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Assessment of subsurface lithology in mountain environments using 2D resistivity imaging
Geomorphology 80 (2006) 32 – 44 www.elsevier.com/locate/geomorph
Assessment of subsurface lithology in mountain environments using 2D resistivity imaging Christof Kneisel Department of Physical Geography, University of Würzburg, Germany Received 13 December 2004; received in revised form 27 September 2005; accepted 27 September 2005 Available online 9 March 2006
Abstract The use of 2D resistivity imaging for the investigation of typical periglacial phenomena is shown on different case studies from a mid-latitude high-alpine and a high-latitude subarctic periglacial environment. Important parameters that need to be determined for geomorphological studies in mountain environments are the location and extent of permafrost and its characteristics, the spatial variability of the active layer as well as the internal structure of different periglacial landforms. The investigations on different typical periglacial features demonstrate that 2D electrical resistivity imaging is a suitable method to detect permafrost with an appropriate accuracy. Delineation of active-layer thickness is possible but depends on the contrast in resistivity between the active layer and the permafrost underneath. The detection of small-scale variability within the internal structure of some typical periglacial landforms can be limited because of the smoothing effect of the Wenner array and the high ground resistances and weak signal strength which do not allow the successful application of the Dipole–Dipole array. Hence, limitations of application of geoelectric methods can arise from the rugged terrain conditions in periglacial mountain environments since good electrical coupling between the electrodes and the ground is a prerequisite for geoelectrical surveys. Nevertheless, it is demonstrated that even on rough terrain conditions 2D electrical resistivity imaging could be used successfully to obtain reproducible results which are required to use changes in the apparent resistivity measurements to monitor changes in the subsurface resistivity distribution, and hence, permafrost aggradation and degradation. © 2006 Elsevier B.V. All rights reserved. Keywords: 2D electrical resistivity imaging; Mountain permafrost; Subarctic periglacial environment; Alpine periglacial environment; Permafrost monitoring
1. Introduction Geophysical methods are particularly suitable for geomorphological investigations since the knowledge of structure, layering and composition of the subsurface at different scales are key parameters for geomorphological problems. In periglacial environments the permafrost distribution can be a further important parameter influencing periglacial morphodynamics. Various geoE-mail address: [email protected]. 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.09.012
physical techniques have been used to study permafrost and characterise areas of permanently frozen ground for many years (see Scott et al., 1990 for a review concerning polar permafrost). Since geoelectrical methods are most suitable for investigating the subsurface with distinct contrasts in conductivity and resistivity, respectively, DC resistivity soundings constitute one of the traditional geophysical methods which have been applied in permafrost research to confirm and characterise mountain permafrost. During recent years, advances have been achieved in
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using the traditional methods but with more powerful, state-of-the-art instruments and modern data processing algorithms, which enable two-dimensional surveys and data processing (e.g. Ishikawa et al., 2001; Isaksen et al., 2002; Hauck and Vonder Mühll, 2003a; Kneisel and Hauck, 2003). Due to the large contrast between the electromagnetic properties of ice and water groundpenetrating radar (GPR) is also an effective tool for mapping the location and depth of ice bodies and thermal interfaces within subarctic permafrost terrain (e.g. Arcone et al., 1998; Hinkel et al., 2001; Moorman et al., 2003). Application of GPR in mountain permafrost has been rare and most of the few surveys were performed on rock glaciers (Berthling et al., 2000; Isaksen et al., 2000; Vonder Mühll et al., 2000). The increase of application of modern geophysical methods also in mountain geomorphology is due to the fact that geophysical methods are comparatively fast and non-destructive compared to conventional drilling and even more important, information of the whole survey area can be obtained rather than results only from the drilling sites. With the more effective data acquisition a geophysical mapping of the near-surface lithology is enabled even in heterogeneous mountain environments. This contribution aims to provide an insight into the application of 2D resistivity tomography for imaging periglacial conditions with a special view to the aims and demands of the SEDIFLUX network (Sedimentary source-to-sink-fluxes in cold environments) especially detection and characterisation of mountain permafrost and monitoring of the spatio-temporal evolution of mountain permafrost. A further intention of this work is to evaluate the possibility of an assessment of the internal structure of typical periglacial landforms – as a further key point to improve the process understanding – through the application of 2D resistivity tomography. Applicability, limitations and perspectives of 2D resistivity imaging for the investigation of periglacial permafrost affected environments are shown based on case studies from the mid-latitude high-alpine and highlatitude subarctic periglacial mountain environments in the Swiss Alps and northern Sweden. 2. Electrical resistivity imaging — data acquisition and processing The basic principle for the successful application of geoelectrical methods in geomorphology/Quaternary geology is based on the varying electrical conductivity of minerals, solid bedrock, sediments, air and water and consequently their varying electrical resistivity. The resistivity of rock for example depends on water
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saturation, chemical properties of pore water, structure of pore volume and temperature. The large range of resistivity values for most materials is due to varying water content. Resistivity values of permafrost or ground ice can vary over a wide range depending on the ice content, the temperature and the content of impurities. The dependance of resistivity on temperature is closely related to the amount of unfrozen water. Perennially frozen silt, sand, gravel or frozen debris with varying ice content show a wide range of resistivity values between 1 kΩ m to several hundred kΩ m (e.g. Haeberli and Vonder Mühll, 1996). A differentiation between sedimentary ice and congelation ice is enabled according to the genetic/petrographic classification after Shumskii (1964). Congelation ice (interstitial and segregation ice), which is the predominant form of ice in the ground, shows much lower resistivities than sedimentary ice which forms by a firnification process. Characteristic values for sedimentary ice from temperate alpine firn zones are several MΩ m to more than 100 MΩ m and for congelation ice 10 kΩ m to a few MΩ m (cf. Table 1). Resistivity measurements are made by injecting a direct current into the ground via two current electrodes. The resulting voltage difference is measured at two potential electrodes. Resistivity surveys give an image of the subsurface resistivity distribution. Knowing the resistivities of different material types, it is possible to convert the resistivity image into an image of the subsurface consisting of different materials. The conventional method of plotting the results for the interpretation is the so called pseudosection, which gives an approximate image of the subsurface resistivity distribution. The shape of the contours depend on the array geometry and the subsurface resistivity. For the case studies shown in this contribution the most commonly used arrays for Table 1 Range of resistivities for different materials (compiled mainly after Telford et al., 1990; Reynolds, 1997) Material
Range of resistivity [Ω m]
Clay Sand Gravel Granite Gneiss Schist Ground water Frozen sediments a / ground ice a / mountain permafrost a Glacier ice (temperate) Air
1–100 100–5 × 103 100–4 × 102 5 × 103–106 100–103 100–104 10–300 1 × 103–106
a
Different sources, cf. Kneisel (2003b).
106–108 Infinity
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2D resistivity surveys the so called Wenner, Wenner– Schlumberger and Dipole–Dipole configurations were used. The Wenner configuration has a moderate investigation depth and has a good resolution for horizontal structures with vertical changes of resistivity. Since the total number of measurements required is less than for other configurations the time to complete a survey is comparatively short, however, also the obtained information of the subsurface is less than that derived from other arrays. The Dipole–Dipole array comprises two dipoles formed by the current electrodes on one side and the potential electrodes on the other side. This array type has a better horizontal, but weaker vertical resolution and a shallower investigation depth than the Wenner array. Furthermore, the largest number of readings of all three presented configurations are required to complete a survey. The Wenner–Schlumberger array is a combination of the Wenner array and the Schlumberger array (commonly used for vertical resistivity soundings) with constant potential electrode spacing but logarithmically increased current electrode spacings leading to a better depth resolution compared to the Wenner configuration. The number of measurements is more than for a Wenner survey but less than for a Double–Dipole array. Choice of the appropriate electrode configuration for a field survey has to be determined from case to case. Special characteristics of the different array geometries should be considered, above all the investigation depth and the
sensitivity of the array to vertical and horizontal changes in the subsurface resistivity distribution. The Dipole– Dipole array provides superior lateral resolution and will often be the first choice for geomorphological applications with expected lateral heterogeneity. However, on rough surface conditions with high ground resistance and weak signal strength which frequently occur in periglacial terrain, the application of Dipole–Dipole surveys might be critical because reception voltage values drop quickly with the increasing of the spacing between the injection and reception dipoles. In these cases it might be necessary to use the Wenner configuration because it is less sensitive to weak signal strength. Since the Wenner–Schlumberger configuration is a combination of Wenner and Schlumberger array types, it is useful for horizontal and vertical geomorphological structures and can be the best choice as a compromise between the Wenner and the Dipole–Dipole array. Application of two different array geometries at the same survey site such as Wenner and Dipole–Dipole enables a more accurate and reliable interpretation of the subsurface. Further details on different array geometries are given for instance in Telford et al. (1990) and Reynolds (1997). Applicability of different arrays to various geomorphological studies is described in Kneisel (2003a). For the two-dimensional surveys a SYSCAL Junior Switch system was applied. Application of geoelectrical surveys in periglacial environments often implies one major problem which is the coupling between the
Fig. 1. Resistivity-meter, multi-electrode cable and resistivity survey layout across a solifluction terrace with sorted stone stripes.
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electrodes to the sometimes heterogeneous and rocky ground surface. Because of high contact resistances of the coarse blocky surface material in some of the presented case studies, sponges soaked in water were used at each 40–50 cm long stainless steel electrode to establish sufficient electrical contact to the ground. Sometimes the application of sponges soaked in salt water is recommended to overcome this problem, however experience has shown that it is sufficient to pour water over the sponges and the vicinity of the electrodes. The data processing of the measured sets of apparent resistivities was performed using the software package RES2DINV. This inversion software tries to reduce the difference between the calculated and measured apparent resistivity values by adjusting the resistivity of the model blocks. A measure of this difference is given by the rootmean-square error (RMS). However, the best model from a geomorphological or geological perspective might not be the one with the lowest possible RMS. Thus, it is essential to perform the interpretation with
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consideration of the local geomorphological setting. This enables unrealistic images of the subsurface structure to be excluded. A further advantage of the 2D inversion software is the possibility to incorporate topographic corrections into the inversion algorithm which is an essential point for studies in alpine periglacial environments with often complex and heterogeneous topography. 3. Applications and case studies 3.1. Case studies from a high-latitude subarctic periglacial environment In a subarctic periglacial environment in northern Sweden (Pallenvagge, Abisko mountains, 68°10′N, 18°45′E), the spatial distribution of permafrost and its characteristics related to typical periglacial landforms was investigated using different methods. The mean annual air temperature in the periglacial environment at
Fig. 2. Resistivity tomogram of a Wenner survey over a solifluction terrace (a) and across sorted stone stripes (b).
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1200 m a.s.l. can be inferred from an adjacent mountain field station (− 2.3 °C at Latnajaure 981m a.s.l., 68°20′ N, 18°30′E) to be about − 3.5°C. Bedrock consists of amphibolite, gneiss and mica schists. In order to evaluate the permafrost distribution in the investigation area on a larger scale, measurements of the bottom temperature of the snow cover were carried out (socalled BTS-method introduced by Haeberli, 1973). For the characterisation of the subsurface lithology 2D resistivity surveys were performed at various places of which the surveys on an ice-cored moraine, on a solifluction/gelifluction terrace and on sorted polygons are presented in this contribution. Fig. 1 shows the survey line across a solifluction terrace at a north exposed slope. Because of the marked terrace step topography was included. The image of resistivities against depth shows a general decrease in resistivity with depth (Fig. 2). A large high-resistive anomaly is detected extending over the whole survey area indicating a permafrost body. Air-filled voids or cavities which could cause similarly high resistivities can be excluded at this site. From the range of resistivities, it can be assumed that the subsurface consists of ice-rich layers and that the ice content is decreasing with depth. The active layer appears to be comparatively thin which is probably due to the fact that the surface has become snow-free only recently. To obtain a reliable interpretation of this survey site a second profile at the foot of the solifluction terrace was
performed at right angles to the profile displayed in Fig. 2; intersecting point is marked with an arrow. The results show consistency, leading to the geomorphological interpretation that this slope is underlain by permafrost and that the periglacial morphodynamics are clearly related to the presence of active permafrost forming the terrace and the sorted stone stripes through the process of intensive gelifluction. Depth to bedrock is difficult to delineate from the results of this survey except in the right part of the cross-profile in Fig. 2b where bedrock comes closer to the surface (between horizontal distances 120 and 160). In the upper parts of the longitudinal profile in Fig. 2a even the bottom resistivities are still values indicating permafrost, the distinct resistivity decrease with depth is interpreted as the lower boundary of the ice-rich layer. However, in case of large resistivity gradients the reliability of the inversion result below a high-resistive layer can be significantly diminished (Marescot et al., 2003). In the same investigation area a survey on a terminal moraine complex was performed in order to figure out whether this moraine contains massive ice (Fig. 3). The survey line started in front of the moraine crossing a perennial snow patch at the foot of the moraine complex. Because of high contact resistances of the coarse blocky surface material, sponges soaked in water were used at each electrode on the moraine to establish sufficient electrical contact to the ground. Massive ground ice of considerable depth could be confirmed in
Fig. 3. Terminal moraine complex with the location of the survey line, base camp in the foreground.
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the moraine which is visible in the high resistivities in the left and central parts of the tomogram (Fig. 4). The range of resistivities with values of several MΩ m allows the assumption that the massive ground ice consists mainly of sedimentary ice from a firnification process (typical glacier ice). Between horizontal distances 120 and 135 the perennial snow patch causes near-surface high-resistive values. Similar profiles have been measured also on moraines in the Swiss Alps (cf. Hauck and Vonder Mühll, 2003b; Kneisel, 2004; Reynard et al., 2003). Ishikawa et al. (2001) used the significantly different distribution of resistivity values in the subsurface of two rock glaciers obtained through electrical resistivity imaging to infer genetic differences between glacierderived (originated from glacial dead ice) and talus derived rock glaciers (ice-cemented rock glacier) in Nepal. However, the interpretation of maximum resistivities has to be done carefully. Marescot et al. (2003) showed that the resistivity within the high-resistive zones cannot be determined accurately. For that reason it can be problematic to compare resistivity values from different field sites concerning ice origin or ice content. High-resistivity values may be caused by a high ice content at one site and a glacial origin of the ice at another site. However, the comparison of resistivity surveys which are performed in the same investigation area and in the same period under similar measurement conditions and using the same inversion parameters as in the case studies presented in Figs. 2 and 4 are possible to the authors opinion and experience from numerous
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measurements on surface ice and ground ice occurrences. Nevertheless, the interpretation of resistivity tomograms concerning ice content must be done carefully. Further particularities of inversion and interpretation of 2D resistivity surveys on mountain permafrost are given in Hauck and Vonder Mühll (2003b) and Marescot et al. (2003). A further point which has to be considered especially in periglacial mountain environments is the choice of the appropriate array configuration. Different array geometries of the same profile can lead to significant differences in the model results. In order to evaluate the sensitivity of the survey results to the chosen electrode configuration, both Wenner and Dipole–Dipole arrays were tested and compared for a resistivity imaging on patterned ground with sorted polygons using 2.5m spacing of the electrodes (Fig. 5a, b). Since a large heterogeneity could be expected in the subsurface underneath the polygon field, the Dipole–Dipole electrode geometry was additionally applied at the survey site. Compared to the examples in Fig. 2, the resistivity values are much lower but still in the range of permafrost and frozen sediments, respectively (cf. Table 1). The near-surface lithology is interpreted as unconsolidated sediments which consist of frozen material of different grain-size in which the fine-grained parts are probably composed of segregation ice layers. The near-surface high-resistive anomalies between horizontal distances 20 and 30 which appear in both tomograms are most likely due to poor electrode
Fig. 4. Resistivity tomogram of a Wenner survey on a terminal moraine complex. Dashed line indicates inferred depth of active layer.
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Fig. 5. Resistivity tomogram of a Wenner (a) and a Dipole–Dipole survey (b) on patterned ground with sorted polygons. Note the different depth axes.
coupling. Active-layer thickness seems to be variable over the survey area, but cannot be delineated clearly because resistivity gradients are small in most parts of the profile. A comparison of the results of the Wenner array with the results of the Double–Dipole array indeed shows that the latter provide superior horizontal resolution, but a smaller penetration depth as for the
Wenner array. In the example in Fig. 5 the results of the Wenner array do not seem to be sufficient for the overall characterization of the near-surface lithology at this site. The smoothing effect of this array type prohibits the detection of small-scale variability. The results of the Dipole–Dipole array provide more information although the data of this array leads to noisier models, which is
Fig. 6. Rock glacier Muragl with the location of the survey lines.
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expressed in the higher RMS error. For a detailed interpretation of the subsurface the results of both surveys are insufficient. Whether this polygon field is underlain by active permafrost cannot be inferred from these tomograms alone. A combination of several overlapping surveys using different electrode spacings such as 5, 2.5 and 1m to get reasonable vertical and horizontal resolution would be necessary to be able to characterise the subsurface below this polygon field in more detail. Consequently, a large number of measurements would be required. Furthermore, time-lapse resistivity surveys would be necessary to monitor timedependent resistivity changes and hence, the evolution of the active layer. Mapping the extent of thawed zones and detection of the variability of their thickness and shape changes over time is an important component of
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monitoring periglacial processes in order to improve the process understanding with a view to the potential effects of climate change in permafrost affected cold environments. 3.2. Case studies from a mid-latitude high-alpine periglacial environment In an alpine periglacial environment in the Upper Engadine, eastern Swiss Alps investigations on different aspects of mountain permafrost distribution and characteristics have been performed over recent years with the main focus on permafrost in recently deglaciated forefields (Kneisel, 2003b, 2004). In this contribution surveys from the Muragl glacier forefield and the adjacent Muragl rock glacier are presented. The
Fig. 7. Resistivity tomogram of two Wenner surveys on rock glacier Muragl, steep rock glacier front (top panel) and cross-profile (bottom panel). Dashed line indicates inferred depth of active layer.
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investigated glacier forefield extends from 2600 to 2950 m a.s.l.; for this altitudinal range a mean annual air temperature of − 2 to −4 °C can be inferred from two nearby mountain climate stations at 2250 m a.s.l. (− 0.3°C) and 3300 m a.s.l. (− 6°C) and own measurements of the mean annual ground surface temperature at different sites within the glacier forefield ranging from 0 and − 1.35 °C (Kneisel, 2003b). The bedrock consists mainly of gneiss and mica schists. The Muragl rock glacier was recently intensively investigated using seismic refraction tomography and georadar cross-hole tomography indicating zones of degrading permafrost (Maurer et al., 2003). Fig. 6 shows rock glacier Muragl and the survey lines from the alpine meadows up to the rock glacier across the steep front as well as a cross-profile. These kind of profiles belong to the most challenging surveys in alpine permafrost since the steep front consists of loose debris and much care has to be taken for rockfalls which can hit the multi-electrode cable as happened during the setup of this survey line. Both Wenner profiles, also the crossprofile are of comparatively good quality, again sponges were used at each electrode to establish sufficient contact to the ground (Fig. 7). The cross-profile supports the findings from the seismic refraction tomography and the georadar cross-borehole tomography, indicating a larger area without ground ice (Maurer et al., 2003). A further motivation for performing these two surveys was to find out whether this method could be successfully used to confirm general concepts about dynamic effects and resistivity structure within active rock glaciers, namely the assumed flow-induced thickening of the permafrost and active layer in areas of longitudinal compression towards the front and the lowering of the
resistivities towards the front (cf. Haeberli and Vonder Mühll, 1996). The results obtained on this extremely rugged terrain are promising, showing variable activelayer thicknesses in the area of the furrow and ridges (between horizontal distances 40 and 100 in the top panel) and different resistivities for the longitudinal and the cross-profile. Delineation of active-layer thickness through resistivity surveys alone can be difficult, especially if blocky surface substratum which leads to high ground resistances is underlain by ice-rich permafrost. Air-filled voids or cavities could cause equally high resistivities as massive ground ice. Especially in such cases, knowledge of the topographical and geomorphological setting as well as measurement and data analysing experience is essential for the interpretation. During the survey on this rock glacier, the sound of flowing water at the permafrost table gave a further hint to the depth of the active layer. Thus, a semiquantitative interpretation concerning the variability of the active-layer thickness over the survey area is enabled. Using a greater number of 2D geoelectric surveys on several rock glaciers within a comparative study, it appears to be possible to support the above mentioned general concepts about dynamic effects and resistivity structure within active rock glaciers and to achieve new findings towards the understanding of rock glacier internal structure and dynamics. Fig. 8 presents survey results from an area where glacial flutes have developed in the Muragl glacier forefield (cf. Fig. 9). In the upper parts of the survey profile down to the middle parts, a resistivity anomaly is detected which can be interpreted as a shallow permafrost occurrence as the resistivity values are
Fig. 8. Resistivity tomogram of a Wenner survey within the recently deglaciated glacier forefield Muragl.
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Fig. 9. Glacier forefield Muragl with the location of the survey lines.
decreasing below the roughly 15 m thick resistive anomaly. With the special conditions in glacier forefields, permafrost occurrences can be assumed to be shallow, “warm” and thus, rich in unfrozen water, which can explain the comparatively low apparent resistivity values. The occurrence of fluted moraines and push moraines (as in the Muragl glacier forefield for instance) provides geomorphological evidence for a complex thermal regime of the former glacier with cold marginal parts frozen to the bed and warm-based ice in more central parts where fluted moraines could develop. Perennially frozen ground encountered in areas where glacial flutes occur is interpreted as a permafrost aggradation after the disappearance of the temperate surface ice (Kneisel, 2003b). High gradients in the subsurface resistivity distribution usually indicate interfaces between different layers. Since the coupling of the electrodes at this survey site was comparatively good and the ground resistance on this fine to mediumgrained glacial till was low, the variable depth of the marked interface as inferred from the resistivity survey can be related to variable active-layer thickness at this site. The thick active layer between stations 60 and 90 could point to a melting/degrading permafrost body. Monitoring of aggrading and degrading permafrost is work which is currently in progress; two survey sites have been established in the Muragl glacier forefield on different surface substratum, one on fine to medium
grained glacial till and one on a blocky moraine. In order to study the changes of subsurface resistivity with time, 2D resistivity surveys are repeated over the same survey line at different times, therefore a fixed electrode array was installed with 2 m electrode spacing. Hauck (2002) has shown that frozen ground monitoring can successfully be realized using resistivity tomography, however, the monitoring site was installed on fine to medium grained weathered sediments overlying bedrock. For the monitoring of time-dependent processes (so-called timelapse experiments) changes in the subsurface resistivity are estimated by using changes in the apparent resistivity measurements. This requires accurate and reproducible results even on rough terrain. Thus, first test measurements on the blocky moraine in the Muragl glacier forefield were performed using the Wenner and Wenner–Schlumberger arrays (cf. Fig. 10). These configurations are well suited to detect horizontally layered structures; vertical structures are more accurately detectable by using the Dipole–Dipole array. However, the latter is often difficult to apply in periglacial mountain terrain with rough surface conditions and high ground resistance because of relatively low signal-tonoise ratios. The three measurements at the beginning of August, end of August and middle of September show consistency in the results. In consideration of the rugged topography the inversion results are of good quality and
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Fig. 10. Resistivity tomograms of a Wenner–Schlumberger survey on a moraine consisting partly of coarse debris. Upper panel represents the survey on August 4th, middle panel the survey on August 27th, bottom panel the percentage change in resistivity. Dashed line indicates inferred depth of active layer.
reproducible. The data were first inverted independently. This approach has given only limited information on subsurface resistivity changes with time since the inversion routine tries to minimize the difference between the measured and calculated apparent resistivity values. Thus, differences in the resistivity values are not necessarily related to actual changes in the subsurface resistivity distribution. To overcome this problem a joint inversion technique (time-lapse inversion method within RES2DINV) was applied. Hereby, the model obtained from the inversion of the first measurement is used as a reference model to constrain the inversion of the later time-lapse measurements. Fig. 10 displays the data set of the surveys on August 4th (upper panel) and on August 27th 2004 (middle panel) as well as the percentage change in resistivity (bottom panel). In the upper parts of the tomogram down to 3 m no significant changes in resistivity are visible, except an area between horizontal distances 28 and 30 and more prominent between 52 and 64, where there is a decrease in resistivities recognizable of up to 20% and 25%. In deeper parts below 3 m, a distinct decrease of resistivities is
visible more or less over the whole survey area and especially between horizontal distances 26–44 and 52– 64. The decrease of resistivities can most likely be related to a thickening of the active layer due to melting of permafrost at the lower boundary of the active layer towards the underlying ice-rich permafrost. A significant influence of temperature effects on the decrease of the resistivity values or heavy rainfall within this time span can be disregarded. The variability over the survey area is probably due to the variability of the surface substratum which consists of medium grain sizes from horizontal distances 1 to 24 and coarser grain sizes between 24 and 48 and is even blocky between 48 and 68. The apparently thin active layer in the right part of the tomogram (cf. upper panel and middle panel in Fig. 10) is due to the blocky surface substratum which in turn leads to high ground resistances. The high percentage change in the resistivity values at this side of the survey line leads to the assumption that there is a remarkable thickening of the active layer between the two measurement dates whereas the minor changes on the left side of the
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survey line indicate that active-layer thickness was not increased within this period. The resistivity changes below the high-resistive anomaly are not included in the interpretation because these data are not well constrained as the sensitivity of the inversion results to the input data is low for high-resistive model regions (cf. Marescot et al., 2003 see above; Hauck et al., 2003). These test measurements show that the spatiotemporal evolution of the active-layer thickness can be monitored also on rugged terrain with rough surface conditions using repeated measurements of the subsurface resistivity distribution. Future measurements will be carried out over a longer time span and will be combined with measurements of the near-surface ground temperatures. 4. Conclusions and perspectives The illustrated case studies show that electrical resistivity imaging is a valuable method for studies in periglacial mountain environments. Limitations of application can arise from extremely rugged terrain conditions which can occur in alpine periglacial terrain. Good data quality is largely dependant on a well designed survey which includes good coupling of the electrodes to the ground. Insufficient coupling of the electrodes and great heterogeneity of the surface terrain can lead to bad data quality resulting in noisy model interpretations of the subsurface. In spite of these limitations two-dimensional resistivity tomography is considered as the most multifunctional geophysical method and could be first choice for geomorphologists working in periglacial mountain environments if only one single method can be applied. Ideally, two-dimensional electrical resistivity surveys should be used in conjunction with for instance refraction seismics or GPR as they can provide complementary information about the subsurface. However, the application and interpretation of GPR can be difficult in mountain terrain, because the heterogeneous subsurface can cause scattering of the electromagnetic waves. A valuable aspect of modern geophysical techniques and their application in geomorphology is the ability to image the subsurface. Resistivity surveys give an approximate image of the subsurface resistivity distribution. For a reliable interpretation of that image a geomorphological background and some experience is required. The case studies presented in this paper illustrate the application of 2D resistivity imaging for a number of permafrost related problems. The detection of permafrost and mapping of the horizontal extent of permafrost
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or massive ground ice bodies is enabled with an appropriate accuracy. Delineation of the permafrost base at least for shallow permafrost occurrences and active-layer thickness is possible in a semi-quantitative way in many cases. On coarse scree slopes where airfilled cavities occur which can cause high-resistive anomalies, interpretation of electrical resistivity surveys can be difficult in terms of permafrost delineation. In order to get unambiguous results seismic refraction tomography should be used additionally also in cases where a more accurate determination of the sediment thickness is required. An assessment of the ice origin and ice content can be possible, but has to be done carefully. For a reliable interpretation of the survey results, knowledge of the geomorphological and geological setting is essential and limitations of data inversion have to be considered (e.g. reliability of the inversion results below a very-high resistive massive ice body, assessment of ice content in the subsurface). Application of two different array geometries at the same survey site such as Wenner and Dipole–Dipole enables a more accurate interpretation of the subsurface. However, on rough surface conditions with high ground resistance and weak signal strength which frequently occur in periglacial mountain terrain, the application of Dipole–Dipole surveys might be critical. Hence, the detection of the small-scale variability of the internal structure of some typical periglacial landforms can be limited (e.g. case study on sorted polygons). On the other hand and for larger periglacial landforms such as rock glaciers a future perspective of two-dimensional resistivity imaging is the possibility of performing a detailed mapping of the subsurface resistivity distribution using overlapping profiles to obtain reliable results even on rough surface conditions with high ground resistances. In the future, 3D resistivity surveys can even enhance the potential of this method. To date, survey speed is too slow and geophysical field work in remote high mountain areas is also dependent on battery capacity and time, and good weather conditions have to be used efficiently. Currently, the most promising perspective is the approach of monitoring subsurface resistivity changes through repeated measurements of the electrical resistivity. The described case study shows that even on rugged terrain with rough surface conditions 2D electrical resistivity imaging could be used to monitor permafrost aggradation and degradation. Such a geoelectrical monitoring in combination with continuous measurements of the nearsurface ground temperatures could comprise two important components of a low cost approach to monitor effects of climate change in different permafrost affected cold environments.
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Acknowledgements The author acknowledges a grant for the research in Northern Sweden funded by the University of Würzburg (“Jubiläumsstiftung zum 400-jährigen Bestehen der Universität Würzburg 2003–2004). Furthermore, the author would like to thank M. Friedlein for support during fieldwork and O. Sass and an anonymous reviewer for critical comments on an earlier version of this manuscript. References Arcone, S., Lawson, D., Delaney, A., Strasser, J.C., Strasser, J.D., 1998. Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost. Geophysics 63, 1573–1584. Berthling, I., Etzelmüller, B., Isaksen, K., Sollid, J.L., 2000. Rock glaciers on Prins Karls Forland. II: GPR soundings and the development of internal structures. Permafrost and Periglacial Processes 11 (4), 357–369. Haeberli, W., 1973. Die Basis-Temperatur der winterlichen Schneedecke als möglicher Indikator für die Verbreitung von Permafrost. Zeitschrift für Gletscherkunde und Glazialgeologie 9, 221–227. Haeberli, W., Vonder Mühll, D., 1996. On the characteristics and possible origins of ice in rock glacier permafrost. Zeitschrift für Geomorphologie N.F. Suppl. 104, 43–57. Hauck, C., 2002. Frozen ground monitoring using DC resistivity tomography. Geophysical Research Letters 21, 2016, doi:10.1029/ 2002GL014995. Hauck, C., Vonder Mühll, D., 2003a. Evaluation of geophysical techniques for application in mountain permafrost studies. In: Schrott, L., Hoerdt, A., Dikau, R. (Eds.), Geophysical Methods in Geomorphology. Zeitschrift für Geomorphologie, Suppl., vol. 132, pp. 161–190. Hauck, C., Vonder Mühll, D., 2003b. Inversion and interpretation of two-dimensional geoelectrical measurements for detecting permafrost in mountainous regions. Permafrost and Periglacial Processes 14, 305–318. Hauck, C., Vonder Mühll, D., Maurer, H., 2003. Using DC resistivity tomography to detect and characterise mountain permafrost. Geophysical Prospecting 51, 273–284. Hinkel, K.M., Doolittle, J.A., Bockheim, J.G., Nelson, F.E., Paetzold, R., Kimble, J.M., Travies, R., 2001. Detection of subsurface permafrost features with ground-penetrating radar, Barrow, Alaska. Permafrost and Periglacial Processes 12, 179–190. Isaksen, K., Oedegaard, R.S., Eiken, T., Sollid, J.L., 2000. Composition, flow and development of two tongue-shaped rock glaciers in the permafrost of Svalbard. Permafrost Periglacial Processes 11, 241–257. Isaksen, K., Hauck, C., Gudevang, E., Oedegaard, R.S., Sollid, J.L., 2002. Mountain permafrost distribution in Dovrefjell and Jotunhei-
men, southern Norway, based on BTS measurements and 2D tomography data. Norsk Geografisk Tidsskrift 56, 122–136. Ishikawa, M., Watanabe, T., Nakamura, N., 2001. Genetic difference or rock glaciers and the discontinuous mountain permafrost zone in Kanchanjunga Himal, eastern Nepal. Permafrost and Periglacial Processes 12, 243–253. Kneisel, C., 2003a. Electrical resistivity tomography as a tool for geomorphological investigations — some case studies. In: Schrott, L., Hoerdt, A., Dikau, R. (Eds.), Geophysical Methods in Geomorphology. Zeitschrift für Geomorphologie, Suppl., vol. 132, pp. 37–49. Kneisel, C., 2003b. Permafrost in recently deglaciated glacier forefields — measurements and observations in the eastern Swiss Alps and northern Sweden. Zeitschrift für Geomorphologie 47, 289–305. Kneisel, C., 2004. New insights into mountain permafrost occurrence and characteristics in glacier forefields at high altitude through the application of 2D resistivity imaging. Permafrost and Periglacial Processes 15, 221–227. Kneisel, C., Hauck, C., 2003. Multi-method geophysical investigation of a sporadic permafrost occurrence. In: Schrott, L., Hoerdt, A., Dikau, R. (Eds.), Geophysical Methods in Geomorphology. Zeitschrift für Geomorphologie, Suppl., vol. 132, pp. 145–159. Maurer, H.R., Springman, S.M., Arenson, L.U., Musil, M., Vonder Mühll, D., 2003. Characterisation of potentially unstable mountain permafrost — a multidisciplinary approach. Proceedings of the 8th International Conference on Permafrost, Zurich, Switzerland, pp. 741–746. Marescot, L., Loke, M.H., Chappellier, D., Delaloye, R., Lambiel, C., Reynard, E., 2003. Assessing reliability of 2D resistivity imaging in mountain permafrost studies using the depth of investigation index method. Near Surface Geophysics 1, 57–67. Moorman, B.J., Robinson, S.D., Burgess, M., 2003. Imaging periglacial conditions with ground-penetrating radar. Permafrost and Periglacial Processes 14, 319–329. Reynard, E., Lambiel, C., Delaloye, R., Devaud, G., Baron, L., Chappellier, D., Marescot, L., Monnet, R., 2003. Glacier/ permafrost relationships in forefields of small glaciers (Swiss Alps). 8th International Conference on Permafrost, Zurich, Switzerland. Proceedings, pp. 947–952. Reynolds, J.M., 1997. An Introduction to Applied and Environmental Geophysics. Wiley, Chichester. Scott, W., Sellmann, P., Hunter, J., 1990. Geophysics in the study of permafrost. In: Ward, S. (Ed.), Geotechnical and Environmental Geophysics. Soc. Expl. Geophys.,Tulsa, pp. 355–384. Shumskii, P.A., 1964. Principles of Structural Glaciology. Dover, New York. Telford, W.M., Geldart, L.P., Sheriff, R.E., 1990. Applied Geophysics, 2nd ed. Cambridge University Press, Cambridge. Vonder Mühll, D., Hauck, C., Lehmann, F., 2000. Verification of geophysical models in Alpine permafrost using borehole information. Annals of Glaciology 31, 300–306.
Assessment of subsurface lithology in mountain environments using 2D resistivity imaging Christof Kneisel Department of Physical Geography, University of Würzburg, Germany Received 13 December 2004; received in revised form 27 September 2005; accepted 27 September 2005 Available online 9 March 2006
Abstract The use of 2D resistivity imaging for the investigation of typical periglacial phenomena is shown on different case studies from a mid-latitude high-alpine and a high-latitude subarctic periglacial environment. Important parameters that need to be determined for geomorphological studies in mountain environments are the location and extent of permafrost and its characteristics, the spatial variability of the active layer as well as the internal structure of different periglacial landforms. The investigations on different typical periglacial features demonstrate that 2D electrical resistivity imaging is a suitable method to detect permafrost with an appropriate accuracy. Delineation of active-layer thickness is possible but depends on the contrast in resistivity between the active layer and the permafrost underneath. The detection of small-scale variability within the internal structure of some typical periglacial landforms can be limited because of the smoothing effect of the Wenner array and the high ground resistances and weak signal strength which do not allow the successful application of the Dipole–Dipole array. Hence, limitations of application of geoelectric methods can arise from the rugged terrain conditions in periglacial mountain environments since good electrical coupling between the electrodes and the ground is a prerequisite for geoelectrical surveys. Nevertheless, it is demonstrated that even on rough terrain conditions 2D electrical resistivity imaging could be used successfully to obtain reproducible results which are required to use changes in the apparent resistivity measurements to monitor changes in the subsurface resistivity distribution, and hence, permafrost aggradation and degradation. © 2006 Elsevier B.V. All rights reserved. Keywords: 2D electrical resistivity imaging; Mountain permafrost; Subarctic periglacial environment; Alpine periglacial environment; Permafrost monitoring
1. Introduction Geophysical methods are particularly suitable for geomorphological investigations since the knowledge of structure, layering and composition of the subsurface at different scales are key parameters for geomorphological problems. In periglacial environments the permafrost distribution can be a further important parameter influencing periglacial morphodynamics. Various geoE-mail address: [email protected]. 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.09.012
physical techniques have been used to study permafrost and characterise areas of permanently frozen ground for many years (see Scott et al., 1990 for a review concerning polar permafrost). Since geoelectrical methods are most suitable for investigating the subsurface with distinct contrasts in conductivity and resistivity, respectively, DC resistivity soundings constitute one of the traditional geophysical methods which have been applied in permafrost research to confirm and characterise mountain permafrost. During recent years, advances have been achieved in
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using the traditional methods but with more powerful, state-of-the-art instruments and modern data processing algorithms, which enable two-dimensional surveys and data processing (e.g. Ishikawa et al., 2001; Isaksen et al., 2002; Hauck and Vonder Mühll, 2003a; Kneisel and Hauck, 2003). Due to the large contrast between the electromagnetic properties of ice and water groundpenetrating radar (GPR) is also an effective tool for mapping the location and depth of ice bodies and thermal interfaces within subarctic permafrost terrain (e.g. Arcone et al., 1998; Hinkel et al., 2001; Moorman et al., 2003). Application of GPR in mountain permafrost has been rare and most of the few surveys were performed on rock glaciers (Berthling et al., 2000; Isaksen et al., 2000; Vonder Mühll et al., 2000). The increase of application of modern geophysical methods also in mountain geomorphology is due to the fact that geophysical methods are comparatively fast and non-destructive compared to conventional drilling and even more important, information of the whole survey area can be obtained rather than results only from the drilling sites. With the more effective data acquisition a geophysical mapping of the near-surface lithology is enabled even in heterogeneous mountain environments. This contribution aims to provide an insight into the application of 2D resistivity tomography for imaging periglacial conditions with a special view to the aims and demands of the SEDIFLUX network (Sedimentary source-to-sink-fluxes in cold environments) especially detection and characterisation of mountain permafrost and monitoring of the spatio-temporal evolution of mountain permafrost. A further intention of this work is to evaluate the possibility of an assessment of the internal structure of typical periglacial landforms – as a further key point to improve the process understanding – through the application of 2D resistivity tomography. Applicability, limitations and perspectives of 2D resistivity imaging for the investigation of periglacial permafrost affected environments are shown based on case studies from the mid-latitude high-alpine and highlatitude subarctic periglacial mountain environments in the Swiss Alps and northern Sweden. 2. Electrical resistivity imaging — data acquisition and processing The basic principle for the successful application of geoelectrical methods in geomorphology/Quaternary geology is based on the varying electrical conductivity of minerals, solid bedrock, sediments, air and water and consequently their varying electrical resistivity. The resistivity of rock for example depends on water
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saturation, chemical properties of pore water, structure of pore volume and temperature. The large range of resistivity values for most materials is due to varying water content. Resistivity values of permafrost or ground ice can vary over a wide range depending on the ice content, the temperature and the content of impurities. The dependance of resistivity on temperature is closely related to the amount of unfrozen water. Perennially frozen silt, sand, gravel or frozen debris with varying ice content show a wide range of resistivity values between 1 kΩ m to several hundred kΩ m (e.g. Haeberli and Vonder Mühll, 1996). A differentiation between sedimentary ice and congelation ice is enabled according to the genetic/petrographic classification after Shumskii (1964). Congelation ice (interstitial and segregation ice), which is the predominant form of ice in the ground, shows much lower resistivities than sedimentary ice which forms by a firnification process. Characteristic values for sedimentary ice from temperate alpine firn zones are several MΩ m to more than 100 MΩ m and for congelation ice 10 kΩ m to a few MΩ m (cf. Table 1). Resistivity measurements are made by injecting a direct current into the ground via two current electrodes. The resulting voltage difference is measured at two potential electrodes. Resistivity surveys give an image of the subsurface resistivity distribution. Knowing the resistivities of different material types, it is possible to convert the resistivity image into an image of the subsurface consisting of different materials. The conventional method of plotting the results for the interpretation is the so called pseudosection, which gives an approximate image of the subsurface resistivity distribution. The shape of the contours depend on the array geometry and the subsurface resistivity. For the case studies shown in this contribution the most commonly used arrays for Table 1 Range of resistivities for different materials (compiled mainly after Telford et al., 1990; Reynolds, 1997) Material
Range of resistivity [Ω m]
Clay Sand Gravel Granite Gneiss Schist Ground water Frozen sediments a / ground ice a / mountain permafrost a Glacier ice (temperate) Air
1–100 100–5 × 103 100–4 × 102 5 × 103–106 100–103 100–104 10–300 1 × 103–106
a
Different sources, cf. Kneisel (2003b).
106–108 Infinity
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2D resistivity surveys the so called Wenner, Wenner– Schlumberger and Dipole–Dipole configurations were used. The Wenner configuration has a moderate investigation depth and has a good resolution for horizontal structures with vertical changes of resistivity. Since the total number of measurements required is less than for other configurations the time to complete a survey is comparatively short, however, also the obtained information of the subsurface is less than that derived from other arrays. The Dipole–Dipole array comprises two dipoles formed by the current electrodes on one side and the potential electrodes on the other side. This array type has a better horizontal, but weaker vertical resolution and a shallower investigation depth than the Wenner array. Furthermore, the largest number of readings of all three presented configurations are required to complete a survey. The Wenner–Schlumberger array is a combination of the Wenner array and the Schlumberger array (commonly used for vertical resistivity soundings) with constant potential electrode spacing but logarithmically increased current electrode spacings leading to a better depth resolution compared to the Wenner configuration. The number of measurements is more than for a Wenner survey but less than for a Double–Dipole array. Choice of the appropriate electrode configuration for a field survey has to be determined from case to case. Special characteristics of the different array geometries should be considered, above all the investigation depth and the
sensitivity of the array to vertical and horizontal changes in the subsurface resistivity distribution. The Dipole– Dipole array provides superior lateral resolution and will often be the first choice for geomorphological applications with expected lateral heterogeneity. However, on rough surface conditions with high ground resistance and weak signal strength which frequently occur in periglacial terrain, the application of Dipole–Dipole surveys might be critical because reception voltage values drop quickly with the increasing of the spacing between the injection and reception dipoles. In these cases it might be necessary to use the Wenner configuration because it is less sensitive to weak signal strength. Since the Wenner–Schlumberger configuration is a combination of Wenner and Schlumberger array types, it is useful for horizontal and vertical geomorphological structures and can be the best choice as a compromise between the Wenner and the Dipole–Dipole array. Application of two different array geometries at the same survey site such as Wenner and Dipole–Dipole enables a more accurate and reliable interpretation of the subsurface. Further details on different array geometries are given for instance in Telford et al. (1990) and Reynolds (1997). Applicability of different arrays to various geomorphological studies is described in Kneisel (2003a). For the two-dimensional surveys a SYSCAL Junior Switch system was applied. Application of geoelectrical surveys in periglacial environments often implies one major problem which is the coupling between the
Fig. 1. Resistivity-meter, multi-electrode cable and resistivity survey layout across a solifluction terrace with sorted stone stripes.
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electrodes to the sometimes heterogeneous and rocky ground surface. Because of high contact resistances of the coarse blocky surface material in some of the presented case studies, sponges soaked in water were used at each 40–50 cm long stainless steel electrode to establish sufficient electrical contact to the ground. Sometimes the application of sponges soaked in salt water is recommended to overcome this problem, however experience has shown that it is sufficient to pour water over the sponges and the vicinity of the electrodes. The data processing of the measured sets of apparent resistivities was performed using the software package RES2DINV. This inversion software tries to reduce the difference between the calculated and measured apparent resistivity values by adjusting the resistivity of the model blocks. A measure of this difference is given by the rootmean-square error (RMS). However, the best model from a geomorphological or geological perspective might not be the one with the lowest possible RMS. Thus, it is essential to perform the interpretation with
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consideration of the local geomorphological setting. This enables unrealistic images of the subsurface structure to be excluded. A further advantage of the 2D inversion software is the possibility to incorporate topographic corrections into the inversion algorithm which is an essential point for studies in alpine periglacial environments with often complex and heterogeneous topography. 3. Applications and case studies 3.1. Case studies from a high-latitude subarctic periglacial environment In a subarctic periglacial environment in northern Sweden (Pallenvagge, Abisko mountains, 68°10′N, 18°45′E), the spatial distribution of permafrost and its characteristics related to typical periglacial landforms was investigated using different methods. The mean annual air temperature in the periglacial environment at
Fig. 2. Resistivity tomogram of a Wenner survey over a solifluction terrace (a) and across sorted stone stripes (b).
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1200 m a.s.l. can be inferred from an adjacent mountain field station (− 2.3 °C at Latnajaure 981m a.s.l., 68°20′ N, 18°30′E) to be about − 3.5°C. Bedrock consists of amphibolite, gneiss and mica schists. In order to evaluate the permafrost distribution in the investigation area on a larger scale, measurements of the bottom temperature of the snow cover were carried out (socalled BTS-method introduced by Haeberli, 1973). For the characterisation of the subsurface lithology 2D resistivity surveys were performed at various places of which the surveys on an ice-cored moraine, on a solifluction/gelifluction terrace and on sorted polygons are presented in this contribution. Fig. 1 shows the survey line across a solifluction terrace at a north exposed slope. Because of the marked terrace step topography was included. The image of resistivities against depth shows a general decrease in resistivity with depth (Fig. 2). A large high-resistive anomaly is detected extending over the whole survey area indicating a permafrost body. Air-filled voids or cavities which could cause similarly high resistivities can be excluded at this site. From the range of resistivities, it can be assumed that the subsurface consists of ice-rich layers and that the ice content is decreasing with depth. The active layer appears to be comparatively thin which is probably due to the fact that the surface has become snow-free only recently. To obtain a reliable interpretation of this survey site a second profile at the foot of the solifluction terrace was
performed at right angles to the profile displayed in Fig. 2; intersecting point is marked with an arrow. The results show consistency, leading to the geomorphological interpretation that this slope is underlain by permafrost and that the periglacial morphodynamics are clearly related to the presence of active permafrost forming the terrace and the sorted stone stripes through the process of intensive gelifluction. Depth to bedrock is difficult to delineate from the results of this survey except in the right part of the cross-profile in Fig. 2b where bedrock comes closer to the surface (between horizontal distances 120 and 160). In the upper parts of the longitudinal profile in Fig. 2a even the bottom resistivities are still values indicating permafrost, the distinct resistivity decrease with depth is interpreted as the lower boundary of the ice-rich layer. However, in case of large resistivity gradients the reliability of the inversion result below a high-resistive layer can be significantly diminished (Marescot et al., 2003). In the same investigation area a survey on a terminal moraine complex was performed in order to figure out whether this moraine contains massive ice (Fig. 3). The survey line started in front of the moraine crossing a perennial snow patch at the foot of the moraine complex. Because of high contact resistances of the coarse blocky surface material, sponges soaked in water were used at each electrode on the moraine to establish sufficient electrical contact to the ground. Massive ground ice of considerable depth could be confirmed in
Fig. 3. Terminal moraine complex with the location of the survey line, base camp in the foreground.
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the moraine which is visible in the high resistivities in the left and central parts of the tomogram (Fig. 4). The range of resistivities with values of several MΩ m allows the assumption that the massive ground ice consists mainly of sedimentary ice from a firnification process (typical glacier ice). Between horizontal distances 120 and 135 the perennial snow patch causes near-surface high-resistive values. Similar profiles have been measured also on moraines in the Swiss Alps (cf. Hauck and Vonder Mühll, 2003b; Kneisel, 2004; Reynard et al., 2003). Ishikawa et al. (2001) used the significantly different distribution of resistivity values in the subsurface of two rock glaciers obtained through electrical resistivity imaging to infer genetic differences between glacierderived (originated from glacial dead ice) and talus derived rock glaciers (ice-cemented rock glacier) in Nepal. However, the interpretation of maximum resistivities has to be done carefully. Marescot et al. (2003) showed that the resistivity within the high-resistive zones cannot be determined accurately. For that reason it can be problematic to compare resistivity values from different field sites concerning ice origin or ice content. High-resistivity values may be caused by a high ice content at one site and a glacial origin of the ice at another site. However, the comparison of resistivity surveys which are performed in the same investigation area and in the same period under similar measurement conditions and using the same inversion parameters as in the case studies presented in Figs. 2 and 4 are possible to the authors opinion and experience from numerous
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measurements on surface ice and ground ice occurrences. Nevertheless, the interpretation of resistivity tomograms concerning ice content must be done carefully. Further particularities of inversion and interpretation of 2D resistivity surveys on mountain permafrost are given in Hauck and Vonder Mühll (2003b) and Marescot et al. (2003). A further point which has to be considered especially in periglacial mountain environments is the choice of the appropriate array configuration. Different array geometries of the same profile can lead to significant differences in the model results. In order to evaluate the sensitivity of the survey results to the chosen electrode configuration, both Wenner and Dipole–Dipole arrays were tested and compared for a resistivity imaging on patterned ground with sorted polygons using 2.5m spacing of the electrodes (Fig. 5a, b). Since a large heterogeneity could be expected in the subsurface underneath the polygon field, the Dipole–Dipole electrode geometry was additionally applied at the survey site. Compared to the examples in Fig. 2, the resistivity values are much lower but still in the range of permafrost and frozen sediments, respectively (cf. Table 1). The near-surface lithology is interpreted as unconsolidated sediments which consist of frozen material of different grain-size in which the fine-grained parts are probably composed of segregation ice layers. The near-surface high-resistive anomalies between horizontal distances 20 and 30 which appear in both tomograms are most likely due to poor electrode
Fig. 4. Resistivity tomogram of a Wenner survey on a terminal moraine complex. Dashed line indicates inferred depth of active layer.
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Fig. 5. Resistivity tomogram of a Wenner (a) and a Dipole–Dipole survey (b) on patterned ground with sorted polygons. Note the different depth axes.
coupling. Active-layer thickness seems to be variable over the survey area, but cannot be delineated clearly because resistivity gradients are small in most parts of the profile. A comparison of the results of the Wenner array with the results of the Double–Dipole array indeed shows that the latter provide superior horizontal resolution, but a smaller penetration depth as for the
Wenner array. In the example in Fig. 5 the results of the Wenner array do not seem to be sufficient for the overall characterization of the near-surface lithology at this site. The smoothing effect of this array type prohibits the detection of small-scale variability. The results of the Dipole–Dipole array provide more information although the data of this array leads to noisier models, which is
Fig. 6. Rock glacier Muragl with the location of the survey lines.
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expressed in the higher RMS error. For a detailed interpretation of the subsurface the results of both surveys are insufficient. Whether this polygon field is underlain by active permafrost cannot be inferred from these tomograms alone. A combination of several overlapping surveys using different electrode spacings such as 5, 2.5 and 1m to get reasonable vertical and horizontal resolution would be necessary to be able to characterise the subsurface below this polygon field in more detail. Consequently, a large number of measurements would be required. Furthermore, time-lapse resistivity surveys would be necessary to monitor timedependent resistivity changes and hence, the evolution of the active layer. Mapping the extent of thawed zones and detection of the variability of their thickness and shape changes over time is an important component of
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monitoring periglacial processes in order to improve the process understanding with a view to the potential effects of climate change in permafrost affected cold environments. 3.2. Case studies from a mid-latitude high-alpine periglacial environment In an alpine periglacial environment in the Upper Engadine, eastern Swiss Alps investigations on different aspects of mountain permafrost distribution and characteristics have been performed over recent years with the main focus on permafrost in recently deglaciated forefields (Kneisel, 2003b, 2004). In this contribution surveys from the Muragl glacier forefield and the adjacent Muragl rock glacier are presented. The
Fig. 7. Resistivity tomogram of two Wenner surveys on rock glacier Muragl, steep rock glacier front (top panel) and cross-profile (bottom panel). Dashed line indicates inferred depth of active layer.
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investigated glacier forefield extends from 2600 to 2950 m a.s.l.; for this altitudinal range a mean annual air temperature of − 2 to −4 °C can be inferred from two nearby mountain climate stations at 2250 m a.s.l. (− 0.3°C) and 3300 m a.s.l. (− 6°C) and own measurements of the mean annual ground surface temperature at different sites within the glacier forefield ranging from 0 and − 1.35 °C (Kneisel, 2003b). The bedrock consists mainly of gneiss and mica schists. The Muragl rock glacier was recently intensively investigated using seismic refraction tomography and georadar cross-hole tomography indicating zones of degrading permafrost (Maurer et al., 2003). Fig. 6 shows rock glacier Muragl and the survey lines from the alpine meadows up to the rock glacier across the steep front as well as a cross-profile. These kind of profiles belong to the most challenging surveys in alpine permafrost since the steep front consists of loose debris and much care has to be taken for rockfalls which can hit the multi-electrode cable as happened during the setup of this survey line. Both Wenner profiles, also the crossprofile are of comparatively good quality, again sponges were used at each electrode to establish sufficient contact to the ground (Fig. 7). The cross-profile supports the findings from the seismic refraction tomography and the georadar cross-borehole tomography, indicating a larger area without ground ice (Maurer et al., 2003). A further motivation for performing these two surveys was to find out whether this method could be successfully used to confirm general concepts about dynamic effects and resistivity structure within active rock glaciers, namely the assumed flow-induced thickening of the permafrost and active layer in areas of longitudinal compression towards the front and the lowering of the
resistivities towards the front (cf. Haeberli and Vonder Mühll, 1996). The results obtained on this extremely rugged terrain are promising, showing variable activelayer thicknesses in the area of the furrow and ridges (between horizontal distances 40 and 100 in the top panel) and different resistivities for the longitudinal and the cross-profile. Delineation of active-layer thickness through resistivity surveys alone can be difficult, especially if blocky surface substratum which leads to high ground resistances is underlain by ice-rich permafrost. Air-filled voids or cavities could cause equally high resistivities as massive ground ice. Especially in such cases, knowledge of the topographical and geomorphological setting as well as measurement and data analysing experience is essential for the interpretation. During the survey on this rock glacier, the sound of flowing water at the permafrost table gave a further hint to the depth of the active layer. Thus, a semiquantitative interpretation concerning the variability of the active-layer thickness over the survey area is enabled. Using a greater number of 2D geoelectric surveys on several rock glaciers within a comparative study, it appears to be possible to support the above mentioned general concepts about dynamic effects and resistivity structure within active rock glaciers and to achieve new findings towards the understanding of rock glacier internal structure and dynamics. Fig. 8 presents survey results from an area where glacial flutes have developed in the Muragl glacier forefield (cf. Fig. 9). In the upper parts of the survey profile down to the middle parts, a resistivity anomaly is detected which can be interpreted as a shallow permafrost occurrence as the resistivity values are
Fig. 8. Resistivity tomogram of a Wenner survey within the recently deglaciated glacier forefield Muragl.
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Fig. 9. Glacier forefield Muragl with the location of the survey lines.
decreasing below the roughly 15 m thick resistive anomaly. With the special conditions in glacier forefields, permafrost occurrences can be assumed to be shallow, “warm” and thus, rich in unfrozen water, which can explain the comparatively low apparent resistivity values. The occurrence of fluted moraines and push moraines (as in the Muragl glacier forefield for instance) provides geomorphological evidence for a complex thermal regime of the former glacier with cold marginal parts frozen to the bed and warm-based ice in more central parts where fluted moraines could develop. Perennially frozen ground encountered in areas where glacial flutes occur is interpreted as a permafrost aggradation after the disappearance of the temperate surface ice (Kneisel, 2003b). High gradients in the subsurface resistivity distribution usually indicate interfaces between different layers. Since the coupling of the electrodes at this survey site was comparatively good and the ground resistance on this fine to mediumgrained glacial till was low, the variable depth of the marked interface as inferred from the resistivity survey can be related to variable active-layer thickness at this site. The thick active layer between stations 60 and 90 could point to a melting/degrading permafrost body. Monitoring of aggrading and degrading permafrost is work which is currently in progress; two survey sites have been established in the Muragl glacier forefield on different surface substratum, one on fine to medium
grained glacial till and one on a blocky moraine. In order to study the changes of subsurface resistivity with time, 2D resistivity surveys are repeated over the same survey line at different times, therefore a fixed electrode array was installed with 2 m electrode spacing. Hauck (2002) has shown that frozen ground monitoring can successfully be realized using resistivity tomography, however, the monitoring site was installed on fine to medium grained weathered sediments overlying bedrock. For the monitoring of time-dependent processes (so-called timelapse experiments) changes in the subsurface resistivity are estimated by using changes in the apparent resistivity measurements. This requires accurate and reproducible results even on rough terrain. Thus, first test measurements on the blocky moraine in the Muragl glacier forefield were performed using the Wenner and Wenner–Schlumberger arrays (cf. Fig. 10). These configurations are well suited to detect horizontally layered structures; vertical structures are more accurately detectable by using the Dipole–Dipole array. However, the latter is often difficult to apply in periglacial mountain terrain with rough surface conditions and high ground resistance because of relatively low signal-tonoise ratios. The three measurements at the beginning of August, end of August and middle of September show consistency in the results. In consideration of the rugged topography the inversion results are of good quality and
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Fig. 10. Resistivity tomograms of a Wenner–Schlumberger survey on a moraine consisting partly of coarse debris. Upper panel represents the survey on August 4th, middle panel the survey on August 27th, bottom panel the percentage change in resistivity. Dashed line indicates inferred depth of active layer.
reproducible. The data were first inverted independently. This approach has given only limited information on subsurface resistivity changes with time since the inversion routine tries to minimize the difference between the measured and calculated apparent resistivity values. Thus, differences in the resistivity values are not necessarily related to actual changes in the subsurface resistivity distribution. To overcome this problem a joint inversion technique (time-lapse inversion method within RES2DINV) was applied. Hereby, the model obtained from the inversion of the first measurement is used as a reference model to constrain the inversion of the later time-lapse measurements. Fig. 10 displays the data set of the surveys on August 4th (upper panel) and on August 27th 2004 (middle panel) as well as the percentage change in resistivity (bottom panel). In the upper parts of the tomogram down to 3 m no significant changes in resistivity are visible, except an area between horizontal distances 28 and 30 and more prominent between 52 and 64, where there is a decrease in resistivities recognizable of up to 20% and 25%. In deeper parts below 3 m, a distinct decrease of resistivities is
visible more or less over the whole survey area and especially between horizontal distances 26–44 and 52– 64. The decrease of resistivities can most likely be related to a thickening of the active layer due to melting of permafrost at the lower boundary of the active layer towards the underlying ice-rich permafrost. A significant influence of temperature effects on the decrease of the resistivity values or heavy rainfall within this time span can be disregarded. The variability over the survey area is probably due to the variability of the surface substratum which consists of medium grain sizes from horizontal distances 1 to 24 and coarser grain sizes between 24 and 48 and is even blocky between 48 and 68. The apparently thin active layer in the right part of the tomogram (cf. upper panel and middle panel in Fig. 10) is due to the blocky surface substratum which in turn leads to high ground resistances. The high percentage change in the resistivity values at this side of the survey line leads to the assumption that there is a remarkable thickening of the active layer between the two measurement dates whereas the minor changes on the left side of the
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survey line indicate that active-layer thickness was not increased within this period. The resistivity changes below the high-resistive anomaly are not included in the interpretation because these data are not well constrained as the sensitivity of the inversion results to the input data is low for high-resistive model regions (cf. Marescot et al., 2003 see above; Hauck et al., 2003). These test measurements show that the spatiotemporal evolution of the active-layer thickness can be monitored also on rugged terrain with rough surface conditions using repeated measurements of the subsurface resistivity distribution. Future measurements will be carried out over a longer time span and will be combined with measurements of the near-surface ground temperatures. 4. Conclusions and perspectives The illustrated case studies show that electrical resistivity imaging is a valuable method for studies in periglacial mountain environments. Limitations of application can arise from extremely rugged terrain conditions which can occur in alpine periglacial terrain. Good data quality is largely dependant on a well designed survey which includes good coupling of the electrodes to the ground. Insufficient coupling of the electrodes and great heterogeneity of the surface terrain can lead to bad data quality resulting in noisy model interpretations of the subsurface. In spite of these limitations two-dimensional resistivity tomography is considered as the most multifunctional geophysical method and could be first choice for geomorphologists working in periglacial mountain environments if only one single method can be applied. Ideally, two-dimensional electrical resistivity surveys should be used in conjunction with for instance refraction seismics or GPR as they can provide complementary information about the subsurface. However, the application and interpretation of GPR can be difficult in mountain terrain, because the heterogeneous subsurface can cause scattering of the electromagnetic waves. A valuable aspect of modern geophysical techniques and their application in geomorphology is the ability to image the subsurface. Resistivity surveys give an approximate image of the subsurface resistivity distribution. For a reliable interpretation of that image a geomorphological background and some experience is required. The case studies presented in this paper illustrate the application of 2D resistivity imaging for a number of permafrost related problems. The detection of permafrost and mapping of the horizontal extent of permafrost
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or massive ground ice bodies is enabled with an appropriate accuracy. Delineation of the permafrost base at least for shallow permafrost occurrences and active-layer thickness is possible in a semi-quantitative way in many cases. On coarse scree slopes where airfilled cavities occur which can cause high-resistive anomalies, interpretation of electrical resistivity surveys can be difficult in terms of permafrost delineation. In order to get unambiguous results seismic refraction tomography should be used additionally also in cases where a more accurate determination of the sediment thickness is required. An assessment of the ice origin and ice content can be possible, but has to be done carefully. For a reliable interpretation of the survey results, knowledge of the geomorphological and geological setting is essential and limitations of data inversion have to be considered (e.g. reliability of the inversion results below a very-high resistive massive ice body, assessment of ice content in the subsurface). Application of two different array geometries at the same survey site such as Wenner and Dipole–Dipole enables a more accurate interpretation of the subsurface. However, on rough surface conditions with high ground resistance and weak signal strength which frequently occur in periglacial mountain terrain, the application of Dipole–Dipole surveys might be critical. Hence, the detection of the small-scale variability of the internal structure of some typical periglacial landforms can be limited (e.g. case study on sorted polygons). On the other hand and for larger periglacial landforms such as rock glaciers a future perspective of two-dimensional resistivity imaging is the possibility of performing a detailed mapping of the subsurface resistivity distribution using overlapping profiles to obtain reliable results even on rough surface conditions with high ground resistances. In the future, 3D resistivity surveys can even enhance the potential of this method. To date, survey speed is too slow and geophysical field work in remote high mountain areas is also dependent on battery capacity and time, and good weather conditions have to be used efficiently. Currently, the most promising perspective is the approach of monitoring subsurface resistivity changes through repeated measurements of the electrical resistivity. The described case study shows that even on rugged terrain with rough surface conditions 2D electrical resistivity imaging could be used to monitor permafrost aggradation and degradation. Such a geoelectrical monitoring in combination with continuous measurements of the nearsurface ground temperatures could comprise two important components of a low cost approach to monitor effects of climate change in different permafrost affected cold environments.
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Acknowledgements The author acknowledges a grant for the research in Northern Sweden funded by the University of Würzburg (“Jubiläumsstiftung zum 400-jährigen Bestehen der Universität Würzburg 2003–2004). Furthermore, the author would like to thank M. Friedlein for support during fieldwork and O. Sass and an anonymous reviewer for critical comments on an earlier version of this manuscript. References Arcone, S., Lawson, D., Delaney, A., Strasser, J.C., Strasser, J.D., 1998. Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost. Geophysics 63, 1573–1584. Berthling, I., Etzelmüller, B., Isaksen, K., Sollid, J.L., 2000. Rock glaciers on Prins Karls Forland. II: GPR soundings and the development of internal structures. Permafrost and Periglacial Processes 11 (4), 357–369. Haeberli, W., 1973. Die Basis-Temperatur der winterlichen Schneedecke als möglicher Indikator für die Verbreitung von Permafrost. Zeitschrift für Gletscherkunde und Glazialgeologie 9, 221–227. Haeberli, W., Vonder Mühll, D., 1996. On the characteristics and possible origins of ice in rock glacier permafrost. Zeitschrift für Geomorphologie N.F. Suppl. 104, 43–57. Hauck, C., 2002. Frozen ground monitoring using DC resistivity tomography. Geophysical Research Letters 21, 2016, doi:10.1029/ 2002GL014995. Hauck, C., Vonder Mühll, D., 2003a. Evaluation of geophysical techniques for application in mountain permafrost studies. In: Schrott, L., Hoerdt, A., Dikau, R. (Eds.), Geophysical Methods in Geomorphology. Zeitschrift für Geomorphologie, Suppl., vol. 132, pp. 161–190. Hauck, C., Vonder Mühll, D., 2003b. Inversion and interpretation of two-dimensional geoelectrical measurements for detecting permafrost in mountainous regions. Permafrost and Periglacial Processes 14, 305–318. Hauck, C., Vonder Mühll, D., Maurer, H., 2003. Using DC resistivity tomography to detect and characterise mountain permafrost. Geophysical Prospecting 51, 273–284. Hinkel, K.M., Doolittle, J.A., Bockheim, J.G., Nelson, F.E., Paetzold, R., Kimble, J.M., Travies, R., 2001. Detection of subsurface permafrost features with ground-penetrating radar, Barrow, Alaska. Permafrost and Periglacial Processes 12, 179–190. Isaksen, K., Oedegaard, R.S., Eiken, T., Sollid, J.L., 2000. Composition, flow and development of two tongue-shaped rock glaciers in the permafrost of Svalbard. Permafrost Periglacial Processes 11, 241–257. Isaksen, K., Hauck, C., Gudevang, E., Oedegaard, R.S., Sollid, J.L., 2002. Mountain permafrost distribution in Dovrefjell and Jotunhei-
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