RECURRENCE QUANTIFICATION ANALYSIS OF BOREHOLE TEMPERATURES: EVIDENCE OF FLUID CONVECTION

Abstract

To describe intra-hole fluid convection, fine-scale temperature-time variations were monitored at 25 depth levels within a 50 m long interval (80—130 m depth) in a slim experimental borehole in Sporilov (the Czech Republic). High-resolution temperature data were sampled with a 15 sec time interval for 1.6 up to 2.7 days. The time series obtained were from 10 000 to 16 000 data points. In the upper part of the investigated depth interval, the temperature time series showed a complex oscillation pattern with amplitudes of up to 45 mK (milliKelvin). This variation pattern is alternated with a "quiet" regime below 110 m depth, where temperature oscillations fall within 4–10 mK range. Convection produced temperature signal was studied by means of recurrence plots and their quantification analysis. This method was proven to be effective to detect transitions in system dynamics. The analysis confirmed a relevant deterministic part in the measured signal (quasi-periodic skeleton) and enabled to quantify the periodic—periodic and periodic—chaotic transitions in the borehole convection dynamics. Thermal dynamics is richer and cooler at shallower depths. The results were confirmed with temperature time series registered at the bottom hole, where the probe penetrated into soft mud debris, which prevented convection, and restricted heat transport to a pure conduction.