Thermal instability of the fluid column in a borehole: application to the Yaxcopoil hole (Mexico)

Abstract

Observational evidence proved that even when a borehole is in ''fully'' stabilized conditions, temperature data may exhibit certain unrest resembling irregular oscillations in the order of hundredths or (in the extreme case) even tenths of degree. Temperature was monitored in complicated hydrogeological conditions in the Yaxcopoil-1 hole (Chicxulub impact structure, Mexico). Two experiments are reported: (a) 20-day monitoring when a logger was located in the center of the high temperature gradient anomaly produced by the cold wave slowly propagating downwards and (b) simultaneous three-loggers 18-day monitoring with loggers located above, in and below the anomaly. All observed temperature-time series displayed intermittent oscillations of temperature with sharp gradients and large fluctuations over all observed time scales. While the ''upper'' and ''lower'' records revealed quasiperiodic temperature variations, the ''central'' record shows fast temperature oscillations with strong up-and-down reversals, all with amplitudes up to a few tenths of degree. The observed temperature-time series were processed by recurrence and recurrence interval quantification as well as by spectral analyses. It is shown that fluid in a borehole, subject to thermal gradient, is stable, as far as the gradient remains below a certain critical value. At higher Rayleigh numbers, the periodic character of oscillations typical for ''quiescent'' regime is superseded by stochastic features. This ''oscillatory'' convection occurs due to instability of the horizontal boundary layers. In the specific case of the Yaxcopoil hole, the time series above and below the cold wave (characterized by relatively lower temperature gradients between 20 and 50 mK/m) contain a clear low frequency component produced by tidal forcing. This component dominates over the high frequency domain (periods from 10-15 to 1 min), which exhibit a scaling behavior. This pattern conspicuously changes in the center part of the cold wave, where the local temperature gradient exceeds 200 mK/m and where tidal forcing composes only *3% of the signal.