Hydraulic characterization of unconfined aquifers with oscillatory excitations: A numerical analysis

Abstract

We present a synthetic hydraulic analysis of the propagation of oscillatory perturbations in a two-dimensional saturated zone bounded above by a capillary fringe. Unsaturated flows are simulated in a physical model using a hysteretic form of Richards' equation parameterized using the van Genuchten water retention curve and the Mualem hydraulic conductivity equation for a sand, loam, and intermediate soil. Soil type affects periodic responses as a function of their hydrodynamic and van Genuchten properties. We first analyze the effect of various hydraulic or signal properties on the oscillatory responses simulated at different locations and depths. We then test the imaging potential of oscillatory responses recorded in the saturated zone to spatially charaterize homogeneous or heterogeneous soils, also considering the effect of period of oscillation. We find that a separation of these responses into amplitude/phase couples permits to dissociate the characterization of different hydraulic properties. Periodic amplitudes decay with distance mainly as a function of hydraulic conductivity and phase lags decrease with frequency due to storage in the capillary fringe. Thus, responses amplitude appears to be especially sensitive to conductivity K, which allows to isolate this property from the others. Phases can then be used to better characterize the vertical flows effect, through the Van Genuchten parameters α and n. Higher periods will tend to accentuate the unsaturated flows effect on the responses and can therefore be adopted for a better characterization of α and n. However, using low-period responses can also be useful, as they provide better estimates of conductivity values in media with low heterogeneities. We consider this work as a useful preliminary analysis for future sandbox or field real cases applications and provide advices for such applications in the discussion.