Abstract. Understanding of single-phase and multiphase flow and transport in fractures can be greatly enhanced through experimentation in transparent systems (analogs or replicas) where light transmission techniques yield quantitative measurements of aperture, solute concentration, and phase saturation fields. Here we quantify aperture field measurement error and demonstrate the influence of this error on the results of flow and transport simulations (hypothesized experimental results) through saturated and partially saturated fractures. We find that precision and accuracy can be balanced to greatly improve the technique and present a measurement protocol to obtain a minimum error field. Simulation results show an increased sensitivity to error as we move from flow to transport and from saturated to partially saturated conditions. Significant sensitivity under partially saturated conditions results in differences in channeling and multiple-peaked breakthrough curves. These results emphasize the critical importance of defining and minimizing error for studies of flow and transport in single fractures. While light transmission methods for measuring aperture fields have been applied previously [e.g., Glass and Nicholl, 1995; Persoft and Pruess, 1995; Brown et al., 1998], they have not been thoroughly evaluated with respect to error and therefore yield data of ambiguous quality. Additionally, the influence of this error on our interpretation of the underlying physics within a particular single-phase or multiphase experiment has yet to be considered. The appropriate design of experiments required to advance our understanding of the fundamental physics demands that both of these evaluations be accomplished. In this paper, we evaluate aperture field measurement error for the light transmission technique, outline a measurement protocol to obtain a minimum error field (optimal field), and demonstrate the influence of accuracy on hypothesized experimental results using simulations of flow and transport through saturated and partially saturated fractures.To enhance our understanding of the light transmission technique, we independently evaluate each source of error that contributes to the total measurement error. We find reducing measurement error requires balancing precision (random error) and accuracy (systematic or bias error) to minimize the total error for a particular fracture and light transmission apparatus. With this understanding we formulate a general protocol for measuring aperture fields. For our system, measurements on a representative "baseline" rough-walled fracture yielded an estimated root-mean-square (

[1] We conducted two laboratory experiments at the meter scale in which water was applied to the top of an initially dry, uncemented wall composed of porous bricks. One experiment (Experiment 1) encouraged evaporation and resulting mineral precipitation, while the other (Experiment 2) was designed to minimize these processes. In both cases, processes acting within the fracture network controlled early time behavior, forming discrete pathways and demonstrating fractures to act as both flow conductors and capillary barriers. At a later time, evaporation-mineral precipitation in Experiment 1 constrained flow, strengthening some pathways and starving others. In Experiment 2, the wetted structure took on the appearance of a diffuse plume; however, individual pathways persisted within the wetted structure and interacted, displaying erratic outflow over a wide range of timescales, including switching between pathways. Thus, under conditions of constant supply and both with and without evaporation-mineral precipitation, unsaturated flow through fractured rock can create dynamic preferential pathways.

Double-diffusive finger convection is studied experimentally in a transparent Hele-Shaw cell for a two-solute system. A less dense sucrose solution is layered on top of a more dense salt solution using a laminar flow technique, and convective motion is followed photographically from the static state. We systematically increase solute concentrations from dilute to the solubility limit of the salt solution while maintaining a fixed buoyancy ratio of approximately 1.08. Across the 14 experiments conducted, the convective motion shows considerable variation in both structure and time scale. We find that new finger pairs form continuously within a finger generation zone where complexity increases with Rayleigh number, reaches a peak, and then decreases for highly concentrated solutions. The vertical fnger length scale grows linearly in time across the full concentration range. The vertical finger velocity also increases linearly with Rayleigh number, but as the concentrations increase, deviation from linearity and asymmetrical convection occur. The horizontal length scale grows as a power law in time with the exponent constant over most of the range; again, deviations are observed for highly concentrated solutions. The observed deviations at high concentrations are attributed to the increasing nonlinearity in the governing equations as the solutions approach their solubility limits. There, the fluid properties become functions of solute concentration and vary significantly within the experimental fields suppressing structural complexity, imparting asymmetry to the convective motion, and influencing emergent vertical and horizontal length scales and their growth.

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