Introduction

Variations in ground temperature, which follow theseasonal warming-cooling cycle in cold regions, affectthe physical properties of permafrost such as unfrozenwater content and ice content. Moreover, aggradationand degradation of permafrost induce changes in phy-sical properties and in the cryostratigraphic profile.According to Allard et al. (1996), the reconstruction ofthe evolution of permafrost can be made by using infor-mation from cryostratigraphic profiles.

Geophysical logs, collected for this study, provideinformation on the variation of physical properties inpermafrost and in the cryostratigraphic profile. Resultsfrom induced polarization (IP) and resistivity loggingin permafrost are presented. They demonstrate thatthese are useful geophysical tools for mapping per-mafrost discontinuities, for monitoring changes in elec-trical properties of permafrost due to warming or coo-ling, and for understanding the dynamics of permafrost.

Study site

The study site is located in a deep valley, calledÒVall�e des TroisÓ, at the north end of Lac Guillaume-Delisle and near the Inuit community of Umuijaq,Northern Qu�bec, Canada. The mean annual air tem-perature of Umiujaq is about -6.5¡C. Temperature, IPand resistivity logs were carried out in a permafrostplateau. The permafrost has aggraded after emergencein a marine deposit of silt of the postglacial Tyrrell Sea.

Induced polarization

IP EFFECT

The effect of IP can be illustrated with a standardfour-electrode resistivity array. It occurs by interruptingabruptly the current flow between the current elec-trodes. The voltage across the potential electrodes gen-erally does not drop to zero instantaneously (Figure 1).After an initial large decrease from the original steadystate value, the voltage decays rather slowly. The decaytime is in the order of a few seconds or even minutes. If

Abstract

Induced polarization (IP) and resistivity logs were carried out in a permafrost plateau near Umiujaq,Northern Qu�bec, to outline the permafrost dynamics. A permanently buried multiconductor cable into per-mafrost, with regularly spaced electrodes and thermistors, was used for IP, resistivity and temperature logging.

The cryofronts in the active layer and at the permafrost base corresponded to resistivity values around 500 ohm-m. A basal cryopeg was clearly delimited between 21 and 22.7 m in depth by a sharp decrease in resis-tivity from 2000 to 500 ohm-m close to 21 m and a sharp increase in total chargeability from -4 mV/V in per-mafrost to values over -16 mV/V in the basal cryopeg. The freezing-point depression due to overburden pres-sure at the base of perennially frozen layer is evaluated to be -0.2¡C. Resistivity values higher than 60,000 ohmm were measured in permafrost. Decrease in resistivity with time was monitored due to permafrostwarming during summer 1990.

Richard Fortier, Michel Allard 275

INDUCED POLARIZATION AND RESISTIVITY LOGGING IN PERMAFROST

Richard Fortier1, Michel Allard2

1. D�partement de g�ologie et de g�nie g�ologiquePavillon Pouliot, Universit� Laval

Sainte-Foy (Qu�bec)Canada

G1K 7P4e-mail: rfortier@ggl.ulaval.ca

2. Centre dÕ�tudes nordiquesPavillon Abitibi-Price, Universit� Laval

Sainte-Foy (Qu�bec)Canada

G1K 7P4e-mail: michel.allard@cen.ulaval.ca

the current is turned on again, the potential increasesover a similar time interval to the original steady stateafter a sudden initial increase (Figure 1).

The IP effect is evaluated by comparison of the resid-ual voltage V(t) existing at time t after the current flowis interrupted with the steady voltage Vp during thecurrent flow (Figure 1). Chargeability is the parameterdefining the IP effect. The time-domain chargeability m(expressed in mV/V) is given by the following integralover time ta to tb after the cut off of current:

[1]

The time limits (ta and tb) define a given fraction ofthe entire off-time cycle for the measurement of partialchargeability (Figure 1).

Three sources of the IP effect may be recognized:membrane polarization, electrode polarization and sur-face polarization. The membrane polarization and thesurface polarization are the most important sources inIP logging within permafrost. The decay curve ofpotential represents a return to the original state follo-wing the disturbance induced by the current flow.During the period of current flow, storage of chemicalenergy occurs due to the variations in the mobility ofions in fluids throughout the material structure (mem-brane polarization and surface polarization) and thevariations between ionic and electronic conductivity

where metallic minerals are present (electrode polariza-tion). The electrode polarization is not discussed furthersince this IP effect is insignificant in the study of permafrost.

The membrane polarization takes place in the capil-lary pores of material filled with electrolyte. Most mi-nerals such as clay have a net negative charge and,therefore, the positive ions in the electrolyte are attract-ed towards the mineralÕs surface producing a diffusecloud of cations in the vicinity of the minerals. When acurrent flows in the material, the remaining positiveions in the electrolyte easily pass through the diffusecloud but the negative ions accumulate around it.Therefore, an ion-selective membrane exists. This ionconcentration impedes the current flow. When the cur-rent flow is interrupted, the ions move back to their ini-tial steady state. Membrane polarization is most pro-nounced in the presence of clay minerals. In the case offrozen soil where the pores are filled with ice and theunfrozen water films around soil particles are discon-tinuous, the electrolytic conduction can be very low andthe IP effect is almost non-existent (values of char-geability are close to zero).

The surface polarization of cryogenic boundaries isanother important IP effect in permafrost environmentas first mentioned by MelÕnikov (1973). At the contactbetween thawed and frozen layers, the frozen layer actsas a barrier and when current flows across this boun-dary, ionic charges accumulate and polarization occurs.MelÕnikov (1973) established the existence of surfacepolarization at the boundary of the active layer and per-mafrost. The surface polarization is characterized bynegative values of chargeability. IP effects normallyproduce positive values of chargeability (Figure 1). Thenegative chargeability occurs at the boundary betweenpolarizable layers of materials with a large contrast inresistivity. Thus, the surface polarization should occurnot only at the boundary between permafrost and theactive layer but also at the boundary between a resistiveice-rich layer and a more conductive ice-poor layer.

IP AND RESISTIVITY LOGGING

The dipole-dipole array was used for the measure-ment of resistivity and IP (Figure 2). This array consistsof four electrodes along the multiconductor cable. Thetop pair of electrodes forms the current dipole AB,while the bottom pair forms the potential dipole MN.The spacing (a) between each pair of electrodes is con-stant. Spacing (na) between the two dipoles is variableand the spacing increment (n) is varied from 1 to 5. Thepolarity of electrodes is shown in Figure 2.

At the beginning of the dipole-dipole IP log, the firstfour electrodes are selected on the multiconductorcable. Initially, the current dipoles AB remains fixed and

The 7th International Permafrost Conference276

Figure 1. Time domain IP waveform.

mt t

V t

Vdt

b a pt

t

a

b

=-( ) ò1

( )

the potential dipole MN is moved downwards for eachmeasurement by increment of one electrode from one tofive increments. Then the current dipole is moveddown one electrode spacing, the spacing betweendipoles is reduced to one electrode spacing, and thesequence is repeated until the last electrode is reachedat the end of the cable.

The dipole-dipole array combines logging and profil-ing aspects. This array serves to analyze not only resis-tivity and chargeability variations as a function ofdepth but also the lateral variations as a function of dis-tance from the hole. Results are plotted at the intersec-tion of the midpoint of the spread and the radial dis-tance of investigation from the multiconductor cable.The radial distance (r) is controlled in part by the spa-cing between each pair of electrodes (a) and betweenthe dipoles (na). In first approximation, the radial dis-tance of investigation (r in meters) is given by (Barker,1989):

[2]

Since the midpoint of the spread varies in depth andthe radial distance of investigation varies with the spa-cing between dipoles, the dipole-dipole IP log providespseudo-sections of resistivity and chargeability asshown by the grid of x symbols in Figure 2.

Values of apparent resistivity depend on the true elec-trical resistivities of all the components in the materialsinside a ellipsoidal volume of half-axis equal to theradial distance and roughly delimited by the electrodesA and M (Figure 2). For the dipole-dipole array, theapparent resistivity is given by:

[3]

whererdd apparent resistivity (ohm-m),ÆV potential difference across the electrodesMN (V), andI current flow between the electrodes AB (A).

Electrical resistivity of permafrost depends on thetemperature, pore water salinity, porosity, materialtypes, unfrozen water and ice contents, cryostructureand electrode array geometry (Fortier et al., 1994).

INSTRUMENTATION

A two-dipole electrical receiver, SYSCAL R2E manu-factured by BRGM, and a 100 W transmitter were usedto measure resistivity and time-domain chargeability. Apulse-current waveform (+I, 0, -I, 0) is transmitted witha pulse length of 2000 ms for a complete cycle of 8000ms (Figure 1). Four partial chargeabilities (m1, m2, m3and m4) are measured after the interruption of currentflow (Figure 1). From the four partial chargeabilities,two new parameters are defined:

1. the total chargeability (a weighting of the four par-tial chargeabilities in mV/V):

[4]

2. the gradient of chargeability (a average slope of thevariation of chargeability with time in mV/V/s):

[5]

Negative values of the gradient of chargeability arecharacteristic of a decrease in chargeability (decrease inV(t) as illustrated in Figure 1) with time after the inter-ruption of current flow between the current dipole elec-trodes. This decrease in chargeability is the normal IPeffect.

MULTICONDUCTOR CABLE

A 25 m long multiconductor cable with 51 stainlesssteel electrodes spaced every 0.5 m was permanently

Richard Fortier, Michel Allard 277

Figure 2.IP logging Ð Dipole-dipole array.

r L n a= = +( )0 25 0 25 2. .

p an n nV

Idd = +( ) +( )2 1 2pD

mm m m m

t =+ + +120 220 420 820

15801 2 3 4

Dmm m m m m m

=--

+--

+--

æè

öø

1000390 220 710 390 1330 1710 3

2 1 3 2 4 3

buried into permafrost. The electrodes consisted ofstainless steel bands fixed around the cable. Each elec-trode was linked to a contact box at the ground surfaceby an individual electric conductor. In parallel with theelectrode cable, 11 thermistors regularly spaced alonganother cable provided the temperature control.

The access hole for the insertion of the multiconductorcable into permafrost was bored in August 1989 bywater jet drilling. A hook at the end of the multiconduc-tor cable fastened to the bottom of the first pipe allowedto drive the multiconductor cable to be driven into per-mafrost. At the end of drilling, the train of pipes wasremoved.

Drilling rates were highly variable and dependent onthe ice content. A slower rate was associated with ice-rich layers encountered during drilling. The drillingrate was very slow close to the surface, particularlybetween 2 and 4 m, and around 10 m depth. A depth of23.5 m was reached. At a depth of about 21 m, the watercirculation at the surface was lost because the water jetbroke through the base of the perennially frozen layer,and water injection occurred in the water table belowthe permafrost base.

METHODOLOGY

Temperature, IP and resistivity were first logged twoweeks after the installation of the multiconductor cableto evaluate the thermal disturbance of water jet drilling.During the study period in 1990, the same logs werecarried out weekly over a two-month period from May5 to July 4 to monitor the variations in electrical proper-ties and temperature with time. The time intervalbetween the cable installation and the study period was

enough to refreeze the slurry in the borehole and toreach a new thermal equilibrium in the permafrostplateau after one complete winter.

Discussion

THERMAL DISTURBANCE OF DRILLING AND

CRYOSTRATIGRAPHIC PROFILE

Pseudo-sections of resistivity are given in Figure 3 forthe IP logs of August 19, 1989, and May 24, 1990. TheÒ+Ó symbols in Figure 3 indicate the intersection whereparameter values were measured. The temperature pro-files are also given. For the first profile (Figure 3A), theground temperature is close to 0¡C due to the largethermal disturbance induced by the water injected du-ring drilling. At radial distances from the multiconduc-tor cable less than 0.5 m in the pseudo-section, the resis-tivity is below 500 ohm-m for the major part of the logexcept for zones at 6, 11 and between 16 to 21 m indepth. This resistivity value marks the boundarybetween unfrozen and frozen zones. Therefore, the soilclose to the cable (or drilling axis) was still unfrozentwo weeks after drilling. The cone shape of theunfrozen soil due to a larger disturbance increasingtoward the surface is perceptible in this pseudo-sectionwith smaller resistivity values covering a larger areaclose to the surface. At 10 m in depth, the diameter ofthe unfrozen soil can be estimated to be about 1.2 m.

Nine months after drilling, after one full freezingcycle, the permafrost plateau reached a new thermalequilibrium and recovered from the drilling disturbance (Figure 3B). The cooler permafrost is char-acterized by higher resistivity. However, the resistivityvalues for small radial distance remain lower than fur-

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Figure 3. Temperature and resistivity logs(Dipole-dipole array, a = 0.50 m, n = 1 to 5) (A- 08/19/89; B- 05/24/90).

ther away from the cable. These resistivity values arecharacteristic of the disturbed zone. Close to the drillingaxis, the water jet destroyed entirely the sequence ofcryofacies and the original cryostratigraphic profile waslost. For radial distance larger than 0.60 m or for spa-cing increment larger than 3, two layers with resistivity

value in excess of 60 000 ohm-m are visible at a depthof 4 and 11 m. They are associated with ice-rich layersas detected during drilling. They are also visible inFigure 3A but to a lesser extent. The first layer near thetop of permafrost is composed of aggradational iceslowly formed by downward migration of water

Richard Fortier, Michel Allard 279

Figure 4. Temperature, resistivity and chargeability as a function of depth and time (Dipole-dipole array, a = 0.50 m, n = 3).

(Allard et al., 1996). The deeper layer is associated witha stabilization of the thermal gradient over a long pe-riod of time allowing the formation of ice lenses duringthe period of permafrost aggradation. Between theselayers, the resistivity values are lower and, therefore,the ice content is also lower. These ice-poor layersformed when thermal gradients were too large and thecooling rate was too fast to allow much ice lensing du-ring permafrost aggradation (Allard et al., 1996).

RESISTIVITY AT CRYOFRONTS AND FREEZING-POINT DEPRESSION

Figure 4 gives the distributions of temperature, resis-tivity, total chargeability and chargeability gradientwith depth and time over a two-month period duringthe study period in 1990. These distributions were plot-ted from the IP and resistivity logs and for a spacingincrement (n) of 3. The variation of air temperature isalso plotted in Figure 4A. The cryofronts in the activelayer and at the permafrost base are underlined bythick contour lines. The warming of shallow permafrostis clearly shown. The depth of zero annual amplitudechange is approximately 10 m and the permafrost baseis located at a depth of 22.7 m (Figure 4A).

The resistivity at the cryofront in the active layer,delimiting the unfrozen from the frozen zones, is evalu-ated at 500 ohm-m (Figure 4B). Therefore, using thisresistivity value as a reference, a basal cryopeg can bedelimited between 21 m, where a contrast of resistivityfrom 2000 to 500 ohm-m occurs over 0.5 m (from 20.5 to21 m), and 22.7 m at the permafrost base delimited withthe temperature profile (Figure 4A). This resistivity con-trast is due to the phase change from frozen to unfrozenlayers at the base of the perennially frozen layer even ifthe temperature is still below 0¡C. The freezing-pointdepression at the base of the perennially frozen layer (at21 m depth) due to the overburden pressure is thenevaluated to be -0.2¡C.

The warming of shallow permafrost is also percepti-ble through a decrease in resistivity from 60 000 to 30 000 ohm-m for a warming from -6.5 to -3.5¡C at adepth of 3 m over the two-month period. Below 5 m, asmall increase of resistivity was monitored with time.For example, the layer at a depth of 11 m defined byhigh values of resistivity increases in thickness withtime due to the delayed penetration of the cold wave.

Variations of chargeability with time are visible in theactive layer and below the base of perennially frozenlayer (Figures 4C and 4D). In the active layer, they aredue to the increase in unfrozen water before thaw andto the advance of the thawing front in the active layer.Large variations of chargeability and chargeability gra-dient in the basal cryopeg and below the permafrostbase between May 10 and June 4 are associated withleaching by groundwater flow in the water table. The

migration by gravity of snowmelt water (the thawingperiod began around May 15 as observed with the va-riations of air temperature in Figure 4A) along the sideof the valley and in the marine deposit washed andmodified the ionic content of the groundwater. Afterthe snowmelt period, the conditions in water table sta-bilized and slight chargeability variations wereobserved after June 4.

Negative values of chargeability (Figure 4C) and posi-tive values of chargeability gradient (Figure 4D) weremeasured in permafrost. This effect has the oppositesign of normal IP effect. It is due to the surface polariza-tion previously discussed. The boundary between theice-poor layer (10 000 ohm-m at a depth of 7.5 m) andice-rich layer (40 000 ohm-m at a depth of 11 m) isdelimited by a total chargeability close to 0 mV/V(Figure 4C) and a negative chargeability gradient of -4 mV/V/s (Figure 4D) at a depth of 9 m.

Conclusions

Based on the results from temperature, IP and resistiv-ity logs carried out in a permafrost plateau in NorthernQu�bec, the following conclusions can be drawn:

(1) IP and resistivity logging allows detection andmapping of permafrost discontinuities such as alternat-ing ice-rich and ice-poor layers, and the basal cryopeg,and evaluation of the freezing-point depression at thepermafrost base.

(2) The repeated measurement of IP and resistivitylogging can provide information on the permafrostdynamics.

(3) The surface polarization at the boundarybetween frozen and unfrozen layers, or between ice-rich and ice-poor layers, is an important IP effect in per-mafrost environment allowing boundaries betweenthese layers to be located.

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Richard Fortier, Michel Allard 281

References

Allard, M, Caron, S. and B�gin, Y. (1996). Climatic and eco-logical controls on ice segregation and thermokarst: thecase history of a permafrost plateau in Northern Qu�bec.Permafrost and Periglacial Processes, 7, 207-227.

Barker, R. D. (1989). Depth of investigation of colinear sym-metrical four-electrode arrays. Geophysics, 54, 1031-1037.

Fortier, R., Allard, M. and Seguin, M.-K. (1994). Effect ofphysical properties of frozen ground on electrical resistivi-ty logging. Cold Regions Science and Technology, 22, 361-384.

MelÕnikov, V. P. (1973). Concerning the nature of an inducedpolarization field in connection with the surface polariza-tion at the boundary between thawed and frozen soils. In:Proceedings 2nd International Conference on Permafrost,Yakutsk, USSR, USSR Contribution Volume, pp. 497-500.