|Titre||Validation of current ground thermal conditions, Alaska Highway Corridor, Yukon Canada|
|Auteur||Duguay, M; Smith, S L; Lewkowicz, A G|
|Source||Extended abstracts of the Tenth International Conference on Permafrost; par Drozdov, D S (éd.); 2012 p. 129-130|
|Séries alt.||Secteur des sciences de la Terre, Contribution externe 20110388|
|Réunion||10th International Conference on Permafrost; Salekhard; RU; juin 25-29, 2012|
|SNRC||105D/13; 105D/14; 115A/13; 115A/14; 115A/15; 115A/16; 115B/16; 115F/15; 115F/16; 115G/01; 115G/02; 115G/05; 115G/06; 115G/07; 115G/11; 115G/12; 115G/13; 115K/02; 115K/07; 115K/10|
|Lat/Long OENS||-141.0000 -134.0000 62.7500 60.7500|
|Sujets||congélation du sol; glace fossile; températures au sol; pergélisol; analyses thermiques; géologie des dépôts meubles/géomorphologie; géologie de l'ingénieur; Cénozoïque; Quaternaire|
|Programme||pipeline nordiques, Géosciences environnementales|
|Résumé||(disponible en anglais seulement)|
Permafrost presents significant challenges to northern development. Information on permafrost distribution and ground thermal conditions is
required for engineering design of northern infrastructure to ensure its long-term integrity and to minimize environmental impacts.
There is potential for construction and operation of a natural gas pipeline in the Alaska Highway Corridor (Figure
1). Much of the available information on ground thermal conditions in the corridor was collected 30 years ago. Analysis of air temperature records from Environment Canada stations in the corridor indicate that mean annual air temperatures have
in-creased 0.4 to 0.5°C per decade since the 1970s. Recent studies in the corridor (James 2010) and from other regions (Smith et al. 2010) indicate that permafrost has warmed over the last 30 years and in some places it may have completely degraded.
There is a need therefore, to validate current ground thermal conditions within the region to improve the regional characterization of permafrost conditions. In summer 2011, a suite of instrumented field sites were established to provide new
information on ground temperatures for the region and to characterize the changes in the ground thermal regime that have occurred over the last three decades.
In the late 1970s and early 1980s, ground temperatures were
measured in 17 boreholes throughout the corridor by the Geological Survey of Canada (Burgess et al. 1982). Most of these boreholes were 6 to 10 m deep. Field investigations in summer 2011 focused on locating these boreholes and installation of
temperature cables. The goal was to acquire information that could be used to characterize current ground thermal conditions and also to compare with the earlier measurements to deter-mine how conditions have changed.
Fifteen of the boreholes were
found in summer 2011. All of these were located between Whitehorse and the Alaska border. In most cases the boreholes were found to be blocked by ice and had to be opened through steaming. Where this process was successful, a new PVC casing (25 mm
diameter) was installed to preserve the borehole for installation of a multi-thermistor cable.
Eight boreholes were successfully opened to depths of at least 5 m and cased (Figure 1) and instrumented. The temperature cables were connected to
eight-channel data loggers manufactured by RBR Ltd. to provide a continuous record of ground temperatures. A manual reading was also taken from each cable shortly after installation to provide preliminary information on the ground temperatures. Frost
probing was also conducted during the site visit. Site visits are planned in summer 2012 to acquire data from the loggers.
It was not possible to unblock some boreholes beyond a depth of 3 or 4 m and it was therefore not practical to install
multi-sensor cables. However, instantaneous manual temperature measurements were made at some sites (M2 to M5 in Figure 1). Additional instrumentation, including four-channel HOBO data loggers will be installed at some of these sites in summer 2012
to acquire additional information on the shallow ground thermal regime.
Measurements made between 1978 and 1981 in boreholes between Whitehorse and the Alaska border indicated that ground temperatures at depths of 6 to 10 m
were generally above -2°C with exception of three boreholes (located between Burwash and the Alaska border) where the temperature was between -2 and -3°C (Burgess et al. 1982). The earlier measurements also indicate that frozen conditions existed at
13 of the 17 boreholes. Manual measurements were made in August 2011 at 7 of the 8 boreholes instrumented with cables and loggers. Conditions however, may not have completely stabilized following the disturbance associated with unblocking the
boreholes. These preliminary measurements do indicate that permafrost exists at 5 boreholes (R3 to R7) where frozen conditions were also found three decades earlier. Frost probing at R8 indicates that permafrost still persists at this borehole.
Unfrozen conditions were found at the other two boreholes (R1 and R2) which was also the case in the late 1970s.
Although permafrost has continued to persist at these sites over the last 3 decades, it is likely to have warmed in response to
increasing air temperatures. Ground temperatures for warm permafrost in the Mackenzie Valley Northwest Territories for example have increased by about 0.2°C per decade since the mid 1980s (Smith et al. 2010). Sufficient data is not yet avail-able for
the Alaska Highway Corridor boreholes to adequately describe the current conditions and to compare to earlier measurements to determine the magnitude of change that may have occurred.
Fifteen boreholes, in which ground temperatures
were measured between 1978 and 1981 were found in the Alaska Highway Corridor between Whitehorse and the Alaska border. Eight of these were successfully instrumented with temperature cables and loggers. The preliminary ground temperature measurements
indicate that at sites where frozen conditions existed 30 years ago, permafrost generally continues to persist. Data collection for at least one year will be required to characterize the current ground thermal regime and to gain insights into how
conditions are changing. Continued data collection will ensure an adequate baseline is available for the identification of environmental effects that may be associated with development within the corridor.