Characterization of Subsurface Heterogeneity Using Transient Hydraulic and Tracer Tomography

Abstract

Transient hydraulic tomography (THT) is a cost-effective technique for characterizing the heterogeneity of hydraulic parameters in the subsurface. In this study we developed an efficient sequential successive linear estimator (SSLE) for interpreting head data from transient hydraulic tomography to estimate three dimensional hydraulic conductivity and specific storage fields. We first analyzed the cross correlation between transient head data and hydraulic parameters and covariance of transient heads using a hypothetical one dimensional aquifer. This analysis led to an efficient way to interpret transient heads. The SSLE was then tested using a well-posed problem and an ill-posed problem. To affirm the robustness of our approach, we applied transient hydraulic tomography to a hypothetical three-dimensional heterogeneous aquifer.Our SSLE approach involves solving adjoint equations during the sensitivity analysis for transient flow, which creates greater computational cost than steady state hydraulic tomography. To reduce the computational cost, we developed an estimation approach that utilizes the zeroth and first temporal moments of well hydrographs, instead of drawdown itself. The governing equations and adjoint equations for the temporal moments are Poisson's equations. These equations demand less computational resources as opposed to the parabolic equation that governs drawdown evolution. Therefore, a temporal moment approach is expected to expedite the interpretation of THT surveys. Based on this premise, we extended our sequential successive linear estimator (SSLE) to use the zeroth moment and characteristic time of the drawdown-recovery data generated by THT surveys. We subsequently investigated computational efficiency and accuracy of the moment approach using a synthetic aquifer.We further extended the hydraulic tomography concept to tracer tomography for characterizing NAPL (Non-aqueous phase liquid) source zones. Similar to a hydraulic tomographic survey, a tracer tomography survey sequentially injects tracers at a selected well and monitors tracer breakthroughs at other wells in a NAPL source zone to detect the distribution of NAPL's. To quantitatively interpret the breakthroughs from the tracer tomography, a joint stochastic estimation technique was developed. The method is an extension of the SSLE used for interpreting hydraulic tomography surveys. The technology was tested and investigated using a synthetic aquifer contaminated with a single component NAPL.