WELLOG                                         SUSCEPTIBILITY

 

REVISED 3-03-2007

 

© 2007 WELLOG

 

INDUCED MAGNETISM:

 

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 it’s magnetism.

 

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 remaining magnetism in a substance after the initial magnetizing force (H) is removed is called remanant magnetism. Remanant induced intensity (I) can be calculated using the equation:

 

                                                                                    I = kH

 

            The factor (k) in this equation is referred to as the volume susceptibility of the material.

 

                         

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:

 

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 orebody using magnetic methods depends on the costrast 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.

 

For more information on susceptibility contact info@wellog.com