by CJH Hartnady and A Mlisa, Umvoto Africa
The advent of the space age (with the launch of Sputnik, the first artificial satellite, on 4 October 1957), together with the development of atomic clocks (first crystal oscillators in the 1950s, then later atomic clocks) to measure epochs and time intervals precisely, initiated a scientific revolution in geodesy.
In essence, the new “space-geodetic” observation techniques voided the traditional “astrometric” methods and replaced them by the measurement of distances or distance differences, with a gain in accuracy of about two to three orders of magnitude (from 0,5 – 5 m to 1 – 5 mm over a 1000 km baseline).
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Fig.1: Andiswa Mlisa (Umvoto) and George Roux (CD:NGI) installing GPS receiver and antenna (on blue pillar monument) at the HGW1 borehole site. |
Prior to ~1968, only four decades ago, a static or “fixist” view of the solid Earth prevailed. Geodetic concepts, including terrestrial reference frames, were based on fixed coordinates of points on the Earth’s surface. Over the last four decades, however, a “mobilist” perspective has become firmly established. The development of plate-tectonic theory in the late 1960s, along with the subsequent invention and rapid improvement of the space-geodetic technologies since the mid-1980s, has made it clear that, over a wide range of time scales, there is continual change in the relative position of all points on solid Earth’s surface.
GNSS (Global Navigation Satellite Systems) technology is now used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third, Europe's Galileo, is expected to be operational by 2013. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a network of ground stations.GPS became fully operational in 1994, with the completion of a full constellation of 24 satellites. The fundamental principle is that precise timing information is transmitted on radio waves from the GPS satellite, enabling the user’s receiver to measure the range to each satellite in view, and hence calculate its position. (Hence, it is said that the speed of light is the new “ruler of the Earth”). Positions can be calculated at every measurement epoch, which may be once every 30 seconds, or at even greater frequencies between 1 – 50 Hz. Kinematic parameters such as velocity and acceleration are secondary, in that they are calculated from the measured time series of positions (Blewitt, 2007).
Interferometric Synthetic Aperture Radar (InSAR) is a radar-geodetic technique that generates maps of surface deformation from two or more images, using differences in the phase of the radar waves reflected from ground or sea to the satellite or aircraft. InSAR displacement maps derived from spaceborne imagery, e.g., such as acquired by the European Remote Sensing satellites ERS-1 and ERS-2, can yield surface displacements accurate to subcentimetre levels at a spatial resolution of 20 m, over swaths 100 km in extent and over timespans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability and the creation of better models of the elastic properties of the Earth’s crust.
Modern space-geodetic methods of high precision and accuracy were developed because of the benefits that they could bring to geophysics, and now have broad application to almost all branches of geophysics. Geodetic measurement systems involving GNSS and/or InSAR methods are particularly valuable to scientific investigations of the water resources.
The vigorous cycling and replenishing of abundant water throughout the global environment characterises the Earth as a unique, living planet. This operation of the “hydrological cycle” over a range of spatial and temporal scales exchanges large amounts of energy within the “climate engine” as water undergoes phase changes between liquid, solid and vapour states, and moves physically from one part of the Earth system to another. Water is also essential to life and is central to social welfare, progress, and sustainable economic growth. Because it underpins humankind’s capacity to produce sufficient food to support a burgeoning population, clean and fresh (non-saline) water is arguably the most important resource to society.
Lakes, streams, artificial (dammed) reservoirs, and groundwater provide freshwater resources for human usage. Of these, groundwater represents by far the greatest volume, constituting 96% of Earth’s unfrozen fresh water. As such it is a vital resource for irrigation, industry, and domestic consumers. By supplying “baseflow”, it maintains the surface flow of streams between storm events and seasonal rains. Along with soil moisture, it determines the ratio of infiltration to surface runoff, thus affecting the timing, duration, and intensity of floods. Groundwater also feeds back to atmospheric processes and the terrestrial carbon cycle by enabling plants with deep taproots (“phreatophytes”) to continue to transpire during droughts.
Contrary to the common assumption that the net change in groundwater storage over the course of a year is zero, the inter-annual variability of aquifer storage is actually substantial. Accordingly, if the effects of artificial abstraction (pumping) and injections can be removed, groundwater may be an important indicator of natural and human-induced climate variations. Despite the importance of the natural variability of groundwater – and its evident vulnerability to overproduction due to unsustainable rates of pumping, long-term contamination, and climate change, which alone or in combination may lead to future conflicts and human hardship – this largely invisible or hidden resource remains inadequately characterised by the scientific and water resources communities.
The importance of the hydrological cycle, as a major element of the climate engine, for the functioning of most near-surface processes and also the entire biosphere, emphasises the need for innovation in monitoring technologies applicable to water-mass movements. Many of its parameters, such as soil moisture, effects of infiltration on groundwater renewal, percolation of groundwater, subsurface discharge of groundwater into the ocean are known only within large uncertainty limits. Scientists complain about insufficient observations of water transport within nearly all components of the hydrological cycle, which lack is largely caused by the absence of technologies that allow sufficient monitoring of the relevant parameters within economic constraints.
Through detection of small changes in land-surface elevation (GNSS and InSAR) and gravitational potential, properly validated with ground-based physical or geophysical measurements of groundwater level, there is a prospect of the economic long-term monitoring of groundwater-storage changes, at large scales and over long observational periods, as an indicator of climate and water-cycle evolution. These geophysical technologies are therefore important in resource assessment of groundwater because they acquire data through nondestructive means (i.e. without expensive drilling or excavation).
To use GPS to constrain large-scale water storage changes in extensive aquifers such as the Table Mountain Group (TMG) aquifers through the Western Cape, and to make use of GPS measurements to recover the elastic properties of the deeper confined aquifers at particular sites, requires an extensive array of permanent, continuously-recording stations. The Chief Directorate: National Geospatial Information (CD:NGI) (formerly Chief Directorate: Surveys and Mapping (CD:SM)) operates a nationwide "TrigNet" array of such beacons, often co-located with South African Weather Service stations. Apart from tracking crustal movements to millimetre-per-year precision, and thus contribute to understanding of plate tectonics, strain/stress patterns, and earthquake hazard in the subcontinent, TrigNet provides a convenient platform for developing a new space- and groundbased system for monitoring the seasonal or abstraction-induced fluctuations in aquifer storage through detection of associated, small surface deformations.
In a groundwater development study for the Overstrand Municipality (Hermanus, Western Cape Province) a local network of three new continuously-recording GPS stations has been established on the borehole infrastructure of the Gateway Wellfield (see Figure 1). In conjunction with observations at the permanent TrigNet station HNUS, their purpose is to quantify the vertical and horizontal surface deformations related to groundwater abstraction, related to a phase of aquifer test-pumping that commenced early in 2009. Using the emerging GNSS technology, and linking it to water-level monitoring systems established in the course of wellfield development, we aim to determine fundamental hydromechanical properties of the deep confined aquifer, such as its bulk compressibility, improve the estimated values used to calculate aquifer storage, and monitor storage changes over the annual abstraction-recovery/recharge cycle.
The intensive groundwater and environmental monitoring system implemented by the Overstrand Municipality around the Gateway Wellfield covers both the shallow, unconfined Bredasdorp Aquifer and the deep, confined Peninsula Aquifer in the TMG. Elsewhere in the TMG aquifer regions, the network of groundwater measurement stations is spatially too limited. Furthermore, despite the current aim towards integrated water management and observation, the few specialised groundwater networks that do exist to serve local or regional water supply are often poorly integrated with other hydrological observations (e.g. streamflow). Their adequacy to provide a reliable estimate of the available groundwater resource or to measure either its depletion, or degradation of quality, is therefore doubtful. Together with the long-term weather station facility at the Hermanus Magnetic Observatory, the Gateway Wellfield provides an ideal "natural laboratory" setting for initial development and trial of GNSS-based space-geodetic tools for water-resource monitoring and assessment. The application of InSAR technology to the same area awaits the acquisition of new radar imagery over a range of different times and seasonal conditions.
The research project has been undertaken by Umvoto Africa, in collaboration with Professor Eric Calais of the Department of Earth and Atmospheric Sciences, Purdue University (Indiana, USA) and Richard Wonnacott of CD:NGI, and with technical assistance from the African Institute for Mathematical Sciences (AIMS). It is funded by the Water Research Commission (WRC) of South Africa, with additional support from the national Department of Science and Technology (DST).
