  Non-invasive geophysical prospecting and a thermodynamic model were used to
examine the structure, depth and lateral extent of the frozen core of a palsa
near Lake Peeraj\"arvi, in northwest Finland. A simple thermodynamic model
verified that the current climatic conditions in the study area allow
sustainable palsa development. A ground penetrating radar (GPR) survey of the
palsa under both winter and summer conditions revealed its internal structure
and the size of its frozen core. GPR imaging in summer detected the upper
peat/core boundary, and imaging in winter detected a deep reflector that
probably represents the lower core boundary. This indicates that only a
combined summer and winter GPR survey completely reveals the lateral and
vertical extent of the frozen core of the palsa. The core underlies the active
layer at a depth of ~0.6 m and extends to about 4 m depth. Its lateral extent
is ~15 m x ~30 m. The presence of the frozen core could also be traced as
minima in surface temperature and ground conductivity measurements. These field
methods and thermodynamic models can be utilized in studies of climate impact
on Arctic wetlands.
