird' and recorded. This allows deposits as deep as 300 metres beneath the surface to be identified, mapped and made into 3-D computer models. This technique greatly increases our chances of finding deeply buried deposits. Geochemists can determine the composition of what lies below the Earth's surface by sampling soil. Soil at the surface can carry a chemical signature of what lies below, because of the movement of chemicals through the rise and fall of the water table .