DTS as a Hydrostratigraphic Characterization Tool

Background/Need: Subsurface heterogeneity in hydraulic properties and processes is a fundamental challenge in hydrogeology. Most hydrogeologic problems are complicated by uncertainty in permeability, which is often difficult or impossible to fully characterize. The usefulness of heat as a tracer has been limited by thermometry that only records temporal changes in temperature at a single fixed or moving point. Distributed temperature sensing (DTS) is a powerful new method that allows for the nearly continuous measurement of temperature in time and space along fiber-optic cables. The fine spatial and temporal monitoring ability of DTS is creating new and unprecedented opportunities to study hydraulic heterogeneity at a wide range of scales. Despite numerous recent applications of DTS applications in surface water investigations, down-hole uses in hydrogeology have been limited. Recent studies on the Sandstone Aquifer system of Wisconsin have shown preferential flow through laterally continuous bedding plane fractures to be a defining characteristic of sandstone units that were traditionally assumed to be homogeneous and isotropic. The implication of these findings is that more detailed characterization efforts are necessary to adequately assess flow and transport problems in these units.
Objectives: The purposes of this study were to develop DTS as a down-hole groundwater monitoring and aquifer characterization tool, and to use its novel monitoring capabilities to gain new information on hydraulic heterogeneity in the Sandstone Aquifer system. This study builds on previous work by using DTS to monitor ambient and artificially-stimulated temperatures for the purpose of detailed hydraulic characterization at the borehole scale. In addition, DTS was used to investigate the effects of borehole flow processes on temperatures measured in wells. Finally, the novel monitoring capabilities of DTS allow hydraulic heterogeneity in the Sandstone Aquifer system to be studied at an unprecedented level of detail.