Numerical Modeling of Natural Land Subsidence over Sedimentary Basins Undergoing Large Compaction

Abstract

The. natural compaction driven by unsteady groundwater flow in an accreting isothermal sedimentary basin is investigated by a new numerical compaction model. We assume a process of continuous vertical sedimentation and make use of a I-D model of flow where water flow obeys relative Darcy's law in a porous medium which undergoes a progressive compaction under the effect of an increasing load of the overburden. The time interval spanned by the simulation can be millions of years and soil porosity, permeability and compressibility may vary with the effective intergranular stress according to empirically based constitutive relationships. The model takes correctly into account the geometric non-linearity which arises from the consideration of large solid grain movement and is solved using both the Eulerian and the Lagrangian approach. It is shown that the Eulerian derivative of the total vertical stress is well approximated by the sediment loading rate, thus allowing for the removal of a heavy source of non-linearity in the governing equations with a significant acceleration of the iterative solution procedure. Preliminary results from the non linear model are compared with those of the linear model of Bredehoeft and Hanshaw [8] which neglects the medium compaction. These results indicate that the geometric non-linearity is important in relatively compressible and permeable basins, i.e. in basins which display a significant deformation and are normally or almost normally consolidated.