[asac][almanews] ALMA Memo 480 Released

[Stacy Oliver]

ALMA MEMO #480 

Thermal Properties of subsurface layer at Pampa La Bola 


Seiichi Sakamoto (NAOJ) and Hideharu Ishizaki (NAOJ) 

2004/02/06


Keywords: site testing, soils, subsurface temperature, thermal
diffusivity, frost heaving 

We monitored subsurface temperature for more than 2 years at Pampa La
Bola every 20 cm down to 100 cm, and evaluated hourly temperature
change, apparent thermal diffusivity, freezing points, and depth of
frost line. Hourly temperature changes at 0-40 cm are dominated by
diurnal cycle, whereas longer timescale variation takes over at deeper
layer. Annual temperature range remains 9degC peak-to-peak even at the
depth of 80 cm. The hourly temperature change at 80 cm and deeper
remains at a level of ~ 0.005degC hr-1, and is mostly attributed to the
annual cycle and practically cannot be overcome simply by digging deeper
trenches. The apparent thermal diffusivity estimated from amplitude
suppression of diurnal temperature change between 0 and 60 cm was
smaller than 5.3 * 10^{-7} m2 s-1 for more than 75% of time, with the
median value of 4.6 * 10^{-7} m2 s-1. A significant seasonal variation
was found, with higher values (up to ~ 10 * 10^{-7} m2 s-1) during rainy
summer, and with very low values (~ 2 * 10^{-7} m2 s-1) in autumn and
spring when freezing-thawing transition layer appears near the surface.
The freezing point deduced from statistical plots was -0.2degC, and can
be explained with 0.3% salinity. Annual mean subsurface temperature was
higher than freezing point with slight increasing trend as a function of
depth, and no permafrost will exist in the plateau. Depth of frost line
is approximately 1 m at Pampa La Bola, and is estimated to be 2 m or
more at Llano de Chajnantor because of lower mean temperature. 

 

View a pdf version of ALMA Memo #480. 

http://www.alma.nrao.edu/memos/html-memos/alma480/memo480.pdf

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