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<h1><b><font size=2 face="Courier New"><span style='font-size:10.0pt;
font-family:"Courier New";font-weight:normal'>ALMA MEMO #480 <br>
<br>
Thermal Properties of subsurface layer at Pampa La Bola </span></font></b></h1>
<h2><b><font size=2 face="Courier New"><span style='font-size:10.0pt;
font-family:"Courier New";font-weight:normal'>Seiichi Sakamoto (NAOJ) and Hideharu
Ishizaki (NAOJ) <br>
<br>
2004/02/06</span></font></b></h2>
<p class=MsoNormal><font size=2 face="Courier New"><span style='font-size:10.0pt;
font-family:"Courier New"'>Keywords: site testing, soils, subsurface
temperature, thermal diffusivity, frost heaving <br>
<br>
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. </span></font></p>
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face="Times New Roman"><span style='font-size:12.0pt'><span style='text-decoration:
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<p class=MsoNormal style='text-autospace:none'><font size=2 face="Courier New"><span
style='font-size:10.0pt;font-family:"Courier New"'>View a pdf version of ALMA
Memo #480. </span></font></p>
<p class=MsoNormal><u><font size=2 color=blue face="Courier New"><span
style='font-size:10.0pt;font-family:"Courier New";color:blue'><a
href="http://www.alma.nrao.edu/memos/html-memos/alma480/memo480.pdf">http://www.alma.nrao.edu/memos/html-memos/alma480/memo480.pdf</a></span></font></u></p>
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