using the numerical procedure of Anderson (1979)
relation between the EMI-determined conductiv-
should give a good estimate of the instrument
ity reading and the instrument height above the
seawater distance, or the sea-ice thickness, when
seawater, as shown in Figure 2. Because the sea-
the EMI is resting on the ice.
water conductivity under Arctic pack ice does
The Flow Research ice processor module was
not vary significantly from about 2.5 S/m during
designed to use the multilayer analysis of Ander-
the winter, it became clear that a simple plug-in
son (1979), as provided in the Geonics program
processor module for the EMI could be devel-
oped for estimating sea ice thickness. This mod-
PCLOOP, and an interpolation algorithm (Echert
ule would contain a programmed lookup table
1986). Kovacs and Morey (1991) state
listing the apparent conductivity vs. EMI height
This approach assumes that the in-phase and
above the seawater, when the instrument is rest-
quadrature components of the received magnetic
field are unique to specific sea ice thickness and ice
ing on ice. These lookup table values would be
and seawater conductivities. The Flow Research
determined using the PCLOOP Program, a bulk
lookup table was developed using 10 mS/m for the
sea ice conductivity of 10 mS/m and a seawater
bulk conductivity of the sea ice, a seawater conduc-
conductivity of 2.5 S/m. A digital display would
tivity range from 2 to 3 S/m in 0.25-S/m incre-
be provided on the module. After the EMI was
ments, and a sea ice thickness range from 0.25 to 6.0
turned on and a conductivity measurement was
m in 0.25-m increments. The ice thickness displayed
is an interpolation between the tabulated data and
made, this value would be cross-correlated with
the measured EM-31 response.
an instrument height in the lookup table, which
in turn would then be displayed. Only one push
The new plug-in ice thickness processor mod-
button would be required to activate the instru-
ule for the EM-31 was designed and built by
ment for an ice thickness measurement. After fur-
Geonics, Ltd. Provisions were incorporated in the
ther consideration, it was decided to expand the
module to allow for the unit's output to be re-
lookup table to allow for a seawater conductivity
corded at a portable computer stationed up to 30
range from 2 to 3 S/m. This range would be di-
m away or to a small field-portable data recorder.
vided in increments of 0.1 S/m and required a
This provision allows for the continuous record-
second push button to input the appropriate sea-
ing of ice thickness along a survey route when the
water conductivity. This two-button device would
EMI instrument is towed over the ice on a sled or
be a very simple ice measurement module to op-
suspended from a boom off the side of a ship.
erate, and the rigorous procedure needed to cali-
The new ice processor module was based upon
brate the Flow Research ice thickness processor
the 1990 field results, which showed a good cor-
module (Echert et al. 1989) would thus be avoided.
The capability to change the seawater conduc-
tivity would allow the Geonics processor module
300
to be adjusted for unusual seawater salinity con-
ditions. If the operator had reason to believe that
the seawater conductivity was not 2.5 S/m, and a
conductivity bridge was not on hand to make this
measurement, he/she could proceed as follows.
200
A drill hole ice thickness measurement would be
made. Then, with the EMI resting on the ice, an
ice thickness measurement would be made with
the ice processor module. The first measurement
could be made with the seawater conductivity set
100
at 2.2 S/m, a second at 2.3 S/m, and so on until
H
the numerically displayed ice thickness on the
Ice
processor unit agreed with the drill hole mea-
σw = 2.5 S/m
sured value. From this assessment, a measure of
the seawater conductivity would be obtained, and
0
1
2
3
4
5
6
the instrument would be calibrated for this loca-
H, Tape-Measured Distance to Seawater (m)
tion. It should be of interest to note that a seawa-
ter conductivity error of 0.2 S/m has a very
Figure 2. EMI-determined conductivity vs. instrument
height above the seawater (from Kovacs and Morey
small effect (<5%) on the estimated ice thickness
1992) σw = seawater conductivity.
(Kovacs and Morey 1992).
2
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