Methods for Measuring Active-Layer Thickness

F. E. Nelson¹ and K. M. Hinkel ²

1 Department of Geography, University of Delaware, Newark, DE USA 19716

2 Department of Geography, University of Cincinnati, Cincinnati, OH USA 45221

General considerations

The term “active layer” refers to the relatively thin layer of ground between the surface and permafrost that undergoes seasonal freezing and thawing (Muller, 1947; Burn, 1998). Across this layer energy and water are exchanged between the atmosphere and underlying permafrost. Because most biological, physical, chemical, and pedogenic processes take place in the active layer, its dynamics are of interest in a wide variety of scientific and engineering problems.

The definition given above is based entirely on thermal criteria, without regard to material composition or properties. The volume and properties of the active layer are highly variable in time and over space. Idealized diagrams showing smooth trends of active-layer thickness (ALT) across latitudinal gradients (e.g., Brown, 1970, Figure 4, p. 8) can be misleading because variations in vegetation, substrate properties, and water content can result in very large differences in ALT, even over small distances (Nelson et al., 1999; Hinkel and Nelson, 2003). Temporal changes, particularly surface temperature and moisture conditions, can also lead to substantial year-to-year differences in ALT, even at fixed locations. For these reasons, it is necessary to monitor ALT using well-defined measurement and sampling techniques. The Circumpolar Active Layer Monitoring (CALM) program was developed to provide standards of measurement and a comprehensive database describing the history and geography of ALT and other selected parameters at a large number of sites representative of permafrost terrain (Brown et al., 2000).

This note discusses several common methods used to measure the thickness of the active layer, including mechanical probing, frost/thaw tubes, soil temperature profiles, and remote methods such as ground penetrating radar (GPR) and satellite measurements. Only nondestructive methods are discussed. Many of the methods and sampling designs described in this note were developed or refined by various investigators working in association with the CALM program.

The term “active-layer thickness” is used in reference to the maximum development of the thawed layer, reached at the end of the warm season (van Everdingen, 1998). This is distinct from the term “thaw depth,” used here to refer to the thickness of the thawed layer at any time during its development in summer. Stated alternatively, thaw depth is an essentially instantaneous value that is always less than or equal to the thickness of the fully developed active layer. Active-layer thickness can vary substantially on an interannual basis. In general, it is greater in years with warmer summers and thinner in those with cooler temperatures (Brown et al., 2000). Both thaw depth and active-layer thickness display large spatial variability over short lateral distances and, because rates of thaw vary in response to the properties of surface and subsurface materials, the spatial variability of thaw depth increases over the course of a single summer (Nelson et al., 1997). The magnitude of active-layer variability can be quantified and used to characterize various types of permafrost terrain (Nelson et al., 1998; Hinkel and Nelson, 2003).

Observation methods

Probing

Probing of the active layer is performed mechanically with a graduated rod. The typical probe is a 1 m long stainless-steel rod with a tapered point, is 1 cm in diameter, and has an attached handle (Figure 1). At sites where thaw depth is very large (e.g., 1-3 m), the diameter of the probe must be also be greater to withstand the bending stress generated by insertion. It is very difficult, however, to extract a probe in deeply thawed soils, and this problem is exacerbated if the probe’s surface area is very large.

Figure 1:  Steel 1-m probe, graduated in 1-cm increments. This model breaks down into three pieces for transport in handle, and has additional flights. This design was developed at the U.S. Army Cold Regions Research and Engineering Laboratory in the 1970s.

The probe rod is inserted into the ground to the point of resistance. A gentle pumping motion is used to gradually force the rod progressively deeper into the thawed ground without bending. A distinctive sound and feel is apparent when ice-rich frozen ground is encountered. The rod is grasped with the hand, and the hand is slowly slid down the rod to the top of the soil material; i.e., to the base of the vegetation. The rod is grasped firmly and removed, using the fingers to carefully mark the position on the rod. Thaw depth is then read from the graduations and recorded. All measurements are made relative to the surface; in standing water, both thaw depth and water depth are recorded.

Typically, two measurements are made at each location and the average reported. If a standard spacing is maintained between the two samples, the researcher has one metric of thaw variability. Identical probing methods can be used to measure the depth of the snow pack.

Advantages:  Probes are relatively inexpensive and several models and sizes are available (see vendors at http://k2.gissa.uc.edu/~kenhinke/CALM/equipment.html). Measurements are relatively easy to make, and require little time. The primary advantages of probing are: (a) its suitability for collecting large numbers of measurements; (b) its ability to generate samples of data that are statistically representative of local areas; and (c) it can be used in conjunction with vegetation and soil information to estimate the volume of thawed soil over extensive regions (e.g., Nelson et al., 1997; Klene et al., 2001; Shiklomanov and Nelson, 2002).

Although single point measurements have utility, measurements along transects or on grids enable the researcher to plot or map annual thaw patterns, make volumetric estimates, and identify the factors responsible for active-layer  variability (Nelson et al., 1999; Gomersall and Hinkel, 2001). Because measurements can be made rapidly and with little effort, probing is well suited for implementing formal sampling designs (Nelson et al., 1998, 1999; Gomersall and Hinkel, 2001). The length of transects or the dimensions of grids depend on scientific objectives and the scale of the active layer’s local variability, and a program of exploratory sampling may be necessary to establish a grid of dimensions appropriate to the locale and the scientific goals (Nelson et al., 1998, 1999). Grids are typically 10, 100 and 1000 m on a side, with grid nodes spaced evenly at 1, 10, or 100 m, respectively (Brown et al., 2000). Node locations are identified using stakes or other semi-permanent markers, and replicate measurements are made at approximately the same location each year to facilitate interannual comparison.

Disadvantages: Timing is one of the primary limitations with probing. Ideally, measurements should be collected at the time of maximum thaw depth. Field measurements are, however, often constrained by logistical or weather-related considerations. Experience can help the researcher decide when to collect end-of-season measurements, but the date usually varies from year to  year at each site. It is therefore unlikely that measurement of thaw depth coincides perfectly with the actual active-layer thickness. However, because thaw progression is usually proportional to the square root of the time elapsed since snowmelt, late-season thaw-depth measurements generally correspond closely with the maximum thickness of the active layer.

More intractable problems with probing arise when substrate properties prevent accurate determination of the frost table’s position. In some cases, the top of the frozen (ice-bearing) zone does not coincide with the position of the frost table as defined by the 0°C criterion. The relation is dependent on soil salinity, particle size, and temperature. Well-drained sands and gravels may contain too little interstitial ice for adequate resistance to probing to develop. In saline or extremely fine-grained soils probing can yield inaccurate estimates owing to the presence of unfrozen water. Under such conditions it may be possible to calibrate mechanical probing using a thermal probe (Mackay, 1977; Brown et al., 2000, p. 172). Readings may be very difficult to obtain in stony substrates such as glacial till (see paper by S. Hanson, this volume). Probing cannot ascertain if thaw subsidence has occurred.

Frost/thaw tubes

Thaw/frost tubes are devices extending from above the ground surface through the active layer into the underlying the permafrost. They are used extensively in Canada. Construction materials, design specifications, and installation instructions are available for several variants of the basic principle (Rickard and Brown, 1972; Mackay, 1973; Nixon, 2000). A schematic for a recent design is shown in Figure 2. 

Figure 2. Schematic diagram of a recent frost/thaw tube design implemented by the Geological Survey of Canada. This design incorporates a scribing device to record thaw settlement induced by ablation of an ice-rich layer near the interface between the active layer and permafrost table. Diagram courtesy of F. M. Nixon, Geological Survey of Canada. From Nixon et al. (1995).

A rigid outer tube is anchored in permafrost, and serves as a vertically stable reference; an inner, flexible tube is filled with water or sand containing dye. The approximate position of the thawed active layer is indicated by the presence of ice in the tube, or by the boundary of the colorless sand. Each summer the thaw depth, surface level, and maximum heave or subsidence is measured relative to the immobile outer tube. These measurements are used to derive two values for the preceding summer: (1) the maximum thaw penetration, independent of the ground surface and corrected to a standard height above the ground established during installation; and (2) the active-layer thickness, assumed to coincide with maximum surface subsidence. With modifications, the accuracy of the measurements is about 2 cm.

Advantages:  The primary advantage of frost/thaw tubes is that they provide an inexpensive annual record of both maximum thaw penetration and active-layer thickness, although it is not possible to determine the date. Because thaw tubes are durable, a multi-year record is available for comparison. Thaw tubes are especially useful in areas where thaw is too deep to monitor by probing, and in stony, fine-textured, or saline substrates in which probing is not feasible. Because the device is embedded in permafrost, the outer tube serves as a stable reference to determine if thaw subsidence or heave has occurred (Nixon, 2000).

Disadvantages:  Thaw tubes have several limitations. First, each tube records only a single point measurement, which may or may not be representative of a larger area. Second, installation typically entails drilling, and the difficulty and time required may prohibit multiple tube installation at a site. Finally, there may be significant disruption of the surface and subsurface materials during installation, and the structure itself may influence local heat and moisture flow. It is likely that the optimal situation is to install a thaw tube at long-term sites, and collect additional measurements derived from spatially extensive sampling such as probing. Over time, a comparative record can be developed.

Soil temper ature profiles

Soil and air temperature are recorded as basic information at many CALM  sites, especially with the increasing availability of inexpensive, reliable temperature data loggers. Temperature sensors (usually thermistors) are inserted into the active layer and upper permafrost as a vertical array. Several CALM installations currently use an array of thermistors embedded in a small-diameter acrylic cylinder (Figure 3) and connected to a high-capacity data logger.

Figure 3. Thermal probe incorporating one external and 12 internal thermistor sensors in fixed positions within a rigid acrylic tube. The probe employs sophisticated electronics and is usually connected to a large-capacity data logger capable of making and recording high-frequency readings. The probe illustrated here was originally designed by the U.S. Army’s Cold Regions Research and Engineering Laboratory, and is manufactured by MRC (Measurement Research Corporation), Gig Harbor, Washington USA.

Temperature records from a vertical array of  sensors can be used to determine active-layer thickness at a point location. The thickness of the active layer is estimated using the warmest temperatures recorded at the uppermost thermistor in the permafrost and the lowermost thermistor in the active layer. The temperature records from the two sensors are interpolated to estimate maximum thaw depth during any given year. For this reason, the probe spacing, data collection interval, and interpolation method are crucial parameters in assessing the accuracy and precision of the estimate.

Advantages: The advantages are similar to those for frost tubes. However, since temperature monitoring is already being preformed, there is no additional cost. Further, it is possible to estimate the date of maximum thaw with a reasonable degree of accuracy. Depending on whether probes are in a fixed or floating configuration, it may be possible to determine if thaw subsidence has occurred at the location. Numerical methods can be used with high-frequency thermal observations to estimate the thermal properties of the substrate, such as effective thermal diffusivity (e.g., Hinkel, 1997). Thermal records can also be used to identify the operation of non-conductive heat-transfer processes in the active layer, and can be related to meteorological events at the surface (Hinkel et al., 1997, 2001; Kane et al., 2001).

Disadvantages:  Limitations are similar to those of frost tubes; thermistor strings effectively comprise only a single point measurement. They are relatively expensive. They are also subject to surface and installation disruptions, including vandalism and disturbance by animals. The accuracy of the active-layer thickness estimate is fundamentally limited by the vertical spacing of the probes and the data-collection interval. Many field investigators use a simple linear interpolation to estimate maximum seasonal thaw depth, although an exponential best-fit can be employed if there are several thermistors. Estimates of error should be calculated and reported.

Remote sensing methods

Ground penetrating radar (GPR) has been used with some success to map active-layer thickness along transects. Because water effectively absorbs electromagnetic pulses, profiling is most effective in winter, when the ground is frozen and covered with snow. The method relies on the principle that the active layer contains less ice than the permafrost immediately below, resulting in a reflection horizon at the interface. With careful local calibration, usually accomplished through coring, estimates of thaw depth along a continuous profile can be made. The accuracy of the estimates is incompletely known, but appears to be within ± 15% in fine grained soils. The expense, however, is often prohibitive. It is likely that GPR methodologies will continue to develop. Further details on the use of GPR in active-layer investigations are available in Doolittle et al. (1990) and Hinkel et al. (2001). Another ground-based approach was developed by McMichael et al. (1997), who used the Normalized Difference Vegetation Index (NDVI) to exploit known relations between vegetation units and ALT across a toposequence in northern Alaska.

Satellite imaging systems hold promise for monitoring thaw depth across large areas. In particular, synthetic aperture radar, carried at appropriate wavelengths, may have sufficient energy to penetrate the often-saturated active layer and return a signal to the satellite receiver (Kane et al., 1996). Interpretive, convergence-of-evidence approaches have been used by Peddle and Franklin (1993) and Leverington and Duguay (1996) with some success, although the derived classes of ALT were very broad.

All aircraft- or satellite-based systems necessitate collection of training data on the ground for calibration and verification of the signal processing algorithms. The impetus for such a system may come from unmanned missions to Mars.

Other considerations

The issue of thaw subsidence, alluded to at several locations in the preceding text, is a critical issue and forms the focus of several field-based experiments in the CALM program. Thaw subsidence (thaw settlement) refers to downward displacement of the ground surface occurring when ice-rich permafrost thaws. Melting massive ground ice, typically veins and/or lenses, leaves a void; the weight of overlying material, combined with drainage of meltwater, can result in appreciable subsidence at the surface. This consolidation may not be apparent in ALT records obtained exclusively by mechanical probing .

Figure 4: Thaw subsidence results from melting of 3 cm thick ice lens in the upper permafrost. Frost defended bench mark (FDBM) is used to monitor subsidence and heave as base of ALT changes.

Imagine, for example, that the typical active-layer thickness is 30 cm at a site. (Figure 4) Below this is a 5 cm thick layer containing a 3 cm thick ice lens. During one especially warm year, the thaw front penetrates to a depth of 35 cm, causing the ice lens to melt. The measured thaw depth would be only 32 cm, since the 3 cm of ice will have melted and the ground surface subsided by approximately this amount. Thus, the measured active-layer thickness will actually underestimate total thaw penetration. Conversely, the ground surface may heave upward on a seasonal or permanent basis with the growth of segregation ice. This is particularly problematic in medium-textured mineral soils.

Probing may not detect thaw subsidence or heave. For this reason, some recent investigations have employed frost defended bench marks or other methods of tracking changes in surface elevation (Nixon and Taylor, 1998; Little et al., 2003). This may entail installing rigid poles or bars several decimeters into the upper permafrost to ensure that there is no vertical movement. The top of the bar or rod serves as a stable vertical reference point to monitor subsidence and heave. Rock outcrops can also be used for surveying purpose. Simple and inexpensive vertical displacement gauges have been developed to monitor seasonal heave and subsidence. These devices are anchored into the permafrost using a thin, flexible steel cable, and can also be used to monitor long term surface displacement. A description of one of gauge is available on the CALM web site (http://k2.gissa.uc.edu/~kenhinke/CALM/active_layer.html).

To an increasing degree, differential global positioning systems (DGPS) technology is being used to collect vertical control points to monitor and map surface movement (Little et al., 2003). Mobile DGPS units can achieve vertical resolution of less than 1 cm under the proper operating conditions. In many applications, this precision may be sufficient for the user.

Conclusions

Many methods are currently being used to measure thaw depth in the active layer above permafrost. Each method is associated with distinct advantages and disadvantages. Each relies on a specific tool; in the words of Craftsmanä spokesperson Bob Vila, it is a question of “using the right tool for the job.”  The researcher must decide on the most useful and practical method given the scientific objectives, duration of the study, and availability of funds. It is likely, however, that a mixture of methods will be most suitable for long-term sites.

Acknowledgements

This work was supported by the National Science Foundation under grants OPP-9612647, 9896238 and 0095088 to FEN and OPP-9529783, 9732051, and 0094769 to KMH. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Identification of specific products and manufacturers in the text does not imply endorsement by the National Science Foundation.

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Latest update: 7. May 2003.