GDEST 2008 Conference Sessions
Theme 1 - Observing Africa
Emerging Science and Technology for Deep Groundwater Resource Assessment
Andiswa Mlisa, Umvoto Africa Ltd, South Africa
A thick, extensive fractured-rock (FR) aquifer system like the Table Mountain Group (TMG) in South Africa, provides unique opportunity for fundamental advances in understanding interactions between fluid flow and mechanical deformation, through analysis of the "hydromechanical" coupling in FR permeability, fluid transport and deep storage in fracture porosity. For the ~1 km thick Peninsula Aquifer in the TMG, present knowledge of skeletal-framework compressibility, the main unknown used in calculating specific storage, is based on published data from similar rocks elsewhere. The South African Water Research Commission recently obtained laboratory measurements of elastic properties of TMG borehole-core samples, but up-scaling from dry-sample measurements at ~10-cm scale to saturated rock volumes on 100- to 1000-m scale, is methodologically problematic.
These problems are obviated by measuring directly the compaction of, and corresponding surface subsidence above, the pumped aquifer and using these field-experimental measurements to determine the framework compressibility and the specific storage. Historically, such aquifer-deformation measurements have used costly devices (borehole extensometers), but recent advances in GNSS technology (e.g., GPS), and also Interferometric Synthetic Aperture Radar (InSAR), now provide noninvasive methods of geospatial data collection, which can be used in conjunction with borehole hydrograph information to estimate the specific storage and hydraulic conductivity of the aquifers.
This African project contributes to an international effort to develop the Global Geodetic Observation System (GGOS) towards a global-to-regional-scale monitoring of the full hydrological cycle. It supports capacity-building in space-geodetic data-processing, modelling of the hydrological cycle, and interpretation of observations in terms of terrestrial water storage.