AQUIFER STORAGE PROPERTIES GENERATED FROM WELL WATER LEVEL RESPONSES TO SEISMIC RAYLEIGH WAVES

Observation well water levels in confined aquifers may be sensitive to large global earthquakes, and commonly produce hydroseismograms with centimeter scale water level fluctuations. Regional broadband seismographs can generally provide excellent estimates of Rayleigh wave displacement. The sparse amount of hydraulic head data provided by most well recorders relative to seismological data makes direct spectral comparisons impractical because the spectral density of the water deflections cannot be calculated. To account for the sparse data collection interval, we propose that the mean squared water level fluctuation as measured in the time domain in each well be compared to the mean spectral density as predicted from the passing Rayleigh wave. To increase the precision of this method, adjustment for the wavelength and the borehole response is applied in the frequency domain. This reduces the influence of the longer wavelength (and typically higher amplitude) Rayleigh waves on the process and acts as a band-pass filter enhancing components near the resonant frequency of the borehole. This method can be used to assess aquifer properties under several different scenarios. In the simplest case, if storativity and transmissivity of the aquifer have been estimated by previous well tests, these values can be appraised by the direct use of this empirical equation to predict the mean squared water level deflection for comparison to the mean-squared deflection actually observed. Any discrepancies greater than one order of magnitude would suggest problems with the previous estimates of the aquifer properties.