STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

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MWD and LWD 913
A
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DRIVE WINDING
Figure 4-228. Magnetic flux-lines representation in a highly permeable iron
alloy core saturated with an auxiliary magnetic field.
an electrical conductor (“sense winding”), a voltage will be induced each time
they pop in or out of the alloy strip. This induced voltage in the sense windings
is proportional to the number of lines of flux cutting through it, and thus
proportional to the intensity of that component of the earth’s magnetic field
that lies parallel to the alloy strip.
When the alloy strip is saturated, a lot of other lines of flux are created that
are not shown in Figure 4-228. The lines of flux must be sorted out from the
lines of flux due to the earth’s field to enable a meaningful signal to be
produced. A toroidal core as shown in Figure 4-229 will enable this separation
of lines of flux to be accomplished. The material used for the toroidal core is
usually mu metal.
Each time the external lines of flux are drawn into the core, they pass through
the sense windings B-B to generate a voltage pulse whose amplitude is proportional
to the intensity of that component of the external field that is parallel
to the centerline of the sense winding. The polarity, or direction of this pulse,
will be determined by the polarity of the external field with respect to the sense
windings. When the flux lines are expelled from the core they cut the sense
DRIVE
WlNOlNG
A
A
b
SENSE
0 WINDING 8
Figure 4-229. Sketch of principle of a single-axis flux-gate magnetometer.