Prior to commencing coal mining operations, measurements of permeable geological structures are an important task in the mine site exploration to facilitate the understanding of potential mining impacts on the adjacent river and groundwater systems. These potential environmental impacts are likely to be controlled predominately by preferential flow zones provided by faults and fractures, which are known to enhance the transport of gas and water. However, detailed mapping of these geological structures is rarely carried out in mining areas. In order to develop a feasible and rapid survey method, two field campaigns, involving soil gas mapping and gamma ray survey, were designed and carried out a at coal mine site in the Hunter River Valley, NSW, Australia. All measured parameters (radon, gamma ray and CO2) revealed similar anomaly distribution patterns though measured by different techniques over different time periods (1.5 years elapsed between two field campaigns). Geophysical modelling (based on borehole logging results) and soil gas mapping were conducted independently, and both results indicated the existence of geological structures crossing the sampling region. Results of this study confirm the applicability of high intensity soil gas mapping methods for locating active tectonics faults covered by alluvium in the decametre scale. Additionally, compared with labour-intensive soil gas mapping methods, the gamma ray survey is capable of providing relatively reliable results in a fast and convenient way to cover a relatively large field site.
The basic principles and controlling parameters for gas seepage from deep formations to the surface are not fully understood. In order to better understand gas transport mechanisms in the subsurface environment so that the soil gas mapping data can be better interpreted, laboratory experiments were conducted to investigate the flow of discrete microbubbles through a saturated porous medium as part of a rapid gas transport pathway from deep ground formations to the ground surface. During the experiments, bubbles, released from a point source, moved upward through a quasi-2D flume filled with transparent water-based gelbeads and formed a distinct plume that could be well captured by a calibrated camera. Outflowing bubbles were collected on the top of the flume using volumetric burettes for flux measurements. We quantified the scaling behaviours between the gas (bubble) release rates and various characteristic parameters of the bubble plume, including plume tip velocity, plume width and breakthrough time of plume front. The experiments also revealed circulations of ambient pore water induced by the bubble flow. Based on a simple momentum exchange model, we showed that the relationship between the mean pore-water velocity and gas release rate is consistent with the scaling solution for the bubble plume. These findings have important implications for studies of soil gas mapping and gas emission (including hydrocarbon gases) from deep ground formations as well as fundamental research on gas transport in porous media.
During the laboratory experiment, a new image processing method was developed for measuring microbubble sizes under high bubble density and clustered conditions based on a linear relationship between the light centroid and dark outline of bubble projections. The light centroids of individual bubbles in clusters can be measured accurately and used to determine bubble sizes. Tested under a highly clustered condition, the new method was found to be effective and robust.
Microbubble transport, as a possible mechanism for gas transport in the subsurface environment, was further investigated with a continuum model to simulate bubbly two phase flow at the laboratory scale. The Naiver-Stokes equations were solved for bubbly flow in a 2-dimensional (2D) porous medium domain, with bubble released from a point source at pre-set flux rates. The transient bubble transport behaviour and bubble-induced ambient pore water flow were studied to compare with scaling solutions derived from the experimental data. The simulation results provided us with more physical insights into the complex bubble transport behaviour in porous media.