Changes

==Basic concepts ==

==Basic concepts ==

The earth's magnetic field has magnetized certain minerals in the upper part of the crust. Magnetite, an accessory mineral, is by far the most important. This magnetization has two parts: ''remanent magnetism'', which was acquired when the rocks were formed, and ''induced magnetism'', which is proportional to the present earth's field and the magnetic susceptibility of the rocks. The magnetization disappears at temperatures above the Curie point, so that only the topmost 15 to 25 miles of rock are involved. Most sedimentary rocks contain little or no magnetite, and the crystalline basement rocks are by far the most important contributors to local variations in the magnetic field. Aeromagnetic surveys have been enormously successful in mineral exploration as an aid to geological mapping since they help to outline different rock units and allow interpretation of structure. They have been widely applied in the reconnaissance of sedimentary basins for mapping the basement surface. More recently, detailed high resolution aeromagnetic surveys have been aimed at weak anomalies arising from the sedimentary section (Figure 1).

[[file:magnetics_fig1.png|left|thumb|{{figure number|1}}This long aeromagnetic profile from the Canadian Arctic islands illustrates the main use of aeromagnetics in petroleum exploration. Each small circle on the basement cross section shows a depth determined from the magnetic profile. At the south end, the magnetic anomalies are sharp over the shallow basement of Devon Island. Anomalies broaden over Jones Sound where the basement deepens and then become sharper as it rises again. The basin develops fully to the north, and minor sharp anomalies (shown on expanded scale) indicate dikes and sills within the sediments.]]

[[file:magnetics_fig1.png|thumb|{{figure number|1}}This long aeromagnetic profile from the Canadian Arctic islands illustrates the main use of aeromagnetics in petroleum exploration. Each small circle on the basement cross section shows a depth determined from the magnetic profile. At the south end, the magnetic anomalies are sharp over the shallow basement of Devon Island. Anomalies broaden over Jones Sound where the basement deepens and then become sharper as it rises again. The basin develops fully to the north, and minor sharp anomalies (shown on expanded scale) indicate dikes and sills within the sediments.]]

The earth's magnetic field has magnetized certain minerals in the upper part of the crust. Magnetite, an accessory mineral, is by far the most important. This magnetization has two parts: ''remanent magnetism'', which was acquired when the rocks were formed, and ''induced magnetism'', which is proportional to the present earth's field and the magnetic susceptibility of the rocks. The magnetization disappears at temperatures above the Curie point, so that only the topmost 15 to 25 miles of rock are involved. Most sedimentary rocks contain little or no magnetite, and the crystalline basement rocks are by far the most important contributors to local variations in the magnetic field. Aeromagnetic surveys have been enormously successful in mineral exploration as an aid to geological mapping since they help to outline different rock units and allow interpretation of structure. They have been widely applied in the reconnaissance of sedimentary basins for mapping the basement surface. More recently, detailed high resolution aeromagnetic surveys have been aimed at weak anomalies arising from the sedimentary section ([[:file:magnetics_fig1.png|Figure 1]]).

The main magnetic field of the earth is caused by electric currents in the core. The intensity of the field is measured in gammas or the equivalent SI unit of nanoteslas (nT), and it varies from about 60,000 gammas in a vertical direction at the poles to 30,000 gammas in a horizontal direction at the equator. The International Geomagnetic Reference Field (IGRF) is a mathematical approximation of this field, usually removed in processing survey data to make it easier to study the anomalies caused by crustal rocks. The IGRF is regularly updated since the field varies with time.

The main magnetic field of the earth is caused by electric currents in the core. The intensity of the field is measured in gammas or the equivalent SI unit of nanoteslas (nT), and it varies from about 60,000 gammas in a vertical direction at the poles to 30,000 gammas in a horizontal direction at the equator. The International Geomagnetic Reference Field (IGRF) is a mathematical approximation of this field, usually removed in processing survey data to make it easier to study the anomalies caused by crustal rocks. The IGRF is regularly updated since the field varies with time.