Geostatistical inverse modeling for the characterization of aquitard heterogeneity using long-term multi-extensometer data

Abstract

The classical aquitard drainage model COMPAC was used to simulate the deformation of a one-dimensional vertically heterogeneous aquitard (i.e. Heter-COMPAC). By coupling Heter-COMPAC with the quasi-linear geostatistical inversion approach, the vertical heterogeneities of hydraulic conductivity (Kv), elastic specific storage (Sske) and inelastic specific storage (Sskp) fields could be characterized with the availability of long-term multi-extensometer data. The methodology was first tested through a group of ten synthetic cases with randomly generated Kv, Sske and Sskp fields. Results of the synthetic cases revealed that a geostatistical model could characterize the variability of aquitard hydraulic parameter fields, and yields improved fits of simulated versus observed zone deformation than a geological model that treats each zone to be homogeneous. Subsequently, the methodology was applied to a research site situated in Changzhou city, Jiangsu Province, China, where long-term deformation behaviors of the strata within the aquifer-aquitard system have been monitored. Vertical heterogeneities of Kv, Sske and Sskp fields of the investigated aquitard at the site were characterized using the quasi-linear geostatistical approach. The correlation length of the three fields that yielded the best-quality fit was assessed to be approximately 50 m, and the fitting quality was slightly improved in comparison with that produced from calibrating a geological or zonation model. Through Monte Carlo analysis of aquitard deformation utilizing the inversed parameters and corresponding uncertainties, the investigated aquitard is estimated to have compacted −914.8 ± (42.6) mm before the rise in groundwater levels in adjoining aquifers.