STUDIES OF PERMEABILITY IN SHALLOW EOLIAN SEDIMENTS IN THE NEBRASKA SAND HILLS AND GREAT SAND DUNES NATIONAL MONUMENT, COLORADO

A subsurface air permeameter, a device for estimating formation permeability (k), supplies air at a known steady gauge pressure p and flow rate Q to a given location. The developed permeameter delivers air to a given shallow depth in unconsolidated formations. A standard rotameter was used to measure Q, and p was determined from voltage measurements using a calibrated pressure sensor. The theory of operation and subsurface and surface probe components of this system were developed and applied systematically to measure k on vegetation-stabilized dunes at Gudmundsen Sandhills Laboratory (GSL) in the West-Central Nebraska Sand Hills and a barchan dune in the active dune field at Great Sand Dunes National Monument, Colorado. The permeability of the active dune was greatest for grainflow deposits, intermediate for grainfall deposits, and smallest for stoss ripple strata in the active dune. Values of k were determined at five uniformly spaced depths (between 0.10 m and 1.30 m to top of screen) for 34 sites on a large barchanoid dune at GSL. A specific region on the dune displayed an average k up to six times higher than the averages of the other regions, suggesting that it corresponded to the grainflow region at the time the dune was active. A sand core sample was collected at a site with very low k values at 1.0 m measurement depth; examination of the core revealed the presence of lamellae (sand layers enriched in silt and clay). The subsurface permeability on the vegetated dune ranged from 83.8 E-12 m^2 to 0.726 E-12 m^2, and below the detection limit at certain locations.