Stress‐Strain Analyses for Aquifer‐System Characterization

Subsidence data historically have been collected using costly extensometers. Recent advances in InSAR and GPS technology may provide noninvasive methods for collecting these data, which can be used in conjunction with hydrograph information to estimate the skeletal specific storage and hydraulic conductivity values of compressible units. Numerical analysis is used to evaluate the conditions affecting nonrecoverable (inelastic) and recoverable (elastic) skeletal specific storage and to investigate the affects of horizontal strain on the extensometer record. Stress‐strain diagrams represented as subsidence‐drawdown plots are one‐dimensional in scope, yet are typically used for estimating specific storage. Simulation results indicate that nonrecoverable specific can be estimated from these diagrams even when the storage contribution from horizontal strain is significant. Hysteresis effects are common during recovery and recompression of cyclically pumped confined aquifers. Several factors were found to affect the shape of the hysteresis loops; these include (a) the hydraulic diffusivity of the confining unit, (b) the distance from the pumped well, (c) length of the recovery cycle, (d) the ratio of confined aquifer to confining unit hydraulic conductivities, and (e) the thickness of the confining unit. Stress‐strain analyses can supplement classical time‐drawdown data to yield valuable parameter estimation of the confining unit that would be otherwise difficult to quantify.