Determining Fault Permeability from Subsurface Barometric Pressure

Measured barometric pressure fluctuations in a borehole drilled 600 m into the unsaturated zone at a site on the eastern slope of Yucca Mountain, Nevada, are used to compute pneumatic diffusivities and air permeabilities of layered tuffaceous rocks. Of particular interest is the occurrence of increasing pressure amplitude and decreasing phase lags observed at several deep stations. The pressure amplitude increases by 13% and the pressure peak arrives 65 to 72 h earlier as the depth of the unsaturated zone increases from 407 to 436 m. This observation is inconsistent with a conventional 1D downward pressure decay model, but can be explained by hypothesizing the existence of a lateral pneumatic flow path from a nearby fault. A 2D numerical model has been constructed to test this lateral flow hypothesis. The modeling results indicate that a leaky-fault hypothesis can reconcile the measured barometric data with other geologic mapping and permeability measurements at the site. The inferred fault permeability ranges from 0.6 × 10⁻¹² to 0.12 × 10⁻⁹ m², and that of the surrounding tuff layers ranges from 0.4 × 10⁻¹³ to 0.8 × 10⁻¹² m². Our analysis also demonstrates that although the larger-amplitude, longer-period synoptic signals lack coherence and periodicity, determination of air permeability in deep unsaturated zones must rely on these longer-period oscillations of barometric pressure whose periods are on the order of weeks to months because the shorter-period diurnal and semidiurnal barometric oscillations generally are not detectable at deep depths.