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Capacitively Coupled Resistivity Survey of the Sea Ice Near Barrow, Alaska
Dr. Rhett Herman, James Inman
Department of Chemistry and Physics , Radford University, Virginia 24142
Figure 1. Electrical resistivity cross-section of the sea ice near Barrow, Alaska, March 2006, with snow depth data superimposed. Note that snow depths were taken approximately 2/3 meter to
the (seaward) side of the resistivity data. Snow depth data courtesy of Dr. Julienne Stroeve (National Snow and Ice Data Center, http://nsidc.org/ ). All distances are in meters.
Northern end of
survey line
ABSTRACT
BACKGROUND
RESULTS AND DISCUSSION
Capacitively coupled resistivity methods have
the ability to image areas of high resistivity
such as the arctic sea ice. Due to their mobility,
capacitive arrays typically take data faster than
other resistivity methods and can thus cover a
wider survey area in a given amount of time.
Past surveys of arctic ice thickness have used the
EM-31 induction system. Conductivity readings
were obtained along with borehole measurements
of ice thickness to create calibration curves
relating the signal and the ice thickness.[2] This
method is effective for determining average overall
sea ice thickness but can not determine thickness
of snow layer.
We used RES2DINV to invert our data and produce the resistivity cross-section between the depths
z~0.21m-2.72m in Fig. 1. [6] A number of features may be seen, with just a few highlighted below.
This poster shows the preliminary results of a
new survey carried out near Barrow, Alaska
using the OhmMapper™ capacitively coupled
resistivity survey. The data was acquired along
a 300-meter line on the Chukchi Sea ice just
offshore from the BASC research station.[1]
This proof-of-concept survey demonstrates the
ability of the capacitively coupled array to
provide a resistivity image of the sea ice
showing features including the uppermost
snow layer and the ice/water boundary.
Snow depths have been determined by hand using
at first calibrated poles and more recently using
Magnaprobes [3].
Surveys using capacitive systems in the arctic
have been carried out across permafrost terrain
(e.g., Ref. [4]) using dipole-dipole spacings on the
order of tens of meters, giving depths of
penetration of the same magnitude and showing
features on the order of meters in extent.
• The low-resistivity seawater (blue) transitions into the medium-resistivity ice (yellow/orange) and the
high-resistivity snow cover (orange/purple). The undulations of the ice/water boundary are seen all
along the survey line. The ice is clearly thicker on the southern (left) end of the line, and thinner on the
northern end, and matches a private communication of the results of the CRREL- and NSIDC-led team
from their previous week’s survey of the same line.
• Magnaprobe data [3] of the snow depths were taken along the same line, but approximately 2/3 meter
to the seaward side of our survey line (we did not want to walk directly over the test line’s survey
stakes). This data is plotted on the same horizontal and vertical scale as our data, and is seen
superimposed on our cross-section. The Magnaprobe data follows the resistivity contours.
• There is an area at x~241m where we often had some difficulty with very low-resistivity readings,
forcing us to walk even more slowly across this segment. Upon inspection of the image above, this
seems to be a large separation in the ice reaching towards the surface, allowing the seawater upward.
• There is a feature at x~148m that is consistent with a fracture in the ice. This is consistent with the
area’s ice having been recently reformed after being broken up in January 2006 by ice forced up
through the Bering Strait by wind/ocean currents.
Figure 2. Same resistivity data as Fig. 1
but with a sharper color distinction
between the various resistivities to
sharpen the ice/water boundary.
METHOD
This 300-meter-long survey was performed using
the OhmMapper capacitive array in its typical
dipole-dipole spacing.
The data is acquired automatically at 1.0 sec
intervals; the linear data density is determined by
the speed of the operator towing the array along
the surface. Our survey speed was ~1/3 m/s,
giving ~3 data points per horizontal meter.
The effective penetration depth of the 16.5 kHz
signal is determined by the separation between
the transmitter (“Tx”) and receiver (“Rx”) dipoles
(see side panel for illustration). Our dipoles were
L=5.0m long.
The term “n-spacing” refers to the length of the
rope separating the ends of the Tx and Rx . A
spacing of n=1.0 means the rope is 1.0 times the
length of the Tx (or Rx) dipole.
Typical values for n-spacings in other are n=1.0,
2.0, 3.0, etc. This survey used very small nspacings in order to keep the signal as shallow
as possible and image the one- to two-meter
thick sea ice. N-spacings used in this survey
were n=0.25, 0.50, 0.75, 1.00 and 1.25.
Our set of small n-spacings extensively probed
the shallow ice. For example, our L=5.0 meter
dipoles, the effective penetration depth e.g. for
n=1.0 would be z~0.42*L~2.1 meters [Loke pdf].
The 300-m line was surveyed twice (different
days), once with Radford University’s Tx, and
once with another, newer Tx sent to Barrow by
Geometrics. The processed images from these
two data sets were indistinguishable.
ONGOING AND
FUTURE WORK
We are still processing the data obtained in March, 2006. For example, the image in Figure 2 used the
same resistivity data with a sharper distinction between the various resistivities to highlight the ice/water
boundary. This image could be used to calculate the true structure and volume of the sea ice at these
small scales.
Data was also taken in a 2-dimensional grid, size=100m by 10m, with n=0.25, 0.50, 0.75, 1.00, 1.25.
Taking data in this 2-d grid at multiple depths will allow for 3-dimensional data inversion. This will yield a
3-d volumetric image of the sea ice at the same sub-meter scales as the cross-sections above.
More trips to the sea ice for longer and for more 3-D surveys are planned for the coming year.
References
1. The authors would like to thank the members of the team consisting of members of CRREL, NSIDC, et al team for
permission to use the test line they surveyed immediately prior to our survey.
2. Kovacs, A., Diemand, D., and Bayer, J. Jr., 1996, Electromagnetic induction sounding of sea ice thickness. CRREL
Report 96-6.
3. The authors would like to thank Dr. Juliene Stroeve of the Dr. Julienne Stroeve of the National Snow and Ice Data
Center (http://nsidc.org/ ) for permission to use her Magnaprobe data on the Chukchi test line.
4. Calvert, H. T., Capacitive-coupled resistivity survey of ice-bearing sediments, Mackenzie Delta, Canada. Geological
Survey of Canada, 2002.
5. Loke, M. L., Electrical imaging surveys for environmental and engineering studies—a practical guide to 2-D and 3-D
surveys, Minden Heights, Malaysia, 1999. Distributed by http://www.geoelectrical.com.
6. The authors would like to thank Geometrics, Inc. for assistance in data analysis.
7. Kovacs, A., Valleau, N., and Holladay, J. S., 1987, Airborne electromagnetic sounding of sea ice thickness and sub-ice
bathymetry. CRREL Report 87-23.