Instruments and Methods <br>Techniques for measuring high-resolution firn density profiles: case study from Kongsvegen, Svalbard


Journal of Glaciology, Vol. 54, No. 186, 2008 463

Instruments and Methods

Techniques for measuring high-resolution firn density profiles:
case study from Kongsvegen, Svalbard

Robert L. HAWLEY,1 Ola BRANDT,2 Elizabeth M. MORRIS,1 Jack KOHLER,2

Andrew P. SHEPHERD,3 Duncan J. WINGHAM4

1Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER, UK
E-mail: rlh45@cam.ac.uk

2Norwegian Polar Institute, Polar Environmental Centre, NO-9296 Tromsø, Norway
3Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK

4Centre for Polar Observation and Modelling, University College London, Gower Street, London WC1E 6BT, UK

ABSTRACT. Onan11mfirn/icecore fromKongsvegen,Svalbard,wehaveuseddielectricprofiling(DEP)
to measure electrical properties, and digital photography to measure a core optical stratigraphy (COS)
profile. We also used a neutron-scattering probe (NP) to measure a density profile in the borehole from
which the core was extracted. The NP- and DEP-derived density profiles were similar, showing large-
scale (>30cm) variation in the gravimetric densities of each core section. Fine-scale features (<10cm)
are well characterized by the COS record and are seen at a slightly lower resolution in both the DEP and
NP records, which show increasing smoothing. A combination of the density accuracy of NP and the
spatial resolution of COS provides a useful method of evaluating the shallow-density profile of a glacier,
improving paleoclimate interpretation, mass-balance measurement and interpretation of radar returns.

1. INTRODUCTION
Thesnowdensificationprocess isof fundamental importance
in many aspects of glaciology, including regional climat-
ology, watershed runoff forecasting and interpretation of
ice-surface elevation changes. In particular, high-resolution
density profiles are critical for accurate modelling and in-
terpretation of ground-penetrating radar (GPR) returns. In
climatology studies, the thickness and frequency of refrozen
melt layersareusedto infersummerclimateconditions (Oku-
yama and others, 2003), highlighting the importance of
detecting thin ice layers in a density profile. In addition,
the slowly varying depth–density relationship is indicative
of long-term climate (e.g. Herron and Langway, 1980), and
highly accurate density data are needed to exploit this for
paleoclimate interpretation.
In the traditional gravimetric method, the mass and vol-

ume of a sample are measured, with resolution and accu-
racy being dependent upon the sample size. Such samples
are generally obtained in three ways: direct sampling from
a snow-pit wall; bulk sampling of core sections; and sub-
samples cut from core sections. The practical depth limit for
snow-pit sampling isa few metres in most locations. For core
sampling, bulk densities of whole sections of core will not
necessarily reveal fine density stratigraphy or ice layers (as
thin as ∼1mm) that would have an effect on GPR signals
(Kohler and others, 1997; Arcone and others, 2004). Making
smaller subsamples from the core gives finer resolution, but
is time-consuming and thus rarely carried out (Clark and
others, 2007) over the full length of a core. Such sampling
does not coexist easily with chemistry studies on a core, so
non-destructive methods are clearly beneficial.
Alternative methods for measuring the fine-scale density

and stratigraphy in the firn are therefore desirable. Wilhelms
(2005) expanded on the work of Wilhelms and others (1998)
and Wolff (2000) using dielectric profiling (DEP) to infer firn

density from conductivity and permittivity measurements.
Morris and Cooper (2003) described the adaptation of a soil
moisture instrument, the neutron-scattering probe (NP), to
measure snow and firn density in a borehole. The utility of
this method for interpreting firn stratigraphy has been shown
(Hawley and Morris, 2006; Hawley and others, 2006), and
Morris (in press) presented a physically based scattering
model for calibration. Hawley and Morris (2006) demon-
strated a link between firn density and optical properties us-
ing borehole optical stratigraphy (BOS), and many ice-core
processing lines currently use digital photography or scan-
ning to image core sections (McGwire and others, 2007),
yielding data from which to produce a similar core optical
statigraphy (COS) record. Gamma-attenuation profiling, in
which density can be inferred from the absorption and scat-
tering of γ-rays by ice, has also been used (Eisen and others,
2006) to measure densitynon-destructively oncore sections.
Weusedetailed measurementsofdensityacquired byDEP

and NP methods, along with traditional gravimetric meas-
urements and COS, to assess the utility of each technique
for determining detailed firn density profiles for a complex
stratigraphy with snow, firn and ice layers.

2. METHODS
2.1. Study area
Kongsvegen is a 25km long polythermal, surge glacier in
Svalbard, which has been the subject of extensive mass-
balance campaigns (e.g. Hagen and others, 1999). Our study
site is located at mass-balance stake 8 on the glacier, at an
elevation of ∼700m (Fig. 1). Melting or rain events can
occur year-round in Svalbard, and air temperatures during
summer remain above freezing (Førland and Hanssen-Bauer,
2000). Meltwater can therefore percolate deep into the firn,
forming ice layers up to several tens of centimetres thick.

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464 Hawley and others: Instruments and methods

Fig. 1. Map of the Ny-Ålesund region, with the glacier Kongsvegen in the lower right. Our study site is located at stake 8.

2.2. Drilling
At our field site, we collected a firn core to a depth of ∼11m
usingaPolar IceCoringOffice (PICO)10cmhandaugerwith
a power head. We measured and weighed the 35 core sec-
tions in the field to obtain gravimetric densities. Core qual-
ity was consistently poor in the unconsolidated winter snow
comprising the uppermost 2.8m, making it impractical to
return these sections to the laboratory. We transported the re-
maining 23 core sections (listed in Table 1) to the laboratory
for repeat gravimetric measurements, DEP and COS analy-
sis. For registration with borehole measurements, each core
section was located in depth with respect to its neighbours
and several control points, at which we had measured the
depth of the drill cutting head.

2.3. Neutron-probe logging
Once the drilling was complete, we lowered the neutron
probeto thebottomof theboreholeandraised it slowly to the
surface at approximately 7–10cmmin−1, with the probe off-
setandrestingagainst thesideof thehole, logging thedensity
at1cmintervals (MorrisandCooper,2003;HawleyandMor-
ris, 2006). The calibration of the NP measurement depends
on the exact diameter of the borehole, which is larger near
the surface due to the repeated passage of the drill. The NP-
measured density profile shown in Figure 2 was determined
from the measured count rate of neutrons returning to the
detector using three-group neutron-scattering theory (Mor-
ris, in press). The count rate depends on the characteristics
of the probe, the snow/firn/ice density, temperature and the
diameter of the borehole.
The diameter of most boreholes drilled in firn with a hand-

held coring drill is likely to vary; the hole will be larger near

the surface from the repeated removal and insertion of the
drill.Weknowthat, in this case, thediameterof theborehole
was not constant. Specifically, the topmost ∼2m of the hole
were enlarged as a result of repeated raising and lowering
of the drill. This effect is likely to be largest in the upper
few metres and smallest at the bottom, which has seen the
fewest passes of the drill. In the absence of a caliper log of
the hole, we estimated the borehole diameter to be 11cm
(0.5cm on each side of the drill head) throughout most of its
depth, flaring to 14cm at the surface (estimated visually).
As can be seen from the comparison between gravimetric

and NP data in Figure 2, there is good agreement between
thebulkgravimetric densitiesand thosemeasuredbyNP.The
logstopsbefore thebottomof thecorebecause thebottomof
the hole was filled with ice chips that could not be removed
by the drill.

2.4. Dielectric profiling
We measured conductivity and permittivity profiles in the
laboratory using DEP at 250KHz. We made measurements
along thecoresectionsat5mmincrementswith10mmelec-
trodes. For each core section, we made four measurements,
rotating the core by 0, 90, 180 and 270◦. DEP measures the
conductance and capacitance of the ice core, and we use
the capacitance Cp, following Kohler and others (2003), to
estimate the relative permittivity �r:

�r =
Cp
Cair

, (1)

whereCair = 64.5×10−15 Fm−1 isaconstantobtainedusing
a blank reading with an empty instrumental set-up. See Wil-
helms and others (1998) for a more thorough description of

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Hawley and others: Instruments and methods 465

Table 1. Laboratory measurements of the core sections

Piece Length Diameter Mass Density

m m kg kgm−3

1 0.290 0.0750 0.568 444±10
2 0.255 0.0740 0.527 481±12
3 0.285 0.0760 0.761 589±13
4 0.460 0.0760 0.153 733±13
5 0.400 0.0750 0.961 544±10
6 0.500 0.0750 1.191 539±9
7 0.370 0.0750 0.994 608±12
8 0.540 0.0760 2.074 847±14
9 0.570 0.0760 1.391 538±9
10 0.720 0.0755 1.976 613±9
11 0.465 0.0760 1.505 714±12
12 0.335 0.0760 0.929 612±12
13 0.330 0.0770 1.256 818±16
14 0.205 0.0770 0.816 855±24
15 0.155 0.0770 0.593 822±29
16 0.440 0.0755 1.306 663±12
17 0.090 0.0770 0.241 575±33
18 0.625 0.0760 1.706 602±9
19 0.290 0.0760 0.872 663±14
20 0.095 0.0770 0.264 597±33
21 0.110 0.0760 0.329 660±31
22 0.550 0.0760 1.950 782±13
23 0.130 0.0770 0.402 664±27

the instrument and discussion of the technique. We then use
the relationship between density ρ and relative permittivity
�r given by Kovacs and others (1995),

�r = (1 +8.45 × 10−4ρ)2, (2)

to calculate the firn/ice density. Since DEP measures over
a finite volume, the measurements near the ends of each
core section are subject to end effects. We excised these
low-density end-effect anomalies by eye. The full set of DEP
profiles is shown in Figure 2. Note that the DEP-derived den-
sity profile appears to capture a significant amount of high-
frequency variability, agrees well with gravimetric densities
in high-density areas and slightly underestimates the density
of the lower-density layers.

2.5. Core optical stratigraphy
Visible stratigraphy analysis on ice cores has a long history
of success (e.g. Alley, 1988; Alley and others, 1997). More
recently, Hawley and others (2003) have developed BOS,
which takes the visual analysis concept to boreholes and
measures a log of brightness vs depth. To create a COS pro-
file, we imaged the core sections illuminated from the side
with a digital camera. The details of the camera system are
presented by Sjögren and others (2007).
We processed the images to obtain a brightness log by

subsampling the centre portion of the image, avoiding the
edges of the core, and taking the mean value of the pixels at
a given depth of the core. A subsection of the optical profile
with the accompanying imagery is shown in Figure 3. With
side illumination, light detected by the camera is primarily
scattered from within the firn, so the relatively low-scattering
ice layers appear dark.

400 600 800

0

1

2

3

4

5

6

7

8

9

10

11

D
ep

th
 (m

)

a

400 600 800

Density (kg m−3)

b

400 600 800

c d

Fig. 2. Density data (black) and data averaged over core sections
(grey) to facilitate comparison: (a) gravimetric density data; (b) NP
data; (c) DEP data (thin grey lines depict four runs at 0, 90, 180 and
270◦ rotation, and the black linedepicts themean); and (d) imagery
of the core on a black background with side illumination, the basis
of the COS shown in Figure 3.

3. DISCUSSION
3.1. Accuracy of the methods
As can be seen in Figure 2, there are differences between
the absolute densities measured by the three methods. We
evaluate the accuracy of each technique.
The gravimetric technique using core samples has the

longest history and has proven utility. The depth resolution,
however, is usually insufficient to resolve the shorter-scale
spatial fluctuations in density, and the accuracy can be af-
fected by several factors. The diameter of the core is gen-
erally measured at several places along the core, but may
not be consistent. The length of the core is measured, but,

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466 Hawley and others: Instruments and methods

300 600 900

3.5

4

4.5

5

Densityy (kgg m−3)

D
ep

th
 (m

)

50100150
Brightness (digital number)

a b

Fig. 3. (a) NP (grey) and DEP (black) density profiles; (b) COS profile, a mean brightness from the core imagery; and (c) digital imagery of
the core, the basis of the COS profile. In this detailed view, the effect of core breaks on the DEP and core-optical measurement can be seen;
there are short data gaps where the end effects in the data have been eliminated and are delineated with horizontal grey lines. Note that
thin ice layers as detected by COS are smoothed in the NP profile; this is because the 13.5cm neutron detector behaves as a low-pass filter
on the measurement. The smoothing effect is also present in the DEP profile (e.g. depths 4.4m and 4.6m).

in the event of an uneven core break, this length might not
be uniform. This can result in density being either over- or
underestimated. Often a core will be in many pieces upon
extraction from the drill. If a piece is lost, the measured mass
of the core section will be reduced, resulting in an under-
estimation of density. This is particularly troublesome when
core quality is poor or in unconsolidated snow, where care
is required to obtain good results. Systematic density over-
estimation is rare, because this would require excess weight
orvolumetobeunderestimated.Core lossalso introducesan
ambiguity in thedepthpositioningofadensitymeasurement,
although measurements of the drill depth for any given core
can help to resolve this.
Density ismeasuredbyNPbycountingtherateofneutrons

slowed by scattering in the snow and then absorbed by the
detector. The diameter of the borehole has an effect on the
relation between density and count rate, as does the position
of the probe in the hole, i.e. whether the probe is centred
in the hole or lying against the side (Morris, in press). On
Kongsvegen we did not measure borehole diameter but were
careful to align the probe with the side of the hole when we
set up the measurement. Although it is reasonable to assume
the probe did not at any depth losecontact with the wall, we
cannot categorically exclude this possibility. In future tests,
use of a pressure shoe to hold the tool against the borehole
wall would eliminate this source of uncertainty.
We have assumed in section 2.3 that the diameter of the

hole is 11cm through most of its depth, but that it flares
from 11cm at the first hard layer (∼2m) to 14cm at the
surface. We do not know what the error in this estimate is,
but sensitivity calculations by Morris (in press) indicate that
fora10%error inboreholediameter the resultingerror in the
derived density would be of the order 8–10%. A caliper log
of the hole, showing the exact diameter, would allow us to
account for the effect of variation in borehole diameter more
accurately, although the calipers may not work properly in
unconsolidated snow.

In standard practice, when co-registration of NP profiles
with core is not required, the borehole can be drilled using
a 5cm non-coring auger and a rigid guide tube up to several
metres long. The guide tube is made of aluminium which
does not affect the neutrons, and can be left in place during
logging. This prevents collapse of lower-density layers and
ensures the hole is of constant diameter through the upper-
most (generally lowest-density and weakest) part of the firn.
In addition, the accuracy of the method is much improved
by using a small-diameter access hole (Morris and Cooper,
2003; Morris, in press).
DEP uses the electrical properties of the ice, as outlined

above, to calculate density. As can be seen in Figure 2, DEP
appears to underestimate in the lower-density sections of
core but agrees with the gravimetric measurements for the
higher-density sections.
We suspect the underestimation at lower densities is

caused by thinner core diameter (typically 7.3–7.6cm com-
pared to 7.8cm for ice layers). The air gap between the core,
guarding and electrodes in the DEP affects the capacitance
readings. Inessence, byreducing thecorediameterby5mm,
13% less core material will occupy the cradle and thus the
relative permittivity will be underestimated. Propagating this
error through Equation (2) leads to a possible error of up to
20% in the low-density sections and 10–13% in the high-
density sections. However, since the higher-density sections
typically have a smaller air gap, the uncertainty is further
reduced. The actual observed underestimation in the low-
density parts of the core is ∼15%.
In addition, there could also be a change in conductivity

with density which is unaccounted for in the density cal-
culation, or an inaccuracy of the blank measurement. For
the purposes of characterizing the large-scale fluctuations of
this layered (firn/ice) core, the present method is sufficient.
For a polar firn core, where density is more slowly varying,
one would process the DEP using the full permittivity and
conductivity values following Wilhelms (2005).

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Hawley and others: Instruments and methods 467

3.2. Details in the density profile
The three density measurements are plotted together in Fig-
ure 2. Both DEP and NP techniques show finer spatial reso-
lution compared to the gravimetric method. Figure 3 shows
a section of the DEP, NP and COS profiles, along with com-
posite imagery of the core. The sharpest contrasts are seen in
the COS profile. Thin ice layers such as that seen at ∼4.4m
are spatially resolved by COS, and can be seen smoothed in
the NP and DEP profiles. The smoothing of thin ice layers
in the NP data is readily apparent. This is due to the fact
that the active length of the neutron detector (13.5cm) acts
as a low-pass filter (with a cut-off of approximately half the
active length, or 6.75cm). This is particularly noticeable at
∼3.75m in Figure 3. Less obvious but also apparent is the
smoothing effect of DEP with electrodes 10mm long which
sampleafinitevolume.Afirst estimateof the sensingvolume
can be found when excising the core-break end effects from
the DEP record. Generally, ∼2cm was removed from each
end of a core section, implying that the sensing length along
the core is ∼4cm.
In principle, the true density profile could be extracted by

inverse methods, using the NP or DEP densities and geom-
etry constrained by COS. Since optical stratigraphy can be
obtained using down-hole techniques (i.e. BOS), a combin-
ation of NP and BOS would allow a very accurate and de-
tailed density profile to be constructed.

3.3. Pseudo-density from COS
Since the optical signal is affected by factors other than den-
sity, such as grain size and shape and other aspects of firn
microstructure, the inversion of optical brightness to find
density is not straightforward. A true inversion is beyond the
scope of this study. We can, however, exploit the strong cor-
relation between brightness anddensity (Hawley andMorris,
2006; Sjögren and others, 2007) to investigate the potential
foranoptical record (COSorBOS) tobeused incombination
with a high-resolution density profile (such as that from NP
or DEP) to produce a detailed and accurate interpretation of
density.
Simplifying the procedure of Sjögren and others (2007)

for obtaining a density profile from COS, we apply a linear
transformation y = Ax + B to the intensity data, and vary A
and B to minimize the mismatch over a short depth range
between this ‘pseudo-density’ profile and the density profile
measured by NP. The optimum values are A = −2.4 and
B = 1000, and the resulting profile is shown in Figure 4.
Note that the ice layers are very clearly seen in the pseudo-
density profile, and the background density is in agreement
with the NP density profile.

4. CONCLUSIONS
We have made side-by-side density measurements in mixed
firn and ice using NP, DEP, COS and gravimetric density
measurement techniques. Unconsolidated snow near the
surface affects the measurements derived from all the
techniques, and further refinements are needed for these
conditions.
Althoughdependent onan accurate measurement ofbore-

holediameter, theNPmethoddoesnot require thecollection
and shipping of core and is relatively simple to deploy in the
field. This means that the NP method is free of the problems
associated with core depth registration, core breaks, poor
core quality or melting of cores during shipping. In fact, NP

500 550 600 650 700 750 800 850

3.5

4

4.5

5

Density (kg m−3)

D
ep

th
 (m

)

Fig.4.A‘pseudo-density’ profile,derived fromtheCOSprofileanda
simple linear transformation, is shown in black with the NP-density
data ingrey.Note thatwhilenotgivinga truephysicallybasedmeas-
urement of density, the COS-derived pseudo-density profile clearly
captures the details of the ice layers.

can be deployed in a rapidly drilled hole with a 5cm non-
coring auger.
The DEP method has the fine resolution needed to charac-

terize thin (<5cm) ice layers, but suffers from the problems
associated with collecting cores mentioned above. Although
both NP and DEP smooth the density profile to some extent,
both offer a vast improvement over gravimetric methods of
density profiling, with increased spatial resolution and pre-
cision and reduced potential for human errors.
Optical stratigraphy in the borehole or on the core, while

not providing a quantification of density, can be combined
with either NP or DEP to improve the ability of either tech-
nique to resolve thin ice layers. A combined NP, optical and
caliper down-hole tool may prove to be the ideal means of
measuringacontinuous,high-resolution,high-accuracy pro-
file of density from the surface to any depth accessible via
drilling.

ACKNOWLEDGEMENTS
We thank K. Christianson for assistance with drilling. This
work was supported by the European Space Agency and by
the UK Natural Environment Research Council under grant
No. NER/0/S/2003/00620, and by the Norwegian Research
Council. We thank D. Peel (Scientific Editor), C. Shuman and
an anonymous reviewer for insightful comments which im-
proved the manuscript.

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