By monitoring the subsurface temperature at Chajnantor, the thermal diffusivity of the soil and the damping of diurnal temperature fluctuations with depth have been measured. The thermal diffusivity, a = (1-5) x 10^-7 m^2 s^-1, roughly the range expected for sandy soil, and varies daily. For the maximum observed diffusivity the diurnal temperature swing 1 m deep is only 0.02 % of the surface amplitude. Shorter period variations are damped more strongly. This damping is sufficiently strong the overall phase stability of the ALMA optical fibers may be determined not by the 25 km long buried sections, but by shorter, above ground lengths.
Large diffusivity values are correlated with precipitation and soil moisture. At 30 cm depth, soil moisture persisted for about 15 days after a snowfall that melted from the surface in 7 days. Subsurface freezing and melting episodes suggest the soil salinity is sufficiently high to enable drainage and prevent permafrost.
View a PDF version of ALMA Memo #314. Download a PS version of ALMA Memo #314.
Last modified: June 27, 2000
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