Interference pumping tests in a fractured limestone (Poitiers – France): Inversion of data by means of dual-medium approaches

SummaryHydraulic interference pumping tests remain a valuable method for obtaining information on underground flows at a scale compatible with the operation of a reservoir. Combining a dual-medium model with the inversion of interference testing data in fractured rocks is very recent, probably because the model is ruled by very contrasted sensitivities to its main parameters. It is shown that an accurate and stable inversion requires a specific procedure based on re-scaled sensitivities to parameters. Both homogeneous and fractal dual media have been inverted on two sets of data obtained from the fractured limestone of the experimental site in Poitiers (France). The first data set can be homogenized with both homogeneous and fractal approaches, yielding almost constant hydraulic parameters or only weak statistical variations beyond experienced distances of 200 m. The second data set collected in a site perturbed by forced flows from the first campaign shows preferential flow paths with a piston-like propagation of the pressure depletion. Only the fractal dual medium is able to partly homogenize the hydrodynamic behavior. Surprisingly, the hydraulic conductivity is constant but the dimensioning constants of the capacity parameters continue to decrease with the size of the system. Thus, the intrinsic decrease of parameters with the size of the system in the fractal approach is not sufficient to mimic the scale effects recorded by interference tests. Superimposition of piston-like effects onto dual media shall be further investigated to better account for the influence of scarce flow paths with very high velocities.