Narrow Results

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.

In the present work, we focus on the multifractal structure of the microtemperature time series monitored at depth in boreholes, namely in two holes drilled in Kamchatka (Russia). Two monitoring series were performed for approximately two weeks with a 5-second reading interval; thus, each series contains about 230,000 data points. The observed temperatures displayed sharp gradients and large fluctuations over all observed time ranges, reflecting fine structure of the heat transfer process in the shallow subsurface. Recurrence plot (RP) technique was applied for detecting hidden rhythms that generated the time series. The most characteristic feature of the RPs is their web-like structure, indicating quasi-periodic occurrence of the sharp changes. The spectral and the local growth of the second moment techniques were used for distinguishing between the potentially different heat transfer processes. Both spectra show "red noise" behavior, however, exhibit distinct scaling exponents for different frequency domains from near 1.33 for low frequencies to near 3 at the high frequency end of the spectra. Local growth of the second moment technique has revealed the presence of temperature forming process with the characteristic time of approximately 1 minute, that smoothes out generally anti-persistent behavior on shorter time scales, but has a little effect on longer time scales.

The investigation was accomplished by the calculation of universal multifractal indices, which characterize temperature fluctuation upon scales in the range from minutes to weeks. Both time series show very similar multifractal behavior. The Hurst exponent, characterizing the degree of non-stationarity, equals to 0.18–0.20, the measure of intermittence C₁ amounts to 0.10, and the α-index, characterizing the degree of multifractality, amounts to 1.32 and 1.24 for both boreholes. We speculate on the origin of these common features intervening all over the observed range of time scales in a borehole.

A laser-ultrasonic experimental setup was used to study, at a reduced scale, the wave propagation inside and around fluid-filled wells. Simulations tools were also developed and calibrated from comparisons with experimental signals. These tools serve as a connection to realistic scale. A semi-analytical approach, the discrete wave number method was first used to compute signals in a simplified geometrical configuration. This method is fast enough to be used in the identification of the main parameters that describe at best the experimental signals. Then a finite difference scheme was implemented in order to describe accurately the actual well. The two methods describe the attenuation mechanisms by using the Kelvin–Voigt model for the solid and the Maxwell model for the fluid. Comparisons between numerical and experimental waveforms, obtained in the two fundamental elastic configurations: the fast and the slow formations, show very good agreement in arrival times, waveforms and relative amplitudes. This satisfactory result provides insights useful for the recognition and interpretation of wave propagation in complex media. Such is the case of modern sonic-logging technology.

Improper design of landfill sites may cause serious problems to groundwater. Leachate, a very concentrated pollutant generated from the decomposition of waste and by precipitation, may penetrate through the waste layers and go straight to the aquifer. This chapter discusses some background on groundwater resource, its properties, and monitoring at landfill sites. Pulau Burung landfill in Penang, Malaysia was taken as the case study site where boreholes and monitoring wells were sampled and the results discussed.