ALMA-J Site Testing
ALMA-J Site Testing Archive
Last update 2005-01-24

Subsurface Temperature Profiling

    A six-channel thermometer was installed at Pampa la Bola (4800 m a.s.l.) in 2000 February to monitor time variation of the temperature profile of the subsurface layer. 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 5.3x10-7 m2 s-1 for more than 75% of time, with the median value of 4.6x10-7 m2 s-1. A significant seasonal variation was found, with higher values (up to ~10x10-7 m2 s-1) during rainy summer, and with very low values (~2x10-7 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.


Technical Description