3 Measuring and Monitoring Subsidence and Compaction

The analysis and prediction of expected anthropogenic land subsidence due to fluid pumping require a careful reconnaissance study of the area of interest, with a detailed layout of the basin’s geology and geometry and reconstruction of the pumping rate evolution, pressure head, and displacements of the land surface. Geomechanical and hydraulic properties are of the utmost importance. Pre‑consolidation stress; zones of overpressure; and the presence of faults and thrusts along with their extent, orientation and geomechanical properties (that is, friction angle and cohesion) must all be reliably identified. Advanced technology such as 2‑D and 3‑D seismic surveys, airborne‑electromagnetic investigations, well‑logs, exploration boreholes, pumping tests, and specific laboratory analyses can be of great value. Much progress has been made, since the traditional spirit leveling, in accurately monitoring ground surface movements. New techniques include Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) by which land subsidence is measured from space with high accuracy. Advances have also been accomplished in measuring shallow and deep aquifer system compaction by single‑level cable and multi‑level magnetic borehole extensometers.

Anthropogenic land subsidence modeling and forecasting tools are continuously improved. They take advantage of both enhanced computer devices (for example, parallel hardware) and advanced measurements technology applied to horizontal and vertical ground movements (for example, Differential Global Positioning System (DGPS) and InSAR technologies). Modeling tools are helpful in determining and distinguishing among multiple causes, and can be effectively combined with measurement techniques. Once the models have been calibrated to the observed history of the aquifer, they can be used in their predictive capacity to evaluate various future scenarios of groundwater use. They can be used to develop integrated resource management programs that should take environmental and socio‑economic impacts into account. The models are used to evaluate the adverse consequences of fluid extraction in a medium/long time range, in particular for urban flood management of coastal areas and in other cases of environmental vulnerability. The following sections present methods used to measure land subsidence.