(Image courtesy Alaska DNR DGGS gpr2003_001_01a_sh001)
All substances that are subjected to a magnetizing force become magnetized to a certain intensity. When the material is removed from the magnetic force, it loses all or part of its magnetism.
The image above is a contour map of Residual Magnetic Intensity (RMI). The world around us has varying magnetic intensity that is directly related to Magnetic Susceptibility.
Some types of ore, nickel, chromite, iron, magnetite, exhibit a magnetic force when an external magnetic force is not present. These substances have a property known as permanent magnetism. The induced magnetic intensity in a substance having susceptibility (k) subjected to a weak magnetizing force (H) is called remanant magnetism. Remanant induced intensity (I) (per unit volume) can be calculated using the equation:
I = kH
The factor (k) in this equation is referred to as the volume susceptibility of the material.
RELATIVE MAGNETIC PERMEABILITY:
The factor 1+k is denoted by mr and is called relative magnetic permeability.
m = mr * m0 = absolute permeability expressed in units of Ohm-sec/m.
MAGNETISM IN ROCKS:
Reference: “Magnetic Properties in Rocks and Minerals ”
Ferromagnetic mineral content is the largest factor controlling magnetism in rocks. Ferromagnetic minerals have a relatively high susceptibility and are capable of becoming permanently magnetized. Iron oxides, Magnetite, hematite, ilmenite, pyrrhotite are common examples of ferromagnetic minerals. In the case of sulphide ores and certain igneous rocks, pyrrhotite mineralization affects susceptibility.
Susceptibilities of rocks may vary to a large extent. In broad terms, pyrrhotite, magnetite, ilmenite and various chromite and manganese ores show large susceptibilities and in contrast, pyrite, hematite, zinc blende, and galena show low susceptibilities. Basalts, diabases, skarns and a few granulites are much more magnetic than limestones, sandstones, and shales. Pyrite and hematite have very low susceptibilities to the extent that they acquire very weak magnetization in the earth’s magnetic field and cannot sufficiently distort it so that their detection is not feasible using magnetic methods.
Susceptibility is relevant because detection of a magnetic ore body using magnetic methods depends on the contrast of its high susceptibility compared to the surrounding host rocks.
EXAMPLES OF SUSCEPTIBILITY IN ROCKS (x 106 ):
Limestone (Germany) 500 -700
Diabase 10,000 – 150,000
Slates 0 – 1,250
Chromite ore 7,500 – 1,200,000
Iron ore 650,000 – 1,530,000
Reference: Puzicha, 1942; Mooney, 1952; Mooney & Bleifuss, 1953, Werner, 1945
Rough measurements of susceptibilities and remanance can be performed with a vertical field magnetometer.
Portable instruments are used on the surface and other borehole instruments are available for measurement of rocks in situ.
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REVISED 11-26-2018 © 2018 WELLOG All Rights Reserved