Comparison of geophysical investigations for detection of massive ground ice (pingo ice)



[1] Six different geophysical investigations, (1) ground-penetrating radar, (2) DC resistivity 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 also compared from similar sites near one of the study pingos. The geophysical investigations were undertaken, along with core sampling and permafrost drilling, to enable measurement 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 profiling are useful for ice detection under heterogeneous conditions. However, the DC resistivity sounding investigation required high-quality ground contact and less area coverage. The active layer thickness and the homogeneous horizontal structure of the overburden material are important parameters influencing detection of massive ice in permafrost for most methods such as seismic, TEM, or surface NMR.

1. Introduction

[2] Many geophysical variables, such as dielectric constant, electrical conductivity, or P wave velocity, display strong contrasts between frozen and unfrozen soil phases. This presents a useful distinction to enable the study of permafrost and to characterize areas of permanently frozen ground [Scott et al., 1990; Dallimore and Davis, 1992; Moorman et al., 2003]. Thus, geophysical techniques are widely accepted in permafrost and periglacial process studies such as those of the active layer [Doolittle et al., 1990; Hinkel et al., 2001], rock glaciers [e.g., Vonder Mühll and Schmid, 1993; F. Lehmann et al., unpublished manuscript, 1999], palsas [Doolittle et al., 1992], and pingos [Kovacs and Morey, 1985; Ross et al., 2005].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, much groundwater will exist as brine or in the solid phase at the near 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 is an ice-cored mound that formed in response to sub-, or intra-permafrost artesian water pressure. Before water can reach the ground surface it freezes, forming a core of relatively pure and clear ice. In this paper, we examine three open system pingos near Fairbanks (Figure 1). These pingos have different internal and overburden structure and ice conditions. In cases of inhomogeneous ice distributions, geophysical investigations are often difficult to apply because of the invalid approximation of lateral homogeneity used in many modeling approaches. The goal of this study is to examine the feasibility of using different geophysical techniques to detect massive ground ice in Martian permafrost.

Figure 1.

General information and permafrost conditions for the study of pingos around Fairbanks Alaska.

2. Methods

2.1. Ground-Penetrating Radar

[4] Ground-Penetrating Radar (GPR) investigations have been conducted at three pingos around Fairbanks. We tested two types of radars to detect a pingo's massive ice body: a swept-frequency radar (5–120 MHz), and an impulse type radar (GSSI, SIR-2000 with the bistatic antenna model 3200MLF (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 to characterize 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 signal after receiving a trigger from the system controller, and the reciever digitized the radar response using a 50 kSPS 16-bit A/D converter. Two large (2.5 m length) bowtie antennas were constructed for initial testing using an aluminum insect screen that was easily rolled up for storage and transportation. 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 the center frequency of 40 or 80 MHz. The resultant stack data (0.5-m interval) was processed with the cross-correlation method [Yilmaz, 1987]. This method uses the Radan advanced geophysical function (velocity analysis) programs from 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 soil depends on the soil type, temperature, water content, porosity, and salinity. In general, the resistivity (ρ) values of frozen soil are 10–1000 times greater than those of unfrozen or brine soils [Harada and Yoshikawa, 1996]. DC resistivity sounding used four electrodes for measurement. A current (I) was delivered and received between the outer electrodes, and the resulting potential difference (V) was measured between the inner electrodes. For this array on the ground surface, an apparent resistivity (ρa) is calculated from

equation image

where “a” is the distance separating the electrodes. The inversion analysis was performed with changing values of resistivity and layer thickness by using the linear filter method [Das and Verma, 1980] for a one-dimensional investigation.

[8] For the acquisition of the two-dimensional apparent resistivity data we used multichannel, equally spaced electrodes 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 performed using the software package RES2DINV (Geotomo software), which performs smoothing and constrained inversion 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 (1500–4700 m/s) is much higher than that of the dry active layer (300–800 m/s) [e.g., Hunter, 1973].

[10] The seismic refraction measurements were conducted at three study pingos around Fairbanks. A three-channel seismograph, McSEIS-3 (Oyo Co., Ltd.) was used for these measurements. A 4-kg sledgehammer produced a seismic 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 a large pingo (Cripple Creek Pingo). The shotpoints were placed at 3-m intervals for the short survey lines and 5-m intervals for the long line. In the latter case, the shotpoints were placed at 2.5-m intervals near the receivers. Refractor depth and P wave velocities were calculated using the Intercept Time Method [Palmer, 1986]. Since seismic refraction technique presumes that the ground had a layered structure with higher P wave velocity in a deeper layer, the method 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 (WADI Abem) were evaluated for their ability to delineate and measure the thickness of shallow permafrost. The WADI VLF system is a two-component magnetic receiver operating in the frequency range of 15–30 kHz. The sources for these frequencies are powerful radio transmitters, similar to those used for submarine radio-communication. In Alaskan studies presented here, typically three different transmitters operating at different frequencies were used. When these transmitter signals propagate from the source positions to the position of the measurements site, they interact in a complex way with the two electrical conductors: the Earth at the bottom and the ionosphere at the top. However, owing to their small penetration (400 meters in normal granites) compared with the distance to the sources, we can regard the signals as plane waves propagating downward into the earth at any receiver site. The plane wave assumption enables a relatively simple and fast interpretation of the data using two-dimensional models. Two magnetic components (Hx, Hz) are measured with a single antenna consisting of two mutually orthogonal coil sensors. We used the software package RAMAG to process data from the WADI and 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 a weighted running average smoothing function.

2.5. Helicopter-Borne Electromagnetic (HEM)

[12] HEM investigations were conducted at Cripple Creek Pingo. In the early 1970s, several studies were conducted to map the location and thickness of permafrost zones with ground and airborne geophysics [Hoekstra et al., 1975]. While these studies confirm that electromagnetic geophysical methods (EM) could be used to detect permafrost, results indicated additional progress was needed to establish operational capabilities. Single space frequency domain ground EM systems (e.g., EM31) were not able to map the vertical distribution (thickness) of the permafrost [Hoekstra, 1978]. The airborne EM (Dighem I) used in the early 1970s was not very effective at mapping the permafrost. It had only one frequency, 918 Hz, which would have an upper resistivity limit of about 1000 ohm-m. In the area of the survey, very little ground data was collected, but the available data suggests that the permafrost distribution was very sparse. Current Dighem (HEM) systems have five frequencies, 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 the Cripple 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 Cripple Creek 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 small receiver coil after shutting off the direct current. Owing to ohmic losses in the resistive earth, the secondary field gradually collapses. As the resistivity of the background is increased, the collapse rate of the response increases. In a high-resistivity environment like permafrost, these quick responses and primary field effects (IP effects, magnetic relaxation, Rx box, direct coupling with primary field) create difficulties in estimating the resistivity structure through a standard central induction measurement. Permafrost surveys have been conducted using TEM with a large loop of more than 100 m [e.g., Rozenberg et al., 1985; Todd et al., 1991; Todd and Dallimore, 1998]. Here a measurement configuration outside the transmitter loop was utilized, to reduce the primary field effects, with a small loop for a high production rate.

[14] For our measurements, the transient data were recorded using a PROTEM 47 TEM system of Geonics Limited with a receiver coil having an effective area of 31.4 m2. Decay voltages were recorded in two overlapping time 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 amperes of loop current, established the precise timing by a reference cable 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 technique capable of direct detection of groundwater resources. The surface NMR method has similarities and differences with the NMR measurements commonly made in controlled laboratory experiments and in the medical field. In both experimental setups, the fact that a hydrogen molecule (proton) has a magnetic moment and an angular momentum is exploited [Hoekstra et al., 1998]. Hydrogen atoms are excited by pulses of alternating current at the proper frequency through a 100-m transmitting loop placed on the ground. The depth of investigation is a function of the product of the intensity of the current at the resonance frequency by the duration of the pulse, which gives the moment of the excitation pulse. Hydrogen atoms of water molecules are energized by pulses of alternative current at the proper frequency (Larmor frequency), transmitted into a loop laid on the ground. The magnetic field they produce in return 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 is turned off. The one-dimensional inversion provides estimates of the water content, the mean pore size, and the depth of each layer. The time constant of the relaxation field is related to the pore size. This allows a distinction to be made between pore free water and clay bound water.

3. Results

3.1. Caribou Creek Pingo

[16] Caribou-Poker Creeks Research Watershed (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 permafrost temperature is −0.7°C. The list of the study pingos and their physical properties is shown in Figure 1. The Caribou Creek pingo has a 60- to 80-m base diameter, and is more than 10 m tall. The massive ice core starts 7.35 m below the ground surface and extends down to 23.5 m. Several segregation and inclusive ice layers (a few centimeters to 30 cm thick) occur in the overburden silty permafrost. This pingo survived wildfire several times. A charcoal layer was observed 15 cm below the surface where the radiocarbon age is 820 ± 40 years B.P. (GX-28572). The active layer fluctuated between 1 m (present) and 2 m (post fire disturbance) during the Holocene. The increase in active layer 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 the lower horizontal contact of the massive ice. Figure 2a shows overburden ice content and pingo structure.

Figure 2.

Surface topography of the Caribou Creek pingo and its internal structure as verified by drilling 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) Massive ice core between 155 and 370 ns at the top of pingo.

[17] The radar responses at the top of the pingo are also shown in Figure 2e. A linear gain is applied to accentuate some of the deeper anomalies. Several reflections are present within these waveforms, and there seems to be a general trend of a decrease in center frequency and bandwidth from the deeper reflections. This is not unusual, as one would expect increased attenuation at higher frequencies. From the reflection profile, the ice body's possible locations can be estimated.

[18] Two of the reflections indicate the top and bottom of the pingo ice core (Figure 2e). The first is around 150 to 160 ns and has amplitude of about 5% of the surface response. The second is around 360 to 390 ns and has amplitude of about 4% of the surface response. Figure 3d shows impulse radar profile from the 80 MHZ GSSI SIR 2000 systems. Ice was observed as free of reflection around 155 and 370 ns.

Figure 3.

(a) Field results from Cripple Creek pingo. Two-dimensional resistivity profiling defines the massive 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 ns as the free reflection zone. (d, f) Pingos at the Cripple Creek displaying higher resistivity compared with the 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 ice core; 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.

[19] Seismic refraction measurements are employed between the south-facing slope and the top of the pingo. The direct observation with a thaw probe indicated that the active layer was thicker at the south facing slope (>3 m) and 2 m at the top of pingo. P wave velocity was configured with 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 represents the active layer, although the thickness is different from the direct observation. The thick organic layer on the top of the pingo may weaken the signals, which results in the contradiction. No signal from the massive ice body was observed because the survey line was slightly short (30 m) to enable investigation of the deeper structure. The weak signals through the thick organic layer prevented us from extending the 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 was obtained, which is a reasonable value. Resistivity studies permit better investigation of the detailed sub surface structure, 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's ice core contacts bedrock at 23.5 m.

3.2. Cripple Creek Pingo

[21] Cripple Creek is located about 10 km west of Fairbanks. 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 −0.95°C 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 limited segregation ice and inclusive ice, which are located only 6.3 – 6.5 m below the pingo top. Most of the overburden permafrost contains fluvial sediment. The massive pingo ice contains very pure ice and less bubble structure. Figure 3 shows 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 to the location was difficult and collecting data for the radar surveys over a traverse was practically impossible. As a result, data collection was restricted to a few discrete locations, often separated by a meter or more. GPR measurements were employed, using the GSSI impulse system with 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. The average speed of the wave propagation for the massive ice core was 0.165 m/ns and was estimated at 0.08 m/ns for the overburden material, using the velocity analysis technique. Thus we calculated the dielectric to be 3.3 for the ice core and 14 for overburden.

[24] The refraction seismic results were excellent at this pingo, although the velocity of the third layer is a little higher than typical values of ice rich permafrost (1500 to 4700 m/s). P wave velocity was configured for three layers at 400 m/s, 1900 m/s and 6000 m/s from top to bottom. The thickness of the first layer is 1 m at the top of the pingo and 2 m at the base of the south-facing slope. The number corresponds well with measurements of active layer thickness by thaw probe. The base of the second layer was located 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 the borehole wall depth between 4 and 6 m was 2500 m/s. The third layer was the massive ice body, 6000 m/s. The reasons for the much faster velocity are as follows: (1) very few cracks in the ice, (2) most of the ice was pure and did not contain soil layers holding unfrozen water, and (3) low air bubble content. In addition, the ends of the traveltime curves may include signals from underlying massive bedrock, which can result in the extraordinary high velocity for an ice layer, although the quality of the data is not good enough to calculate the fourth layer.

[25] A resistivity result was reasonable and ice was estimated at more than 7000 ohm-m, which is similar to results from the Caribou Creek pingo. Interestingly, 900-Hz HEM 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 results indicate 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 ice core, and the modeling process between Wenner and Schlumberger arrays (Table 1) showed minor horizontal differences. These differences were caused by heterogeneousness of the horizontal structure of the pingo.

Table 1. Comparison of One-Dimensional DC Resistivity Modeling Between Wenner and Schlumberger Arrays and TEMa
Resistivity, ohm-mDepth, mResistivity, ohm-mDepth, mResistivity, ohm-mDepth, m
  • a

    All measurements were performed on the same day around morning (0900–1300 local time on 9 August 2005). The Schlumberger array was less influenced 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.

Active layer500 542 ND 
Overburden permafrost45001.280001.7245 
Massive ice18,0001.018,0007.7194410.9
Permafrost or bedrock700031800030133096.8

[26] VLF surveys were conducted using WADI. Figure 3g shows 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 from VLF result. In-phase signals are reduced significantly at the top of the pingo (distance 70 m in Figure 3g). The phase change has not been demonstrated in other geophysical investigations. The signal level was lower (6–10) during the investigation.

3.3. Grenac Creek Pingo

[27] Grenac Creek is located about 3 km north of Fairbanks along the Farmer's Loop Road. One pingo is located at the toe of a fluvial fan. The thickness of the permafrost is 38 m, and annual mean upper permafrost temperature is −0.7°C.

[28] The study pingo is 50 m wide, 6 m high, and has a massive ice core extending from 6.5 m to 12.5 m depth from the top of the pingo. The water lenses located between 13.5 m and 18 m at the bottom of the pingo ice. Permafrost was found between 18 m and 38 m depth. The subpermafrost groundwater is under active artesian conditions here. The bedrock was located at a depth of about 48 m, which is 36 m below the pingo's ice body. Figure 4 shows 1D, 2D resistivity soundings, the drill log, 60-MHz GPR signals, and swept-frequency radar results at Grenac Creek pingo.

Figure 4.

Field results from Grenac Creek pingo. (a) Two-dimensional resistivity profiling showing reasonable 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) The GPR 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).

[29] There are two easily detectable reflections occurring at 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, and converting to range places these reflections at about 4–5 m for the top and 7–8 m for the bottom. GSSI impulse system GPR 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 the traveltime curves. The upper layer has a P wave velocity of 390 m/s and the lower layer of 2300 m/s. The calculated depth 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 the frost table (about 2 m). The result is problematic because the upper 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 the traveltime curves shows a large lateral gap at the northern part of the survey line, which possibly results from considerable 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 resistivity sounding (Figure 4e) and two-dimensional resistivity profiling (Figure 4a) for these investigations with the Wenner electrode configuration. Inversion results indicate a reasonable 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 wave velocity. Wet, thawing and frozen soils are favorable and detectable targets. The water lenses beneath the younger pingo ice core would be ideal targets for most of the techniques, with the exception of seismic refraction.

[33] During our investigations, almost all of the techniques had different results at the different pingos. The major interferences were caused by the heterogeneous structure of the horizontal overburden materials. Most of the geophysical investigations use a model of defined uniform horizontal layer structure, except GPR and resistivity profiling. However, the size of the pingo (e.g., volume of the massive ice 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 parts of the pingos are strongly affected by slope aspect and historical active layer fluctuations. The variation of the ice content of the overburden complicates the signal analysis for each method. Thus the Cripple Creek pingo had relatively 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 help to more accurately elucidate subsurface structures. Dielectric (GPR survey) may be affected by bubble orientation and density 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 permafrost (k = 4–8) [Arcone, 1984]. Also, the free reflections from 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 fire disturbance. The Alaskan boreal forest burns every 20–200 years [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 been destroyed by the post-fire disturbance. Most of the pingos that survived wildfire had deeper active layers or talik formation that did not reach to the massive ice. Severe fire removes most of the surface organics and surface thermal offset [Romanovsky and Osterkamp, 1995] changes, thawing permafrost and increasing the active layer moisture content. As a result, severe fire has caused 3 to 4 m of active layer thawing around the Fairbanks region [Yoshikawa et al., 2002]. A pingo may be destroyed if the depth of the massive ice is shallower than the maximum thawing depth. All of the study pingos observed displayed a sharp horizon of ice rich and dry permafrost layers, which indicated the previous maximum active layer. The discontinuous boundary also presents the possibility to negate massive ice core detection by the geophysical investigations. Seismic refraction investigations were strongly affected by the depth of ice rich permafrost. In a seismic survey, if a faster wave velocity layer exists, it is very difficult to quantify the structure of a lower layer.

[36] Recent climatic warming also affected the thermokarst process, especially on south facing slopes. Most of the resistivity surveys show south-facing slopes of the pingo had 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 the nonlinear relationship between the measured signal and the energizing pulse intensity. This means that doubling the pulse current does not mean doubling the signal; instead it 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 permafrost. However, the ice content of the frozen silt did not respond at all, even at conditions warmer than −1°C. One of the difficulties of this method is using the Earth's magnetic 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 time we made the measurements in Caribou Creek, the Earth's magnetic field changed considerably. Tuning to the Lamar frequency was therefore difficult.

[38] Some of the characteristics of Interior Alaskan permafrost include a fairly thick active layer, taliks, and heterogeneous structures. Subsurface investigations have demonstrated a great deal of material contrast.

[39] This research provides results from the several geophysical investigations at the same location to determine the capability of distinguishing distinct features such as permafrost, massive ice and talik formations. Delineating massive ice from frozen ground is useful information for Martian permafrost studies. A wide variety of pingo forms have been tentatively 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 geophysical investigations of terrestrial pingos would provide support in correct identification of pingos on Mars. Identifying Martian pingos would provide information on ground ice distribution, groundwater potentials, and climate change. In the reality of the planetary exploration, the free ground contact investigation methods will be a very important factor in these investigations. Table 2 shows a summary of an ideal system for Martian subsurface exploration. Both radars are excellent candidates for implementation of these methods. However, a high brine layer may exist near surface at higher latitudes on Mars. High brine content will attenuate radar signals. Airborne EM will be a secondary choice of the free contact methods and capable of larger area coverage. Recently developed prototype airborne TEM will be an interesting technique for future planetary explorations.

Table 2. Summary of Geophysical Investigations for Pingosa
 ParameterIce Value at Caribou CreekIce Value at Cripple CreekIce Value at Grenac CreekGround ContactIce DetectionSpatial Sampling, m
  • a

    ND, no data.

One-dimensional resistivityresistivity, ohm mND18,0005000needgood>100
Two-dimensional resistivityresistivity, ohm m860010,0004555needexcellent5
Impulse-type radardielectric constantND3.34.6noexcellent<5
Swept-frequency radardielectric constant8ND8noexcellent<5
Seismicp-wave velocity, m/s280060002300needgood60
HEMresistivity, ohm mND650NDnogood135
TEMresistivity, ohm mND1944NDnofair25–50
VHFresistivity, ohm mNDhigh noiseNDnofair75
SNMRhydrogen spin, nV<10 nVNDNDnofair100–200

5. Conclusion

[40] Massive ice and overburden permafrost can be discriminated by the resistivity sounding and GPR investigations. The seismic refraction method is useful and reasonable, 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 massive ice detection using seismic method. Our results indicate that seismic velocities of permafrost vary between 1900 m/s and 4200 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 methods suffered 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 boundary between the pingo ice and frozen soil is detectable; however, it is not sharp. One of the biggest benefits of the radar method is that it does not require ground contact and the system is relatively lightweight.

[42] Resistivity profiling is one of the most powerful tools for detecting massive ice bodies, but its need for superior ground contact makes it useful in limited areas. The weakness of the resistivity method is its ability to distinguish between ice body and the bedrock underneath the massive ice. The resistivities of these materials are very high compared with surface materials. One-dimensional DC resistivity sounding was relatively difficult to obtain reasonable subsurface structure information, because of the lacking of horizontal homogeneity. However, two-dimensional resistivity profiling works well to distinguish heterogeneous conditions., A recent inversion model provides 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 to distinguished bedrock and ice.

[43] In case of resistivity parameters, TEM and HEM can be a better method for mapping ice distribution in variable areas because of free ground contact and much deeper sounding. However, these methods are poor for detailed study of heterogeneous conditions, especially near-surface sounding (Table 2). It is possible that a large amount of water exists in a solid phase beneath the surface of Mars and the geophysical methods presented here present may present useful tools for Martian exploration.


[44] This research was funded in part under the FWI CHAMP (NSF0229705) program and the International Arctic Research Center (IARC) through a Cooperative Agreement with the National Science Foundation. We would also like to thank WERC staff and Gary Schikora for their field support, and L. Burn (DGGS, State of Alaska), and Paul Costello (Alaska North Star Borough) for their help with HEM data and land use permission and an anonymous reviewer for their review comments. The Fugro Airborne Surveys Dighem system was funded by the State of Alaska Department of Geological and Geophysical Surveys for mapping mine resources.