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The drift shadow is a region below a cavity in an unsaturated environment that is sheltered from downward-percolating water, and is caused by capillary forces being too weak to immediately draw percolating water into this region. The drift shadow has been predicted using numerical and analytical models, but has not been identified in field sites. We are performing a field investigation to find the drift shadow. (Natural Analogue Studies of the Drift Shadow Effect- Powerpoint file).
In this figure, flow paths through a coal sample are identified by the darker colors. This image sequence contains x-ray CT slices of a 1.5 inch diameter coal core. Each image is the difference between two x-ray CT images. In one, liquid CO2 was present in the larger pores, and in the other, a strongly x-ray attenuating potassium iodide brine was present in the connected porespace. When the two images are subtracted, the difference shows where the flow is occurring.
The ridges identified in these photos are amorphous silica deposited in a fracture that was heated at the bottom. Water containing dissolved silica flowed from the top. The implication is that only a small band of deposited mineral is required to disrupt flow. Scale bars are 0.5 mm long.