Electromagnetic Induction

Sounding of Sea Ice Thickness

Austin Kovacs, Deborah Diemand and John J. Bayer, Jr. June 1996

CRREL REPORT 96-6
Abstract: Field results from a man-portable electromagnetic induction sounding instrument, with special

plug-in data processing modules for the remote measurement of sea ice thickness, are discussed. The field

trials indicate that the instrument was capable of estimating undeformed sea ice thickness, with a snow

cover, generally within about 5% of the drill hole

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measured thicknesses from about 1.25 to 4.5 m. No

ice under 1.25 m was sounded in this study. Instrument thickness determinations of multiyear sea ice

over about 4.5 m thick showed larger deviation from

the drill hole snow and ice thickness measurement. It

is proposed that the undulating multiyear sea ice relief

is the major cause of the EM deviation.
CRREL Report 96-6

Electromagnetic Induction

Sounding of Sea Ice Thickness

Austin Kovacs, Deborah Diemand and John J. Bayer, Jr. June 1996

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared by Austin Kovacs, formerly a Research Civil Engineer

at the U.S. Army Cold Regions Research and Engineering Laboratory, Hanover,

New Hampshire (CRREL), by Deborah Diemand, Research Physical Scientist, Applied Research Division, and by John J. Bayer, Jr., Civil Engineering Technician,

Civil and Geotechnical Engineering Research Division, Research and Engineering

Directorate, CRREL.

The authors thank Rexford Morey and Dr. Patrick Black for technically reviewing the manuscript of this report.

The contents of this report are not be used for advertising or promotional

purposes. Citation of brand names does not constitute an official endorsement or

approval of the use of such commercial products.

ii
CONTENTS

Page

Preface ..................................................................................................................... ii

Introduction ............................................................................................................ 1

Instrumentation ..................................................................................................... 1

Test results .............................................................................................................. 3

Discussion ............................................................................................................... 7

Literature cited ....................................................................................................... 7

Abstract ................................................................................................................... 9

ILLUSTRATIONS

Figure

1. EMI instrument shown resting on lead ice as used in this study ...... 1

2. EMI-determined conductivity vs. instrument height above the

seawater .................................................................................................. 2

3. The Geonics and Flow Research ice thickness modules ..................... 3

4. Aerial view of ~10-km-long survey line................................................ 4

5. EMI conductivity reading vs. instrument boom orientation to the

survey line .............................................................................................. 4

6. On surface EMI conductivity measurement vs. drill hole measured

snow plus ice thickness ........................................................................ 5

7. Comparison of the EMI snow and ice thickness measurements

made parallel vs. perpendicular to the survey line ......................... 5

8. Model of the sea ice crystal structure, showing the transition from

columnar ice to the skeletal layer at the bottom of an ice sheet ..... 6

9. EMI determined vs. drill hole measured snow and ice thickness ..... 6

10. A one-to-one linear curve drawn through the data shown in

Figure 9 ................................................................................................... 7

TABLE

Table

1. Apparent conductivity vs. EMI boom alignment with the

preferred c-axis alignment ................................................................... 6

iii
INTRODUCTION

In a 1990 field study, a hand-held electromagnetic induction sounding instrument with a special plug-in data processing module for the remote measurement of sea ice thickness was

evaluated (Kovacs and Morey 1992). The processor module, used to convert the measured secondary electromagnetic field in-phase and quadrature phase response to an ice thickness, was found

to be defective. The electromagnetic instrument

(EMI) would not work after a short period at

temperatures below 10°C. Indications were that

the source of the problem was battery-related.

The EMI was then used without the processor

module. In this operation, the instrument was

used to measure an apparent conductivity as a

function of instrument standoff distance above

the seawater. The results showed that a good correlation existed between the EMI-determined conductivity reading and the sea ice thickness. This

led to the conclusion that a simple graph or lookup

table could be used to estimate sea ice thickness

from the conductivity reading. In short, the technically elaborate processor module built by Flow

Research, Inc. (Echert 1986 and Echert et al. 1989)

was overly sophisticated for the measurement of

undeformed sea ice thickness. This conclusion was

based on the knowledge that the seawater under

winter Arctic pack ice has a relatively uniform

conductivity of about 2.5 S/m and an overriding

influence on the conductivity determined by the

EMI. In addition, because undeformed sea ice is

relatively resistive, it does not have a significant

influence on the EMI’s conductivity measurement

(Kovacs and Morey 1992).

This report gives an assessment of a new

Geonics processor module for determining sea

ice thickness, based upon the above findings, as

well as a second field trial of the Flow Research

processor module.

INSTRUMENTATION

The primary sensor is the 9-kg man-portable

Geonics Ltd., EM-31-D electromagnetic induction

sounding system (Fig. 1). This device is designed

to measure the magnitude of the in-phase and

quadrature components of the secondary electromagnetic field induced in the ground by the

instrument’s 9.8-kHz transmitted (primary) electromagnetic field (Geonics Ltd. 1984). Since sea

ice is relatively transparent at this frequency, the

response measured by the instrument is a strong

function of its height above and the conductivity

of the seawater. Therefore, an accurate measurement of the secondary electromagnetic field response and a full solution analysis of the data

Electromagnetic Induction Sounding

of Sea Ice Thickness

AUSTIN KOVACS, DEBORAH DIEMAND,

AND JOHN J. BAYER, JR.

Figure 1. EMI instrument shown resting on

lead ice as used in this study.
using the numerical procedure of Anderson (1979)

should give a good estimate of the instrument–

seawater distance, or the sea-ice thickness, when

the EMI is resting on the ice.

The Flow Research ice processor module was

designed to use the multilayer analysis of Anderson (1979), as provided in the Geonics program

PCLOOP, and an interpolation algorithm (Echert

1986). Kovacs and Morey (1991) state

This approach assumes that the in-phase and

quadrature components of the received magnetic

field are unique to specific sea ice thickness and ice

and seawater conductivities. The Flow Research

lookup table was developed using 10 mS/m for the

bulk conductivity of the sea ice, a seawater conductivity range from 2 to 3 S/m in 0.25-S/m increments, and a sea ice thickness range from 0.25 to 6.0

m in 0.25-m increments. The ice thickness displayed

is an interpolation between the tabulated data and

the measured EM-31 response.

The new plug-in ice thickness processor module for the EM-31 was designed and built by

Geonics, Ltd. Provisions were incorporated in the

module to allow for the unit’s output to be recorded at a portable computer stationed up to 30

m away or to a small field-portable data recorder.

This provision allows for the continuous recording of ice thickness along a survey route when the

EMI instrument is towed over the ice on a sled or

suspended from a boom off the side of a ship.

The new ice processor module was based upon

the 1990 field results, which showed a good correlation between the EMI-determined conductivity reading and the instrument height above the

seawater, as shown in Figure 2. Because the seawater conductivity under Arctic pack ice does

not vary significantly from about 2.5 S/m during

the winter, it became clear that a simple plug-in

processor module for the EMI could be developed for estimating sea ice thickness. This module would contain a programmed lookup table

listing the apparent conductivity vs. EMI height

above the seawater, when the instrument is resting on ice. These lookup table values would be

determined using the PCLOOP Program, a bulk

sea ice conductivity of 10 mS/m and a seawater

conductivity of 2.5 S/m. A digital display would

be provided on the module. After the EMI was

turned on and a conductivity measurement was

made, this value would be cross-correlated with

an instrument height in the lookup table, which

in turn would then be displayed. Only one push

button would be required to activate the instrument for an ice thickness measurement. After further consideration, it was decided to expand the

lookup table to allow for a seawater conductivity

range from 2 to 3 S/m. This range would be divided in increments of 0.1 S/m and required a

second push button to input the appropriate seawater conductivity. This two-button device would

be a very simple ice measurement module to operate, and the rigorous procedure needed to calibrate the Flow Research ice thickness processor

module (Echert et al. 1989) would thus be avoided.

The capability to change the seawater conductivity would allow the Geonics processor module

to be adjusted for unusual seawater salinity conditions. 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.

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

at 2.2 S/m, a second at 2.3 S/m, and so on until

the numerically displayed ice thickness on the

processor unit agreed with the drill hole measured value. From this assessment, a measure of

the seawater conductivity would be obtained, and

the instrument would be calibrated for this location. It should be of interest to note that a seawater conductivity error of ±0.2 S/m has a very

small effect (<5%) on the estimated ice thickness

(Kovacs and Morey 1992).

0 1 2 3 4 5 6

100

200

300

H, Tape-Measured Distance to Seawater (m)

EMI Measured Conductivity (mS/m)

σ = 2.5 S/m w

Ice

H

Figure 2. EMI-determined conductivity vs. instrument

height above the seawater (from Kovacs and Morey

1992) σw = seawater conductivity.

2
The Geonics and Flow Research plug-in modules are shown in Figure 3. Both units are configured to fit into the space normally occupied by

the EMI’s internal battery pack. Note that the

Flow Research unit has numerous switch settings

and a series of small push buttons, most needed

to calibrate the unit. The Geonics unit has two

push buttons and an auto-manual selector switch.

The latter allows for continuous (auto.) or manual

(man.) sounding. When on manual, a reading is

made by pressing the bottom start button. Pressing the center input button simply changes the

seawater conductivity value, which is automatically shown in the digital displayed during this

procedure. Both units display ice thickness to 1

cm. The ice thickness display range of the Geonics

unit is 0 to 9.99 m, while the Flow Research module has a larger range. Nevertheless, the practical

ice thickness measurement range of the EMI is

about 5 m.

The Geonics plug-in module was designed to

be operated from either a rechargeable battery

belt pack or a portable rechargeable battery. The

Flow Research unit was designed to work with

internal disposable lithium batteries.

TEST RESULTS

The EMI-31-D, with the two processor units,

was tested on the pack ice north of Alaska (Cross

Island) in April 1992. Problems immediately developed with use of the Flow Research processor

module. After being removed from the warm helicopter, this unit would soon stop operating. Repeated efforts to make the unit work at field temperatures of –15° to –25°C were of no avail.

When the Flow Research unit was returned to

CRREL, it was disassembled and the electrical

components retested. No faulty components were

found. Nevertheless, when the unit was placed in

a cold box, it would stop functioning when the

temperature in the box was lowered below –10°C;

above this temperature the unit worked. After a

long sequence of testing and probing, the mystery was finally solved. It turned out to be a poor

solder joint at a hidden connector. When warm,

the wire at the joint was physically in contact

with the connector, but when the processor was

cooled below –10°C, the wire would contract and

no longer be in contact at the solder joint. Unfortunately, as a result of this problem, no field test

data are available from use of this module.

The Geonics processor module was shipped

directly from the company to Deadhorse, Alaska.

We had no prior instructions for, or experience

with, the unit. When first used, it, too, did not

work properly. After lengthy discussion with the

manufacturer’s engineer over the phone, two

problems were identified. A resistor and an incorrect calibration setting needed to be changed.

Once these changes were made, the Geonics processor module worked at field temperatures as

low as –25°C.

Over 300 EMI soundings were made on both

first-year and multiyear ice. Most of the soundings were made at stations located at 5-m intervals along a 1-km-long survey line established for

another purpose (Fig. 4). At each station the snow

and ice thicknesses were determined by drill hole

measurement. First-year pressure ridges were not

sounded because of the electromagnetic field distortion that occurs in the seawater–ice block keel

structure. This distortion cannot be properly accounted for. Abrupt changes in ice thickness, such

as at the transition of thick ice to thin lead ice,

were also not sounded because of the electromagnetic field distortion that occurs at such sharp

transitions.

EMI conductivity and ice thickness measurements were made parallel and perpendicular to

Figure 3. The Geonics (left) and Flow Research ice thickness modules.

3
the survey line. The conductivity measurements

are shown in Figure 5. The regression curve shown

passing through the data indicates that the conductivity normal and parallel to the line has a

slight bias. This bias indicates that the ice was

slightly thicker parallel to the line. The EMI conductivity readings vs. ice thickness are shown in

Figure 6. As expected, these results show an exponential decrease in conductivity with increasing ice thickness. The slope of the curve passing

through the data becomes rather small beyond

about 5 m, suggesting that the instrument’s reliable sounding limit has been reached.

The EMI thickness determinations made parallel vs. perpendicular to the survey line are shown

in Figure 7. As expected from the conductivity

measurements, the ice thickness results indicate

the ice parallel to the line was about 2–3 cm thicker

than perpendicular to the line. However, no drill

hole measurements were made to confirm this

apparent thickness variation. Possibly that the

small conductivity and therefore ice thickness

variation were caused by ice structure effects.

Kovacs and Morey (1978) showed that currents

under the Beaufort Sea pack ice induce selective

ice platelet growth in which the c-axis of the sea

ice crystals become aligned with the current. This

finding was verified in an extensive field study

by Weeks and Gow (1979). The significance of

this alignment is that it renders the ice anisotropic and thus affects the electromagnetic (Kovacs

Figure 4. Aerial view of ~1.0-km-long survey line. The

line extends from multiyear sea ice, in the foreground, to

first-year sea ice in the background. The dark spot to the

right of the survey line is the shadow of the helicopter

from which the photo was taken.

Figure 5. EMI conductivity reading vs. instrument boom orientation (parallel

vs. perpendicular) to the survey line.

4
and Morey 1978, 1979; Morey et al. 1984) and

mechanical (Payton 1966, Wang 1979, Timco and

Frederking 1990, Kovacs 1993) properties of the

ice.

The bottommost 0.2-m portion of the ice sheet

causes the largest change in the sea ice electromagnetic properties. Here the entrapped brine,

which governs the ice conductivity, increases exponentially with depth. The brine inclusions reside between the vertical basal plane of the numerous fresh ice platelets that make up the sea ice

crystal structure as depicted (Fig. 8). When the caxes of the ice crystals are all aligned, then the

brine inclusions reside in parallel rows. Radar

reflection measurement has shown that when the

antenna E-field is aligned parallel with the preferred horizontal c-axis direction of the ice crystals at the bottom of the ice sheet, maximum reFigure 6. On surface EMI conductivity measurement vs. drill hole measured

snow plus ice thickness.

Figure 7. Comparison of the EMI snow and ice thickness measurements made

parallel vs. perpendicular to the survey line.

5
flected energy from the ice bottom is recorded.

However, when the antenna is oriented perpendicular to the preferred c-axis direction, the reflected energy was significantly reduced or eliminated (Kovacs and Morey 1979).

The ordered ice platelet structure at the bottom of the thick (>0.5 m) sea ice has been shown

to be an effective polarizer of traverse UHF and

VHF electromagnetic waves. This ice structure

may have been the reason for the conductivity

anisotropy noted in Figure 5 and the related ice

thickness offset shown in Figure 9. Nine conductivity measurements were made on new lead ice

with the EMI’s boom aligned parallel and then

perpendicular to the preferred sea-ice c-axis crystal direction, and two measurements were made

at ~45° to the preferred c-axis alignment, as determined from ice core samples. The conductivity

measurements, without gain correction, are listed

in Table 1. The data clearly indicate a strong conductivity anisotropy at this location when the EMI

was rotated on the ice. But, when the instrument

was elevated 1 m above the ice surface at sites 2

and 3 and the measurements repeated, the conductivity anisotropy was not discernible. While

this field program was not intended to explore

sea ice anisotropy using the EMI, the limited field

measurements do suggest that the sea ice structure may have affected the apparent ice conductivity measurements and therefore the ice thickness estimate. Further study should resolve this

issue.

A one-to-one curve was passed through the

data presented in Figure 9 along with ±5% deviation limits as shown in Figure 10. As may be

inferred from Figure 10, the EMI ice thickness

determination for the most part fell within the 5%

variation of the drill hole measured snow and ice

thickness measurements up to about 4.5 m. Above

4.5 m the undulating ice relief and therefore the

ice thickness in the area of the drill hole probably

adversely affected the correlation between the EMI

and drill hole measurements.

d


– 















   

   

 

   

  













    













Fresh Ice


Platelet

a


– Gas

Brine

SK Ice


~ 1 to 5 cm c-axis


Direction

Basal


 

Plane

  



 Columnar


Ice 









b


–















































c


–

Brine


Pockets

Root


Zone

Figure 8. Model of the sea ice crystal structure, showing the transition from columnar ice to the skeletal

(SK) layer at the bottom of an ice sheet. The root zone

depicts the uneven contact between ice platelets in the

zone of platelet separation. The brine pockets are shown

to vary in size and shape but are always located along

the basal plane between the fresh ice platelets.

7

6

5

4

3

2

1

0 246

Drill Hole Measured Snow and Ice Thickness (m)

EMI Snow and Ice Thickness (m)

y = 0.20 + 0.92x

r = 0.970 2

Table 1. Apparent conductivity vs. EMI boom

alignment with the preferred c-axis alignment.

Conductivity (mS/m)

 Thickness* Boom Boom Boom

Site (m) parallel perpendicular @ 45°

1 0.48 838 855 840

2 0.48 870 880

3 0.43 878 893

4 0.43 860 875

5 0.45 870 880

6 0.45 860 875

7 0.45 840 860 852

8 0.46 840 855

9 0.47 838 855

*Drill hole measured snow and ice thickness.

Figure 9. EMI determined vs. drill hole measured snow

and ice thickness.

a ≤ b < c
 – – –

d ~ 0.25 to 1.25 mm (avg 0.7)


–

a ~ 0.1 to 0.3 mm; b ~ 1 to 5 × a; c > 5 × a
 – – – – –  



Frozen Interface

Seawater Interface

6
DISCUSSION

From the results presented in this report, conductivity measurements made with a manportable EMI can apparently be used to estimate

Arctic sea ice thickness. This is based on field

conductivity measurements that were found to

systematically decrease with increasing ice thickness. The data indicate that the conductivity readings can be used to estimate sea ice thickness using a simple lookup table or a graph. An EMI

plug-in processor module may also be used to

convert the measured conductivity directly into a

numerically displayed ice thickness. The ice thickness estimates obtained with the use of the EMI

were found to be in good agreement with the drill

hole measurements. For sea ice from about 1.5 to

4.5 m thick, the deviation between the EMI instrument estimated and the drill hole measured ice

thickness was on the order of ±5%.

At the 12th International Conference on Port

and Ocean Engineering Under Arctic Conditions,

held in Hamburg, Germany, during 17–20 August 1993, S. Gerland and C. Hass of the Alfred

Wegener Institute for Polar and Marine Research

presented a poster display of the EM-31 conductivity measurements they made on sea ice. They

too found good correlation between EM-31 conductivity reading and the drill hole measured ice

thickness in areas of nonridged sea ice. However,

their EM-31 instrument was found to be temperature sensitive. In their test, the conductivity reading drifted to lower values as the instrument

cooled from room temperature to the –40°C outside temperature. After cold soaking the instrument for 30 minutes, Gerland and Hass determined that the conductivity reading had

apparently decreased to where the estimated ice

thickness was about 5% too low. After an hour of

cold soaking, instrument drift was no longer of

significance. It was determined that the drift was

largely due to cooling the antenna coils. Our field

measurements were made at much warmer temperatures and after the EM-31 had been outside

for well over an hour before being used. While

the results of Gerland and Hass are instructive,

their findings are not unexpected.

The EM-31 operator’s manual (Geonics 1984)

states that “the EM-31 is temperature compensated and set to read correctly, but due to its high

sensitivity, fine adjustment of the instrument gain

in the field may be helpful, particularly in the

case of the large changes in ambient temperature.” In short, the instrument should not be used

until it has thermally stabilized, and a calibration

adjustment may also be in order if a large temperature change has occurred.

In the EM-31 ice thickness module program, a

bulk value of 10 mS/m was used for the conductivity of the sea ice or the combined snow and sea

ice layer. Slight variations in this bulk value will

not significantly affect the EMI’s determined snow

and ice thickness, but estimated snow and ice

thickness will be in error where the snow load

has depressed the sea ice below sea level and a

portion of the snow is now saturated with highly

conductive seawater. The same would be true for

rafted ice sheets separated by ice blocks or slush

ice. In this situation, the layer of seawater or high

conductivity slush layer between the ice sheets is

not accounted for by the ice thickness module

program and the thickness estimate will be in

error.

LITERATURE CITED

Anderson, W.C. (1979) Computer programs; numerical integration of related Hankel transforms

of orders 0 and 1 by adaptive digital filtering.

Journal of Geophysical Research, 44: 1287–1305.

Echert, D.C. (1986) Electromagnetic induction remote sensing of sea ice thickness. Flow Research

Co., Inc., Kent, Washington, Flow Research Report 388.

Echert, D.C., G.B. White and A. Becker (1989)

Electromagnetic induction sensing of sea ice thick7

6

5

4

3

2

1

0 246

Drill Hole Measured Snow and Ice Thickness (m)

± 5%

EMI Snow and Ice Thickness (m)

Figure 10. A one-to-one linear curve drawn through the

data shown in Figure 9. Also shown is the ±5% variance for the curve.

7
ness and conductivity. Flow Research Co., Inc.,

Flow Technical Report 492.

Geonics, Ltd. (1984) Operating manual for EM31-D non-contacting terrain conductivity meter,

Geonics, Ltd., Toronto, Ontario, Canada.

Kovacs, A. (1993) Comparison of axial doubleball and uniaxial unconfined compression tests on

freshwater and sea ice samples. In Proceedings of

the 12th International Conference on Port and Ocean

Engineering Under Arctic Conditions, Vol. 1, p. 72–84.

Kovacs, A. and R.M. Morey (1978) Radar anisotropy of sea ice due to preferred azimuthal orientation of the horizontal c-axis of ice crystals. Journal of Geophysical Research, 83(C12): 6037–6049.

Kovacs, A. and R.M. Morey (1979) Anisotropic

properties of sea ice in the 50- to 150-MHz range.

Journal of Geophysical Research, 84(C9): 5749–5759.

Kovacs, A. and R.M. Morey (1992) Sounding sea

ice thickness using a portable electromagnetic induction instrument. Journal of Geophysical Research,

56(12): 1992–1998.

Morey, R.M., A. Kovacs and G.F.N. Cox (1984)

Electromagnetic properties of sea ice. Cold Regions

Science and Technology, 9: 53–75.

Payton, H.R. (1966) Sea ice strength. Geophysical

Institute, University of Alaska, Report UAG-182.

Timco, G.W. and R.M.W. Frederking (1990) Compressive strength of sea ice sheets. Cold Regions

Science and Technology, 17: 227–240.

Wang, Y.S. (1979) Sea ice properties. Technical

Seminar on Alaska Beaufort Sea Gravel Island Design. Exxon Company, Houston, Texas.

Weeks, W.F. and A.J. Gow (1979) Crystal alignment in the fast ice of Arctic Alaska. USA Cold

Regions Research and Engineering Laboratory,

CRREL Report 79-22.

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June 1996

Electromagnetic Induction Sounding of Sea Ice Thickness

Austin Kovacs, Deborah Diemand and John J. Bayer, Jr.

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL 96-6

Hanover, New Hampshire 03755-1290

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road

Hanover, New Hampshire 03755-1290

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16

Conductivity Remote Sensing Thickness

Electromagnetic instruments Sea ice

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

Field results from a man-portable electromagnetic induction sounding instrument, with special plug-in data

processing modules for the remote measurement of sea ice thickness, are discussed. The field trials indicate that

the instrument was capable of estimating undeformed sea ice thickness, with a snow cover, generally within

about 5% of the drill hole measured thicknesses from about 1.25 to 4.5 m. No ice under 1.25 m was sounded in

this study. Instrument thickness determinations of multiyear sea ice over about 4.5 m thick showed larger

deviation from the drill hole snow and ice thickness measurement. It is proposed that the undulating multiyear

sea ice relief is the major cause of the EM deviation.