Improving Aquifer Imaging and Long-term Monitoring with Oscillating Signals: Oscillatory Hydraulic Tomography

Pumping tests provide direct information about hydrologic properties - especially hydraulic conductivity (K) - that can be used to understand 3-D spatial heterogeneity and improve aquifer flow and transport predictions. A state-of-the-art method for analyzing pumping tests, Hydraulic Tomography (HT), uses information from sets of constant-rate pumping tests to tomographically estimate K throughout an aquifer. In this presentation, we discuss a modified version of HT, multi-frequency Oscillatory Hydraulic Tomography (OHT), in which the pressure responses from a series of oscillating pumping tests of varying frequency are analyzed. In particular, we present a computationally efficient framework for tomographic analysis of data from such tests, which: 1) Compresses data and extracts relevant information using signal processing routines 2) Performs inversion using a fast-running and physically-based steady-periodic forward model and 3) Jointly analyzes multi-frequency data in order to reduce uncertainty associated with parameter estimates. We demonstrate many of the benefits of the multi-frequency OHT concept using a 3D transient synthetic application. While the transient forward model requires 10s of minutes to generate each pumping test's synthetic data, we show that this data can effectively be inverted using a separate, steady-periodic inverse model that requires only seconds to run. Thus, large datasets from many pumping tests can be analyzed, and large problem domains can be studied, using an inversion framework that requires only modest computational resources. Because OHT involves no net water extraction, it can also be used in a "passive" configuration over long time-periods to monitor changes in aquifer properties over time (e.g., due to infiltration of a high-viscosity NAPL). Forward modeling of signal response changes due to such processes will be presented in a separate synthetic application, which tests the ability of OHT signals of various magnitudes and frequencies to detect transient processes.