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Comparison of geophysical investigations for detection of massive

ground ice (pingo ice)

K. Yoshikawa,

1

C. Leuschen,

2

A. Ikeda,

1

K. Harada,

3

P. Gogineni,

4

P. Hoekstra,

5,6

L. Hinzman,1 Y. Sawada,7 and N. Matsuoka8

Received 3 August 2005; revised 1 November 2005; accepted 2 December 2005; published 10 June 2006.

[1] Six different geophysical investigations, (1) ground-penetrating radar, (2) DCresistivity sounding, (3) seismic refraction, (4) very low frequency (VHF) electromagnetic,(5) helicopter borne electromagnetic (HEM), and (6) transient electromagnetic (TEM)techniques, were employed to obtain information on the ice body properties of pingos near Fairbanks, Alaska. The surface nuclear magnetic resonance (NMR) data were alsocompared from similar sites near one of the study pingos. The geophysical investigationswere undertaken, along with core sampling and permafrost drilling, to enablemeasurement of the ground temperature regime. Drilling (ground truthing) results support field geophysical investigations, and have led to the development of a technique for

distinguishing massive ice and overburden material of the permafrost. The two-dimensional DC resistivity sounding tomography and ground-penetrating radar profilingare useful for ice detection under heterogeneous conditions. However, the DC resistivitysounding investigation required high-quality ground contact and less area coverage. Theactive layer thickness and the homogeneous horizontal structure of the overburdenmaterial are important parameters influencing detection of massive ice in permafrost for most methods such as seismic, TEM, or surface NMR.

Citation: Yoshikawa, K., C. Leuschen, A. Ikeda, K. Harada, P. Gogineni, P. Hoekstra, L. Hinzman, Y. Sawada, and N. Matsuoka

(2006), Comparison of geophysical investigations for detection of massive ground ice (pingo ice), J. Geophys. Res., 111, E06S19,

doi:10.1029/2005JE002573.

1. Introduction

[2] Many geophysical variables, such as dielectric con-stant, electrical conductivity, or P wave velocity, displaystrong contrasts between frozen and unfrozen soil phases.This presents a useful distinction to enable the study of

permafrost and to characterize areas of permanently frozenground [Scott et al., 1990; Dallimore and Davis, 1992;

Moorman et al., 2003]. Thus, geophysical techniques arewidely accepted in permafrost and periglacial processstudies such as those of the active layer [ Doolittle et al.,1990; Hinkel et al., 2001], rock glaciers [e.g., Vonder Muhll and Schmid , 1993; F. Lehmann et al., unpublished manu-

script, 1999], palsas [ Doolittle et al., 1992], and pingos

[ Kovacs and Morey, 1985; Ross et al., 2005].[3] Recently, interest in water and H2O ice in permafrost has increased substantially, owing to the discovery of

possible life on Mars. Detecting groundwater is a high priority goal for NASA’s decadal research. However, muchgroundwater will exist as brine or in the solid phase at thenear surface in these extraterrestrial terrains. The focus of this study is to detect massive ice bodies in terrestrial

permafrost regions. An open (hydraulic) system pingo isan ice-cored mound that formed in response to sub-, or intra-

permafrost artesian water pressure. Before water can reachthe ground surface it freezes, forming a core of relatively

pure and clear ice. In this paper, we examine three opensystem pingos near Fairbanks (Figure 1). These pingos have

different internal and overburden structure and ice condi-tions. In cases of inhomogeneous ice distributions, geophys-ical investigations are often difficult to apply because of theinvalid approximation of lateral homogeneity used in manymodeling approaches. The goal of this study is to examinethe feasibility of using different geophysical techniques todetect massive ground ice in Martian permafrost.

2. Methods

2.1. Ground-Penetrating Radar

[4] Ground-Penetrating Radar (GPR) investigations have been conducted at three pingos around Fairbanks. We tested

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, E06S19, doi:10.1029/2005JE002573, 2006

1Water and Environmental Research Center, University of Alaska,

Fairbanks, Fairbanks, Alaska, USA.2Applied Physics Laboratory, Johns Hopkins University, Laurel,

Maryland, USA.3Department of Environmental Sciences, School of Food, Agricultural

and Environmental Sciences, Miyagi University, Miyagi, Japan.4Center for Remote Sensing of Ice Sheets, University of Kansas,

Lawrence, Kansas, USA.5Blackhawk Geosciences, Golden, Colorado, USA.6Retired.7Institute of Low Temperature Science, Hokkaido University, Sapporo,

Japan.8Department of Earth Science, University of Tsukuba, Ibaraki, Japan.

Copyright 2006 by the American Geophysical Union.0148-0227/06/2005JE002573$09.00

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two types of radars to detect a pingo’s massive ice body: aswept-frequency radar (5–120 MHz), and an impulse typeradar (GSSI, SIR-2000 with the bistatic antenna model3200MLF (15–80 MHz)).

[5] The swept-frequency radar unit was developed by the

University of Kansas as a prototype [ Leuschen et al., 2003; Arcone et al., 2001] for possible deployment on a rover tocharacterize local stratigraphy and detect subsurface water/ ice. The transmitter subsystem used a 300-MHz direct-digital synthsizer to generate a 5- to 120-MHz chirp signalafter receiving a trigger from the system controller, and thereciever digitized the radar response using a 50 kSPS 16-bit A/D converter. Two large (2.5 m length) bowtie antennaswere constructed for initial testing using an aluminum insect screen that was easily rolled up for storage and transporta-tion. The antennas operated over the frequency range 10– 120 MHz.

[6] Also, a commercial impulse radar unit was used for reference. The velocity analysis was attempted at the drill

sites using the common midpoint (CMP) method at thecenter frequency of 40 or 80 MHz. The resultant stack data(0.5-m interval) was processed with the cross-correlationmethod [Yilmaz , 1987]. This method uses the Radan ad-vanced geophysical function (velocity analysis) programsfrom Geophysical Survey Systems, Inc.

2.2. DC Resistivity Sounding

[7] DC resistivity sounding was conducted at three study pingos around Fairbanks. We employed classical one-dimensional DC resistivity sounding with the Oyo Co.,Ltd., McOHM model-2115 and two-dimensional resistivity

profiling (IRIS instruments; Syscal pro R1 48 – 72 channel)

for this investigation at 5-m intervals with the Wenner electrode configuration. The electrical resistivity of soildepends on the soil type, temperature, water content, poros-ity, and salinity. In general, the resistivity (r) values of frozensoil are 10–1000 times greater than those of unfrozen or

brine soils [ Harada and Yoshikawa, 1996]. DC resistivitysounding used four electrodes for measurement. A current (I)was delivered and received between the outer electrodes, andthe resulting potential difference (V) was measured betweenthe inner electrodes. For this array on the ground surface, anapparent resistivity (ra ) is calculated from

ra ¼ 2pa V=I; ð1Þ

where ‘‘a’’ is the distance separating the electrodes. Theinversion analysis was performed with changing values of resistivity and layer thickness by using the linear filter method [ Das and Verma, 1980] for a one-dimensionalinvestigation.

[8] For the acquisition of the two-dimensional apparent resistivity data we used multichannel, equally spaced elec-trodes with a standard spacing of 5 m. Each measurement was repeated up to 16 times, depending on the variance of the results. Two-dimensional model interpretation was per-formed using the software package RES2DINV (Geotomosoftware), which performs smoothing and constrained in-version using finite difference forward modeling and quasi-

Newton techniques [ Loke and Barker , 1994].

2.3. Seismic Refraction

[9] The seismic refraction technique permits us to detect the frost table because the P wave velocity of frozen ground

Figure 1. General information and permafrost conditions for the study of pingos around FairbanksAlaska.

E06S19 YOSHIKAWA ET AL.: GEOPHYSICAL SURVEYS FOR PINGO ICE

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(1500–4700 m/s) is much higher than that of the dry activelayer (300–800 m/s) [e.g., Hunter , 1973].

[10] The seismic refraction measurements were con-ducted at three study pingos around Fairbanks. A three-channel seismograph, McSEIS-3 (Oyo Co., Ltd.) was usedfor these measurements. A 4-kg sledgehammer produced aseismic pulse. The pulses were summed at the same point

by multiple hits to improve the signal-noise ratio. The

length of survey lines depended on the size of the pingos.The survey lines were 30–36 m for relatively small pingos(Caribou and Grenac Creeks Pingos) and 50 m long for alarge pingo (Cripple Creek Pingo). The shotpoints were

placed at 3-m intervals for the short survey lines and 5-mintervals for the long line. In the latter case, the shotpointswere placed at 2.5-m intervals near the receivers. Refractor depth and P wave velocities were calculated using theIntercept Time Method [ Palmer , 1986]. Since seismicrefraction technique presumes that the ground had a layeredstructure with higher P wave velocity in a deeper layer, themethod is generally not applicable to map permafrost or ice

body base.

2.4. Very Low Frequency (VLF) Electromagnetics[11] VLF EM investigations were conducted at Cripple

Creek Pingo. VLF electromagnetic techniques (WADIAbem) were evaluated for their ability to delineate andmeasure the thickness of shallow permafrost. The WADIVLF system is a two-component magnetic receiver operat-ing in the frequency range of 15–30 kHz. The sources for these frequencies are powerful radio transmitters, similar tothose used for submarine radio-communication. In Alaskanstudies presented here, typically three different transmittersoperating at different frequencies were used. When thesetransmitter signals propagate from the source positions tothe position of the measurements site, they interact in acomplex way with the two electrical conductors: the Earth

at the bottom and the ionosphere at the top. However, owingto their small penetration (400 meters in normal granites)compared with the distance to the sources, we can regardthe signals as plane waves propagating downward into theearth at any receiver site. The plane wave assumptionenables a relatively simple and fast interpretation of thedata using two-dimensional models. Two magnetic compo-nents ( H

x , H

z ) are measured with a single antenna consisting

of two mutually orthogonal coil sensors. We used thesoftware package RAMAG to process data from the WADIand transform field data. The linear filtering process used is

based on the Karous and Hjelt algorithm [ Karous and Hjelt ,1983]. This filtering algorithm has been combined with aweighted running average smoothing function.

2.5. Helicopter-Borne Electromagnetic (HEM)

[12] HEM investigations were conducted at CrippleCreek Pingo. In the early 1970s, several studies wereconducted to map the location and thickness of permafrost zones with ground and airborne geophysics [ Hoekstra et al.,1975]. While these studies confirm that electromagneticgeophysical methods (EM) could be used to detect perma-frost, results indicated additional progress was needed toestablish operational capabilities. Single space frequencydomain ground EM systems (e.g., EM31) were not able tomap the vertical distribution (thickness) of the permafrost

[ Hoekstra, 1978]. The airborne EM (Dighem I) used in theearly 1970s was not very effective at mapping the perma-frost. It had only one frequency, 918 Hz, which would havean upper resistivity limit of about 1000 ohm-m. In the areaof the survey, very little ground data was collected, but theavailable data suggests that the permafrost distribution wasvery sparse. Current Dighem (HEM) systems have fivefrequencies, ranging from 400 Hz to 102,000 Hz. The depth

of penetration for each frequency is an inverse function of frequency squared, so the subsurface zone investigated can

be five different depths. We applied HEM data at theCripple Creek Pingo site from State of Alaska Department of Geological and Geophysical Surveys [ Burns et al.,2004a, 2004b, 2004c, 2004d].

2.6. Transient Electromagnetic (TEM)

[13] TEM investigations were conducted in both CrippleCreek and Caribou Creek pingos. The TEM sounding was

performed by laying a 60-m loop of wire on the ground or ice and driving current through it. The transient response of the secondary magnetic field was measured in a smallreceiver coil after shutting off the direct current. Owing to

ohmic losses in the resistive earth, the secondary fieldgradually collapses. As the resistivity of the background isincreased, the collapse rate of the response increases. In ahigh-resistivity environment like permafrost, these quick responses and primary field effects (IP effects, magneticrelaxation, Rx box, direct coupling with primary field)create difficulties in estimating the resistivity structurethrough a standard central induction measurement. Perma-frost surveys have been conducted using TEM with a largeloop of more than 100 m [e.g., Rozenberg et al., 1985; Todd et al., 1991; Todd and Dallimore, 1998]. Here a measure-ment configuration outside the transmitter loop was utilized,to reduce the primary field effects, with a small loop for ahigh production rate.

[14] For our measurements, the transient data wererecorded using a PROTEM 47 TEM system of GeonicsLimited with a receiver coil having an effective area of 31.4 m2. Decay voltages were recorded in two overlappingtime ranges (0.006813 – 0.06959 ms and 0.03525 – 2.792 ms after current is turned off) at two transmitter-waveform base frequencies (nominally 285 and 75 Hz,respectively). A transmitter, supplied with 1.5 to 2.5 amperesof loop current, established the precise timing by a referencecable with a receiver.

2.7. Surface Nuclear Magnetic Resonance (NMR)

[15] Surface NMR investigations were conducted at Caribou Creek. A NUMIS NMR (IRIS instruments) was

used for these measurements [ Hoekstra et al., 1998].Surface NMR is presently the only geophysical techniquecapable of direct detection of groundwater resources. Thesurface NMR method has similarities and differences withthe NMR measurements commonly made in controlledlaboratory experiments and in the medical field. In bothexperimental setups, the fact that a hydrogen molecule(proton) has a magnetic moment and an angular momentumis exploited [ Hoekstra et al., 1998]. Hydrogen atoms areexcited by pulses of alternating current at the proper frequency through a 100-m transmitting loop placed onthe ground. The depth of investigation is a function of the


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product of the intensity of the current at the resonancefrequency by the duration of the pulse, which gives themoment of the excitation pulse. Hydrogen atoms of water molecules are energized by pulses of alternative current at the proper frequency (Larmor frequency), transmitted into aloop laid on the ground. The magnetic field they produce inreturn is measured and analyzed for various energizing

pulse moments. The relaxation field of the protons is

measured in the same loop after the excitation current isturned off. The one-dimensional inversion provides esti-mates of the water content, the mean pore size, and thedepth of each layer. The time constant of the relaxation fieldis related to the pore size. This allows a distinction to bemade between pore free water and clay bound water.

3. Results

3.1. Caribou Creek Pingo

[16 ] Car i bo u - Po k er Cr eeks Resear ch Wat er sh ed(CPCRW) is located about 48 km northeast of Fairbanks.CPCRW contains two pingos along the Caribou Creek at the

base of the north facing slopes. The thickness of the

permafrost is about 120 m and annual mean upper perma-frost temperature is 0.7C. The list of the study pingos andtheir physical properties is shown in Figure 1. The CaribouCreek pingo has a 60- to 80-m base diameter, and is morethan 10 m tall. The massive ice core starts 7.35 m below theground surface and extends down to 23.5 m. Severalsegregation and inclusive ice layers (a few centimeters to30 cm thick) occur in the overburden silty permafrost. This

pingo survived wildfire several times. A charcoal layer wasobserved 15 cm below the surface where the radiocarbonage is 820 ± 40 years B.P. (GX-28572). The active layer fluctuated between 1 m (present) and 2 m (post firedisturbance) during the Holocene. The increase in activelayer depth during climatic optimum and/or post wild fire

disturbance decreases the ice content of the upper part of permafrost. The bedrock (schist) appeared just below thelower horizontal contact of the massive ice. Figure 2a showsoverburden ice content and pingo structure.

[17] The radar responses at the top of the pingo are alsoshown in Figure 2e. A linear gain is applied to accentuatesome of the deeper anomalies. Several reflections are

present within these waveforms, and there seems to be ageneral trend of a decrease in center frequency and band-width from the deeper reflections. This is not unusual, asone would expect increased attenuation at higher frequen-cies. From the reflection profile, the ice body’s possiblelocations can be estimated.

[18] Two of the reflections indicate the top and bottom of

the pingo ice core(Figure 2e).The first is around150 to 160 nsand has amplitude of about 5% of the surface response. Thesecond is around 360 to 390 ns and has amplitude of about 4%of the surface response. Figure 3d shows impulseradar profilefrom the 80 MHZ GSSI SIR 2000 systems. Ice was observedas free of reflection around 155 and 370 ns.

[19] Seismic refraction measurements are employed be-tween the south-facing slope and the top of the pingo. Thedirect observation with a thaw probe indicated that theactive layer was thicker at the south facing slope (>3 m)and 2 m at the top of pingo. P wave velocity was configuredwith a two-layer structure at 360 m/s and 2800 m/s. The first

layer (360 m/s) is 3 m thick on the south-facing flank of the pingo and 4 m thick at the top. The first layer representsthe active layer, although the thickness is different from thedirect observation. The thick organic layer on the top of the

pingo may weaken the signals, which results in the contra-diction. No signal from the massive ice body was observed

because the survey line was slightly short (30 m) to enableinvestigation of the deeper structure. The weak signals

through the thick organic layer prevented us from extendingthe survey line more than 30 m on this site.

[20] DC resistivity profiling results were useful for (Figure 2b) identifying the massive ice core, except at the

bedrock interface. An ice resistivity of 8600 ohm-m wasobtained, which is a reasonable value. Resistivity studies

permit better investigation of the detailed sub surfacestructure, especially distinguishing the frozen silt layer and the upper boundary of the ice core but not bedrock contact. Results from drilling a borehole indicate the pingo’sice core contacts bedrock at 23.5 m.

3.2. Cripple Creek Pingo

[21] Cripple Creek is located about 10 km west of Fair-

banks. The upstream basin has three pingos at the bottom of the Cripple Creek valley. The thickness of the permafrost varies between 25 m and 200 m and annual mean upper

permafrost temperature is 0.95C at the study site.[22] The study pingo is 120 m wide and 10 m high, with a

massive ice core located 10.2 m to 26 m below the top of the pingo. The pingo’s overburden has a very limitedsegregation ice and inclusive ice, which are located only6.3 – 6.5 m below the pingo top. Most of the overburden

permafrost contains fluvial sediment. The massive pingo icecontains very pure ice and less bubble structure. Figure 3shows the resistivity profiling, drill log, 40 MHz GPR signals, HEM data, and VLF result at Cripple Creek Pingo.

[23] The area surrounding the pingo site was dense with

trees and shrubs. Owing to the dense vegetation, access tothe location was difficult and collecting data for the radar surveys over a traverse was practically impossible. As aresult, data collection was restricted to a few discretelocations, often separated by a meter or more. GPR mea-surements were employed, using the GSSI impulse systemwith a 40-MHz bi-static antenna as well as velocity analysis

by CMT. Radar signals had strong refection at the top(180 ns) and the bottom (480 ns) of the ice core. Theaverage speed of the wave propagation for the massive icecore was 0.165 m/ns and was estimated at 0.08 m/ns for theoverburden material, using the velocity analysis technique.Thus we calculated the dielectric to be 3.3 for the ice coreand 14 for overburden.

[24] The refraction seismic results were excellent at this pingo, although the velocity of the third layer is a littlehigher than typical values of ice rich permafrost (1500 to4700 m/s). P wave velocity was configured for three layersat 400 m/s, 1900 m/s and 6000 m/s from top to bottom. Thethickness of the first layer is 1 m at the top of the pingo and2 m at the base of the south-facing slope. The number corresponds well with measurements of active layer thick-ness by thaw probe. The base of the second layer waslocated at 10 m at the top of pingo and 9.5 m at the base of the south facing slope. These are determined by overburden

permafrost layer. Actual measurement of P wave velocity at


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the borehole wall depth between 4 and 6 m was 2500 m/s.The third layer was the massive ice body, 6000 m/s. Thereasons for the much faster velocity are as follows: (1) very

few cracks in the ice, (2) most of the ice was pure and didnot contain soil layers holding unfrozen water, and (3) lowair bubble content. In addition, the ends of the traveltimecurves may include signals from underlying massive bed-rock, which can result in the extraordinary high velocity for an ice layer, although the quality of the data is not goodenough to calculate the fourth layer.

[25] A resistivity result was reasonable and ice wasestimated at more than 7000 ohm-m, which is similar toresults from the Caribou Creek pingo. Interestingly, 900-HzHEM results did not correspond with DC resistivity results.The 900-Hz HEM apparent resistivity values were down to

300 ohm-m at the top of pingo, compared with surroundings(>500 ohm-m). the 56,000-Hz and 7200-Hz HEM resultsindicate massive ice core. The 900-Hz signal is most likely

responding to the water lens at the bottom of the pingo ice(Figure 3f). 1D DC resistivity results indicated a massive icecore, and the modeling process between Wenner andSchlumberger arrays (Table 1) showed minor horizontaldifferences. These differences were caused by heteroge-neousness of the horizontal structure of the pingo.

[26] VLF surveys were conducted using WADI. Figure 3gshows 9 m (30 feet) filtered results using the same transect of the HEM results (Figure 3f). However, no significant information was obtained either in-phase or quadrature fromVLF result. In-phase signals are reduced significantly at thetop of the pingo (distance 70 m in Figure 3g). The phase

Figure 2. Surface topography of the Caribou Creek pingo and its internal structure as verified bydrilling operations. (a) Results of two-dimensional resistivity profiling. (b) Pingo ice and overburden

permafrost (7.3 m) interface; bedrock interface not shown. (c) Drill log at the top of the pingo. (d) LF(80 Mhz) GPR image over the same cross-section as shown in Figure 2a and the resistivity transect (Figure 2b). The GPR image clearly indicates the massive ice core as the free reflection zone. (e) Massiveice core between 155 and 370 ns at the top of pingo.


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change has not been demonstrated in other geophysicalinvestigations. The signal level was lower (6–10) duringthe investigation.

3.3. Grenac Creek Pingo

[27] Grenac Creek is located about 3 km north of Fair- banks along the Farmer’s Loop Road. One pingo is located

at the toe of a fluvial fan. The thickness of the permafrost is38 m, and annual mean upper permafrost temperature is0.7C.

[28] The study pingo is 50 m wide, 6 m high, and has amassive ice core extending from 6.5 m to 12.5 m depth fromthe top of the pingo. The water lenses located between13.5 m and 18 m at the bottom of the pingo ice. Permafrost

Figure 3. (a) Field results from Cripple Creek pingo. Two-dimensional resistivity profiling defines themassive ice core. (b) Drill operation log at the top of the pingo showing massive clear ice starting 10.2 m

below the ground surface. (c) GPR wiggle curve indicating the massive ice core between 180 and 480 nsas the free reflection zone. (d, f) Pingos at the Cripple Creek displaying higher resistivity compared withthe surrounding valley at the 7200 Hz HEM image. (g) No significant signal obtained either in-phase or

quadrature from VLF. (e) TEM and (h) one-dimensional DC resistivity results indicating the massive icecore; minor horizontal differences observed by the modeling process between Wenner and Schlumberger arrays (Table 1). These differences may be caused by the diverse horizontal structure of the pingo.


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was found between 18 m and 38 m depth. The subperma-frost groundwater is under active artesian conditions here.The bedrock was located at a depth of about 48 m, which is36 m below the pingo’s ice body. Figure 4 shows 1D, 2Dresistivity soundings, the drill log, 60-MHz GPR signals,and swept-frequency radar results at Grenac Creek pingo.

[29] There are two easily detectable reflections occurringat about 100 ns and 150 ns extending from the top of the

hill to about 25 m using the swept-frequency system(Figure 4c). Shifting time zero to the surface response,assuming an average dielectric constant of about 8, andconverting to range places these reflections at about 4–5 mfor the top and 7–8 m for the bottom. GSSI impulse systemGPR measurements are employed using the 40-MHz

bistatic antenna as well as velocity analysis by CMT. Radar signals have strong reflection at the top (200 ns) and the

bottom (450 ns) of the ice core (Figure 4d).[30] A seismic refraction measurement was employed at

the top of the pingo. Two layers were identified from thetraveltime curves. The upper layer has a P wave velocity of 390 m/s and the lower layer of 2300 m/s. The calculateddepth of the boundary is about 6 m, which is close to the top

of the massive ice core (6.5 m) and much deeper than thefrost table (about 2 m). The result is problematic because theupper part of permafrost only showed P wave velocity lower than 1000 m/s, even if a theoretically undetectable layer from the traveltime curve was considered. One of thetraveltime curves shows a large lateral gap at the northern

part of the survey line, which possibly results from consid-erable melting of the permafrost, except for the northern

part of the pingo.[31] Resistivity investigations have been conducted at this

pingo. We employed classic one-dimensional resistivitysounding (Figure 4e) and two-dimensional resistivity pro-filing (Figure 4a) for these investigations with the Wenner electrode configuration. Inversion results indicate a reason-

able ice body (>6000 ohm-m) with sub pingo ice talik (<100 ohm-m).

4. Discussion

[32] Geophysical surveys depend upon specific material properties such as dielectric constant, resistivity, or P wavevelocity. Wet, thawing and frozen soils are favorable anddetectable targets. The water lenses beneath the younger

pingo ice core would be ideal targets for most of thetechniques, with the exception of seismic refraction.

[33] During our investigations, almost all of the techni-ques had different results at the different pingos. The major interferences were caused by the heterogeneous structure of the horizontal overburden materials. Most of the geophys-ical investigations use a model of defined uniform horizon-tal layer structure, except GPR and resistivity profiling.However, the size of the pingo (e.g., volume of the massiveice core) is, for most of the investigations, not big enough to

be considered a uniform structure and allow the solution to be resolved. The geophysical properties of the upper partsof the pingos are strongly affected by slope aspect andhistorical active layer fluctuations. The variation of the icecontent of the overburden complicates the signal analysisfor each method. Thus the Cripple Creek pingo had rela-tively simple results for all of the geophysical investigations

because of the uniform overburden structure and size.[34] Some combinations are difficult to distinguish, such

as dry bedrock and cold massive ice core. Applications of the multiple geophysical survey techniques definitely helpto more accurately elucidate subsurface structures. Dielec-tric (GPR survey) may be affected by bubble orientation anddensity in the ice. The presence of highly dense air bubbles

in the ice yields a lower dielectric constant (k = 1– 3).However, this would help distinguish ice rich silty perma-frost (k = 4–8) [ Arcone, 1984]. Also, the free reflectionsfrom massive ice were observed at all of the study pingos.

[35] One of the complex structures of the upper part of the permafrost in interior Alaska is the presence of forest firedisturbance. The Alaskan boreal forest burns every 20–200years [Yarie, 1981; Kasischke et al., 2000; Dyrness et al.,1986]. Most of the study pingos developed in the Holocene,and experienced fire. Some pingos may have beendestroyed by the post-fire disturbance. Most of the pingosthat survived wildfire had deeper active layers or talik formation that did not reach to the massive ice. Severe fireremoves most of the surface organics and surface thermal

offset [ Romanovsky and Osterkamp, 1995] changes, thaw-ing permafrost and increasing the active layer moisturecontent. As a result, severe fire has caused 3 to 4 mof active layer thawing around the Fairbanks region[Yoshikawa et al., 2002]. A pingo may be destroyed if thedepth of the massive ice is shallower than the maximumthawing depth. All of the study pingos observed displayeda sharp horizon of ice rich and dry permafrost layers,which indicated the previous maximum active layer. Thediscontinuous boundary also presents the possibility tonegate massive ice core detection by the geophysicalinvestigations. Seismic refraction investigations were

Table 1. Comparison of One-Dimensional DC Resistivity Modeling Between Wenner and Schlumberger Arrays and TEMa

Structure

Schlumberger Wener TEM

Resistivity, o hm-m Depth, m R esistivity, ohm -m Depth, m R esistivity, ohm -m Dep th , m

Active layer 500 542 NDOverburden permafrost 4500 1.2 8000 1.7 245Massive ice 18,000 1.0 18,000 7.7 1944 10.9Talik 50 28 20 25 238 26.6Permafrost or bedrock 7000 31 8000 30 1330 96.8

a All measurements were performed on the same day around morning (0900–1300 local time on 9 August 2005). The Schlumberger array was lessinfluenced by horizontal structural homogeneity than the Wenner array. As compared with drilling (ground truthing), the Schlumberger array or TEM

performs better at detecting local massive ice bodies. ND, no data. Boldface denotes ice bodies.


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strongly affected by the depth of ice rich permafrost. In aseismic survey, if a faster wave velocity layer exists, it isvery difficult to quantify the structure of a lower layer.

[36] Recent climatic warming also affected the thermo-karst process, especially on south facing slopes. Most of theresistivity surveys show south-facing slopes of the pingohad lower resistivity (wet) than the north-facing slope of

pingo.[37] Surface NMR is a direct method for groundwater

detection, as it directly measures the response of the water

itself (H protons). One feature of surface NMR is thenonlinear relationship between the measured signal andthe energizing pulse intensity. This means that doublingthe pulse current does not mean doubling the signal; insteadit increases the depth of investigation. On the other hand,the surface NMR signal is linearly related to the water content of the layers. In investigations at the Caribou Creek site, surface NMR indicated a reasonable aquifer location

between the bedrock and eolian silt interface in the perma-frost. However, the ice content of the frozen silt did not

Figure 4. Field results from Grenac Creek pingo. (a) Two-dimensional resistivity profiling showingreasonable results at both the upper and lower boundaries of the ice. (b) Drill operation log indicating that a water saturated layer (talik) was observed at the bottom of the ice core (13.5–18 m depth). (c, d) TheGPR wiggle curve at the LF (60 MHz) signal indicating the massive ice core as free reflection zone.(e) Wenner array one-dimensional resistivity result showing the pingo ice and the overburden permafrost

(6.5 m).


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respond at all, even at conditions warmer than 1C. Oneof the difficulties of this method is using the Earth’smagnetic field to rotate the dipoles of water molecules.That is a very small intensity field compared, for example,with that used in medical imaging. Particularly at the timewe made the measurements in Caribou Creek, the Earth’smagnetic field changed considerably. Tuning to the Lamar frequency was therefore difficult.

[38] Some of the characteristics of Interior Alaskan per-mafrost include a fairly thick active layer, taliks, andheterogeneous structures. Subsurface investigations havedemonstrated a great deal of material contrast.

[39] This research provides results from the several geophysical investigations at the same location to determine thecapability of distinguishing distinct features such as perma-frost, massive ice and talik formations. Delineating massiveice from frozen ground is useful information for Martian

permafrost studies. A wide variety of pingo forms have beententatively identified on Mars [ Lucchitta, 1981; Cabrol et al., 2000; Rice et al., 2002; Tanaka et al., 2003; Sakimoto,2005; Soare et al., 2005]. An analogue study and geophys-ical investigations of terrestrial pingos would provide sup-

port in correct identification of pingos on Mars. IdentifyingMartian pingos would provide information on ground icedistribution, groundwater potentials, and climate change. Inthe reality of the planetary exploration, the free groundcontact investigation methods will be a very important factor in these investigations. Table 2 shows a summaryof an ideal system for Martian subsurface exploration. Bothradars are excellent candidates for implementation of thesemethods. However, a high brine layer may exist near

surface at higher latitudes on Mars. High brine content willattenuate radar signals. Airborne EM will be a secondarychoice of the free contact methods and capable of larger areacoverage. Recently developed prototype airborne TEM will

be an interesting technique for future planetary explorations.

5. Conclusion

[40] Massive ice and overburden permafrost can be dis-criminated by the resistivity sounding and GPR investiga-tions. The seismic refraction method is useful andreasonable, if the size of the massive ice is big enough(>50 m). The presence of the heterogeneous horizon of the

pingo presents a complex structure and complicates massiveice detection using seismic method. Our results indicate that seismic velocities of permafrost vary between 1900 m/s and4200 m/s and are dependent on ice content and temperature.The seismic velocity for massive ice was found to be

between 3200 m/s and 6000 m/s (Table 2).VLF methodssuffered from a lack of resolution of the near-surface effects.

[41] GPR can detectable the pingo’s massive ice structure by using a lower-frequency (<80 MHz) system. The bound-ary between the pingo ice and frozen soil is detectable;however, it is not sharp. One of the biggest benefits of theradar method is that it does not require ground contact andthe system is relatively lightweight.

[42] Resistivity profiling is one of the most powerful toolsfor detecting massive ice bodies, but its need for superior ground contact makes it useful in limited areas. Theweakness of the resistivity method is its ability to distin-guish between ice body and the bedrock underneath themassive ice. The resistivities of these materials are veryhigh compared with surface materials. One-dimensional DCresistivity sounding was relatively difficult to obtain rea-sonable subsurface structure information, because of thelacking of horizontal homogeneity. However, two-dimensional resistivity profiling works well to distinguishheterogeneous conditions., A recent inversion model pro-vides dramatically improved subsurface two- or three-dimensional resistivity profiles. Massive ice and water lenses shapes are clearly visualized using this method that includes elevation correction, but it is still difficult todistinguished bedrock and ice.

[43] In case of resistivity parameters, TEM and HEM can

be a better method for mapping ice distribution in variableareas because of free ground contact and much deeper sounding. However, these methods are poor for detailedstudy of heterogeneous conditions, especially near-surfacesounding (Table 2). It is possible that a large amount of water exists in a solid phase beneath the surface of Mars andthe geophysical methods presented here present may present useful tools for Martian exploration.

[44] Acknowledgments. This research was funded in part under theFWI CHAMP (NSF0229705) program and the International Arctic Re-search Center (IARC) through a Cooperative Agreement with the NationalScience Foundation. We would also like to thank WERC staff and GarySchikora for their field support, and L. Burn (DGGS, State of Alaska), and

Table 2. Summary of Geophysical Investigations for Pingosa

Parameter

IceValue at

Caribou Creek

IceValue at

Cripple Creek

IceValue at

Grenac Creek GroundContact

IceDetection

SpatialSampling,

m

One-dimensionalresistivity

resistivity, ohm m ND 18,000 5000 need good >100

Two-dimensionalresistivity

resistivity, ohm m 8600 10,000 4555 need excellent 5

Impulse-type radar dielectric constant ND 3.3 4.6 no excellent <5Swept-frequency

radar dielectric constant 8 ND 8 no excellent <5

Seismic p-wave velocity, m/s 2800 6000 2300 need good 60HEM resistivity, ohm m ND 650 ND no good 135TEM resistivity, ohm m ND 1944 ND no fair 25 – 50VHF resistivity, ohm m ND high noise ND no fair 75SNMR hydrogen spin, nV <10 nV ND ND no fair 100 – 200

a ND, no data.


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Paul Costello (Alaska North Star Borough) for their help with HEM dataand land use permission and an anonymous reviewer for their reviewcomments. The Fugro Airborne Surveys Dighem system was funded by theState of Alaska Department of Geological and Geophysical Surveys for mapping mine resources.

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11100 Johns Hopkins Road, Laurel, MD 20723, USA. (carl.leuschen@ jhuapl.edu)

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