Investigation of hydraulic properties in fractured aquifers using cross-well travel-time based thermal tracer tomography: Numerical and field experiments

Hydraulic properties are well known to be essential in controlling fluid flow, solute migration, and heat transport in fractured subsurface. However, accurate characterization of hydraulic properties including the locations of conductive fracture and their hydraulic conductivities as well as their cross-well connectivity is still challenging. In this work, we modified the inversion framework of travel-time based thermal tracer tomography (TT) firstly based on a numerical study. Considering the features of heat transfer in fractured media, a regularization term and an irregular triangular mesh are introduced. A distortion of thermal travel times caused by annular wall flow at an observation well is eliminated by assigning a specific well zone in the inversion model. The performance of this modified inversion framework in characterizing hydraulic properties of fractured aquifers is firstly analyzed through nine numerical tests. Results indicate that the modified TT method can efficiently identify directly connected or interconnected fractures, even with the presence of an ambient hydraulic gradient, observation well annular wall flow, or measurements based on conventional thermal sensors with less precision. Accordingly, a tomographic thermal tracer test is performed and analyzed at a fractured rock experimental site located at the University of Göttingen, Germany. It can be inferred that there is a directly connected fracture, two interconnected fractures, and an overburden zone, with hydraulic conductivities between 5 × 10⁻⁴ m/s and 1 × 10⁻⁵ m/s. The outcomes demonstrate that the proposed inversion framework is efficient and robust in characterizing hydraulic properties of fractured aquifers.