Tracer Tomography: Design Concepts and Field Experiments Using Heat as a Tracer

Abstract

Numerical and laboratory studies have provided evidence that combining hydraulic tomography with tomographic tracer tests could improve the estimation of hydraulic conductivity compared with using hydraulic data alone. Field demonstrations, however, have been lacking so far, which we attribute to experimental difficulties. In this study, we present a conceptual design and experimental applications of tracer tomography at the field scale using heat as a tracer. In our experimental design, we improve active heat tracer testing by minimizing possible effects of heat losses, buoyancy, viscosity, and changing boundary conditions. We also utilize a cost-effective approach of measuring temperature changes in situ at high resolution. We apply the presented method to the 8 m thick heterogeneous, sandy gravel, alluvial aquifer at the Lauswiesen Hydrogeological Research Site in Tübingen, Germany. Results of our tomographic heat-tracer experiments are in line with earlier work on characterizing the aquifer at the test site. We demonstrate from the experimental perspective that tracer tomography is applicable and suitable at the field scale using heat as a tracer. The experimental results also demonstrate the potential of heat-tracer tomography as a cost-effective means for characterizing aquifer heterogeneity.

Synthetic fracture network characterization with transdimensional inversion

“…Equivalent to the hypothetical case, three sources and three receiver points are used, located at the right and left edge of the domain at known fracture ends. The same initial and boundary conditions are used as in the previous case, but the experiment is only performed in one direction without swapping the injection and observation well, which is considered to be a more realistic scenario [ Doro et al ., ; Djibrilla Saley et al ., ; Jiménez et al ., ]. The experimental time was set to 2000 s and the concentrations were measured every 20 s. Given the longer time of the experiment, much more data points are collected than in the hypothetical case.…”

Travel-time-based thermal tracer tomography

“…In all of our models, the ambient groundwater temperature is considered uniform and 10 • C. Viscosity and density effects increase with the temperature difference, T , in comparison to the ambient groundwater. These effects may distort the results of inversion, and thus a maximal difference of T = 8-15 • C has been suggested for thermal tracer testing (Doro et al, 2015;Ma and Zheng, 2010;Russo and Taddia, 2010). This severely constrains the applicability of heat as an active tracer, because it complicates interpretation of BTCs influenced by buoyancy forces.…”

“…The solution to this equation can be formulated as a series of wave equations (Fatemi et al, 1995). Using the complex wave functions as an asymptotic expansion, the solution becomes T (x, t) = e iωσ (x,t) ∞ n=0 τ n (x, t)(iω) −n ,…”

“…Travel times of traditional solute tracers are related to the hydraulic properties of aquifers, assuming that the main transport process is advection. This is the case given a sufficient ambient hydraulic or forced gradient during the experiment (Doro et al, 2015;Saar, 2011). One important difference of heat tracer transport over traditional tracers is that diffusion takes place not only in the pore fluid but in the rock matrix as well.…”

“…This method provides cross-well connectivities. The work by Doro et al (2015) is dedicated to the experimental design of cross-well forced gradient thermal tracer tomography. In their approach, a special multi-level injection system is necessary to induce the tracer into a horizontal layer.…”