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ELECTROMAGNETIC
INDUCTION (EM)
Electromagnetic induction (EM), as the name
implies, uses the principle of induction to
measure the electrical conductivity of the
subsurface. A primary alternating electric current
of known frequency and magnitude is passed through
a sending coil creating a primary magnetic field
in the space surrounding the coil, including
underground. The eddy currents generated in the
ground in turn induce a secondary current in
underground conductors, which results in an
alternating secondary magnetic field that is
sensed by the receiving coil. The secondary field
is distinguished from the primary field by a phase
lag. The ratio of the magnitudes of the primary
and secondary currents is proportional to the
terrain conductivity. The depth of penetration is
governed by the coil separation and orientation.
Unlike conventional resistivity techniques, no
ground contact is required. This eliminates direct
electrical coupling problems and allows much more
rapid data acquisition. |
| For shallow
profiling (up to 20 feet), a Geonics, Inc. EM-31
Terrain Conductivity meter is used. An EM-34 is
used for depths of investigation between 30 and
180 feet. These tools are extremely sensitive and
accurate, capable of detecting variations in
conductivity of as little as 3%. Data are
automatically stored in an electronic data logger
for later transfer to a computer. |
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The EM-61 instrument
is a high resolution, time-domain device for
detecting buried conductive objects. It
consists of a powerful transmitter that generates
a pulsed primary magnetic field when its coils are
energized, which induces eddy currents in nearby
conductive objects. The decay of the eddy
currents, following the input pulse, is measured
by the coils, which, in turn, serve as receiver
coils. The decay rate is measured for two
coils, mounted concentrically, one above the
other. By making the measurements at a relatively
long time interval (measured in milliseconds)
after termination of the primary pulse, the
response is nearly independent of the electrical
conductivity of the ground. |
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Thus,
the instrument is a super-sensitive metal
detector. Due to its unique coil arrangement, the
response curve is a single well defined positive
peak directly over a buried
conductive object. This facilitates quick and
accurate location of targets. Conductive objects,
to a depth of approximately 11 feet, can be
detected. |
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EM31 Data |
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EM61 Data |
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