Introduction to Magnetic Exploration - Historical Overview
Unlike
the gravitational observations described in the previous section, man has been
systematically observing the earth's magnetic field for almost 500 years. Sir
William Gilbert (left) published the first scientific treatise on the earth's
magnetic field entitled De magnete. In this work, Gilbert showed that
the reason compass needles point toward the earth's north pole is because the
earth itself appears to behave as a large magnet. Gilbert also showed that the
earth's magnetic field is roughly equivalent to that which would be generated
by a bar magnet located at the center of the earth and oriented along the earth's
rotational axis. During the mid-nineteenth century, Karl Frederick Gauss confirmed
Gilbert's observations and also showed that the magnetic field observed on the
surface of the earth could not be caused by magnetic sources external to the
earth, but rather had to be caused by sources within the earth.
Geophysical exploration using measurements of the earth's magnetic field was employed earlier
than any other geophysical technique. von Werde located deposits of ore by mapping variations in the
magnetic field in 1843. In 1879, Thalen published the first geophysical manuscript entitled The
Examination of Iron Ore Deposits by Magnetic Measurements.
Even to this day, the magnetic methods are one of the most commonly used geophysical tools. This
stems from the fact that magnetic observations are obtained relatively easily and cheaply
and few corrections must be applied to the observations. Despite these obvious advantages, like
the gravitational methods, interpretations of magnetic observations suffer from a lack of uniqueness.
