Micro gravity and cross-hole seismic to monitor water storage changes in aquitards

Abstract

Aquitards surround many aquifers. Due to their comparatively high storage capacity, they will control the hydraulics of aquifers in the long term. Long term management of aquifers therefore requires a good understanding of aquitard characteristics. To quantify water storage changes in an alluvial sequence a micro gravity station has been set up adjacent to an agricultural field in the Liverpool Plains in NSW, Australia. Gravity changes over time will provide an integrated signal of the total water storage changes at the site. Piezometers at the site will allow to determine at which depth the storage change is occurring. To derive water storage changes from the water level changes in the aquitards and confined aquifers at the site, the specific storage (Ss) of the formation needs to be known. Two insitu methods to derive Ss are applied. Firstly, Ss is derived based on the bulk modulus, which is obtained in a cross hole seismic survey and secondly Ss is derived based on pressure analysis in the lysimeters. Ss values derived from the pressure analysis in the lysimeters are one magnitude higher than those based on the cross hole seismic. This difference needs to be further investigated and might be due to the thick unsaturated zone (20 m) at the site, which appears to interfere with the pressure analysis in the lysimeters.