Utility of Ambient Groundwater Temperature Profiling within Sealed Bedrock Boreholes for Fracture Flow Characterization in Seasonally Dynamic Environments

Abstract

Temperature profiles were collected periodically over a 12 month period in 3 angled and 3 vertical FLUTe™ lined boreholes, drilled to 24-32 mbgs, in a dolostone bedrock aquifer adjacent to and extending beneath a bedrock river to study system hydrodynamics of a bedrock groundwater-surface water interface. Ambient borehole temperature data were shown to be effective at identifying zones of hydraulic activity during periods of intra-seasonal stability and whose seasonal atmospheric effects provided evidence of a strong vertical flow component in the upper 8-10 mbgs of the system. Analysis of thermal deviation logs’ Fourier spectra provided a novel way to observe the shallow bedrock flow system’s temperature evolution, identify noise caused by free convection, and represents a diagnostic tool to improve confidence in identifying hydraulic activity from thermal data sets. Free convection extended the depth of detection to 26 mbgs, as opposed to 14 mbgs when free convection was not present.