Magnetic Resonance in the Subsurface – 5th International ... - LIAG

Detection Coils for Near-Surface Earth’s Field NMR
E. Fukushima, A. F. McDowell, T. Z. Zhang, S. A. Altobelli
ABQMR, Albuquerque, New Mexico, USA
mcdowell@abqmr.com
At present, Earth’s Field NMR from the
surface of the Earth (that is, Surface NMR) is
used to detect significant volumes of water or
organic pollutants at depths roughly
comparable to the coil’s horizontal extent.
However, there are applications that require
the detection signals from shallower depths.
For example, the detection of oil that has
escaped under Artic sea ice, or the
determination of soil water content in the
vadose zone. The usual Surface NMR
detection coils are not particularly suited for
these shallower depths; improvements that
yield increases in both SNR and depth
resolution are needed.
We have developed a new flat coil design to
address these issues. In traditional high field
NMR, there have been many attempts to
optimize coils designs for unilateral
applications and, in addition, to flat, thin,
samples or sampled regions. The common
simple loop is optimal for a sample region that
is approximately a radius away from the loop,
thus not ideally suited for a very thin sample
region, close to the coil. The meanderline coil
can have good senetivity and position
resolution, but it is designed for flat samples
oriented along the static field, not the
orientation that is common for EFNMR. An
improved coil should have a flat, sensitive
region that is close to the coil with high and
flat specific sensitivity profile.
Our coil consists of physically parallel wires
connected in series and spread out over a flat
substrate. The current return wires are placed
on the same substrate but displaced to the sides
to minimize their contributions to the sensitive
region of the coil. We have built an example
of such a coil having 82 parallel wires in the
center with the return wires bundled at the
edges of the ~1 meter square plywood
substrate. A single turn of the coil is
topologically a figure-8, with the loops of the 8
taking the form of rectangles. The two straight
segments in the center constitute two of the
multiple parallel wires of the flat coil. A
Detection Coils for Near-Surface Earth’s Field NMR
simple extension to a two-turn figure-8 results
in four parallel wires in the central region of
the coil. For the 82 wire model coil, each
straight wire is displaced on the plywood so
nearly the entire board is covered.
This 1 m 2 coil has been used in the field to
detect EFNMR signals from water placed
directly on top of the coil structure. These
detections typically require only a few minutes
to achieve. The sentivitiy of the coil drops
with distance from the coil structure, so the
detection of signals from the sub-surface will
require larger coils. The sensitivity profile of
the coil is relatively flat for depths small
compared to the lateral extent of the coil, a
property which will enable depth profiling.
Alternatively, the flat sensitivity profile can be
used to optimize the coil for the detection of a
thin layer of oil floating on water under ice.
A virtue of the flat coil consisting of multiple
figure-8 windings is its relative immunity to
magnetic interference such as from distant
power lines. The sensitivity to interference can
be adjusted after placement of the coil by
either moving the wires or by adjusting
conducting paddles in the spaces between the
main wires and the returns.
This project is funded by Exxon Mobil Upstream
Research Company to develop technology to detect
spilled/leaked oil caught under Arctic sea ice. We
also acknowledge assistance in coil winding and
testing by J. Bench, D. Kuethe, and N. Sowko.
Magnetic Resonance in the Subsurface – 5 th International Workshop on Magnetic Resonance
Hannover, Germany, 25 – 27 September 2012
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