PERMAFROST AND PERIGLACIAL PROCESSESPermafrost and Periglac. Process. 19: 157–178 (2008)Published online in Wiley InterScience

(www.interscience.wiley.com) DOI: 10.1002/ppp.616

Advances in Geophysical Methods for Permafrost Investigations

Christof Kneisel ,1* Christian Hauck ,2 Richard Fortier 3 and Brian Moorman 4

1 Department of Physical Geography, University of Wurzburg, Germany2 Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Germany3 Centre d’etudes nordiques, Universite Laval, Quebec, Canada4 Department of Geography, University of Calgary, Canada

* CoGeogGerm

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ABSTRACT

Geophysical techniques can be used to examine the spatial distribution of subsurface geophysicalproperties to delineate horizontally and vertically the active layer, permafrost and taliks. Spatialand temporal changes in subsurface geophysical properties due to permafrost cooling, warming,aggradation or degradation can also be assessed through geophysical monitoring. This paper reviewsthe geophysical methods most frequently applied in mountain and arctic/subarctic lowland permafrostinvestigations. Key results and recommendations based on the analysis of the applicability andreliability of different geophysical techniques for permafrost studies are summarised. Emphasis isput on the tomographic capabilities of geophysical methods. Recent advances in application and datainterpretation are shown in relation to five case studies, and future perspectives are highlighted.Copyright # 2008 John Wiley & Sons, Ltd.

KEY WORDS: geophysics; permafrost; geoelectrical methods; seismic refraction; ground penetrating radar

INTRODUCTION

Frozen ground is a widespread phenomenon in alpineand arctic/subarctic cold environments where peri-glacial processes have a significant influence onlandscape evolution. Climate warming and its impacton periglacial environments are research topics ofincreasing importance, due to the growing concern ofwarming-induced permafrost degradation and itsconsequences regarding slope instability, constructionfailure and other hazards related to the melting ofground ice. In a warming climate, permafrost is likelyto disappear first from marginal areas with meanannual air temperatures close to 08C, while the

rrespondence to: Christof Kneisel, Department of Physicalraphy, University of Wurzburg, Wurzburg D-97074,any. E-mail: kneisel@uni-wuerzburg.de

right # 2008 John Wiley & Sons, Ltd.

greatest terrain disturbance is likely to occur in areaswith the largest amount of near-surface ground ice.

The surface and subsurface conditions of periglacialenvironments can be highly variable. The occurrenceof permafrost depends strongly on elevation, incomingradiation, local climatic conditions and surface andsubsurface factors (e.g. organic layers, characteristicsof unconsolidated sediments such as coarse blockymaterial). These heterogeneous permafrost conditionsrequire effective investigation methods to remotelysense and resolve shallow subsurface conditions atscales between a few metres and several kilometres.Geophysical methods can be used to characterise thesubsurface continuously over large areas, often withinvestigation depths down to a few tens of metres, andcan be applied rapidly and at low cost. Because deepdrilling in permafrost is expensive, time-consumingand logistically demanding, direct examination of thesubsurface is seldom possible in remote locations and

Received 20 December 2007Revised 19 March 2008

Accepted 19 March 2008

158 C. Kneisel et al.

this constitutes one of the main reasons for usinggeophysical methods instead.

Geophysical methods provide information on thephysical properties of the subsurface and on the spatialdistribution of these properties, and by inference, onthe structure of the subsurface. They have been widelyused to characterise areas of perennially frozen groundand locate massive ice. Their successful applicationwithin cold environments is based on changes inphysical properties that occur following the phasetransition from an unfrozen to a frozen state. Until thelate 1980s, they were mostly applied in polar regions,where seismic, electromagnetic (EM) and electricalmethods were particularly suitable for exploration andengineering purposes (see the review by Scott et al.,1990).

In the early years of geophysical investigationsof mountain permafrost, one-dimensional (1D) directcurrent (DC) resistivity soundings and seismicrefraction surveys were used to detect ground ice.Recent advances have been achieved with morepowerful and state-of-the-art instruments and moderndata processing algorithms allowing two-dimensional(2D) and even three-dimensional (3D) surveys, andrapid data processing. These non- or minimally-invasive geophysical methods can rapidly provideinformation over an entire survey area in contrast tothe point-source information available from the drillsites. Modern data acquisition techniques permitgeophysical mapping of the subsurface conditionseven in heterogeneous mountain environments. Drillsites remain important, however, to improve theinterpretation of the data from the geophysicalsurveys.

The present paper reviews the geophysical methodsmost frequently applied in permafrost research.Advances in application and data interpretation overthe past few years are highlighted using case studiesand future prospects are discussed.

RELEVANT GEOPHYSICAL PROPERTIES

Permafrost detection and characterisation withgeophysical methods depends on the subsurfacegeophysical properties differing from those of thesurrounding noncryotic ground. These differencesmainly relate to the physical properties of cryoticground materials containing ice or unfrozen water.The degree of variation depends on the water/icecontent, pore size, pore water chemistry, ice structure,ground temperature and overburden pressure (Scottet al., 1990). Three useful geophysical properties fordifferentiating between frozen and unfrozen material

Copyright # 2008 John Wiley & Sons, Ltd.

are the electrical resistivity, the dielectric permittivityand the velocity of seismic waves. Typical values ofthese parameters for different periglacial environ-ments are summarised in Hauck and Kneisel (2008).

Electrical Resistivity

Electrical and EM techniques are based on themeasurement of electrical resistivity or its reciprocal,electrical conductivity. Resistivity increases stronglyat the freezing point due to the phase change fromelectrically conductive water to electrically non-conductive ice. For many permafrost materials,resistivity increases exponentially until most of thepore water is frozen (Pearson et al., 1983). Resistivityvalues of frozen ground depend on grain size, pore sizeand void ratio, water content, degree of saturation,pore water salinity, temperature and water phase. Thedependence of resistivity on temperature is closelyrelated to the material type and amount of unfrozenwater (Olhoeft, 1978). Generally, resistivities offrozen material can range through a few orders ofmagnitude with significantly higher values in moun-tain permafrost terrain (Kneisel and Hauck, 2008). Inmarine sediments, the resistivity of permafrost can beexceptionally low due to the higher conductivity ofsaline pore water. As a consequence of the depressionof the freezing point, the unfrozen pore water contentcan be comparatively high even at temperatures wellbelow 08C enabling a significant electrolytic conduc-tion (cf. Scott et al., 1990; Harada and Yoshikawa,1996; Yoshikawa et al., 2006; Ross et al., 2007).

Dielectric Permittivity

In contrast to DC resistivity techniques, which dependon electrical resistivity, EM techniques such as groundpenetrating radar (GPR or georadar) are sensitive tovariations in dielectric permittivity. However, the highelectrical resistivity of most permafrost terrain givesfavourable conditions for GPR techniques becauseattenuation of the EM signal propagating into theground increases with decreasing resistivity. Dielec-tric permittivity and especially its real part, thedielectric constant, are markedly different for frozenand unfrozen materials, with values for the latter of1 for air, 3 to 4 for ice, around 6 for frozen sediments,around 25 for unfrozen sediments and 80 forfresh water (Moorman et al., 2003). The dielectricpermittivity governs the propagation speed of GPRwaves, which varies significantly for periglacialmaterials (Maurer and Hauck, 2007), while thecontrast in dielectric permittivity at a stratigraphiccontact within the ground controls the reflection of

Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

Advances in Geophysical Methods for Permafrost Investigations 159

energy back to the surface and the propagation of theEM signal further into the ground.

Dynamic Properties

Seismic techniques are based on changes in thecompressional- and shear-wave velocities of rocks andsoils relating to their elastic properties and dynamicbehaviour. Compressional- and shear-wave velocitiesof most earth materials increase sharply followingfreezing but this increase is most pronouncedfor coarse-grained sediments. An increase in porewater salinity reduces this effect near the freezingpoint, as freezing occurs over a range of temperaturesrather than at a single value (Pandit and King, 1978).As with resistivity, the increase in velocity on freezingis closely related to the decrease in unfrozen watercontent (e.g. King et al., 1988; Leclaire et al., 1994).At some point during the freezing process, however,the seismic velocities reach a plateau and any furthercooling produces very little change in them (Panditand King, 1978; Pearson et al., 1983) while theelectrical resistivity continues to increase even whenpore spaces are nearly filled with ice. This illustrates afundamental difference in the mechanisms controllingthe transmission of electrical and acoustic energy infrozen ground. Electrical conduction takes place in theunfrozen portion of the pore water, so electricalproperties remain sensitive to the amount of unfrozenwater present, even if the unfrozen water contentbecomes very small. In contrast, seismic wave energyis transmitted primarily through the solid matrix, soonce the pore volume is largely filled with ice, afurther decrease in the already small unfrozen watercontent produces a negligible change in velocity(Pearson et al., 1983). If both the compressional- andshear-wave velocities and density of a permafrostenvironment are known, Young’s modulus, the shearmodulus and Poisson’s ratio can be assessed.

GEOPHYSICAL TECHNIQUES AND THEIRAPPLICATION IN PERMAFROSTENVIRONMENTS

Permafrost problems resolvable by geophysicaltechniques include:

� th

Co

e measurement of subsurface geophysical proper-ties (electrical resistivity and conductivity, dielec-tric permittivity, seismic wave velocity) through theapplication of geophysical methods to infer perma-frost conditions and the physical properties of fro-zen ground;

pyright # 2008 John Wiley & Sons, Ltd.

� th

e study of the spatial distribution of subsurfacegeophysical properties through geophysical map-ping for delineating the active layer, permafrost andtaliks both horizontally and vertically;

� th

e assessment of spatio-temporal changes in subsur-face geophysical properties due to permafrost cool-ing, warming, aggradation or degradation throughgeophysical monitoring (i.e. time-lapse measure-ments).

Table 1 provides an overview of the most importantgeophysical methods, their characteristics and poten-tial applications.

Geoelectrical Methods

Due to the great sensitivity of electrical resistivity tothe transition from unfrozen to frozen materials,electrical resistivity measurements constitute one ofthe standard geophysical methods that is widely usedin permafrost investigation. Among the availablenear-surface geophysical methods, the tomographicvariant of the electrical resistivity sounding (electricalresistivity tomography, ERT) is a powerful tool inpermafrost investigation. Following the recent devel-opment of multi-electrode resistivity systems, ERTis relatively easy to apply even in heterogeneousmountain and arctic/subarctic terrain. Moreover,commercially available 2D inversion software allowssophisticated data processing. ERT is considered to bean important multi-functional method in periglacialgeomorphology since comprehensive characterisationof the subsurface lithology can be obtained, differ-entiation between genetic ice types is allowed in manycases and changes in subsurface properties over timecan be monitored (time-lapse measurements) (Kneiseland Hauck, 2008).

In ERT surveys, a set of apparent resistivity data isobtained through measurements of multiple electrodeconfigurations with different spacings and centrepoints yielding a 2D data set along a profile line.Choice of an appropriate electrode configuration isdependent on the difficult surface conditions oftenassociated with periglacial environments. Since themaximum current injected into the ground canbe quite low, geometrical factors of the electrodeconfigurations may be critical. For this reason, Wenneror Wenner-Schlumberger configurations are oftenemployed, even though a Dipole-dipole configurationmay provide superior lateral resolution (Loke, 1996;Kneisel, 2006). Further details on different arraygeometries and data processing are given in a numberof publications (e.g. Reynolds, 1997; Kneisel, 2003;Hauck and Vonder Muhll, 2003). A range of possible

Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

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Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

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Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

Advances in Geophysical Methods for Permafrost Investigations 161

162 C. Kneisel et al.

applications of ERT in periglacial geomorphology indifferent permafrost areas is given in Kneisel (2006).A recent case study for inferring the internal structureof permafrost mounds in the discontinuous permafrostzone from ERT is presented in Fortier et al. (2008).

ERT surveys conventionally use galvanic couplingof the electrodes but electrical properties of thesubsurface can also be measured using a capacitively-coupled electrical resistivity meter. A simple coaxial-cable array with transmitter and receiver sections ispulled along the ground. Current injection into theground and potential measurements are conductedthrough line antennas placed horizontally on thesurface. An alternating current (AC) is induced inthe earth at a frequency of 16.5 kHz and AC voltagemeasurements are made at the receiver’s dipole,yielding an apparent resistivity profile for a specifictransmitter-receiver spacing. Data are collected inDipole-dipole electrode configuration. As with stan-dard ERT systems, an apparent resistivity is calculatedby multiplying the appropriate geometric factor by thevoltage measured at the receiver dipole and normal-ised by the transmitter current. To obtain a 2D data setfor tomographic inversion, an array of severalreceivers at different distances can be employed,and/or the survey line must be traversed several timesusing different transmitter-receiver spacings. Sincecapacitively-coupled systems do not require galvaniccontact with the ground, they are especially well suitedfor study sites where metallic electrodes cannot easilybe driven into the ground, such as hard frozen groundsurfaces, paved or gravel roads and airstrips andsurveys on snow. This technique is especially useful inregions with highly resistive surface characteristics,such as arctic/subarctic and mountain environments(Timofeev et al., 1994; Calvert et al., 2001; de Pascaleet al., 2008). In flat or easy accessible terrain, datacollection with capacitively-coupled systems is fasterthan galvanic systems. However, in steep alpineterrain and/or rugged topography an efficient appli-cation of the capacitively-coupled system is moredifficult (Hauck and Kneisel, 2006).

EM Techniques

EM techniques also measure electrical conductivitywithout galvanic coupling with the ground. EMmethods have been widely employed in studies oflowland arctic permafrost (e.g. Harada et al., 2000,2006; Ingeman-Nielsen, 2005; Cockx et al., 2006;Yoshikawa et al., 2006). Although less common inmountain regions, the use of EM methods has recentlyincreased (Hauck et al., 2001; Kneisel and Hauck,2003; Bucki et al., 2004; Hauck et al., 2004). EM

Copyright # 2008 John Wiley & Sons, Ltd.

techniques include frequency-domain EM systems(FEM), time-domain EM systems (TEM), very- lowfrequency systems and the radiomagnetotelluricmethod. A detailed description of these methodsand their application characteristics in periglacialenvironments is given in Hordt and Hauck (2008).Here we focus on FEM and TEM which are the mostcommonly used methods.

EM methods are based on electrical currents that areinduced in the earth by a varying current in atransmitter loop. The current is varied either byalternating (operating in the frequency domain, FEM)or by terminating it (transient methods, operating inthe time domain, TEM). The induced currents dependon the electrical conductivity distribution in thesubsurface, and generate a secondary EM field, whichcan be measured at the surface by a receiver loop. Themore conductive the subsurface, the larger the eddycurrents, producing a greater secondary field, which inturn allows the ground conductivity distribution to bedetermined by a simple proportional relation(McNeill, 1980). In FEM surveys, no further dataprocessing is usually required.

FEM systems are usually small and lightweight, andsurveys can be conducted by one or two persons. Dueto the fast and simple application, FEM systems areoften used for horizontal conductivity mapping oflarge areas. Only the spatial variability of the bulkconductivity of the uppermost subsurface layer isdetermined without any information concerning depth(e.g. Hauck et al., 2001).

Time-domain or transient EM methods (TEM) userapid closure of the transmitter current. The receivermeasures the signal immediately after the transmittercurrent has been switched off, yielding a signal whichdepends only on conductivity of the subsurface. Theresponse of the subsurface in terms of the decayingamplitude of the induced magnetic field can then bemeasured as a function of time and therefore of depth,because later responses originate at greater depths.The typical decay curve of the signal is similar to anapparent resistivity sounding curve generated by 1Dvertical electrical sounding surveys. Consequently,TEM surveys are commonly used for the determi-nation of vertical conductivity variations. Differentmeasurement configurations can be utilised. Centralloop soundings where the receiver coil is located in thecentre of a large transmitter coil (50 m � 50 m or100 m � 100 m) or outside configurations (offset TEMsoundings) where the receiver is placed outside thetransmitter loop to reduce the noise level throughprimary field effects have been employed (Toddand Dallimore, 1998; Harada et al., 2000, 2006).On mountain slopes or rock glaciers, such large

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Advances in Geophysical Methods for Permafrost Investigations 163

transmitter loops are often impractical due to theblock-covered and irregular terrain. A flexible eight-turn 5 m� 5 m transmitter loop can be used instead,which allows for reasonably easy handling whilemaintaining sufficient depth penetration (Hauck et al.,2001; Bucki et al., 2004).

For the majority of the frequency ranges commonlyused in EM surveys, the characteristic decrease inconductivity with the phase change from unfrozento frozen state is independent of the measured EMfrequency. However, absolute conductivity values arefrequency-dependent and differ strongly for differentmaterials and unfrozen pore water contents (Olhoeft,1978). In addition, EM surveys may be influenced in anumber of unwanted ways, such as through variablesnow cover thickness, electrical power lines, atmos-pheric lightning in the vicinity or instrument drift. Theresponse to metallic objects, power lines or lightningin an otherwise extremely resistive environment isvery pronounced and can usually be distinguishedfrom the subsurface target signal, even though it mayprohibit data acquisition. The influences of changingsnow cover or instrument drift are much smaller andcan easily be mistaken for a subsurface target signal(see Hordt and Hauck, 2008).

GPR

GPR is the general term applied to EM techniquesemploying short radio waves, typically in thefrequency range between 10 and 1000 MHz, to mapstructure and features buried in the ground. It is ahigh-resolution tool comprising two antennas, onetransmitting an EM signal into the ground and theother receiving reflected EM energy propagating backto the surface, and is mainly used for mapping soil androck stratigraphy (Davis and Annan, 1989).

GPR can be deployed in three basic modes:1) single-or multi-fold, fixed-offset reflection profil-ing, 2) common mid-point sounding (CMP sounding)for the estimation of velocity of the EM signal into theground and 3) transillumination or EM tomography.While the antennas are moved across the groundsurface for the two first modes, transillumination iscarried out with antennas in surface-to-borehole orborehole-to-borehole configurations.

GPR Reflection Profile.A high-resolution stratigraphic profile can be

produced by moving the antennas with a fixed offsetdistance along a survey line over the ground surface.Reflector position along the survey and the travel timecan be identified on the travel time profile. These areassociated with reflection of the EM signal back to the

Copyright # 2008 John Wiley & Sons, Ltd.

surface from interfaces characterised by contrastingdielectric permittivity. Single-fold reflection profilingis currently the most common mode of GPR operation,but with the recent development of multichannel GPRsystems, multi-fold profiling is starting to allow moreinformation to be obtained on the lateral and depthvariations in propagation velocity.

CMP Sounding.The velocity of the EM signal through the ground

needs to be measured in order to transform the traveltime profile into an accurate depth profile. Thepropagation velocity can be measured from multi-folddata or by varying the antenna spacing but keeping afixed CMP between the antennas and identifying thetime move out of the EM signal as a function of theantenna separation for different reflectors.

EM Tomography.The spatial distribution of radar wave slowness (the

reciprocal of the velocity) and electrical conductivityin the ground between two boreholes can be assessedusing EM tomography. Complete coverage of theground can be achieved by horizontal and obliquetransmission of the EM energy between two bore-holes. For horizontal transmissions, the transmittingand receiving antennas are at the same depth in theboreholes. For a fixed transmission point in oneborehole, the receiving antenna is moved at regulardepth intervals in the other borehole leading tovaryingly oblique transmission paths. The trans-mission point is then moved by one depth incrementin the transmitting borehole and the receiving antennais repositioned at regular depth intervals in the other.The travel time of the first arrivals and attenuation ofthe EM energy can be measured for each travel path.The spatial distribution of the radar wave slowness canbe assessed from the set of travel times using aninversion technique called the algebraic reconstruc-tion technique (Herman et al., 1973) taking intoaccount curvilinear travel paths. Based on these travelpaths and the attenuation of the EM energy, the spatialdistribution of electrical conductivity can be assessedusing the same algebraic reconstruction technique.

Seismic Techniques

The sharp increase in seismic wave velocities at thefreezing point can be used to differentiate betweenfrozen and unfrozen material. This method is especiallyuseful to locate the top of permafrost, as the contrast forthe P-wave velocity between the unfrozen active layer(400–1500 m/s) and the permafrost body (2000–4000 m/s) is usually large. On the other hand, valuesfor most common rock types are similar to those of ice,

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164 C. Kneisel et al.

so the detection of mountain permafrost is often difficultusing seismic refraction data alone.

The propagation of seismic waves through layeredground is determined by the reflection and refractionof the seismic energy at the interfaces. When a seismicwave reaches an interface, some energy is refractedinto the deeper layer while the reflected wavetransmits the energy back to the surface where it isrecorded by geophones. Reflected waves are the basisof the seismic reflection method, which is commonlyused for surveys with great investigation depths, suchas determining the base of permafrost below the sea orlakes (e.g. Schwamborn et al., 2002).

Seismic refraction is based on recording refractedenergy that propagates upwards as head waves. Whenthe angle of incidence of a seismic wave propagatingdownwards to an interface between two layers reachesa critical angle, the angle of refraction becomes 908.Head waves are generated when this criticallyrefracted wave travels along the upper boundary ofthe lower medium. Critical refraction occurs onlyif the seismic velocity in the lower medium is greaterthan in the overlying material. A thorough treatment ofthe principle of seismic refraction is given in standardgeophysical textbooks while a detailed discussion ofthe application in periglacial environments is given inSchrott and Hoffmann (2008). Seismic refraction hasbeen used in several case studies of permafrostconditions (e.g. Zimmerman and King, 1986; VonderMuhll, 1993; Ikeda, 2006). As with ERT, a tomo-graphic variant of the method can be applied yielding2D velocity distributions of the subsurface (Musilet al., 2002; Hauck et al., 2004; Maurer and Hauck,2007; Kneisel and Schwindt, 2008).

Borehole Geophysics

When drilling with or without sampling is performedat a permafrost site, there is an opportunity to increasethe quantity of data gathered by carrying outgeophysical logging. If the borehole is still openand any fluids used during drilling have not frozen,standard geophysical logging (natural gamma activity,self-potential, normal resistivity, borehole diameter,fluid temperature and resistivity) can be performed. Ifpermafrost samples were not obtained, these geophy-sical logs can often adequately replace the missinginformation. Time-lapse geophysical logging can alsobe an interesting way to monitor changes in thegeophysical properties of permafrost during theannual warming-cooling cycle and/or during degra-dation due to the impacts of global warming. For thesemeasurements, plastic or steel access tubing is needed,and the type of casing can seriously limit the options

Copyright # 2008 John Wiley & Sons, Ltd.

for geophysical logging. Electrical resistivity loggingcannot be carried out in either casing since galvaniccontact is needed directly with permafrost or through afluid. However, an electrode cable can be directlydriven and buried in the borehole to provide thenecessary galvanic contact (Fortier and Allard, 1998;Fortier et al., 1993). Plastic casing is prescribed forEM logging such as radar tomography (Delisle et al.,2003; Musil et al., 2006; Larrivee, 2007) while steelcasing is appropriate for natural gamma activity andneutron activation. Either type of casing filled withsilicone oil is adequate for temperature monitoring inthe active layer and permafrost.

FORWARD AND INVERSE PROBLEMS INGEOPHYSICS: AN INTEGRATEDINTERPRETATION STRATEGY

The values of a given geophysical property, measuredduring a geophysical survey carried out at the groundsurface, lead to indirect information on the subsurfacedistribution of this property. For example, the standardrepresentation of an ERT survey is a pseudo-section ofapparent electrical resistivity which gives a distortedpicture of the subsurface distribution of electricalresistivity related to the subsurface geology. Thespecific subsurface distribution of electrical resistivitycan be assessed from inversion of the values ofobserved apparent electrical resistivity. The generalaim of geophysical inversion is to find a subsurfacemodel that best fits the observed data. The terminversion comes from the inverse problem in contrastto the forward problem where a set of geophysical datais predicted from a specific subsurface distribution ofgeophysical properties (Figure 1).

Based on a back-and-forth procedure adopted byFortier et al. (2008) for the interpretation of ERTsurveys on permafrost, an integrated strategy for thedesign, realisation and interpretation of any geophy-sical survey is proposed (Figure 1):

1) A

geological model is developed using the infor-mation available on a given medium.

2) A

synthetic geophysical model is derived from thegeological model using appropriate or known geo-physical properties for each layer constituting thegeological model.

3) F

orward modelling of the synthetic geophysicalmodel leads to a set of predicted geophysical data.

These three first steps of the integrated strategyconcern the forward problem in geophysics. They arevery useful for designing a geophysical survey in orderto optimise the field procedure (e.g. array, location of

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Figure 1 Concept of forward and inverse problems in geophysics (modified and generalised from Fortier et al., 2008). The numbers in theblock diagram refer to the text.

Advances in Geophysical Methods for Permafrost Investigations 165

measurements) in relation to the survey objectives(e.g. features to discriminate, depth of investigation)and the medium characteristics (e.g. expected range ofphysical properties affecting the geophysical propertyof interest, expected subsurface distribution ofphysical properties).

The following three steps of the integrated strategyconcern the inverse problem in geophysics.

4) T

Cop

he geophysical survey is carried out in the fieldbased on the prior design so that observed geo-

yright # 2008 John Wiley & Sons, Ltd.

physical data can be directly compared to thepredicted data from step 3.

5) A

n unconstrained inversion of the observed geo-physical data is performed. In most inversionroutines, an initial geophysical model (e.g. theaverage of the observed geophysical data) is usedto calculate a set of recovered geophysical data byforward modelling. The sum of the squared differ-ences between the observed and recovered data(root mean square (RMS) error) is calculated. Themisfits between the observed and recovered data

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Cop

166 C. Kneisel et al.

are then used to perturb the initial geophysicalmodel and the forward modelling of this newmodel is undertaken. If the least-squares methodis being used, this iterative process is stopped whenthe RMS error is minimised. Topographic correc-tions, which can be incorporated into the inversionalgorithm, are essential for periglacial environ-ments with complex topography. Many softwarepackages are available for the modelling and inver-sion of the ERT survey (e.g. RES2DINV fromLoke, 1996; DCIP2D from UBC-GIF 2001). Theuse of different inversion parameters can lead todifferent geophysical models (e.g. in case study Ibelow). The geophysical models can be directlycompared to the synthetic one (step 2) to assess themodel quality and its ability to reproduce expectedand unexpected features in the medium.

6) T

he best model from a geological perspectivemight not be the one with the lowest possibleRMS error. Thus it is essential to interpret the datawhile taking the local geological setting intoaccount. This is possible through the constrainedinversion of the observed geophysical data takinginto account the synthetic geophysical model(step 2) as a reference model to start the inversionprocess. Unrealistic images of the subsurface struc-ture can then be excluded.

7) F

inally, if the empirical relationships between thegeophysical property of interest and the physicalproperties of the medium are known, the subsur-face distribution of these physical properties can beassessed from the best constrained geophysicalmodel (step 6).

RECENT ADVANCES IN PERMAFROSTINVESTIGATIONS

Recent advances in the application of differentgeophysical methods and the data interpretation forpermafrost research are shown in the context of fiverecent case studies. The first two come from investi-gations carried out in alpine periglacial environmentswhile the three others involve subarctic periglacialenvironment investigations. Emphasis is placed on thetomographic capabilities of geophysical methods.

Case Study I — Joint ERT and SeismicRefraction Survey at a Sporadic PermafrostSite in the Swiss Alps and Effect of GeophysicalInversion on Data Interpretation

The sporadic permafrost site below the timberline islocated in the Bever Valley, a U-shaped valley with

yright # 2008 John Wiley & Sons, Ltd.

basal elevations between 1730 m and 1800 m a.s.l.The occurrence of several isolated permafrost lenseswithin vegetated scree slopes at this low elevation wasconfirmed by several geophysical techniques (Kneiselet al., 2000; Kneisel and Hauck 2003). ERT andseismic refraction surveys were used in tandem to mapthe sporadic permafrost in detail (Kneisel andSchwindt, 2008). Figure 2 shows two isolated high-resistivity anomalies with thicknesses of 10–15 m,horizontal extent between 10–20 m and maximumresistivities of more than 160 kohm-m, values typi-cally associated with permafrost bodies. Thisinterpretation is supported by the results of theseismic refraction tomography with P-wave velocitiesof 4300 m/s for the lower anomaly.

Two potential sources of uncertainty may limit theinterpretability of geophysical data sets: 1) erroneousor difficult data acquisition and 2) data inversion.Whereas the former is often connected with hetero-geneous surface conditions and can usually bedetected quite early during data processing, the lattermay not be recognised without extensive geophysicalexperience since it often exhibits non-linear beha-viour. Data inversion is normally conducted usingcommercially available software requiring the input ofseveral inversion parameters. The choice of theseparameters is not unambiguous and can lead topotentially large sources of misinterpretation. Experi-ence in choosing appropriate parameters has graduallyincreased within the permafrost community and canbe assisted by the application of diagnostic tools toaddress the reliability and sensitivity of the inversionresults on the observed data set (Hauck and Kneisel,2008; Marescot et al., 2003). Using the aboveproposed back-and-forth approach, these potentialuncertainties can be reduced.

The influence of inversion on seismic datainterpretation can be exemplified using a syntheticdata set based on the Bever Valley case study. Hauckand Kneisel (2008) describe a similar analysis usingsynthetic ERT values. The synthetic data set wasgenerated from an idealised permafrost model con-sisting of isolated patches of ice occurrences in anunfrozen host material. A gradient model with aP-wave velocity Vp¼ 500 m/s at the surface, whichincreases by 100 m/s per metre to a maximum value of4500 m/s was used. Two velocity anomalies (Vp¼3500 m/s) were inserted representing the permafrostoccurrences.

Influence of the Choice of the Initial Model.The choice of the initial model strongly affects the

tomographic inversion results (Figure 3). In all cases,24 geophones and 12 shot points were modelled and

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DOI: 10.1002/ppp

Figure 2 Electrical resistivity and refraction seismic tomograms along a vegetated scree slope with isolated permafrost lenses at relativelylow elevation in the Bever Valley, Switzerland.

Advances in Geophysical Methods for Permafrost Investigations 167

inversion results are shown following 20 iterations.Synthetic travel times were calculated using theinversion algorithm developed by Lanz et al. (1998).To simulate field conditions, 5 per cent Gaussian noisewas added to the travel times prior to inversion.

The results for the small gradient initial model showtwo clearly delineated anomalies, but without theirvertical extent being correctly resolved (Figure 3,upper panels). In comparison to the ‘true’ model, allvelocities below 10 m depth are much too low,indicating that they did not change enough duringiterations. This is confirmed by considering thecalculated ray paths, which simulate the wavepropagation and do not reach depths below 10 mindicating that model cells at greater depths could notbe perturbed substantially during the inversion. Incontrast, ray coverage in the second tomogramextends down to 20 m, enabling the inversionalgorithm to perturb the initial velocities. The RMSerror of residuals between observed and calculatedtravel times is much smaller (1.7 ms) than for the smallgradient model (3.1 ms). These results demonstratethat care has to be taken in choosing the appropriateinitial model. If a priori information on the sub-surface velocities is poor, the results suggest that it isbetter to overestimate rather than underestimate the

Copyright # 2008 John Wiley & Sons, Ltd.

initial velocity gradient to ensure sufficient raycoverage (Lanz et al., 1998).

Influence of the Regularisation Parameter.Figure 4 shows inversion results for different values

of the regularisation parameter l, which determines theamount of smoothing (spatial) and damping (changesbetween iterations) applied during the inversionprocedure. A value of l¼ 1 corresponds to equalweights for regularisation and data during the inversionprocess. Too much regularisation can lead to over-damped and over-smoothed models, with the final resultvery similar to the initial model. In contrast, too littleregularisation leads to very noisy inversion results, asthe underdetermined parts of the inversion problembecome dominant and no unique solution can be found.This can be seen for a l-value of 0.1 in Figure 4, wheresmall-scale structures are dominant in the tomogram.The high-velocity anomalies are difficult to delineatedue to the enhanced representation of the artificiallyadded noise. When using too much regularisation, allvariations from the background geologic model aredamped and/or smoothed out and an almost lateralhomogeneous velocity model is obtained duringinversion.

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Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

168 C. Kneisel et al.

Figure 5 Time-lapse electrical resistivity tomography surveys carried out on the Muragl glacier forefield at high elevations in the SwissAlps. Percentage change in the subsurface resistivity values with time obtained from inversion of the data set from 17 September 2006(bottom panel) compared to the reference model from inversion of the first data set from 26 July 2006 (upper panel).

Advances in Geophysical Methods for Permafrost Investigations 169

Case Study II — Geoelectrical Monitoringof Permafrost Dynamics within the MuraglGlacier Forefield at High Elevations in theSwiss Alps

In order to study changes in subsurface resistivity overtime, ERT surveys were repeated at intervals usingfixed electrode arrays installed in the Muragl glacierforefield on different surface substrates. Reproduci-bility is a precondition for monitoring time-dependentprocesses since changes in subsurface resistivity areestimated using changes in the apparent resistivitymeasurements.

Figure 5 displays the percentage change inresistivity of the ERT surveys on 17 September2006 compared to the reference model from inversionof the first data set on 26 July 2006. The high negativepercentage change in resistivity values on the right-hand side of the survey line indicates that thaw occ-urred at the lower boundary of the active layeroverlying ice-rich permafrost. The potential influence

Copyright # 2008 John Wiley & Sons, Ltd.

of heavy rainfall on the decrease in resistivity valueswithin this time span is not regarded as significant. Thehighest negative percentage changes in modelresistivity (up to 25 per cent in the central partsbetween horizontal distances 32 and 48 m) areinterpreted as being due to the thaw of ice-richpermafrost and increasing unfrozen water contentswithin the permafrost body leading to greaterconductivity. Resistivity changes at the bottom ofthe tomograms, where a distinct area of resistivityincrease is located next to an area of resistivitydecrease, require careful interpretation. The limiteddata points at these depths are not well constrainedand the sensitivity of inversion results to inputdata beneath a high-resistivity layer can be signifi-cantly diminished (Hauck and Vonder Muhll, 2003;Marescot et al., 2003). A more sophisticatedanalysis of this geoelectrical monitoring datarequires near-surface temperature and borehole temp-erature measurements which were started in autumn2006.

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170 C. Kneisel et al.

Case Study III — ERT and GPR asComplementary Geophysical Methods forPermafrost Investigation Underneath theAirstrip at Tasiujaq, Northern Quebec, Canada

ERTenables inferences to be developed regarding spatialchanges in permafrost conditions but stratigraphiccontacts are not well constrained because resistivitycontrasts are smoothed by the inversion process. GPRcan precisely map the stratigraphy of permafrost butdoes not reveal directly the changes in permafrostconditions that occur at reflectors. ERT and GPR aretherefore complementary and integration of the datagathered by these two methods can generate astratigraphic section through the permafrost.

Investigations of permafrost conditions beneath theairstrip at Tasiujaq in northern Quebec (Canada)were performed using both ERT and GPR in summer2005 (Figure 6). A capacitively-coupled electricalresistivity meter (OhmMapper) with eight differentDipole-dipole arrays and a pulseEKKO 100 with50 MHz antennas and a fixed offset of 2 m were used tocarry out 1000 m long geophysical surveys along theairstrip. A 300 m long portion of the survey is shown inFigure 6.

Since the embankment thickness is about 2 m, thesmooth resistivity contrast on the model from20 kohm-m at a depth of 1 m to 1.6 kohm-m at adepth of 4 m does not accurately represent the contactbetween the embankment and the natural ground(Figure 6). The resistivity of the underlying fluvialdeposit is similar to that of the embankment. The highvalues close to the surface on the electrical resistivitymodel therefore include the electrical effects of boththe embankment and the fluvial deposit. However, areflector on the GPR reflection profile is associatedwith the contact between the embankment and thenatural ground. A second reflector at a depth of about3 m is related to the thaw front. Between 4 and 7 mdeep, the resistivity values are just greater than1 kohm-m which is quite low for permafrost, while thetemperature is below -28C. Sampling at an exposurelocated 200 m from the airstrip gave pore watersalinities of 5–18 g equivalent sodium chloride (NaCl)per litre in the intertidal unit and the upper layer of thecoarse sand unit, which would be high enough todecrease resistivities. The resistivity of the frozencoarse sand unit at depth is >10 kohm-m. At around14 m, the resistivity decreases to well below1 kohm-m, a smooth change that is actually associatedwith the contact between the coarse sand unit and thedeep unit of marine clay. The stratigraphic contact iswell delineated by a strong reflector on the GPRprofile which shows that it is not totally flat and

Copyright # 2008 John Wiley & Sons, Ltd.

exhibits a topographic high at a distance of 560 m.This topographic high is barely detectable on theelectrical resistivity model.

Case Study IV — Multi-frequency GPRImaging of Shallow Water Permafrost ThermalConditions in Mackenzie Delta, Canada

In the shallow waters of the Mackenzie Delta margin(less than 2 m water depth) the local water depth is thedominant control on the thermal regime of thesubsurface and the presence of permafrost. However,in an active depositional environment such as thiswater depths can dramatically change over a very shorttime period. Moreover, the complexity of an arcticdeltaic environment results in significant localisederosion at freeze up and spring break up. To quantifythis spatial and temporal variability, a series ofGPR transects and grids were run across the delta overa three year period. The GPR interpretations wereverified using data from five 10 m deep boreholes thatwere instrumented with thermistor cables and dataloggers such that daily temperatures could be recordedthroughout the year.

The complex vertical structure and dramaticallycontrasting dielectric properties between layers in theshallow water environment make it difficult to achievean adequate compromise between resolution anddepth of penetration. The general vertical structure is0–30 cm of snow, over 1–2 m of ice, over 0–2 m ofwater, over frozen and/or unfrozen sediments. Waterdepths can increase to 5–7 m when crossing localisedsubaqueous channels. The difference in propagationvelocity between ice (v¼ 0.16 m/ns) and water(v¼ 0.03 m/ns) and the difference in signal attenu-ation between ice (extremely low) and unfrozensediments (high) mean that a single GPR frequencycannot be used to image the subsurface withreasonable resolution and also map the depth to thepermafrost table beneath a shallow suprapermafrosttalik. As a result, multi-frequency GPR profiling wasundertaken. Because the surveys were conducted priorto spring melt over very flat terrain, multiple GPRsystems were easily towed behind a tracked vehicle,enabling single pass surveying and precise relativepositioning of the different GPR sections. A 500 MHzGPR system enabled the accurate measurement ofsnow and ice thickness given its fast propagationvelocity and resulting broad pulse width. Water depthwas measured with a 250 MHz GPR system and thethermal structure of the subbottom was imaged withfrequencies ranging from 100 MHz down to12.5 MHz, depending on the thickness of the unfrozensediment. As illustrated in Figure 7, the pulse widths

Permafrost and Periglac. Process., 19: 157–178 (2008)

DOI: 10.1002/ppp

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DOI: 10.1002/ppp

Advances in Geophysical Methods for Permafrost Investigations 171

)

172 C. Kneisel et al.

of the differing frequencies resulted in differentvertical resolution, while each frequency also pene-trated to a different depth. The dramatic contrast inpermittivity between the ice and water also resulted inreverberations being generated under certain con-ditions (depending on ice thickness and whether it wasfloating or frozen to the bottom).

Case Study V — Geophysical Logging in aPermafrost Mound at Umiujaq, NorthernQuebec, Canada

Seismo-electrical cone penetration tests (SECPT),electrical resistivity logging, vertical seismic profil-

Figure 7 A multi-frequency ground penetrating radar profile in a shalfrequencies provide better resolution in the near surface, while the lo

Copyright # 2008 John Wiley & Sons, Ltd.

ing, seismic tomography and radar tomography werecarried in a permafrost mound at Umiujaq, northernQuebec (Canada), to study its internal structure(Buteau et al., 2005; LeBlanc et al., 2004, 2006;Larrivee, 2007; Delisle et al., 2003). This permafrostmound, with a diameter close to 50 m and a height ofabout 5 m, has formed in frost-susceptible marinesediments. The permafrost table at a depth of 1.7 m ismarked by a sharp increase in ice content, thepermafrost base is about 21.5 m deep and the averageice content is about 60 per cent.

During a cone penetration test, an instrumentedpenetrometer is vertically driven into the groundand different geotechnical parameters such as cone

low water deltaic area, Mackenzie Delta margin, Canada. The higherwer frequencies provide greater depth of penetration.

Permafrost and Periglac. Process., 19: 157–178 (2008)

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Copyright # 2008 John Wiley & Sons, Ltd. Permafrost and Periglac. Process., 19: 157–178 (2008

DOI: 10.1002/ppp

Advances in Geophysical Methods for Permafrost Investigations 173

)

Figure 9 Tomograms of P- and S-waves velocities in a permafrost mound at Umiujaq, northern Quebec, Canada (modified from LeBlancet al., 2004).

174 C. Kneisel et al.

resistance, soil friction and temperature are continu-ously recorded to provide geotechnical stratigraphicprofiles and investigate the penetrated formations(Lunne et al., 1997). Other sensors can be added tothe penetrometer to increase its investigative capa-bilities such as an electrical resistivity module andgeophones or accelerometers for electrical resistivitylogging, vertical seismic profiling and seismictomography.

At first glance, the variations with depth ofelectrical resistivity recorded during the SECPTcarried out in the permafrost mound appear verynoisy (Figure 8). However, these variations reflectthe stratified cryofacies of permafrost comprising acomplex sequence of ice lenses and layers of frozenmarine sediments (Fortier et al., 2004). The ice lensesare characterised by resistivity values in excess of10 kohm-m while the dense layers of silt with onlypore ice and high unfrozen water content haveresistivity values lower than 1 kohm-m. Reticulatedcryofacies show resistivity values between 1 and10 kohm-m.

Two curves appear in the temperature profile(Figure 8). The black dotted curve is the temperatureprofile at equilibrium measured along a thermistorcable permanently installed in a plastic casing filledwith silicone oil while the single line is the quasi-statictemperature measured during the cone penetrationtest.

A multi-offset vertical seismic profiling survey wasalso carried out in the permafrost mound during thedeep SECPT. A survey line of 20 seismic shot points1-m apart centred on the SECPT was used. From the

Copyright # 2008 John Wiley & Sons, Ltd.

thawing front down to a depth of 24 m, the conepenetration was stopped at depth intervals of 1 m toadd a new pushing rod, perform the seismic shots withthe seismic source and record the seismic wavespropagating down to the accelerometers embedded inthe penetrometer. The seismic source was directly onthe thaw front at a depth of about 0.75 m to provide abetter mechanical contact and to avoid signalattenuation within the unfrozen active layer.

The travel time of the first arrivals picked on theseismic records was measured for each travel pathbetween each pair of seismic shots and receivers. Thespatial distribution of the P- and S-wave slowness wasthen assessed from the travel times considering thepositions of the seismic shots and the receivers. Analgebraic reconstruction technique taking into accountthe curvilinear travel paths was used to producethe tomograms of P- and S-wave velocity (Figure 9).From these tomograms, the vertical seismic profiles ofP- and S-wave velocities in the permafrost moundcould be assessed (Figure 8). The P- and S-wavevelocities in ice-rich permafrost are close to 3000 and1500 m/s, respectively (Figures 8 and 9). Thedependence of P- and S-wave velocities on tempera-ture and unfrozen water content is shown in Figure 10.P- and S-wave velocities increase steadily withdecreasing temperatures from 0 to �0.88C as longas the unfrozen water content remains above 11 percent. For permafrost conditions colder than �0.88Cand unfrozen water content below 11 per cent, thevariations in P- and S-wave velocities are negligible.

Young’s modulus, the shear modulus and Poisson’sratio were assessed from the P- and S-wave velocities

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Figure 10 P- and S-wave velocities as a function of temperatureand unfrozen water content in a permafrost mound at Umiujaq,northern Quebec, Canada (modified from LeBlanc et al., 2004,2006).

Advances in Geophysical Methods for Permafrost Investigations 175

and the measured density of undisturbed permafrostsamples in the field. Their overall variations in depthare similar to those of P- and S-wave velocities(Figure 8). However, the shear modulus exhibits lesservariability because unfrozen water content does notvary greatly with depth. As with the P- and S-wavevelocities (Figure 10), these properties also depend onground temperature.

CONCLUSIONS AND PROSPECTS

Geophysical techniques, if properly applied, can be ofgreat utility in investigating both mountain and polar/subpolar lowland permafrost. Our summary of recentadvances leads to the following key results andrecommendations:

Copyright # 2008 John Wiley & Sons, Ltd.

� T

he characteristic of interest (e.g. thermal boundaryor ice content) and the required resolution deter-mine which technique or techniques are suitable.The opportunity for subsurface verification usingboreholes or sections influences whether straight-forward reflection techniques can be used (e.g. GPRor seismic reflection) or whether other methods arerequired that provide information on the materialproperties (e.g. ERT or EM field methods).

� T

he sedimentology of the study area influences thecapability for imaging and subsurface verification.For example, GPR and drilling are of limited appli-cability in bouldery till, whereas directly coupledERT is difficult to use in completely frozen ground.

� E

RT is suitable for a number of permafrost-relatedproblems, such as detection, mapping of horizontalextent, estimation of ice/unfrozen water content andmonitoring purposes. Seismic refraction tomogra-phy is equally well suited for these targets, butinterpretation of the results is more difficult, asthe difference between the measurement signalsof frozen and unfrozen ground is smaller than withDC resistivity. Ideally, a combination of bothmethods should be applied.

� I

f the surface prevents the galvanic (direct) couplingof electrodes and ground surface used in standardERT surveys, the application of capacitive-coupledresistivity systems or the use of EM inductionsystems (FEM or TEM) is recommended. Theapplication of FEM allows mapping of permafrostdistribution and characteristics over larger areas,but without vertical resolution. TEM is recom-mended for vertical soundings, particularly to deter-mine the base of permafrost.

� G

PR is best suited to determining the internalstructure of layered permafrost bodies, as well asfor detecting the spatial extent of individual layers.Ice-rich zones are best delineated on the basis oftheir very high electrical resistivities, whereas inter-faces between loose and denser material (e.g. thebase of the active layer, regions of degraded perma-frost, top of the bedrock) are best seen in seismicimages.

� A

mbiguities in resistivity model inversion andinterpretation must be carefully addressed. Theseinclude model dependence on inversion parameterslike the number of iterations and the dampingfactor, the influence of topography and measure-ment geometry, as well as misinterpretations ofhigh-resistivity values caused by air-filled cavities.

� I

n the case of large resistivity contrasts due tocomparatively high ice contents, reliable distinc-tions can be made between permafrost andnon-permafrost areas. However, the reliability of

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Co

176 C. Kneisel et al.

inversion results diminishes at greater depths andhas to be assessed for each case study (e.g. using therelative sensitivity or the depth-of-investigationmethod).

� C

hoice of appropriate inversion parameters can beimportant especially for noisy data sets. Multipleinversions using different values of the regularis-ation parameters (model smoothing and damping)may help to distinguish model artefacts from realanomalies.

� A

dvances in technology and general computingpower enable rapid data collection and farmore complex analysis than was possible 15 yearsago. These are making possible 3D and four-dimensional surveys where the terrain issuitable.

� T

o date, efficient 3D geophysical mapping of thesubsurface in alpine periglacial environments withrough terrain has not been possible over large areas.As a practical compromise, the results of severalclosely spaced 2D geophysical surveys can bemerged to build up a pseudo 3D image of thesubsurface characteristics and lithology. Thisapproach is currently underway in several studiesand will become increasingly important in thefuture.

� A

utomatic ERT monitoring will be possible in thenear future. This will allow time-lapse measure-ments at a high temporal resolution which in turnwill enable more sophisticated conclusions to bedrawn regarding the influence of atmospherictemperature and snow cover evolution on the sub-surface ground temperature regime and freezingand thawing processes.

ACKNOWLEDGEMENTS

The authors thank the two anonymous referees fortheir suggestions on an earlier version of the paper.

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