Metadata for snow-depth and electromagnetic induction (EM)
ice thickness measurements as part of the SBI project "
Physical-biological control of primary production in
Beaufort and Chukchi Sea ice: Its contribution to shelf-
basin interactions in the western Arctic ", Spring Process
Cruise, USCGC Healy, May/June 2002 (data filename
SBI2002IceSnowThick.xls):

NSF Grant: OPP-0125464

The data set comprises sea-ice thickness and snow depth
measurements, completed in May/June 2002 during the Shelf-
Basin-Interactions (SBI) spring process cruise into the
eastern Chukchi and western Beaufort Sea. Measurements were
carried out with an electromagnetic (EM) induction device at
5 m spacing along a total of 19 profiles of typically
several hundred meters length. Both raw conductivity and
inverted thickness data are given. Direct measurements of
snow depth (and melt pond depth) were obtained as well,
along with a limited number of direct drill-hole
measurements of ice thickness. Data are given as an MS Excel
workbook, with each profile summarized in a single
worksheet, including information on location and other
relevant information about ice characteristics.

Ice thickness and snow depth were determined along linear
profiles at 5 m spacing. Profiles were located in ice
representative of large-scale conditions and no effort was
made to avoid ridged or rubbled areas. Snow depth was
determined at 5 m intervals with a measuring stick
(measurement error =1 cm).

Indirect measurements of ice thickness were completed with a
Geonics EM31 electromagnetic induction (EM) device. The
instrument determines the apparent conductivity of the
underlying medium based on measurements of the secondary
electromagnetic field induced by a transmitter coil in the
ice and the seawater underneath. Owing to low sea ice
conductivities (<50 mS/m) and high seawater conductivities
(>2500 mS/m) the signal is controlled by eddy currents
generated at the ice-seawater interface. The two coplanar
transmitter and receiver antenna coils are mounted at a
spacing of 3.66 m and operate at 9.8 kHz. The entire
instrument was mounted either on a plastic sled or in a
polyethylene kayak hull to allow for towing by hand across
the ice surface. In some rough areas the instrument was hand-
carried.

Ice thicknesses have been derived from apparent conductivity
measurements through inversion of an empirical ice thickness-
conductivity relationship based on a large data set
collected over Arctic first- and multi-year summer sea ice
(details of the method and further information can be found
in Eicken et al., 2001, Haas and Eicken, 2001, and Haas et
al., 1997). For further validation, concurrent ice thickness
and EM measurements have been obtained at selected sites
along each profile.

Direct measurements of ice thickness were carried out by
drilling a 5-cm hole with a thickness auger and lowering a
hinged ice thickness gauge down the hole (measurement error
=2 cm).

Furthermore, based on ice-core and upper ocean conductivity
measurements a one-dimensional, two-layer conductivity model
has been employed to assess sources of error and reliability
of the data set. The accuracy of the EM31 instrument is
specified at better than 1 mS/m. Based on the first
derivative of the empirical thickness-conductivity
relations, the sensitivity of the method has been derived as
0.015 m for 3 m thick level ice and 0.09 m for 5 m thick
level ice. Uncertainties in the distance between the
instrument and the ice surface and in coil orientation
increase the total measurement error to approximately 0.05 m
for 2 m thick ice when compared against profile drill-hole
ice thickness measurements (at a point spacing and profile
length that captures the relevant ice roughness features;
Eicken et al., 2001). Note that drill hole measurements of
thickness may deviate from EM measurements due to different
footprint sizes as well (i.e., <10 cm for drill and >2 m for
EM).

The ice thickness (total thickness of ice and snow cover
minus snow depth) has been derived from semi-empiric ice
thickness (Z, in m) - apparent conductivity (C, in mS/m)
inversion equations. These are based on a one-dimensional 3-
layer model (as described in Eicken et al., 2001) and
exponential regression to larger data sets collected over a
wider range of typical Arctic sea ice (Haas, 1997; Haas and
Eicken, 2001):
Z = 7.71 - ln(C - 79.5) / 0.913 (Haas, 1997)
Z = 8.167 - ln(C - 57.35) / 0.8406 (Eicken et al., 2001).

Note that due to limitations of the instrument and the one-
dimensional modeling approach, derived ice thicknesses from
very rough ice or ice thicker than about 6 m are associated
with a significant error. The same is true for measurements
over melt ponds or ice underlain by freshwater layers (see
Eicken et al., 2001). As the aim of the current measurements
was to arrive at accurate estimates of level ice thickness
and distinguish between the extent of deformed ice of
different thickness and level ice, no derived ice
thicknesses have been provided for thick, ridged ice (data
cells are left blank in the data files).

Data files (MS Excel) contain 14 lines of header
information, three blank lines, followed by the data blocks.
Data are arranged by position along the profile in meters.
Columns provide data on apparent conductivity,
thickness/depth of snow cover, deteriorated ice or melt
puddles (indicated in comments column at end), total
thickness of snow depth and ice thickness as derived from
either of the models described above and the corresponding
ice thickness value (minus snow or pond depth), ice
thickness and freeboard as determined from direct drillhole
measurements and comments.
Cited references and relevant publications describing
sampling techniques etc.:
Eicken, H., R. Gradinger, A. Graves, A. Mahoney, I. Rigor
(2005) Sediment transport by sea ice in the Chukchi and
Beaufort Seas:Increasing importance due to changing ice
conditions? Deep-Sea Res. II, 52, 3281-3302.
Eicken, H., W.B. Tucker III, D.K. Perovich (2001) Indirect
measurements of the mass balance of summer Arctic sea ice
with an electromagnetic induction technique, Ann. Glaciol.,
33, 194-200
Haas, C. (1997) Determination of sea-ice thickness with
seismic and electromagnetic-inductive methods (in German).
Ber. Polarforsch., 223, 161pp.
Haas, C. and H. Eicken (2001) Interannual variability of
summer sea ice thickness in the Siberian and Central Arctic
under different atmospheric circulation regimes. J. Geophys.
Res., 106(C3). 4449-4462
Haas, C., S. Gerland, H. Eicken, H. Miller (1997) Comparison
of sea-ice thickness measurements under summer and winter
conditions in the Arctic using a small electromagnetic
induction device, Geophysics, 62(3), 749-757

Point of contact:
Hajo Eicken, Geophysical Institute, University of Alaska
Fairbanks, Fairbanks, AK 99775-7320, phone: 907-474-7280, e-
mail; hajo.eicken@gi.alaska.edu