Applications
B1-2 ENVI-MAG Manual
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Figure B-1 The magnetic field of the Earth.
The ENVI-MAG is a total-field magnetometer, and optionally a gradiometer, using
the proton-precession technique to measure the scalar amplitude of the magnetic
field vector. This is explained with the aid of the diagram shown in Figure B-2 on
page B1-3. The predominant magnetic field is that of the Earth’s (shown by the
vector in the figure). A local magnetic disturbance has its own magnetic field
(represented by the vector in the figure). This local vector adds to the Earth’s
field vector to produce the total-field vector . What the ENVI-MAG in the total-
field mode measures, is the projected amplitude of the total-field T in the
direction of the dominant Earth’s field . This is displayed as the intensity in
nanoTeslas by the instrument.
Note: No directional information concerning the field of the local magnetic
disturbances can be inferred from a total-field intensity measurement.
With the ENVI-MAG, you can measure not only the total-field intensity, but also
the vertical gradient (or rate of change) of the total field (if you have purchased
the gradiometer option). The vertical gradient, as previously mentioned, is the
rate of change of the total field with vertical distance. This measurement is
accomplished by simultaneously reading the total magnetic field at two
different elevations, and recording the difference. The usefulness of both these
parameters will be explained in the next Chapter.
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