Permafrost distribution

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13

Permafrost distribution

permafrost. Bedrock was also not encountered in

Our data show that the extent and thickness of

any of the boreholes and apparently lies below

permafrost vary greatly across the north-central

50 m depth. Reflectors in the permafrost are

cantonment area (Fig. 15). Seasonal freezing

from features that may include strata of differ-

depths often exceed 3.0 m, while active layers may

ing grain size, sedimentary structure, or higher

have seasonal thaw depths as little as 0.3 m. The

ice volume.

depth of thaw apparently varies with drainage,

vegetative cover, organic content of the sediments,

Transect 93-11 is 1600 m long, trends west to

and other factors (e.g., Hopkins et al. 1955, Pw

east, and passes just north of the landfill (see

1958, Ferrians and Hobson 1973). Deeply thawed

Fig. 6). Bedrock is a prominent reflector at depths

materials, ranging from several meters to over 20

of 20 to 30 m along nearly its entire length (Fig.

m in depth, commonly exist beneath disturbed

14). Only where signals are attenuated by water,

areas, such as roads, and in former swales and

such as locations with standing water or where

deeply thawed sediments are near the surface, is

channels (Fig. 15a). Deep thaw may also be

expected beneath the landfill; borehole and GPR

this reflector absent. A subpermafrost aquifer is

a. Elevations of the top of permafrost from borehole records. Active layer

thickness (more than 2 m deep) was also measured in August and Sep-

tember 1994 using a hand-driven steel probe.

Figure 15. Elevation of permafrost (U.S. Army Engineer District,

Alaska, unpublished borehole logs). Bold numbers show wells drilled

between 1 March 1994 and 30 June 1995. All others were drilled be-

tween 1 July and 1 December 1995.