The earth's magnetic field strength was measured by Carl Friedrich Gauss in 1835 and has been repeatedly measured since then, showing a relative decay of about 5% over the last 150 years.  Magnetic surveys are undertaken in order to detect anomalies in the natural background, which might be caused by the presence of certain rock types, significant metallic mineral deposits and buried objects such as a pipes or submerged submarine. Typically, magnetometers are flown in aircraft or towed as an instrument or an array of instruments from surface ships.

Magnetic surveys are not only amongst the most useful types of airborne work but are also, because of the low weight and simplicity of the equipment, the cheap­est.  The standard instrument, the proton magnetometer, is self-orienting and can be mounted either on the aircraft or in a towed bird.  Sensors on aircraft are usually mounted in specially constructed non-magnetic booms or stingers placed as far from the main aircraft sources of magnetic field as possible. The aircraft field will vary slightly with heading and is usually compen­sated by a system of coils and permanent magnets.  The magnitude of any heading error will need to be checked on a regular basis.

Magnetic anomalies are caused by maghemite (a form of hematite with the crystal form of magnetite), pyrrhotite and magnetite, magnetite being by far the commonest.  Ordinary hematite, the most plentiful ore of iron, does not produce anomalies detectable in aeromagnetic sur­veys.  Because all geologically important magnetic rnin­erals lose their magnetic properties at about 600o C, a temperature reached near the base of the continental crust, local features on magnetic maps are virtually all of crustal origin.  Magnetic field variations over sedimen­tary basins are often only a few nT (nano Tesla) in amplitude, but changes of hundreds and even thousands of nT are common in areas of exposed basement.  The largest anomalies known reach to more than 150 000 nT, sev­eral times the strength of the earth's normal field.

Magnetic anomaly maps are often interpreted qualitatively since they contain a wealth of information about rock types and structural trends which can be appreciated without mathematical analysis, and auto­matic (e.g. Euler deconvolution) image processing methods are increasingly being used in analysing mag­netic data.  Depths, shapes and magnetization intensities of sources of individual anomalies can be estimated, but it would seldom be useful, or even practicable, to do this for all the anoma­lies in an area of basement outcrop.  In particular, mag­netization intensities are not directly related to any economically important parameter, even for magnetite ores, and are therefore rarely of interest.

The Digital Magnetic Anomaly Map of the World (WDMAM) is an international scientific joint effort to compile and publish a reliable world map of magnetic anomalies that are attributable to the Earth’s uppermost lithosphere.  The map, first edition of which was released in July 2007, is compiled from the wealth of magnetic anomaly information derived from more than 50 years of aeromagnetic surveys over land areas, research vessel magnetometer traverses at sea, and observations from earth-orbiting satellites, supplemented by anomaly values derived from oceanic crustal ages.  The work is aimed at attracting the geological community towards the value of magnetic anomaly data in the exploration of the earth, its tectonics and resources. 

Total intensity magnetic anomaly map derived from a compilation of aeromagnetic and marine magnetic data. The nominal observation altitude is defined at 5 km above the WGS84 ellipsoid. Wavelengths longer than about 2600 km have not been included.

The magnetic anomaly map provides an interpretive dimension to surface observations of magnetic variations in terms of the Earth's composition and geologic structure. The magnetic anomalies represented on this map originate primarily in igneous and metamorphic rocks, in the Earth's crust and possibly, uppermost mantle. Magnetic anomalies represent an estimate of the short-wavelength (<2600 km) fields associated with these parts of the Earth, after estimates of fields from other sources have been subtracted from the measured field magnitude. The natural increase of temperature with depth in the Earth means that rocks below a certain depth will be essentially non-magnetic. This depth is typically in excess of 20 km in stable continental regions, and may be as shallow as 2 km in young oceanic regions. Studies of crustal magnetism have contributed to geodynamic models of the lithosphere, geologic mapping, and natural resource exploration. Inferences from crustal magnetic field maps such as these, interpreted in conjunction with other information, can help delineate geologic provinces, locate impact structures, dikes, faults, and other geologic entities which have a magnetic contrast with their surroundings. The World Digital Magnetic Anomaly Map (WDMAM) is available in both digital and map form.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals



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