Measuring horizontal pore gas velocity profiles in porous media in response to near-surface wind speed and gustiness

Summary: A simple method for experimental determination of horizontal wind-induced, near-surface pore gas velocities in porous media is presented. This method uses traditional tracer gas tracking methodology, but is designed for applications where mass loss of tracer gas from the experimental domain occurs at an unknown rate (as is the case in near-surface, wind-exposed porous media), making traditional inverse transport modelling inapplicable. The method was applied to a dry, granular porous medium consisting of crushed basalt (2–4 mm grain size), using CO₂ as the tracer gas. Experiments were conducted under controlled (wind tunnel) conditions, at average near-surface wind speeds ranging from 0 to 5.6 ms⁻¹, for both gusty and non-gusty winds. Velocity profiles of near-surface horizontal pore gas in the top 15 cm of the porous medium were determined for 10 different combinations of average wind speed and wind gust frequency. Pore gas velocities of up to 0.5 cms⁻¹ were observed that depended on depth and near-surface wind conditions. Results indicate that horizontal gas flow can occur in wind-exposed porous media under both non-gusty and gusty wind conditions, and that wind gustiness has a significant effect on the velocity profiles of horizontal pore gas inside porous media. Results show further that pore gas velocities were almost linearly proportional to near-surface average wind speed, regardless of porous medium depth. Highlights: Developed a novel method for measuring wind-induced pore gas velocity in porous media. Near-surface wind action induced horizontal pore gas velocities. Pore gas velocity profiles depended strongly on wind gustiness (gust frequency). Pore gas velocities were directly proportional to average near-surface wind speed.