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Date: | Sun, 1 Apr 2001 17:57:27 -0400 |
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Any magnetometer measures the earth's magnetic field. You
normally put two of them together, as is the common practice
as the EMF can vary by 100% over an hour. You can't tell
whether you're measuring archaeology or sunspot activity
with only one. They're set in series opposition such that in
"normal" conditions, one reads the same as the other, but
because they're in opposition, the reading on the meter is
0. One mag reads the overall EMF and the one nearer the
ground reads the EMF as well as any variation to the MF
caused by near-surface anomalies. The result is either a
positive or negative number. Soil tends to be relatively
homogenous, so that things cut into it will cause the MF to
be different in the localized area of disturbance.
Readings are taken on a grid to enable the output to be
comparable to neighboring readings. Output varies: contour
lines, dot density plots, grayscale and color plots are all
taking the readings and manipulating them. What you see on
the printout is the result of all of the readings. The
graphic of the readings are then processed by our brains for
the patterns which we relate to archaeological features.
If the anomaly has less iron in it than the surrounding
soil, it will appear as a negative number. If it has more,
then it will be positive. If it has nearly the same, then
there may not be enough difference to differentiate it from
the surrounding matrix.
It's the relativity that counts. Absolute EMF values may in
some instances be useful, but in general, it's the relative
difference between an "anomaly" and the general background.
But an anomaly can be of geological origin as well as
archaeological. One has to know the geology of the area and
know the universe of potential archaeological anomalies.
It's also faster, better and cheaper than digging lots of
little holes.
Lyle
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