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The [[Wikipedia:Magnetic field#Earth's magnetic field|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 [[Wikipedia:Curie temperature|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.  

The [[Wikipedia:Magnetic field#Earth's magnetic field|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 [[Wikipedia:Curie temperature|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.  

Magnetic disturbances caused by rocks are localized effects superimposed on the normal [[Wikipedia:Magnetic field#Earth's magnetic field|magnetic field of the earth]]. The distribution of [[magnetite]] in rocks is the primary cause of the local variations in the magnetic field observed in magnetic surveys. Magnetite is not the only magnetic mineral, but it is the dominant cause of magnetic anomalies.<ref name=ch14r7>Nettleton, L. L. 1962, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series 1, 121 p.</ref> The magnetite content of [[basement]] rocks can be two orders of magnitude greater than the magnetite content of sedimentary rocks. Consequently, variations in the magnetic field result mainly from basement rocks underlying the sedimentary section.

Magnetic disturbances caused by rocks are localized effects superimposed on the normal [[Wikipedia:Magnetic field#Earth's magnetic field|magnetic field of the earth]]. The distribution of [[magnetite]] in rocks is the primary cause of the local variations in the magnetic field observed in magnetic surveys. Magnetite is not the only magnetic mineral, but it is the dominant cause of magnetic anomalies.<ref name=ch14r7>Nettleton, L. L., 1962, Elementary Gravity and Magnetics for Geologists and Seismologists: Society of Exploration Geophysicists Monograph Series 1, 121 p.</ref> The magnetite content of [[basement]] rocks can be two orders of magnitude greater than the magnetite content of sedimentary rocks. Consequently, variations in the magnetic field result mainly from basement rocks underlying the sedimentary section.

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 [[Basement|crust]]al 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 [[Basement|crust]]al rocks. The IGRF is regularly updated since the field varies with time.

Magnetic anomalies for an object of the same size, composition, and depth have different signatures at different magnetic latitudes because the magnetic inclination—the angle at which the magnetic force field is oriented with the earth's surface—changes with latitude. [[:file:using-magnetics-in-petroleum-exploration_fig14-1.png|Figure 1]] shows profiles of magnetic total intensity anomalies for the same object at different latitudes in the northern hemisphere. In the southern hemisphere the profiles would be the opposite [north and south would reverse south of the equator and the inclination angles (''i'') would be negative].

Magnetic anomalies for an object of the same size, composition, and depth have different signatures at different magnetic latitudes because the magnetic inclination—the angle at which the magnetic force field is oriented with the earth's surface—changes with latitude. [[:file:using-magnetics-in-petroleum-exploration_fig14-1.png|Figure 1]] shows profiles of magnetic total intensity anomalies for the same object at different latitudes in the northern hemisphere. In the southern hemisphere the profiles would be the opposite [north and south would reverse south of the equator and the inclination angles (''i'') would be negative].

==Gravity vs. magnetic anomalies==

==Gravity vs. magnetic anomalies==