Vertical movements of the ground surface due to frost heave and thaw subsidence are common in permafrost regions, and are extremely important periglacial geomorphic processes. The magnitude of annual frost heave can be substantially larger than simple expansion of soil porewater on freezing because water migrates to freezing fronts, often resulting in thick accretions of segregation ice. Frost heaving is of major concern to operators of chilled gas pipelines in cold regions because ice segregation could have disruptive effects on such structures. Differential thaw settlement occurs annually in permafrost environments as the active layer develops; annual heave and settlement are important drivers of sediment transfer on slopes. At longer temporal scales, melting of ground ice in response to climate warming may create thermokarst terrain, with serious consequences for human infrastructure.

Many techniques have been employed to measure frost heave and thaw settlement in the field. These traditional methods include optical leveling relative to a stable benchmark, mechanical devices measuring movements at set horizontal distances (“motometers”), and scribers recording vertical movement. Disadvantages associated with these methods include: (a) their limited ability to determine geographic patterns of heave and settlement; (b) difficulties in placing their results into geodetic coordinate systems; and (c) the bulk and/or complexity of such instrumentation.

The Circumpolar Active Layer Monitoring (CALM) program incorporates a network of more than 150 observatories at which active-layer dynamics are monitored. The active layer is an extremely dynamic part of the permafrost system and experiences freezing/thawing at sub-annual scales. Active layer monitoring is an important component of efforts to assess the effects of global change in permafrost environments. The North Slope of Alaska is currently occupied by 29 CALM sites representative of a variety of regional environmental conditions. The majority of sites are regular 1 ha and 1 km² grids in which the measurement of active-layer thickness (ALT) is performed annually at the end of the thawing season. The measurement procedure involves pushing a metal rod, calibrated in cm, to the point of refusal, interpreted in most cases to be the frost table. Results of 13 years of measurements show that ALT is decreasing at almost all sites located in the region.

Recent finding by Shur et al. (2005) developed the concept of the transient layer, a layer of earth materials between the active layer and permafrost that cycles through freezing and thawing at frequencies ranging from decadal to millennial. Penetration of thaw into or through the transient layer can result in pronounced differential settlement at the surface, jeopardizing structures at the surface.

Penetration of thaw into an ice-rich layer, referred to here as “thaw penetration,” is accompanied by loss of volume (thaw consolidation). Straightforward measurement of active-layer thickness by such methods as repeated probing with a rigid metal rod, currently used at a majority of the CALM studies, may not yield accurate estimates of changes in the system.

The University of Delaware Permafrost Group developed an instrumental technique to study frost heave and thaw subsidence that combines a hierarchical nested sampling design (NSA) with the advantages of high-precision georeferenced DGPS measurements (Figure 2). The methodology was successfully applied to study spatial patterns of frost/heave and subsidence in tundra environments at two CALM sites located on Alaska's North Slope (Little et al., 2003; Streletskiy et al., 2005).

Measurements at West Dock and Sagwon were performed based on the hierarchical NSA design. Each site is represented by 32 sampling points spaced at intervals of 1, 3, 10, and 30 m. DGPS/ ALT measurements were performed in June and August of every year over the period 2001-2007. All sites were equipped with temperature loggers measuring air and ground surface temperature at hourly intervals.

Results show that DGPS measurement error does not exceed 1.6 cm in this study. Other small sources of error stem from disturbance of targets. Total subsidence for the 2001-2007 period at the Sagwon site was 9.3 cm, 11.8 cm at West Dock upland, and 11.5 cm at West Dock lowland. On average, winter heave did not compensate for summer ground subsidence at any of the studied landscapes during the 2001-2006 observation period (measurements in 2007 were taken only in August).

Significant heave/subsidence occurred at all study sites. Although there has been no significant increase in ALT over the study period, thaw penetration and subsidence at the surface both increased substantially (Figure 3). Because ALT showed no comparable trend during this period, a clear need exists for revisions in the CALM measurement protocol.

Differential GPS is an effective tool for measuring frost heave and thaw settlement in permafrost environments. The methodology offers distinct advantages, including a high degree of accuracy, geographic coverage at virtually any scale, and automatic placement of survey networks into geodetic networks with first-order accuracy. Although use of DGPS as described here is expensive and time consuming, both of these constraints are diminishing. Owing to technological improvements in successive generations of field instruments, the time required to execute our annual surveys at West Dock and Sagwon diminished significantly between 2001 and 2007. The cost of the instruments has also decreased during this period. Such improvements are likely to continue. DGPS may become the preferred method by which to monitor frost heave and thaw subsidence within the next decade.

This research was sponsored by the U.S. National Science Foundation grants OPP-0352957 and OPP-009588. We are grateful to J. Little, M. Walegur, M. Schimek, A. Klene, and H. Sandall for their assistance in collecting data, and to VECO Polar Resources and the Barrow Arctic Science Consortium for providing logistical support.