Imaging the hydraulic properties of a contaminated alluvial aquifer perturbed with periodic signals

Imaging characterization of a heterogeneous alluvial aquifer at a decametric scale is presented. The characterization relies on responses to oscillatory pumping tests led in two different wells and at two different periods of oscillation (5 and 10 min). These specific oscillatory responses are extracted from the hydraulic pressure values, measured in 13 boreholes during the pumping tests, through a data processing procedure. Then, a deterministic inversion process, led with a two-dimensional hydraulic properties model, aims to reproduce this set of oscillatory responses, in a frequency domain, by optimizing the distribution of the property values in the model. Two inversion processes are led separately with each set of responses corresponding to the two periods of oscillation used, and one joint inversion is led with the set of all responses together. The inversion results image the heterogeneities in the distribution of the field properties. The results suggest that longer periods of oscillation (in this case 10 min) permit characterization of larger areas around the pumping wells. In contrast, shorter periods (in this case 5 min) propagate more readily in the conductive zones near the pump and provide less information for the less conductive areas. Therefore, it appeared that performing a joint inversion, combining the information carried by the responses to both periods, provided more information on the heterogeneous distribution of the field properties and resulted in better constrained property maps than the ones obtained from separate inversions.