Download 1.3.2 The Magnetic Method Several minerals containing iron and

1.3.2 The Magnetic Method
Several minerals containing iron and nickel display the property of
ferromagnetism. Rocks or soils containing these minerals can have strong
magnetization and as a result can produce significant local magnetic fields. The
magnetization can be either remnant (a permanent magnetization created by the
earth's magnetic field during some process in the history of formation of the
mineral) or induced magnetization created by the presence of the earth's magnetic
field. In most rocks both are present. The goal of the magnetic method is to map
changes in the magnetization which are in turn related to the distribution of
magnetic minerals.
The simple model of Figure 1.3.2 illustrates the fundamental idea.
Figure 1.3.2
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An object in this case, an iron ore deposit, has been magnetized with a
magnetization M in the direction of the earth's field H. The magnetized body has
its own magnetic field Hsec, which for this body has the roughly dipolar form
shown by the dashed lines in the figure. These secondary fields add vectorally to
the inducing (Earth's) field. Accurate measurements of the magnetic field along a
profile over the body will reveal a characteristic pattern or anomaly caused by the
body. For the illustration of Figure 1.3.2 a profile of the vertical component of the
magnetic field is plotted above the figure. Instruments used to measure the
magnetic field are called magnetometers. An important distinction between the
magnetic and gravity methods is that magnetization depends on the inducing field
so that the resulting field from an object depends, in a rather complex way, on how
the induced field interacts with the inducing field to alter it and hence to change the
magnetization. These are the so called demagnetization effects. For gravity the
effect of a body is simply the Newtonian gravitational attraction of the point
masses which make it up - the force of attraction has no effect on the density. It
will be seen later that for most practical situations the magnetization of rocks is
weak and a simple approximation does allow magnetic anomalies to be calculated
in a manner equivalent to the linear summation used in gravity.
The magnetic and gravity fields that have been described share a common
mathematical property in that they are curl free. This means that work functions,
integrals of force along paths in space are independent of the path. Such fields are
defined as conservative. Any curl free field can be derived from a potential, , via
the gradient operation and so gravity and magnetic methods are called potential
field methods.
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