Understanding time-lapse thermal deviation logs in a shallow fractured bedrock groundwater flow system
Abstract
Temperature has long been recognized as an effective tracer for groundwater flow in porous and fractured media. Recent technological advancements have enabled the collection of high-resolution spatial and temporal temperature measurements with unprecedented accuracy, precision and frequency. The ability to detect subtle, yet measurable changes in groundwater flow paths based on discrete thermal deviations is fundamental to understand groundwater flow and solute transport through heterogeneous fracture networks. Until recently, groundwater temperature measurements in bedrock were collected along long open boreholes. Although these cross-connecting or "open" holes improved detection of high-flow zones and vertical hydraulic gradients, these data provide minimal insight into the natural flow regime. Advancements in borehole liner technologies that hydraulically seal the borehole water column from the adjacent formation provide a means of measuring centimetre-scale changes in temperature associated with active groundwater flow in discrete fractures under natural flow conditions. We present the results of a 1 year long borehole temperature profiling experiment conducted across a cluster of hydraulically-sealed vertical and angled boreholes positioned along the shoreline of a river. Time-series and Fourier analysis was applied to thermal deviation logs to evaluate intra- and inter-seasonal spatiotemporal transients in groundwater flow, and differentiate between forced-convection (gradient-driven) and free-convection (density-driven) flow outside and inside the borehole, respectively. Although free-convection within the boreholes during cooler seasonal periods led to higher thermal noise, the formation of these convection cells induced a thermal-disequilibrium within the homothermic zone, enabling the characterization of previously undetected fracture flow features deeper in the system. Our results demonstrate the utility of time-lapse temperature profiling in vertical and angled boreholes exposed to extreme seasonal thermal variability and hydraulic activity. Quantitative time-series and frequency analysis was necessary to assess the dynamic thermal signatures associated with groundwater fluxes in a near-surface fractured sedimentary rock environment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.H33I..07S
- Keywords:
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- 1805 Computational hydrology;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1832 Groundwater transport;
- HYDROLOGY;
- 1835 Hydrogeophysics;
- HYDROLOGY