Tomographic Characterization of Aquifer Heterogeneity

Abstract

The Darcy's Law proportionality constant, hydraulic conductivity, describes the relative ease or rate at which water can move through a permeable medium and its fine-scale heterogeneity determines preferential flow rates and pathways. Traditional aquifer tests, such as slug and pumping tests, predict hydraulic conductivity values without detailed information about aquifer heterogeneity. The multiple source and receiver signals of a hydraulic tomography aquifer test can estimate interwell heterogeneity, but it requires extensive time to collect and then invert large amounts of tomographic data. An innovative adaptation of an oscillatory pressure signal was used to reduce the data collection and processing time associated with a tomography test. The phase shift of the sinusoidal pressure signal is related to the hydraulic conductivity. Multiple offset gathers (MOG) ray paths were estimated with a spatially weighted straight ray approximation method and analyzed with data processing programs that extend the 3D homogenous spherical radial equation to the heterogeneous case. A numerical model was used to check the heterogeneous extension for accuracy. High quality zero-offset profile ray paths (ZOP) were used to determine hydraulic conductivity, K, at a relatively fine scale and interpreted into representative aquifer models between different tomographic well pairs. The aquifer models were used with MOG data to evaluate the anisotropy ratio and lateral heterogeneity of the aquifer. Two different oscillatory periods, 3 and 30-sec, were evaluated and compared to previous work at the site. Analysis indicates that the 3-sec period data were more sensitive to different anisotropy ratios and both periods are capable of resolving K zones of about one meter.