Storage in confined aquifer: Spectral analysis of groundwater responses to seismic Rayleigh waves

It is frequently found that the fluctuations observed in aquifer-well systems demonstrate high coherence to nearby boundary disturbances, or more specific, in response to the vertical motion of seismic Rayleigh waves. This study derives the spectral relationship between groundwater head and vertical displacement disturbed by Rayleigh waves in the time-frequency domain. Essentially, the groundwater storage property, e.g. storage coefficient or specific storage, representing the amount of water released from storage when the head changes from water expansion and aquifer compression occur. They can be determined using the spectral representation of the factors associated to groundwater head and Rayleigh waves. For this study, the Sumatra-Andaman Islands Earthquake with magnitude 9 at UT 2004/12/26 00:58:53.45 is used as the seismic source while a groundwater monitoring well and a seismological station located on the east of Taiwan acted as the receivers. The time series data associated with the observed groundwater head and vertical displacement of Rayleigh waves are used to evaluate spectral response analyzing the autospectral density, cross-spectral density, and coherence between them. The resultant spectral estimates can be used to determine the storage coefficient and specific storage of the case site to be on the order of 10(-3) and 10(-4) (m(-1)), respectively. In addition, the matrix bulk modulus also demonstrated a linear trend with porosity by taking the value of standard water modulus. It suggests that the storage of an aquifer can be robustly estimated using spectral representations and responses from head changes in groundwater and vertical displacement of Rayleigh waves. (C) 2009 Elsevier B.V. All rights reserved.