On the importance of considering specific storage heterogeneity in hydraulic tomography: Laboratory sandbox and synthetic studies

Over the last two decades, various studies on Transient Hydraulic Tomography (THT) have shown that it is an effective approach to characterize subsurface heterogeneity. Typically, high-resolution hydraulic conductivity (K) distributions were recovered, while the spatial variability of specific storage (S-s) was found to be smooth. In some studies, S-s heterogeneity has been intentionally ignored due to the belief that S-s is less variable than K. Therefore, one may question the importance of considering S-s heterogeneity during THT and its impact on the reliability of estimated hydraulic parameters. To investigate these issues, three modeling approaches (i.e., effective parameters, geological, and geostatistics-based) were used to obtain K and S-s estimates of varying spatial resolutions. The reliability of K and S-s estimates were evaluated by comparing their drawdown prediction performances. The values of using different prior K and S-s information for THT analyses were investigated. Our results revealed that: (1) the K distribution estimated from the geostatistics-based steady-state HT analysis accurately predicted the late time drawdowns, while further improvements in transient drawdowns were obtained only after jointly treating the S-s field as heterogeneous; (2) S-s heterogeneity should be considered in addition to K for THT inversions, even when the estimated S-s field is smooth; and (3) using K and S-s estimates from the calibrated geological model as initial mean distributions for the geostatistical inversion approach were helpful in capturing both interlayer and intralayer heterogeneity of K and S-s. These findings suggested that S-s heterogeneity should be properly considered during the implementation of THT.

Automated summary

Transient Hydraulic Tomography (THT) has been shown to be effective in characterizing subsurface heterogeneity by extracting high-resolution hydraulic conductivity (K) distributions. It is important to consider the heterogeneity of specific storage (S-s) in addition to K during THT inversions, even when S-s is found to be less variable than K.