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Magnetic Surveys are similar to gravity surveys in terms of data acquisition and presentation, but their interpretation is more difficult. This particular method depends on the fact that most rocks contain small, but significant quantities of ferro-magnetic minerals, such as hematite and magnetite. Accordingly, rocks have weak magnetisation that is at least partially induced by the earth's magnetic field, and which is partly permanent.

A magnetometer survey measures local variations in the magnetic field. The key element in magnetic exploration for petroleum is that sedimentary rocks are nearly non-magnetic. Therefore, they have very small responses, compared to basement rocks. Accordingly, a magnetic anomaly (deviation or departure from the norm) indicates a lack of sediments, and this survey technique rules out area that have no interest for exploration companies. These surveys are particularly useful in defining the regional extent of sedimentary basins before more-expensive seismic surveys are conducted.

On the other hand, salt domes provide an opposite reaction. Salt is diamagnetic, and it tends to oppose the field that is polarising it, causing a "magnetic low." Magnetometer surveys can be conducted on the surface (with land vehicles or ships), but better coverage can be gained from the air. This assumes that the navigation is accurate, and corrections can be made for the higher speeds of flight. Generally, oil and gas companies spend a very small portion of their exploration budgets on this type of survey.