Download PROTON MAGNETOMETER Operating Instructions

MC5 Proton Magnetometer User Manual
upgraded) versions, a data string for combining with position data from a GPS receiver and
depth information from an echo sounder.
4.0 THE EARTH'S MAGNETIC FIELD
A reasonable understanding of the earth's magnetic field is fundamental to a successful
operation of a proton magnetometer, particularly when used for detection purposes.
The earth's field has a number of parameters, the most important of which is the 'total
intensity', and this is the parameter measured and analysed by the magnetometer. Under
short-term static conditions the total intensity value is maintained to a stability which
allows the magnetometer to measure to a resolution of better than 1 nT (gamma).
4.1 MAGNETIC FIELD STRENGTH & INCLINATION
The earth’s magnetic field is similar to that of a bar magnet with the maximum strength at
the magnetic poles (Fig. 4). However, it is not an ideal bar magnet, but has variations in
field strength due to certain geological effects. Fig. 5 is a magnetic map showing the lines
of constant field strength throughout the world. The change in magnetic field strength
measured by a proton magnetometer is expressed in nanoTesla (nT) or gamma
(1nT=1gamma). As an example - around the UK the earth’s magnetic field strength varies
from approximately 47000nT in the south to 50000nT north of SCOTLAND. The lines of
magnetic flux are not horizontal to the ground but inclined at 70 degrees. Although the
earth’s field varies by 3000nT over the UK it can be considered uniform over the localised
area of search.
The upper part of Figure 5 below shows the distribution of magnetic field strength over
the major part of the earth's surface, The relevance to the operator of this diagram, is
covered in the section that refers to initially setting the correct operating ZONE. The
contours are shown in Gauss (100,000 nT = 1Gauss).
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