1 INTRODUCTION

The study of the dynamic behaviour of frozen soils isfundamental in cold regions engineering for thedesign of vibrating machinery on frozen ground, theresponse of frozen soil to dynamic loading induced byearthquakes and the excavation of frozen ground. Themajor part of the investigation into the dynamic char-acteristics of frozen soils has been conducted in a lab-oratory on artificial frozen samples or on undisturbedpermafrost samples (Nakano & Froula 1973, Stevens1975, King et al. 1982, Zimmerman & King 1986). Inthe field, various investigations have used seismic-refraction and seismic-reflection surveys (Barnes1963, Hunter 1973, MacAulay & Hunter 1982, Milleret al. 2000) and vertical seismic profiling (Skvortsovet al. 1992) not only for the detection and delineationof permafrost, but also for the measurement ofdynamic properties of frozen ground such as seismicvelocities of both compressional and shear waves.However, in situ studies that consider the cryostrati-graphy and heterogeneity of dynamic properties ofpermafrost are scarce.

Multi-offset vertical seismic profiling (VSP) wascarried out in a permafrost mound near the Inuit com-munity of Umiujaq in Nunavik, Canada, to study thedynamic properties and determine the cryostratigra-phy of permafrost. In this paper, the field methodologybased on a seismic cone penetration test (SCPT) andthe results from both SCPT and VSP in permafrost arereported.

2 STUDY SITE

The fieldwork was conducted on the east coast of theHudson Bay, near the Inuit community of Umiujaq(56°N, 76°W) in Nunavik, Canada, in the discontinu-ous and scattered permafrost zone (Fig. 1). The studysite is a permafrost mound located in a deep valleyand formed in marine sediments of the Tyrell Sea(Allard & Seguin 1987). This permafrost mound has a diameter and height of about 70 and 4 m respectively(Fig. 2). Ostioles and gelifluxion process affect thesummit and sides of the permafrost mound. Previousstudies (Fortier & Allard 1998) have shown that theactive layer thickness is close to 1.5 m and the per-mafrost base is about 21.5 m deep. The permafrost inthe valley is qualified as warm since the temperatureat a depth of 10 m is close to �1°C. The area is char-acterised by a subarctic climate and the mean annualair temperature is about �3.5°C.

3 FIELD METHODOLOGY AND EQUIPMENT

Deep seismic cone penetration tests (SCPTs) includingmulti-offset vertical seismic profiling (VSP) were car-ried out down to a depth of 24 m below the permafrostbase in the permafrost mound. The penetrometer usedis a Vertek cone with a 10 cm2 cross sectional area ofthe tip base and a 60° cone angle, a 100 cm2 frictionsleeve, an electrical resistivity module, an inclinometer,a temperature sensor at the tip and a module of triaxial

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Seismic cone penetration test and vertical seismic profiling in permafrost

A.-M. LeBlanc & R. FortierDépartement de géologie et de génie géologique and Centre d’études nordiques, Université Laval, Sainte-Foy (Québec), Canada

M. AllardDépartement de géographie and Centre d’études nordiques, Université Laval, Sainte-Foy (Québec), Canada

ABSTRACT: Seismic Cone Penetration Tests (SCPTs) were carried out to a depth of 24 m in a permafrostmound near the Inuit community of Umiujaq in Nunavik, Canada, to study its cryostratigraphy in terms of pene-tration resistance and seismic velocities. The tip load, friction, temperature, inclination and electrical resistivitywere measured with sensors embedded in the penetrometer. The cone penetration was stopped periodically to adda new rod and to perform multi-offset Vertical Seismic Profiling (VSP). A Swept Impact Seismic Technique(SIST) was used as a seismic source directly in contact with the thawing front and the seismic signal was recordedwith a module of triaxial accelerometers, also embedded in the penetrometer, and with a seismograph. The seis-mic velocities were evaluated from the propagation time of the seismic signal in permafrost. The permafrost con-ditions are characterised by variation in seismic velocities with depth, depending on the ground temperature andsequence of frozen soil layers and ice lenses.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

accelerometers. A linear pushing system, recentlydeveloped at Laval University (Buteau & Fortier 2000)for penetration rate-controlled SCPT in permafrost(Fig. 3), was used to control the penetration rate accu-rately. A constant penetration rate of 0.1 cm/s was usedto carry out the deep SCPT below the permafrost base.

During an SCPT, tip load, friction, temperature, incli-nation and electrical resistivity were measured withsensors embedded into the penetrometer and automati-cally recorded at an interval of 5 seconds using a dataacquisition system. A multimeter (Keithley model

2700) was used for the data acquisition and a graphicalprogram developed in LabVIEW (National Instru-ments) was used for the monitoring. The LabVIEWgraphical interface gives data access in real-time.

The cone penetration was stopped at depth intervalsof 1 m to add a new penetration rod and to perform a multi-offset vertical seismic profiling (VSP). A crossconfiguration of seismic source locations at the sur-face, formed by two axial lines of twenty seismic shotpoints each, was used to carry out the VSP. The shotpoints of each axial line were 1 m apart and the SCPTwas at the intersection of axial lines. The seismicsource, VibSIST-20 from Vibrometric, stood directlyon the thawing front at a depth of about 0.75 m for bet-ter mechanical contact and to avoid the signal attenu-ation in unfrozen ground (Figs 4a, b). Forty steelstriking plates were buried in the ground, followingthe cross configuration of seismic source locations,for allowing the VSP to be done faster and increasingthe work efficiency. To avoid any thermal disturbanceof the active layer and the permafrost, the excavatedmaterial and vegetation were put back in place. Plastictubes were used to give permanent access to the strik-ing plates and protect them against the collapse of surrounding material (Fig. 4b). Therefore, the seismicsource (Fig. 4a) could be moved quickly from one shotpoint to another to complete 40 seismic shots in lessthan thirty minutes before continuing the SCPT downto another meter. During a seismic shot, the ball

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Figure 1. Location of study site. Umiujaq, Nunavik,Canada.

Figure 2. The study site is a permafrost mound formed inthe marine sediments of the Tyrell Sea. The photographscale is given by a Logan’s tent in the center.

Figure 3. Penetration rate-controlled SCPT in perma-frost. The linear pushing system is made of two ball screwscontrolled by two electrical servo-motors. The overallheight of the system is about 2.5 m.

extremity of the impact rod of the VibSIST (Fig. 4a)rests on the ball joint, a concave cavity in the strikingplate.

The seismic source used in the present study is basedon the Swept Impact Seismic Technique (SIST) devel-oped by Park et al. (1996), which is a combination of the Vibroseis swept-frequency and the Mini-Sosiemulti-impact techniques (Crawford et al. 1960, Barbieret al. 1976). The seismic signal produced by theVibSIST is a series of short pulses according to a deter-ministic coding scheme 10 seconds long, in which therate of impacts increases linearly with time. Low-powerand high-frequency broadband seismic pulses aretransmitted by the VibSIST-20 for the high resolutionsurvey. Each impact has a frequency content between40 and 2000 Hz. The rate of impact is swept from 10 to30 pulses per second. Because of the deterministic coding scheme, the decoded process of the seismic sig-nal is accomplished by a “shift-and-stacking” methodidentical to a cross-correlation operation (Park et al.1996). The coding scheme is monitored using animpact sensor fixed on the VibSIST-20. The SIST pro-vides a higher signal-to-noise ratio and a better resolu-tion than a conventional single-pulse technique. TheVibSIST-20 is controlled from a portable computer.

The seismic signal propagating in permafrost wasrecorded using the module of triaxial accelerometersembedded into the penetrometer and a high-resolutionseismograph, StrataVisor NZ from Geometric. A sampling interval of 125 �s and a record length of

8 seconds were used to monitor the seismic signal.The total number of impacts during a sweep variedbetween 125 and 135. The energy delivered by theseismic source was about 2.5 kJ with 20 J/impact.

4 PERMAFROST SAMPLING

On June 8th 2001, one borehole was drilled a fewmeters away from the SCPTs to a depth of 4.6 m in thepermafrost mound to measure the physical propertiesand determine the cryostratigraphy of the permafrost.The marine sediments of the Tyrell Sea are clayey siltwith a few sand beds. The depth of the thawing frontwas 0.75 m and a few ice lenses 0.5–1 mm thick werepresent in the frozen active layer below the thawingfront. The permafrost table at a depth of 1.64 m isclearly marked by an increase in ice content. A com-plex reticulate network of horizontal and vertical icelenses from 1 to 4 cm thick characterise the cryofaciesof permafrost. The average density of the active layerand permafrost is 2.1 and 1.75 kg/m3, respectively,while the total water content is lower than 20% for theactive layer and higher than 40% for the permafrost.

5 CRYOSTATIGRAPHY OF PERMAFROST

Three deep SCPTs were carried out in the permafrostmound in June 2001 to a depth of 24 m below the per-mafrost base. The cryostratigraphy of this permafrostmound has been defined in terms of its mechanical andelectrical properties. The results of the SCPT carriedout on June 16th 2001 are given in Figure 5. Coneresistance or tip load (qc) is a measure of the soil resist-ance to the cone penetration while the friction ratio is a measure of the mobilised friction along the frictionsleeve of the penetrometer normalised by the coneresistance (fs/qc). As a general interpretation rule forunfrozen ground, coarse-grained materials such as sandproduce high cone resistance and low friction ratiowhile fine-grained materials such as clay or silt pro-duce low cone resistance and high friction ratio.According to previous studies (Campanella et al. 1984,Buteau 2002), cone resistance is high and friction ratiois very low for ice. Therefore, the friction ratio for icelayers should be close to zero. This is due to the melt-ing of ice under the high stress induced by the conepenetration and the formation of a thin water filmaround the penetrometer, which decreases the frictionmobilised along the penetrometer shaft (Campanella et al. 1984). The electrical resistivity depends on the soiltype, water content, unfrozen water content and groundtemperature. Permafrost is characterised by electricalresistivity values over 1000 ohm-m and values as highas 100,000 ohm-m can be found for an ice-rich perma-frost layer (Fortier et al. 1993).

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Figure 4. (a) Seismic source VibSIST-20 made of a power-controlled Bosch hammer. (b) A striking plate standing onthe thawing front. The access to the striking plate is ensuredby an plastic tube for protecting against the collapse of thesurrounding soil.

Three temperature curves are given for the tempera-ture graph shown in Figure 5. The full line is thedynamic temperature measured during penetration.The friction mobilised along the penetrometer shaftwarms up the penetrometer, as well as the surroundingsoil, so the dynamic temperature does not reflect the in situ conditions (Dlugach et al. 1995). During theseismic shots, the penetration was stopped but the dataacquisition system was still recording the temperaturefor assessing the equilibrium temperature after the heatdissipation. The dashed line in Figure 5 gives the tem-perature profile about twenty minutes after the penetra-tion stops. This relaxation time is not enough to reachthe equilibrium temperature. An average difference of 0.26°C between the dynamic and the relaxation temperatures has been observed in permafrost in com-parison to 1.45°C in the unfrozen ground. The line con-necting the large black dots is the temperature profilemeasured on a thermistor cable permanently buried inthe permafrost mound a few meters away from theSCPT. This profile is colder than the two others.

Two electrical resistivity curves are given in Figure 5.The full line is the dynamic electrical resistivity mea-sured during the cone penetration with a resistivity

module of four electrodes, 3 cm apart in a Wennerarray. The dotted line is the electrical resistivity profilemeasured on an electrode cable parallel to the thermis-tor cable (Fortier et al. 1998). A Wenner array of fourelectrodes, 50 cm apart, was moved down along theelectrode cable by selecting the appropriate electrodesin the contact box at the ground surface. This resistiv-ity logging was done on July 19th 1998. Since the elec-trode spacing on the electrode cable is larger than theresistivity module of the penetrometer and the volumeof frozen ground for the measurement is also larger,the resistivity values measured on the electrode cablecorrespond to the maximum values of cone resistivity.The cone resistivity is much more sensitive to the permafrost heterogeneity along the cone path.

Due to the complex sequence of frozen soil layersand ice lenses in the permafrost mound, there are sig-nificant variations of tip load, friction ratio and elec-trical resistivity with depth (Fig. 5). Despite this highdegree of heterogeneity, some important characteris-tics have been highlighted in the SCPT profiles.

The depth of the thawing front is clearly marked at0.75 m by a sharp increase in tip load and friction ratio(Fig. 5). At this depth, the temperature is also close to

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Figure 5. Results of the SCPT carried out on June 16th 2001 in a permafrost mound near Umiujaq, Nunavik, Canada.

0°C (Fig. 5). The permafrost table at a depth of 1.71 mis indicated by an increase in resistivity to values inexcess of 1000 ohm-m, which represents the limitbetween unfrozen and frozen clayey silt according toFortier et al. (1993). Moreover, this depth is marked bya dynamic temperature below 0°C. Observations madeon core samples are also in agreement with the SCPTresults.

Below the permafrost table, the complex network offrozen soil layers and ice lenses cause major variationof tip load, friction ratio and electrical resistivity.According to the general rule given previously, thethick ice lenses (more than 0.2 m) have been identifiedon the SCPT profile (Fig. 5). However, these layers canalso be soil layers in which the ice content is high.Those soils behave much like ice. The tip load in permafrost varies mainly between 10 and 30 MPa, withmaximum values near 40 MPa. A particular layer isnoticed at a depth of 13 m, which has been found in allof the SCPTs carried out in this permafrost mound. Justabove 13 m, a low cone resistance (c. 10 MPa) associ-ated with a low electrical resistivity (300 ohm-m) sug-gests that the soil is fine-grained with higher unfrozenwater content, whereas the high values of cone resist-ance and friction indicate a compact sand layer at 13 m.Below this depth, the profile variability is less signifi-cant and the ice content is lower than above this depth.The ice lenses are also thinner since no high value ofcone resistance has been noticed below a depth of 13 m.

At depths greater than 19.5 m, a change in groundconditions has been detected through a sharp decrease inelectrical resistivity to values well below 1000 ohm-m(Fig. 5). The permafrost base at 21.5 m is defined by the isotherm 0°C. Between 19.5 and 21.5 m in depth,the ground is cryotic but unfrozen because the tem-perature is below 0°C and the resistivity is below1000 ohm-m. The freezing-point depression at 19.5 mis about �0.5°C and it is probably due to the over-burden pressure (Fortier & Allard 1998).

6 DYNAMIC PROPERTIES OF PERMAFROST

The seismic velocities were evaluated from the time ofpropagation of the seismic signal and the distance ofseparation between the accelerometers and the seismicsource locations. The multi-offset source-receiver con-figuration adopted in the present study was used tocarry out surface-borehole seismic tomography, wherethe seismic source is moved at the surface (thawingfront) and the receivers are moved downwards(accelerometers embedded into the penetrometer). Thepurpose of the tomography was to find the spatial distribution of seismic velocity by the inversion of the propagation time. A SIRT (Simultaneous IterativeReconstruction Technique) (Gilbert 1972) algorithm

was used for the inversion. Compressional (P) andshear (S) wave velocity profiles from the SCPT carriedout on June 16th were produced from the inversionprocess and are given in Figure 5. The seismic sig-nals monitored with the vertical accelerometers have been used for the analysis of P waves while the signalsfrom horizontal accelerometers have been used for theanalysis of S waves.

Useful correlation between mechanical, thermal andseismic data can be established from Figure 5. Deeperthan 10 m, the velocity of both P and S waves in per-mafrost decrease with depth, which coincides with anincrease in temperature. Mean maximum velocities forthe P and S waves are 3100 m/s and 1500 m/s respec-tively and these occur between 4 m and 11 m. The permafrost conditions for this depth interval are char-acterised by high ice content and thicker lenses. For a temperature of �2°C, the P and S wave velocitiesfound in the permafrost are characteristic of silty frozenground (Barnes 1963, Nakano & Froula 1973, King et al. 1982). The permafrost table is not clearly high-lighted by a sharp increase in velocities. Below the per-mafrost base, low values in S wave velocity of about500 m/s underline the presence of unfrozen soil. Asmall decrease in the P wave velocity is also apparentjust above 13 m (Fig. 5). The presence of a fine-grainedlayer with higher unfrozen water content can explainthis decrease in P wave velocity. The S wave velocity ismuch less affected, since the S waves do not propagatethrough the unfrozen water.

7 CONCLUSION

The field methodology described in this paper to studythe permafrost cryostatigraphy and determine thedynamic properties of permafrost is based on a SeismicCone Penetration Test (SCPT). Multi-offset VerticalSeismic Profiling (VSP) was carried out to measure thecompressional and shear wave velocities. A surface-borehole configuration was selected, where the seismicsource was moved at just below the ground surface(standing on the thawing front) and the receivers (theaccelerometers embedded into the penetrometer) weremoved downwards in the permafrost during the SCPT.The seismic source stood on the thawing front to avoidthe attenuation of seismic signal propagation inunfrozen ground. A Swept Impact Seismic Technique(SIST) was used to increase the signal-to-noise ratio,the frequency content of the seismic signal and the VSPresolution.

The combination of SCPT, VSP and SIST providednew insights into the dynamic properties of perma-frost. The heterogeneity in mechanical and dynamicproperties of permafrost was highlighted. Values of P and S wave velocities for warm silty permafrost vary

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between 2700 and 3100 m/s, and between 800 and1500 m/s respectively, for a temperature range between�0.2 and �2°C.

The analysis of seismic tomography, dynamic shearand Young’s modulus is still in progress and will bepresented later.

ACKNOWLEDGEMENTS

The research has been supported by the NaturalSciences and Engineering Council of Canada and bythe Fonds de la recherche sur la nature et les tech-nologies of the province of Québec. The assistance inthe field of the Centre d’études nordiques members,especially Martin Fleury, Châtelaine Beaudry, FabriceCalmels, Éric Larrivée and Véronique Tremblay, hasbeen very much appreciated. Finally, special thanksare given to Calin Cosma from Vibrometric for hishelp on the use of the VibSIST-20.

REFERENCES

Allard, M. & Seguin, M.K. 1987. Le pergélisol au Québecnordique: bilan et perspective. Géographie physique etQuaternaire 41(1): 141–152.

Barbier, M.G., Bondon, P., Mellinger, R. & Viallix, J.R.1976. Mini-SOSIE for land seismology. GeophysicalProspecting 24: 518–527.

Barnes, D.F. 1963. Geophysical Methods for DelineatingPermafrost. Proceedings of the 1th InternationalConference on Permafrost. Lafayette, Ind., NAS-NRCPubl. 1287: 349–355.

Buteau, S. 2002. Personal communication.Buteau, S. & Fortier, R. 2000. Penetration rate-controlled

CPT in permafrost. Proceedings of the 53rd AnnualConference of the Canadian Geotechnical Society,Montréal, Canada: 1225–1235.

Campanella, R.G., Robertson, P.K., Gillespie, D. & Klohn, E.J. 1984. Piezometric-friction cone investiga-tion at a tailings dam. Canadian Geotechnical Journal21: 551–562.

Crawford, J.M., Doty, W. & Lee, M.R. 1960. Continuoussignal seismograph. Geophysics 25: 95–105.

Dlugach, A., Loktev, A., Okko, O. & Rokos, S. 1995. Soilmechanical properties and in situ temperature fromarctic offshore CPT data. Proceedings of the Inter-national Symposium on Cone Penetration Testing,CPT’95, Linköping, Sweden 2: 167–172.

Fortier, R. & Allard, M. 1998. Induced polarization andresistivity logging in permafrost. Proceedings of the7th International Conference on Permafrost, Sainte-Foy, Québec: Collection Nordicana: 275–282.

Fortier, R., Allard, M. & Seguin, M.K. 1993. Effects ofphysical properties of frozen ground on electrical resis-tivity logging. Cold Regions Science and Technology22: 361–384.

Gilbert, P. 1972. Iterative Methods for the Three-dimensionalReconstruction of an Object from Projections. Jnl.Theor. Biol. 36: 105–117.

Hunter, J.A. 1973. The application of shallow seismic methods to mapping of frozen surficial materiels.North American Contribution 2th International Con-ference on Permafrost, Yakutsk, U.S.S.R: 527–535.

King, M.S., Pandit, B.I., Hunter, J.A. & Gajtani, M. 1982.Some seismic, electrical, and thermal properties ofsub-seabottom permafrost from the Beaufort Sea.Proceedings of the 4th Canadian Permafrost Con-ference, Calgary, Canada: 268–273.

MacAulay, H.A. & Hunter J.A. 1982. Detailed seismicrefraction of ice-bonded permafrost layering in theCanadian Beaufort Sea. Proceedings of the 4thCanadian permafrost conference, Calgary, Canada:256–267.

Miller, R.D., Hunter, J.A., Doll, W.E., Carr, B.J., Burns, R.A.,Good, R.L., Laflen, D.R. & Douma, M. 2000. Imagingpermafrost with shallow P-wave and S-wave reflection.7nd Ann. Internat. Mtg., Soc. Expl. Geophys., ExpandedAbstracts, Calgary, Canada: 1339–1342.

Nakano, Y. & Froula, N.H. 1973. Sound and shock trans-mission in frozen soils. Proceedings of the 2th Inter-national Conference on Permafrost, Yakutsk, U.S.S.R:359–369.

Park, C.B., Miller, R.M., Steeples, D.W. & Black R.A.1996. Swept impact seismic technique (SIST).Geophysics 61(6): 1789–1803.

Skvortsov, A.G., Hunter, J. A., Gorianinov, G.G., Burns, R.A.,Tsarov, A.M. & Pullan, S.E. 1992. High-resolutionshear-wave reflection technique for permafrost engi-neering applications. 62nd Ann. Internat. Mtg., Soc.Expl. Geophys., Expanded Abstracts, New Orleans:382–384.

Stevens, H.W. 1975. The response of frozen soils to vibra-tory loads. U.S. Army Cold Reg. Res. Eng. Lab. Techn.Monogr., Hanover, USA.

Vibrometric. 2001. VibSIST-20 seismic source operatingand maintenance manual.

Zimmerman, R.W. & King, M.S. 1986. The effect of theextent of freezing on velocities in unconsolidated permafrost. Geophysics 51(6): 1285–1290.

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