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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS017 Oral Presentation 2359
Temperature-time series, thermal regime and the instability of the fluid
column in a borehole
Dr. Vladimir Cermak
Geophysical Institute Academy of Sciences of the Czech Republic IASPEI
The increase in sensitivity together with extended data storage capacity of data loggers has opened
new horizons for detailed in-situ borehole experiments. Observational evidence proved that even when
a borehole is in apparently well stabilized conditions, temperature data may exhibit certain unrest
resembling irregular oscillations. (1) We demonstrate the results of monitoring experiments performed
in widely different geological settings: incidental observations from Kamchatka, Finland and Mexico
together with systematic studies performed in the test hole in Prague (Czech Rep.) covering time span
of various length (days to months) with sampling interval varying from seconds to minutes. (2)
Temperature in the borehole fluid (water) exhibits oscillation of the order of several thousandths or
even hundredths of degree. (3) The character of the oscillation may differ depth to depth and
(probably) also time to time. (4) Obtained temperature-time series displayed intermittent, non-periodic
oscillations of temperature with sharp gradients and large fluctuations over all observed time scales. (5)
Local growth of the second moment technique revealed the presence of at least two distinct
temperature forming processes. One of them can be related to heat transfer in the structurally and
compositionally complex subsurface. The second of them, which presents the bulk of the measured
signal, probably reflects intra-hole convection. It can be demonstrated that at higher Rayleigh numbers
the periodic character of oscillations characteristic for quiescent regime is superseded by stochastic
features. The oscillatory convection occurs due to instability of the horizontal boundary layers. In spite
of the fact that convection is characterized by slow motion, the oscillatory intra-hole flow and
corresponding temperature patterns may exhibit features typical of turbulence. The synchronous
arrangement of several data loggers (up to five) located at close depth intervals (a few meters) enabled
to map time variation of the local temperature patterns at depth.
Keywords: geothermics, temperature time monitoring, borehole convection