<P> Commercially, geophysical prospecting companies also use magnetic detectors to identify naturally occurring anomalies from ore bodies, such as the Kursk Magnetic Anomaly . </P> <P> Magnetometers detect minute deviations in the Earth's magnetic field caused by iron artifacts, kilns, some types of stone structures, and even ditches and middens in archaeological geophysics . Using magnetic instruments adapted from airborne magnetic anomaly detectors developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped . Basalt--the iron - rich, volcanic rock making up the ocean floor--contains a strongly magnetic mineral (magnetite) and can locally distort compass readings . The distortion was recognized by Icelandic mariners as early as the late 18th century . More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor . When newly formed rock cools, such magnetic materials record the Earth's magnetic field . </P> <P> Each measurement of the magnetic field is at a particular place and time . If an accurate estimate of the field at some other place and time is needed, the measurements must be converted to a model and the model used to make predictions . </P> <P> The most common way of analyzing the global variations in the Earth's magnetic field is to fit the measurements to a set of spherical harmonics . This was first done by Carl Friedrich Gauss . Spherical harmonics are functions that oscillate over the surface of a sphere . They are the product of two functions, one that depends on latitude and one on longitude . The function of longitude is zero along zero or more great circles passing through the North and South Poles; the number of such nodal lines is the absolute value of the order m . The function of latitude is zero along zero or more latitude circles; this plus the order is equal to the degree l . Each harmonic is equivalent to a particular arrangement of magnetic charges at the center of the Earth . A monopole is an isolated magnetic charge, which has never been observed . A dipole is equivalent to two opposing charges brought close together and a quadrupole to two dipoles brought together . A quadrupole field is shown in the lower figure on the right . </P>

Horizontal component of the earth's magnetic field