STORAGE CONSTANT VALUES FOR THE EDWARDS AQUIFER BALCONES FAULT ZONE AS DETERMINED FROM SEISMIC EFFICIENCY

Determining storativity values for a confined aquifer typically requires data such as (1) observation-well water levels during a drawdown test or (2) calculated values for barometric or tidal efficiency. Data from paired wells suitable for classic storativity calculations are, however, generally sparse in any aquifer. Barometric and tidal efficiency provide estimates of aquifer compressibility, which is used to calculate storativity using the equation developed by Jacob in 1939. Calculating barometric or tidal efficiency require long-term, detailed records on water levels and either atmospheric pressure or Earth-tide fluctuations. Efficiency ratio estimates need data processing to remove trends including but not limited to pumping-related drawdown and weather patterns. Tidal efficiency requires access to geodetic Earth-tide models.

Seismic efficiency calculations provide another approach to estimating aquifer compressibility. Pressure waves generated by seismic events can cause a water-level response in confined aquifers through which the waves pass. The duration of the water-level response is very brief, so detrending is not needed. A method of estimating the magnitude of the Rayleigh waves in the aquifer and a water-level monitoring network are needed. Continuous recording paper charts can capture more seismic-efficiency data than digital recorders with sampling intervals greater than 15 minutes.

A review to date of fifteen years of historical hydrographs for twelve wells in the Edwards Aquifer, Balcones Fault Zone, shows that seismic events of 7.0 magnitude or greater in North America display a characteristic signal in many of the recorded hydrographs. Events have also been recorded on the hydrographs from the December 26, 2004 and March 28, 2005 Indonesian earthquakes. Differences in amplitude on the same well for separate events correlate with the change in the strength of the pressure wave due to event magnitude or source distance. The variation in amplitude seen between wells during the same event may indicate the difference in storage between the regions. Finding storage constants for a variety of wells through out the Edwards Aquifer Balcones Fault Zone can provide hard data of the storage trends through out the region, and assist in the establishment of a more representative model for the region.