Magnetic susceptibility is a measure of the ability of a substance to be magnetized in the presence of an external magnetic field. In principle, magnetic susceptibility measurement is based on the amount and type of magnetic minerals within the rocks. In cases of positive magnetic susceptibility, the material within the rock can be ferromagnetic, antiferromagnetic, or paramagnetic. The material is said to be diamagnetic if the magnetic susceptibility is negative.
Magnetic susceptibility can be defined by the following equation:
M = kH
where M is the induced magnetization vector, k is the magnetic susceptibility, and H is the magnetic field intensity vector.
The magnetic susceptibility sensor includes an oscillator circuit, which produces a low intensity, alternating magnetic field. Any sample material with a magnetic susceptibility placed near the sensor will induce a change in the oscillator frequency. Other electronics within the senor will convert this pulsed frequency information into magnetic susceptibility values.
Magnetic susceptibility of granite rocks varies between the order of 10-6 in leucocratic granites, and 10-2 in tonalities and granodiorites. Most of the volcanic rocks are strongly magnetic in nature, and changes in their susceptibility are an important geological factor. These variations can be investigated through the susceptibility measurement. Sedimentary rocks have very low magnetic susceptibility. Some sedimentary rocks containing ankerite or siderite may have higher magnetic susceptibilities.
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