Intra-hole fluid convection: High-resolution temperature time monitoring
2008
Čermák, Vladimír | Safanda, Jan | Kresl, Milan
Intra-hole convection was monitored in detail by a string of five high resolution temperature probes arranged in an 8-m long unit lowered step-by-step into a slim experimental borehole. The experiment was performed in the test hole Sporilov (Prague). A 50-m long depth interval (between 80 and 130m) was covered in five successive steps. The hole itself is 150m deep and 112mm in diameter. The experiment occurred in the internal plastic tube 5cm in diameter which is sealed from the influx of ground water in the surrounding strata. In the studied interval the temperature gradient varies in the range of 0.020-0.0215K/m. The borehole was drilled in 1993 and has been in equilibrium since then. Temperature as a function of time was sampled in 15s intervals and the individual measuring steps took from 1.5 to 2.5 days, each temperature time series thus contained 9000 up to 16,000 data points. The obtained results revealed: (1) temperature-time series present a complex apparently random oscillation pattern with the amplitude of up to 0.045K; (2) the statistical analysis confirmed a quasi-periodic skeleton of a two-frequency oscillation structure. Shorter periods of 10 up to 30min are superposed on longer variations with period of several hours. (3) The quasi-periodicity may be hidden under a considerable amount of noise. (4) Within the studied interval the quasi-periodic convection may alternate with a relatively “quiet” regime when temperature oscillations decreased to only 0.004-0.01K range. (5) Regardless of certain deterministic rules present in the dynamics, the bulk of temperature variations are chaotic.
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