1

Abstract— Ice core drillings have been performed in various zones in Antarctica and Greenland to

obtain climatological information, study ice properties, or analyze air and dust encapsulated in the

ice during the quaternary period. During these procedures, a set of measurements to characterize the

ice and to evaluate its physical and chemical properties are usually performed in situ. In particular,

using known temperature and dielectric profiles (DEP measurements), it is possible to evaluate the

ice electromagnetic power absorption profile, valid at the drilling site. In last decades, bedrock

characterization through Radio Echo Sounding (RES) surveys has been improved by the analysis of

the power of radar echoes. In this way, analysis of the electromagnetic properties of bedrock

interfaces makes it possible to assess the physical characteristics and to distinguish between wet and

dry conditions. Power variation of the received echoes also depends on ice absorption and on bedrock

reflectivity due to specific physical conditions of the ice.

In this paper the propagation of electromagnetic waves through the ice sheet is examined, and in

particular a new method for establishing the electromagnetic absorption profile for ice from core

drilling measurements is proposed and discussed. Variation in the ice absorption is deduced, starting

from analysis of ice core data from EPICA at the Concordia station (Antarctica) and from the GRIP

site (Greenland). This direct method of measurement is proposed with the aim of defining common

characteristics of the ice absorption rate that are valid both in Antarctica and in Greenland.

A. Zirizzotti1, L. Cafarella

1, S. Urbini

1, J.A. Baskaradas

2, A. Settimi

1

1 Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

2 School of Electrical and Electronics Engineering SASTRA University, Thanjavur, India

email: achille.zirizzotti@.ingv.it

ASSESSMENT OF ELECTROMAGNETIC ABSORPTION OF ICE FROM ICE CORE

MEASUREMENTS.

2

INTRODUCTION

The Radio Echo Sounding (RES) technique is widely used in polar ice sheet exploration for mapping

bedrock morphologies and ice properties. It remains an indispensable tool for obtaining information about

the physical conditions of the ice mainly from its electromagnetic properties (Plewes and Hubbard, 2001

[36]). During recent years, the scientific interest has been drawn to the possible existence of water

circulation beneath the ice (Remy et al., 2003 [37], Kapitsa et al., 2006 [19]; Wingham et al., 2006 [46];

Bell et al., 2007 [1], Carter et al., 2009 [6], Tabacco et al., 2006 [43]) but, while differences between rock

and lake surfaces can be identified reasonably well in radargrams, this is not always the case for wet and

dry ice-bedrock interfaces. Certain characteristics of the physical condition of the ice-bottom interface have

been deduced in a number of studies using the power of radar echoes (Corr et al., 1993 [7], Bianchi et al.,

2004 [2], Carter et al., 2007 [5], Paden et al., 2005 [33], Oswald and Gogineni, 2008 [32], Fujita et al., 2012

[13]). Briefly, the information about the physical condition of ice-bottom interfaces (assessed from

electromagnetic reflectivity) was obtained starting from the solution of the radar equation using

electromagnetic power variations in echoes received after passing through the ice (Borogosky, 1995 [3]).

Attenuation of the echoes depends mainly on ice conductivity modulated by acidity, due to the quantity of

impurities originating from the presence of sea salt and erupted volcanic elements and in minor contribution

to the crystal fabric. The critical factor for the solution of the radar equation is determining the power

absorption of the ice, which can be obtained from conductivity measurements.

In the past, laboratory-frozen ice measurements were conducted to establish the electrical characteristics of

ice, and in recent decades the connection between the results and electromagnetic wave propagation through

ice has been investigated (Matzler and Wegmuller 1987 [26], Fujita 2000[12]). The aim was to define the

physical properties of ice in order to create models for its electromagnetic absorption, so that these results

could be extended to larger areas covered by RES measurements. Laboratory measurements of ice

conductivity and permittivity at different impurity concentrations, pressures, and temperatures were carried

3

out (summarised in Fujita 2000 [12]) and the models were improved, although discrepancies still remain

between laboratory measurements and those conducted on natural ice (Stillman 2013 [41]).

Electromagnetic ice absorption can be calculated from conductivity measurements on an ice core, using the

temperature profile, obtaining results valid for a particular drilling site. These measurements in the past have

been made at different sites also applying different methods. These include electrical conductivity

measurements (ECM) using direct current (DC) valid exclusively for shallow ice cores (Hammer et al. 1980

[15]). This depends essentially only on the acidity of the ice, even if a large concentration of neutral salt is

present (Moore et al. 1992 [28]). For deep ice cores, the dielectric profiling method (DEP, Moore and Paren,

1987 [27]) is generally used. In this case measurements are made at AC frequencies (below 300 kHz) and the

results depend on acidity, acidic and neutral salt concentrations, and ice ammonium concentrations (Moore

and Paren, 1987 [27]; Moore et al. 1992b [29]; Moore and Fujita, 1993 [30]). Improved DEP instruments

have been developed to simultaneously measure ice core conductivity and permittivity (Wilhelms et. al., 1998

[45]), both profiles being useful in the study of radar signals and propagation velocity through the ice.

Another approach is to evaluate electromagnetic absorption of ice directly, using the amplitudes of RES

measurements (MacGregor et al. 2007 [22]). In this case the radar equation can be solved by assessing ice

absorption using some simplifications. For example, by only considering RES measurements collected over

subglacial lakes, and assuming a constant value for reflectivity. This hypothesis permits assessment of ice

absorption at different lake depths, also in different areas (Zirizzotti et al. 2014 [53]).

Conversely, ice reflectivity can be measured starting from the RES amplitude, assuming a linear trend for ice

absorption (in logarithmic units) (Jacobel et al. 2009 [17], Zirizzotti et al. 2010 [51]). In fact, by assuming a

linear trend for ice absorption (constant ice absorption rate versus ice depth), it is possible to measure

variations in bedrock reflectivity in order to establish a map of wet/dry zones on the bedrock interface (Carter

et al., 2009 [6]; Gades et al., 2000 [14]; Jacobel, 2010 [17]; Langley et al., 2010 [21]; Peters et al. [35], 2005;

Wright et al., 2012 [49]). Furthermore, by analyzing the amplitude of the signal reflected from internal layers,

4

it is possible to extend the information about ice absorption to measure this contribution to bedrock

reflectivity (Siegert and Fujita 2001 [39], MacGregor et al. 2007 [22], Zirizzotti 2012 [52]).

In this paper, data from the GRIP Ice Core Project in Greenland, and from the EPICA European Ice Coring

Project at the Concordia station in Antarctica, (Wolf 1995 [47], community members 2004 [8], Wolf 2004

[48]), are presented and compared. A new method is proposed to assess the ice absorption rate from ice core

measurements, applied to the data from the two drilling sites. The results are discussed in detail and a

comparison is made between the two sites.

MODELLING ICE CONDUCTIVITY

Starting from the EPICA ice core measurements performed at the drilling site, it is possible to plot the raw

conductivity values (measured using the DEP method) as a function of depth with the corresponding

temperature profile (red and blue lines respectively in figure 1).

0 1 103

2 103

3 103

0

10

20

30

40

100

80

60

40

20

0

DEP

scaled DEP

Temp

Depth [m]

Con

du

ctiv

ity [

µS

/m]

Tem

p.

[°C

]

Figure 1: Measured ice conductivity (258),

scaled conductivity and temperature at the EPICA

drilling site

0 1 103

2 103

3 103

0

10

20

30

40

100

80

60

40

20

0

Depth [m]

Co

nd

uct

ivit

y [

µS

/m]

Tem

p.

[°C

]

Figure 2: Measured ice conductivity (258), scaled

conductivity and temperature at the GRIP drilling

site

During the drilling process the extracted ice at a defined depth was measured at the relatively constant

temperature range of -20 ± 2° C. The ice conductivity values 258 were measured and scaled to –15° C

(assumed as a reference value) using the Arrhenius model reported in equation (1). The 258conductivity

shows a slow reasonably constant trend with a mean value of 12.8 µS/m shifting gradually from 20 to 10

5

µS/m. As a second step the reconstructed in situ conductivity profile (green line shown in figure 1) can be

obtained by scaling these measurements according to the temperature profile T(z) using the equation (1)

(Corr 1993 [7], Paden et al. 2005[33], Stauffer et al. 2004 [40], Kulessa 2007 [20]):

258

258

1 1( ) ( )exp

( )B

Ez z

K T T z

1)

where KB = 8.6173324(78) · 10−5

eV/K is the Boltzmann constant, E=0.22 eV is the applied activation

energy (a parameter that measures the sensitivity of the conductivity to temperature) while T(z) and 258(z)

are the temperature and conductivity measurements at different depths z.

The whole procedure was repeated for the GRIP ice core data and the results are reported in figure 2. In the

latter case, conductivity measurements in situ were performed directly on the extracted ice core at the fixed

temperature of -15°C, making the two data sets comparable. As shown in figure 2, 258 conductivity at the

GRIP site exhibits a sudden step at a depth of about 1600 m, changing values from around 20 µS/m to 10

µS/m. A smaller step is also visible at a depth of about 1000 m where conductivity values change from

around 18 µS/m to 22 µS/m.

As shown in equation (1), lower ice temperatures induce lower ice conductivity values. This behavior can

be clearly observed by plotting ice core conductivity (1) against ice temperature as shown in figure 3. The

data were calculated using the known temperature profiles valid at the selected sites. In the same figure,

laboratory measurements obtained from Fujita et al. 2000 [12] are added for comparison as black-dotted

line. These detailed laboratory measurements were made to analyze variations linked to crystal fabric,

density, impurity concentrations, temperature, pressure, air bubbles, and plastic deformation. The black

dots report the measurements performed on pure ice with an additional component of background acidity of

2 µM. Higher acidity values increase ice absorption, as observed in the field in Antarctica, where this value

reaches local peaks up to 10 µM (Fujita et al. 2000 [12]). Laboratory conductivity measurements are

performed at frequencies below 300 MHz using the DEP method. Ice absorption and electromagnetic

6

scattering increase at frequencies higher than these, because electromagnetic waves of higher frequencies

penetrate the ice with more difficulty, hampering deep RES measurements. As shown in figure 3, the

reconstructed in situ ice conductivity profiles exhibit similar average values in the common interval

between -30° to 0° C. The difference at temperatures around -32 °C is due to a local surface temperature

influencing the conductivity profile in the first 1600 m. Large variations at the bottom of the ice are instead

probably linked to different ice acidity levels due to catastrophic volcanic events recorded at both polar

sites. It is worth noting that, in spite of these differences, the reconstructed in situ ice conductivity

measured at EPICA and GRIP (and the corresponding scaled conductivities) is very similar, and also close

to the laboratory measurements. This is the evidence of strong dependence of conductivity on temperature,

as highlighted by equation (1).

Figure 3: Ice conductivity at the two sites vs. ice temperature.

7

MODELLING ICE ABSORPTION

Evaluation of electromagnetic absorption, in a medium with defined electromagnetic properties, starts

from the solution of the wave equation. In particular, it is possible to evaluate ice absorption from the

solution of the reduced wave equation (Helmholtz differential equation, Ulaby 1981 [44]) in the case of a

sinusoidal electrical field Ex oscillating at the frequency along the direction x orthogonal to the

downward direction of propagation z in the ice, with relative dielectric permittivity εr and relative magnetic

permeability µr,:

)()()( 2

2

2

2

2

zEzncz

zExc

x

,

2)

nc is the complex refraction index, which can be separated into real and imaginary parts (as usual j is the

imaginary unit):

)(

)(

2

)()()(

)()()()()()(

00 z

zzjzz

zjzzzzzn

r

rrrrrcrc

3)

in the case of low-loss media (low conductivity):

)()(

0

zz

r

4)

This condition is valid in the case of ice for frequencies higher than 0.1 MHz. Here c(z) is the complex

relative dielectric permittivity, which is not constant in the ice but depends on depth z (through r(z) and

σ(z) the electrical conductivity), c2= 1/ (ε0µ0) is the velocity of light in the vacuum, and ε0 and µ0 are free

space dielectric permittivity and magnetic permeability respectively. Moreover, in the ice conductivity σ(z)

depends on acidity and temperature, as shown in the previous paragraph. As a consequence of the fact that

the ice temperature varies along the depth, the conductivity profile is not constant. The relative dielectric

permittivity r(z) also depends on the media temperature and on the frequency of the electromagnetic

signal.

8

In the case of constant refractive index values, nc=const. (with all the electromagnetic parameters

constant and independent on the z axis), equation (2) can be resolved in incoming and backwards waves

with the damping and oscillating parameters (Ulaby 1981 [44]):

20

0

r

r

rrc

5)

6)

The general case is a linear combination of the two solutions with coefficients that depend on the initial

boundary conditions.

In the case of ice, a media contaminated with impurities, the constant refractive index is no longer valid,

and neither are the parameters (5) and (6). In this context, the equation (2) was solved using the WKB

method, proposed to solve the Schrödinger equation (applied to the Planck parameter ħ adopting ħ 0) and

also applied in optics (for the wavelength parameter =2πc/ω, adopting 0), as proposed in Settimi et al.,

2003 [38]. Using a WKB-like approximation, the solutions can be written as:

z

c dzznc

j

x eEzE 0

')'(

0)(

7)

As shown in the appendix A of Settimi et al., 2003 [38] the condition of validity of the two solutions (7)

is:

4

)(

1)(2

zndz

zdn

c

c.

8)

where is the wave-length in the vacuum. For ice this condition is generally valid, and the left side of

equation (8) in the case of the EPICA and GRIP data are always less than 10-5

m-1

calculated for the whole

9

ice core depth. Moreover, in ice the condition (8) is always valid throughout the frequency range of validity

for condition (4).

Considering only the minus solution (incoming wave travelling along the positive z axis) and using

equation (3) gives:

)()(

0

')'(

00)( zjz

dzznc

j

x eEeEzE

z

c

.

9)

Here, a damping term is multiplied by an oscillating term, so that:

zz

dzz

r

zrz

dzzzZz00

0

0)'(

)'(

2

)'(')'()'(

2

1)(

10)

z

rr

z

dzzzc

dzznc

z00

')'()'(')'()(

11)

where Z(z)2=µ(z)/(z) is the intrinsic impedance, and n(z) is the real refraction index of the ice. Equation 10

and 11 are important because they allow us to calculate amplitude and phase of a propagating

electromagnetic wave in a media with non constant electromagnetic parameters (r, σ, µr ). This is a general

case when we consider natural non uniform material like ice in glaciers, water in oceans and lakes or

stratified terrain. In the specific case of constant conductivity and constant dielectric permittivity and

permeability, this can be expressed as:

zzzr

r

2)(

0

0

12)

10

zzc

z rr

)(

where and are the factors calculated with Eqs. (5) and (6), generally used in the constant solution of

equation (2) (Ulaby et al., 1981 [44]). These equations are also used in amplitude analysis of radar signals

and also in cases of non constant electromagnetic parameters of a media.

The electromagnetic absorption of ice L[dB]

(z), i.e. the attenuation of a radar impulse passing through an

ice column in dB, can be evaluated from the attenuation of the radar signal using the following equation:

)(686.820)( )(][ zeLogzL zdB , 13)

In order to verify this equation and the solution (9), the ice absorption L[dB]

was compared, considering

three different cases. In the first case a numerical solution to evaluate the amplitude attenuation of a wave

propagating in a medium with physical parameters εr(z) and σ(z) was solved numerically using a specific

algorithm of the commercial software “Mathematica”. The conductivity profile was used to calculate the

propagation of an electromagnetic wave at both EPICA and GRIP. It is possible to extract the wave

amplitude from the numerical solution of equation (2), which changes as function of the distance z, in order

to obtain the absorption rate of a selected medium. In the second case the exact solution (13) of the ice

absorption using equation (10) was calculated with the same conductivity and permittivity parameters used

in the numerical solution. Finally, in the third case, the solution was calculated using the equation (12)

which is only valid in the case of constant electromagnetic parameters. The three solutions are plotted and

compared in figure 4 for the EPICA and GRIP data. The red lines are the numerical solutions of the

amplitude variation for a short sinusoidal pulse (50 ns pulse length at f = 60 MHz). Edge effects are visible

and they are due to the initial boundary conditions of the partial derivative equation (initial amplitude and

speed of the transmitted pulse). Below the red line, a black dashed line represents the exact solution of the

ice absorption (equation 13). As shown in the figures, there is no difference between these two cases. The

11

blue lines report the constant solution of absorption rate (equations 12). As is clear, there is an error up to

10 dB using the constant solution compared to the exact one, which is more evident in the deepest zone.

The ice absorption at EPICA (left plot in the figure) slowly and regularly descends through the ice

thickness while, the GRIP measurements show a faster and regular linear trend up to a first constant step,

clearly visible at about 1600 m (right plot in figure 4). Below 1600 m the ice absorption plots in the two

cases (EPICA and GRIP) have about the same trend with different values.

EPICA site

0 0.8 1.6 2.4 3.240

35

30

25

20

15

10

5

0

Numerical solution

Constant solution

Exact solution

Depth [km]

EM

ice

ab

sorp

tio

n [

dB

]

GRIP site

0 0.8 1.6 2.4 3.240

35

30

25

20

15

10

5

0

Numerical solution

Constant solution

Exact solution

Depth [km]

EM

ice

ab

sorp

tio

n [

dB

]

Figure 4: Electromagnetic absorption of ice compared at two drilling sites

The total absorption rate passing through the ice column is:

)(][][ hLL dBdB

TOT 14)

where h is the ice thickness. The total electromagnetic absorption of the ice calculated by means of the

ice core measurements of conductivity and temperature profile at EPICA is -17.4 dB while the same value

at GRIP is -26.2 dB. The electromagnetic power loss is greater in Greenland because of the higher surface

and inner glacier temperatures compared to Antarctica. The new solution proposed for the electromagnetic

absorption gives reduced ice loss as the result, justifying the excessive received signal obtained from the

radar equation, as noted by several authors (see for example Fujita 2012 [13]). Moreover, this new solution

gives higher values for interface reflectivity than those obtained using other solutions.

Another important parameter for the media is the electromagnetic absorption rate A[dB]

(z) in dB/m which is

12

defined as:

)(

)(

2

)(686.8

)(686.8

)()(

0

0

][][

zr

zrz

dz

zd

dz

zdLzA

dBdB

,

15)

where the minus sign takes into account that A is an absorption loss rate, negative in logarithmic unit. In

these equations there is no difference between exact and constant solution demonstrating the strictly

connection between absorption rate in dB and the media parameters.

0 0.5 1 1.5 2 2.5 320

15

10

5

0

GRIP

EPICA

Depth [km]

EM

ab

sorp

tio

rate

[d

B/k

m]

Figure 5: Ice Absorption rate at the two sites.

In figure 5 the ice absorption rate at the two sites has been plotted using the scaled conductivity profile. It

is clear that the electromagnetic absorption of ice is quite different at the two sites in the first part. This is

probably due to the different surface temperatures and the corresponding difference in temperature profile.

At GRIP a constant absorption rate with an average value of -9.4 ± 0.7 dB/km is observed from 0 to about

1.6 km, while at EPICA a slowly diminishing trend with an average value of -3.2 ± 0.8 dB/km is observed

for the first 1.5 km. From 1.6 km to the bottom, a net diminishing trend is visible, at both the sites. This

particular shape is due to the values of corrected conductivity and temperature that at those ranges have

13

similar values.

In the proposed solutions the dielectric permittivity was assessed using a constant value of r = 3.18 for

the ice, but it would also have been possible to use the more general expression r = 3.1884 + 9.10-4

·T (here

the temperature T is in Celsius) (Mätzler and U. Wegmüller, 1987 [26]). The influence of temperature on

dielectric permittivity, and thus on the electromagnetic absorption of ice, was evaluated for the whole

temperature range at both sites and its contribution was found to be negligible. At EPICA, for example, it is

less than 0.2 dB/km, less than the difference between the constant and exact solutions.

The calculated absorption rate can be tested more accurately by selecting and analyzing RES

measurements collected over subglacial lakes located in the area of the ice core, in the hypothesis of a

constant ice absorption rate. This has been done in the area nearby the Concordia Station (EPICA drilling)

considering subglacial lakes at depths between 2960 m and 4500 m. Taking into account the widespread

locations of the lakes and their wide depth range, the averaged ice absorption rate is Am= -7.2 ± 1.4 dB/km

(Zirizzotti et al. 2014 [53]). Furthermore, these results are also comparable with the value A[dB]

=-8.1 ± 2.4

dB/km obtained using equation 13 from 2900 m (see figure 5). It is also similar to the average value of -7.2

± 0.7 dB/km related to ice absorption from bedrock reflections obtained in the same area by Zirizzotti et al.,

2010 [51]. These values are in quite close agreement, within the errors range, even with the ice absorption

rate value of -4.9 ± 2.6 dB/km (by Zirizzotti et al., 2012[52]) calculated from RES measurements in the

Dome C area also taking into account the attenuation of echoes due to internal layers located at depths

between 500 and 2600 m (from figure 4: A[dB]

(500)= -3.0 dB/km and A[dB]

(2600)= -9.9 dB/km).

In Greenland, based on RES measurements, absorption rate values of 14 dB/km and 24 dB/km have been

found at ice thickness of 3000 m and 1000 m respectively (Oswald and Gogineni 2008 [32]). In particular,

at GRIP camp, using ice absorption electromagnetic measurements, a value of 14 dB/km for the first 100 m

of ice was obtained (Paden 2005 [33]). This last value is quite similar to the value of 9 dB/km obtained by

DEP conductivity measurements at GRIP station (figure 5).

14

SUMMARY AND CONCLUSIONS

The electromagnetic absorption of ice is an important physical parameter that plays a significant role in

the definition of the physical conditions at ice bottom, through RES data analysis. In this paper some

consideration about ice conductivity are briefly reported. The propagation of electromagnetic waves

through the ice sheet is examined and a new method to assess the electromagnetic absorption rate profile of

ice from ice core drilling measurements is proposed and discussed. The rate of variation of the ice

absorption is deduced starting from the analysis of ice core data from two sites where the electromagnetic

properties are known (Concordia station in Antarctica, and the GRIP site in Greenland). First, ice

conductivity profiles coming from the EPICA and GRIP drilling sites were used to calculate the exact

solution of electromagnetic wave propagation using the WKB approximation (generally valid for polar ice

conditions). The exact solution was compared to the numerical solution of the wave equation, and to the

generally used solution, valid only in the case of constant ice electromagnetic parameters. The comparison

revealed no differences between the numerical and exact solutions, while appreciable differences were

observed from the comparison with the generally used solution. The ice absorption obtained using this new

method gives lower attenuation values. Moreover, due to the dependency of ice absorption on temperature,

the attenuation of the radar signal results higher values in Greenland. This can be justified taking into

account that here the ice surface temperature is higher than in Antarctica, maintaining a similar linear trend

between 0 and 1.6 km at both sites, but with a difference in rate absorption of about 10 dB. The total

electromagnetic absorption of the complete ice column is -17.4 dB at the Concordia site, while at the GRIP

site the absorption is -26.2 dB.

Since the research of “oldest ice” (Fisher et al., 2013 [10]) in Dome C area could represent a new important

scientific challenge, this method could be helpful to define the characteristics of the ice absorption, to

establish ice-bottom reflectivity, and to obtain information about dry and wet bedrock interfaces needed for

the ice core site selection.

15

Acknowledgements: This research was conducted and funded as part of the framework of the "Progetto

Premiale ARCA".

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21

FIGURES:

0 1 103

2 103

3 103

0

10

20

30

40

100

80

60

40

20

0

DEP

scaled DEP

Temp

Depth [m]

Con

du

ctiv

ity [

µS

/m]

Tem

p.

[°C

]

Figure 1: Measured ice conductivity (258), scaled conductivity and temperature at the EPICA drilling site

0 1 103

2 103

3 103

0

10

20

30

40

100

80

60

40

20

0

Depth [m]

Con

du

ctiv

ity [

µS

/m]

Tem

p.

[°C

]

Figure 2: Measured ice conductivity (258), scaled conductivity and temperature at the GRIP drilling site

22

55 50 45 40 35 30 25 20 15 10 5 01

10

100

EPICA Concordia

GRIP

Lab. meas.

temp. [°C]

Con

du

ctiv

ity [

µS

/m]

Figure 3: Ice conductivity at the two sites vs. ice temperature.

23

0 0.8 1.6 2.4 3.240

35

30

25

20

15

10

5

0

Numerical solution

Constant solution

Exact solution

Depth [km]

EM

ice

abso

rpti

on

[d

B]

0 0.8 1.6 2.4 3.240

35

30

25

20

15

10

5

0

Numerical solution

Constant solution

Exact solution

Depth [km]

EM

ice

ab

sorp

tio

n [

dB

]

Figure 4: Electromagnetic absorption of ice compared at two drilling sites

24

0 0.5 1 1.5 2 2.5 320

15

10

5

0

GRIP

EPICA

Depth [km]

EM

ab

sorp

tio

rate

[dB

/km

]

Figure 5: Ice Absorption rate at the two sites.