Inverse problem for finding parameters that control land subsidence caused by subsurface fluid withdrawal

Land subsidence attributable to the compaction (consolidation) of aquifer systems is recognized to be a geological hazard. It is an environmental consequence of groundwater withdrawal in many cities and other areas worldwide. Prediction of land subsidence due to subsurface fluid withdrawal (whether the fluid is oil, gas, water, stream, or geopressure brine) depends on the quantitative identification of parameters for a selected theoretical model. A new inverse algorithm (InvCOMPAC) for finding transient land subsidence parameters due to the combined compression and expansion of one or more confined aquifer systems in response to ground fluid withdrawal is developed in this dissertation. It consists of combining the Newton-Raphson adjustment algorithm and Helm's one-dimensional finite-difference compaction (or consolidation) model (COMPAC). The subsidence (or consolidation) model can be replaced by any appropriate model. This inverse code (or algorithm) identifies five parameters that control transient land subsidence at a site of interest: vertical hydraulic conductivity of compressible aquitards, K', nonrecoverable S' skv, and recoverable S'ske specific storage of the aquitards, specific storage of the aquifer, S S, and an initial vertical distribution, p' max0, of maximum past preconsolidation pressure within the confined aquifer system. For computational convenience, p'max0 may or may not be considered to be uniform. K', S'skv, S'ske, and S S are constants for the constant-parameter option or indicate only the initial values for the stress-dependent parameter option of Helm's model.; An initial set of estimated values for these five parameters is found to be necessary in order to apply the inverse algorithm to an idealized compressible confined aquifer system. A new graphical-analytic method is introduced for estimating a realistic initial set of these values. The idealized data for developing this method is from COMPAC's calculation of compression and expansion in response to both long-term nondeclining sinusoidal boundary stress and also long-term declining sinusoidal boundary stress. This methodology is based on delay time constants of clay consolidation, the elastic hysteresis loop of clay stress-strain relationships, and Darcy's law. An investigation of this idealized model shows that the relative error of these five parameters found by applying the inverse model to calculated compaction using initial values of the parameters simply obtained from this methodology is 1.2∼6.3%. (Abstract shortened by UMI.)...