POWER February 2016 - 13

Coal Ash Mapping Using Electromagnetic
Technology
The identification and containment of coal ash has risen to the
top of the priority list for plant managers across the U.S. after
inadvertent releases of coal ash into unintended areas resulted in
adverse environmental impacts and negative publicity. Increased
scrutiny and changing regulatory requirements are forcing plant
operators to work aggressively to manage coal ash.
Looking for Trouble
In the past, utilities used exploratory excavations, hand augering,
and test drilling to determine if coal ash basin containment
structures had failed and to identify the extent of coal ash deposits,
which do occasionally fall outside of intended boundaries.
These traditional methods of coal ash exploration are labor-intensive,
time-consuming, and can be inaccurate due to the physical
constraints of the process.
However, an innovative new method utilizing frequency domain
electromagnetic (FDEM) induction surveys has been employed to
scan soil at coal-fired plants to provide improved underground
characterization of subsurface strata. The response from plant
operators, who have been looking for more efficient and definitive
tools to characterize subsurface conditions, especially when
unintended coal ash is involved, has been very positive.
The FDEM method is attractive for generators because it is less
expensive than traditional physical work and the results are highly
reliable. Some studies have determined that using electromagnetic
technology rather than traditional subsurface exploration
techniques is 85% more economical, and results are available in
less than a quarter of the time.
Conductivity and Magnetism Provide Answers
The utilization of the FDEM method at coal ash sites identifies
the lateral extent of the potential coal ash flow. Coal ash contains
the mineral pyrite (iron disulfide), which is very conductive
and magnetic, thus resulting in lateral changes in subsurface
conductivity when coal ash is present.
FDEM measurements provide a noninvasive method (Figure 1)
for the measurement of subsurface conductivity and magnetic
susceptibility. The mapped conductivity and susceptibility contrasts
(Figures 2 and 3) delineate the approximate lateral extent
of the coal ash.
During initial trials of the FDEM method, hand augering was
conducted both inside and outside of coal ash boundaries to
verify results. The plots generated using FDEM were confirmed to
be highly accurate.
" Our clients have been very pleased with the results of the
FDEM execution as quantitatively verified by the hand augers, "
said Todd Eiler, project manager and president of Eiler Services.
As a result, the use of hand augers has not been required on
subsequent projects.
The FDEM method allows fast data acquisition in a fraction of
the time of previous methods. In short, data can be collected
as quickly as a person holding a conductivity rod can walk. The
depths at which FDEM technology can be deployed are dependent
on the length of the conductivity rod, with the standard length
being 12 feet. The sole prerequisite is that a walkable, unobstructed
route is required to complete the grid pattern.
February 2016 | POWER
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13
3. Seeing results. In-phase data from the frequency domain
electromagnetic survey depicts differing conductivity within the soil.
Courtesy: Paul Boyadjian
2. Painting the picture. Bulk conductivity maps depict, by color,
the magnitude and concentration of coal ash. Courtesy: Paul Boyadjian
1. Mapping an area. The process involves having a technician
walk a grid pattern in order to survey for underground ash. Although
a larger area was surveyed in this example, the area of concern was
inside the red lines. The region shaded blue was partially inaccessible.
Courtesy: Paul Boyadjian
http://www.powermag.com
POWER February 2016

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