Electromagnetic (EM) Ground Conductivity Surveys :-
Electromagnetic surveys are a useful means by
which to detect changes in the sub-surface relating to changes in electrical
conductivity. Electromagnetic ground conductivity surveys are used for environmental and engineering investigations
in order to map out sub-surface changes which can be investigated using traditional intrusive methods. The
method is particularly useful for the delineation of a landfill boundarys, mapping in-situ faoundations or concrete slabs and when attempting to
locate lost landfill.
electromagnetic ground conductivity works by a transmitter
coil inducing eddy currents in sub-surface materials which are detected by a
receiver coil. Depending upon the orientation, spacing of the coils and
frequency a variety of depths can be investigated. A range of instruments exist,
including the Geonics EM38, EM31 and EM34, making it possible to investigate
depths between 1.5m and 60m.
The EM61 produced by Geonics uses
the time-domain principle to discriminate metallic targets from ground
materials. It does this by measuring the length of time that the return signal
from metallic targets persists. The system is frequently used for detection of
unexploded ordnance, pile caps, drums and tanks.
Electromagnetic ground conductivity measurements are made
by walking a series of predefined survey lines with no need for ground -
instrument contact. The EM61 system is trolley based but is capable of being
mounted directly on an operator. This adaptability makes EM methods ideal for
surveying poorly prepared sites. Data loggers such as the Allegro Field Computer
are used to capture instrument readings at set intervals. Using the dual serial
ports it is also possible to capture both the instrument data and the GPS data
with differential correction.
On suitable sites GPS
reduces the need for setting out survey lines. Setting out accurately on some
sites would be nearly impossible but with GPS only a few key fixed points are
needed for position and closure checks.
The image (right) shows the track taken during a conductivity survey.
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The photograph (right)
shows an EM31 survey in progress with real-time Differential GPS positioning.
The Differential GPS antenna is located
behind the operators head.
Laying out survey lines with tapes would be
impossible on sites with so much surface vegetation. |
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At the end of each survey day the
data are downloaded to PC for a rapid assessment of the survey success and
findings. Processed data are often superimposed upon existing topographic
surveys. The image below shows typical final output from a conductivity survey
over an former gravel pit with infill ranging from rock waste to putrescible
waste. The only way to find out the distribution of buried materials without the
geophysical approach is to use trial pits and boreholes and to make considerable
assumptions from their "spot-reading" nature.
Applications :-
- Pollution plumes
- Landfill boundaries (EPA Part IIA
asessments)
- Brownfield sites
- Unexploded ordnance
- Mineshafts
- Quarries and opencast
- Pipelines, reinforced slabs, drums,
infill
- Agricultural soil moisture mapping,
also for sports grounds.
Benefits :-
- Rapid surveying - walking speed or
discrete measurements
- GPS compatible - little setting out
required
- Instruments for a range of depths
- Non-intrusive
- Quick analysis and reporting
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