Proceedings of the Tenth International Conference on PermafrostSalekhard, Yamal-Nenets Autonomous District, RussiaJune 25–29, 2012

Tenth InternationalConference on PermafrostVolume 1: International ContributionsEdited by Kenneth M. Hinkel

The Northern PublisherSalekhard2012

Tenth International Conference on PermafrostVolume 1: International ContributionsEdited by Kenneth M. Hinkel© 2012The State Enterprise of the Yamal-Nenets Autonomous DistrictThe Northern Publisher (Severnoye Izdatelstvo)All rights reserved.
Printed in RussiaElmer E. Rasmuson Library Cataloging in Publication DataInternational Conference on Permafrost (10th : 2012 :Salekhard, Yamal-Nenets Autonomous District, Russia)Tenth International Conference on Permafrost :Resources and Risks of Permafrost Areas in a Changing World /edited by Kenneth M. Hinkel [v.1], Vladimir P. Melnikov [v.2, 3].
Salekhard, Russia: The Northern Publisher; Tyumen, Russia: Pechatnik; Ekaterinburg,Russia: Fort Dialog-Iset, 2012, 5 v. : ill., maps, cm.
Includes bibliographical references and index.
June 25-29, 2012.
Contents: vol.1. International contributions - vol.2. Translations of Russian contribution - vol.3. Russiancontribution - vol.4. Extended Abstracts - vol.5. Extended Abstracts in Russian.
1. Permafrost-Congresses. 2. Frozen ground-Congresses.
I. Title. II. Hinkel, Kenneth M. III. Melnikov, Vladimir P.
Cover Design: The Northern Publisher

GB641.I6.2012УДК 551.34; 502.3; 624.139ББК 26.3Д 37ISBN 978-5-905911-01-9 (v.1)
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ISBN 978-5-91128-052-9 (v.4/2)

Production Editor: Thomas Alton (Fairbanks, Alaska)Hinkel, K.M. (ed.). 2012. Tenth International Conference on Permafrost. Vol. 1: InternationalContributions. The Northern Publisher, Salekhard, Russia, 504 pp.

Improvement of Pulse-NMR Technology for Determining theUnfrozen Water Content in Frozen SoilsSatoshi AkagawaCryosphere Engineering Laboratory, Hachioji, Tokyo, JapanGo IwahanaUniversity of Alaska Fairbanks, USAKunio WatanabeMie University, Mie, JapanEvgeny M. ChuvilinMoscow State University, Moscow, RussiaVladimir A. IstominGAZPROM VNIIGAZ LLC, Moscow, Russia

AbstractSeveral methods are used to observe unfrozen water content in frozen soil. These include pulse-NMR (P-NMR), timedomain reflectometry (TDR), differential scanning calorimetry (DSC), and the contact method. However, a preliminarystudy by the authors reveals that test results depend on the method. This disagreement may be caused by impropertreatment of data in P-NMR analysis. In this paper, free induction decay (FID) followed by a 90° pulse of P-NMR hasbeen reexamined with four different soils in order to confirm the validity of the analytical method. In the discussion, twodifferent methods used to determine the appropriate time for choosing signal intensity of each specific FID, are discussed.
After the appropriate method to determine the rational time for the signal intensity of FID is clarified, the effect of coolingand warming runs (hysteresis) is discussed.
Keywords: unfrozen water content; P-NMR; 90° pulse; FID, hysteresis.

IntroductionThe behavior of water molecules in the liquid phase in frozensoil has drawn attention for a long time. In this paper, theliquid phase H2O in frozen soil is defined as “unfrozen water.”Several methods are used to measure the quantity of unfrozenwater, such as pulse nuclear magnetic resonance (P-NMR)(Tice et al. 1978, Tice et al. 1981), time domain reflectometry(TDR) (Smith & Tice 1988, Hivon & Sego 1990), differentialscanning calorimetry (DSC) (Smith & Tice 1988, Hivon &Sego 1990, Kozlowski 2002), and the contact method (Ershovet al. 1979, Chuvilin et al. 2008). However, results producedfrom these various methods are rarely identical. In order toimprove consistency among these methods, the authors reexamined the data analysis procedure for the P-NMR methodas an initial step. We give special attention to the free inductiondecay (FID) signal after a 90° pulse; in the P-NMR method, theFID signal intensity is directly proportional to water content.

P-NMR MethodConventional P-NMR methodSince the main objective of this paper is the technologicalimprovement of the P-NMR method, the theory of P-NMR(e.g., Hornek http://www.cis.rit.edu/htbooks/nmr/) is notdiscussed. The conventional data analysis procedure issummarized in Figure 1. The P-NMR method utilizes freeinduction decay (FID) followed by a 90° pulse (Fig. 1A).
During and after the 90° pulse, the receiver that monitors FID

Figure 1. Data analysis procedure of P-NMR.

21

22  Tenth International Conference on Permafrostis turned off for a certain period to protect the receiver fromthe strong pulse.
This period is called “dead time” and masks the earlyportion of FID. Generally speaking, the length of the deadtime is chosen to mask the FID of the solid phase.
According to the pioneering work by Tice and Oliphant(1984), the basis of P-NMR is that the FID signal intensityis directly proportional to the amount of unfrozen water inthe frozen soil sample. This means that we can calculate theunfrozen water content of the soil shown in Figure 1B if wecompare the signal intensity at a positive temperature to thewater content of the soil. In other words:SI (0,t ) : w (0) = SI (θ : t ) : w (θ )n(1)where SI (0,t) is the signal intensity at 0°C and time t, w(0)is water content of the soil, SI(θ,t) is the signal intensity atthe negative temperature θ°C, and wn(θ) is the unfrozen watercontent by weight (i.e., weight of unfrozen water / weight ofsoil particles, in percent) at the negative temperature θ°C.
Based on the principle demonstrated by Tice and Oliphant(1984), equation 1 is valid.
A question on the conventional P-NMR methodThe validity of equation 1 requires the following FIDrelationship, as depicted in Figure 1A:R(θ,t ) =SI (θ,t )SI (0,t )R(θ,t ) = Const (θ ).

(2)(3)

where R ( θ ,t) is a ratio at the temperature θ and the timet. The relationships shown in equations 2 and 3 suggest thatthe FID curves shown in Figure 1A should be convertedinto horizontally parallel lines. This conversion is necessaryin order to apply equation 1. We examined the FIDs of fourdifferent soils using this procedure.
P-NMR system usedThe P-NMR system we used (see Fig. 2) consists of aP-NMR machine (MARAN Ultra) and a temperature bathhoused within a walk-in type cold room held at a temperatureof +1°C. The operating frequency of the P-NMR is 23MHz.
The 90° pulse length is 7.6 m Sec followed by a dead time of10 m Sec. For one FID measurement, 90° pulses are applied 10times so as to improve the signal-to-noise ratio.
The temperature of the samples is controlled within thetemperature bath. The temperature control function of theP-NMR probe is not used so as to avoid shifts in resonantfrequency due to changing temperature. Instead, for a reading,the temperature controlled soil samples are moved into theP-NMR probe, in which the temperature is maintained atabout +1°C by the temperature of the cold room.
Since the time required for a P-NMR measurement is lessthan 30 seconds, the effect of sample temperature on the probetemperature is negligible and vice versa.

Figure 2. P-NMR System (MARANUltra with temperature bath).

Soil samples testedThe properties of the FID signal acquired from a soil andwater mixture is not completely understood. In order togeneralize the result of this study, we selected four differentsoils. MZ Kaolin is a typical clay sample, predominantlyconsisting of the clay mineral Kaolinite. NSF Clay is uniqueclay with Pyrophyllite as the predominant mineral. Weexamined a volcanic ash that contains no clay minerals butinstead consists primarily of quartz and plagioclase. Finally,we tested the soft mudstone called Dotan in Japan. This soilis caked Diluvium silt and the predominant clay minerals areillite and chlorite.
Some physical properties of these soils are listed in Table1. All of the specimens were saturated with water and thenconsolidated with an oedometer. The water content and wetdensity reported in Table 1 were measured after the specimenswere consolidated.

MeasurementSoil sample freezingThe focus of this experiment is the measurement methodof unfrozen water content. If ice lenses form in the specimenwhile it is freezing, the unfrozen water content may decrease.
In order to inhibit ice lens formation, samples were frozenrapidly with liquid nitrogen. Then frozen samples were placedinto the bottom of the sample holder, which was made of anon-magnetic material such as Teflon.
Test temperatures and measurementThe sample holders were inserted into the temperature bath,set at -20°C, and brought to thermal equilibrium. For the firstmeasurement following thermal equilibration, the sampleholders were sequentially removed from the bath, wiped dry,measured in the P-NMR analyzer, and then reinserted into thebath. After all samples were measured, the bath temperaturewas adjusted to the next measurement temperature. In our  Akagawa et al.  23Table 1. Soil physical properties.
MZKaolin

Soil particledensity(g/cm3)
Dry density (g/cm )3

Wet density(g/cm3)
Water content (%)
Liquid limit(%)
Plastic limit (%)
Plasticity index (-)

NSF Clay

VolcanicAshSoftMudstone

2.64

2.75

2.65

2.60

1.021.6360.170.235.334.91.081.6553.255.029.026.01.481.7820.328.617.710.91,261.7639.262.341.920.4Figure 4. Typical FID curves after 90° pulse.

Figure 5. Normalized FID curves with SI(10).
Figure 3. Temperature response in the dummy specimen.

case, the suite of measurement temperatures were -20, -15,-10, -5, -2, -1, -0.5, -0.2, -0.1 -0.2, -0.5, -1, -2, -5, -15, -20, +5,and +10°C. The time between measurements at two differenttemperatures was set for at least three hours to ensure thermalequilibrium. The monitored temperature in a dummy sample isshown in Figure 3. Thermal equilibrium is reached well withinthe three-hour period. Using this suite of temperature measurements, we also investigated hysteresis between coolingand warming runs.
Analysis method1) Misgivings with the analysisThe main assumption of the P-NMR method is that the FIDsignal intensity is directly proportional to the water content(or unfrozen water content of a frozen soil sample) as shownin equation 1. Another assumption is that the signal from thewater phase in a soil sample follows the exponential curve.
The FID curves acquired from MZ Kaolin (shown in Fig. 4),however, appear to demonstrate characteristics that differ fromthese assumptions.
The important point is that the signal intensity ratio betweenany two FID curves at any time should be constant, as indicated by equation 2 and equation 3. In the following, two different methods will be demonstrated and verified with FID data.
2) Method AEquation 2 and equation 3 are the necessary conditionsto satisfy equation 1. Thus the rationale behind the use ofequation 2 and equation 3 should be first verified.

As is shown in equation 2, R ( θ ,t) is calculated using FIDdata. In other words, all the FID curves were compared withthe FID acquired at the sample temperature of +10°C by takingthe fraction of each FID over the FID of +10°C. The results areshown in Figure 5.
If all the lines in Figure 5 are horizontally parallel, as isrestricted by equation 3, the assumption is applicable at anyspecific time during the acquisition of the FIDs. However, thehorizontally parallel section becomes shorter as the sampletemperature becomes lower. The time for which the assumptionis true is later than 80  mSec in this case, as indicated by thevertical line in Figure 5.
By completing this procedure, one can determine the timerange that will satisfy the assumption. Therefore, by selectingthe suitable time after the 90° pulse for unfrozen water contentanalysis, equation 1 is applicable as the conventional dataanalysis procedure.
3) Method BGenerally speaking, water and ice in soil have longer FIDs(Quinn et al. 1991), with the FID of water longer than thatof ice. From this it follows that the dead time after the 90°pulse in the conventional method is not long enough to maskthe FID from the ice. Assuming that the disorder that is seenin the early part of the FID curves in Figure 5 is due to theoverlapping of FIDs of unfrozen water and ice:FID(measured) = FID(water) + FID(ice)(4)

24  Tenth International Conference on Permafrost -t FID(water) = A • Exp T water (5)- 2FID(ice) = B • Exp t 
 T ice (6)where, A, B, Twater, and Tice are constants. The measured FIDwas discriminated into FID(water) and FID(ice) with theSOLVER function in Excel.
The typical result is shown in Figure 6. As is seen in thefigure, FID(water) and FID(ice) are well discriminated usingSOLVER. The FID(water)s of MZ Kaolin are shown in Figure7. If these straight lines satisfy equation 2 and equation 3, thenequation 1 is applicable.
Twater is the time constant of the decay function of equation5, and the values of Twater of the each FID(water) acquired fromFIDs at different temperatures are shown in Figure 8. Theshape of the FID curves of each soil sample indicates that theTwater of the four soils tested has temperature dependence.
Mathematically, a condition required by equation 3 isthat Twater is constant over the specimen temperature range.
Therefore, it will be difficult to determine the appropriatetime t which fulfills the requirement. However, MethodB is applicable for the soils in cases where the temperaturedependence of Twater is negligible.
4) Comparison of Method A and Method BMethod A and Method B were applied to the soilssummarized in Table 1, and the results are shown in Figures9-1 to 9-4.
For Method A, 100 m Sec was selected as the time t requiredto fulfill equation 3. For Method B, Twater in equation 5 ofdifferent FIDs were assumed to be equal. With this assumption,the requirement is fulfilled at any time t. So t=0 was chosen forequation 5, resulting in SI( θ ,0)=A.
As shown in Figure 9, Method A and Method B providethe same trends in unfrozen water content versus temperature(i.e., unfrozen water content decreases with temperature).
However, the difference between the two methods increasesas temperature decreases. The differences between Method Aand Method B in the low temperature range, such as below-1°C, in the four soil samples are considerably large.
According to the data shown in Figure 5, Method A fulfillsthe requirement to choose the appropriate time t for FID signalintensity analysis. The data shown in Figure 9 indicates thatMethod B consistently provides higher unfrozen water contentper soil sample. Therefore, the authors recommend MethodA for finding the appropriate time t for the unfrozen watercontent analysis using FID signal intensity data.
HysteresisUnfrozen water content curves of warming and coolingtemperature runs are shown in Figure 9. Hysteresis is clearlyvisible in each of these figures. All of the cooling curvesdemonstrate higher unfrozen water content than warmingcurves. It is noteworthy that the predominant temperature

Figure 6. Discriminated FID(water) and FID(ice).

Figure 7. Typical FID(water)s at different temperatures.

Figure 8. Temperature dependence of Twater .

range of the hysteresis depends on soil type. Therefore, it isimportant to indicate which process (either the cooling orwarming process) is used when discussing unfrozen watercontent curves.

Conclusions1) None of the FID curves that were acquired from the fourdifferent soils follow a simple exponential curve.
2) The early part of the FID curves especially demonstratethe tendency mentioned in conclusion 1.
3) Therefore, it is important to choose the appropriate time“t” for the analysis of unfrozen water content.
4) Method A is a simple and effective technique.

  Akagawa et al.  25

1) Comparison of Method A and Method B with MZ Kaolin frozen by LN25) Method B may discriminate between the FIDs fromice and water, assuming equation 5 and equation 6 areapplicable.
6) Since the time constant, Twater, of four soils demonstratestemperature dependency, equation 2 and equation 3 maynot be satisfied.
7) Therefore, Method B may not be the universal techniquefor the unfrozen water analysis.
8) The predominant temperature range of hysteresis dependson soil type. The description of cooling or warmingprocesses is important when discussing unfrozen watercurves.
9) An appropriate data analysis procedure for the P-NMRmethod is demonstrated in this paper. The consistencyof the P-NMR method with other methods such as TDR,DSC, the contact method, and so forth can now be studiedfurther.

AcknowledgmentsThe authors would like to express their appreciation to Dr.
M.M. Darrow of the University of Alaska Fairbanks for heruseful advice in preparing this manuscript.

References2) Comparison of Method A and Method B with NSF Clay frozen by LN23) Comparison of Method A and Method B with volcanic ash frozen by LN24) Comparison of Method A and Method B with Dotan frozen by LN2

Figure 9. Comparison of Method A and Method B for differentsoils (data also show hysteresis).

Chuvilin, E., Guryeva, O., Istomin, V., & Safonov, S. 2008.
Experimental method for determination of the residualequilibrium water content in hydrate-saturated naturalsediments, Proc. of the Sixth Inter. Conf. on Gas Hydrates(ICGH 2008): 8.
Ershov, E.D., Akimov, Y.P., Cheverev, V.G., & Kuchukov, E.Z.
1979. Water phase composition in frozen soils. Moscow:MSU, 192 (in Russian).
Hivon, E. & Sego, D.C. 1990. Determination of the unfrozenwater content of saline permafrost using time-domainreflectometry (TDR). Proc. of the Fifth CanadianPermafrost Conference: 257-262.
Hornek, J.P. The basics of NMR. http://www.cis.rit.edu/htbooks/nmr/.
Kozlowski, T. 2002. A comprehensive method of determiningthe soil unfrozen water curves 1. Application of the termconvolution. Cold Regions Science and Technology 36: 7179.
Quinn, F.X., Weldon, V.L., McBrierty, V.J., Kruijsen, A.A.T.J.J.M,& Unsworth, J.F. 1991. Cryogenic properties of soils androcks 1. Anomalous behaviour of water. Geotechnique 41No.2: 195-209.
Smith, M.W. & Tice, A.R. 1988. Measurement of the unfrozenwater content of soils: A comparison of NMR and TDRmethod. Proc. Of the Fifth Inter. Conf. on Permafrost: 473477.
Tice, A.R., Burrous, C.M., & Anderson, D.M. 1978.
Determination of unfrozen water in frozen soil by usingnuclear magnetic resonance. Third Int. Conf. on Permafrost:149-155.

26  Tenth International Conference on PermafrostTice, A.R., Anderson, D.M., & Sterrett, K.F. 1981. Unfrozenwater contents of submarine permafrost determined bynuclear magnetic resonance. Engineering Geology 18: 135146.
Tice, A.R. & Oliphant, J.L. 1984. The effect of magnetic particleson the unfrozen water content of frozen soils determined bynuclear magnetic resonance. Soil Science 138-No.1: 63-73.