AzWSC Capabilities: Hydrologic Gravity Monitoring

Science Center Objects

Gravity is a measurement of mass: the greater an object's mass, the stronger its gravitational pull. By measuring changes in gravity over time, inferences can be made about changes in mass. In hydrology, this can be used to study water in the subsurface. If the amount of groundwater in a particular area increases over time, through processes such as infiltration of rainfall or aquifer recharge, gravity will also increase. Likewise, losses of groundwater storage, resulting from processes such as pumping, discharge to streams, and evaporation, will cause gravity to decrease.

The U.S. Geological Survey's Arizona Water Science Center (AzWSC) is a worldwide leader in the development and use of gravity methods for hydrology. The AzWSC has developed gravity methods for monitoring recharge near an ephemeral-stream channel, monitoring aquifer-storage change in a compressible aquifer, estimating recharge at an artificial-recharge facility, and estimating specific yield through the correlation of gravity and water-level change in wells.

Applications of Hydrologic Gravity Methods

Gravity monitoring has many applications including geodetic purposes as well as monitoring geothermal energy, petroleum, energy storage, magma, volcanoes, and subsurface water. Hydrologic gravity monitoring can be used in multiple ways to observe variations in water storage in soils, unsaturated zones (the zone above saturated material), and aquifer. Water mass change is the most prevalent and primary cause of gravity variation on the surface of the Earth over periods of decades or less. The basic observation of gravity change (corrected for atmospheric, earth-tide, and elevation variations) among repeated observations is caused by variation in subsurface water mass, unless other mass changes occurred. 

There are three primary applications of hydrologic gravity measurement:

♦ Monitoring individual stationsMonitoring at individual stations provides a measure of subsurface water-mass change below the station.  Any gravity record at an individual site can be viewed as a measure of groundwater-storage change. Another way to describe individual gravity records is as a noninvasive monitor well that only measures storage change but does not provide information about hydraulic head. 

Monitoring at wells where water levels are also monitoredMonitoring at wells with coincident water-level monitoring provides information about the local hydrogeology, including aquifer-storage properties (specific yield) where the storage change occurs in a single aquifer. Gravity records when combined with coincident water-level records at well sites can be used to improve understanding of the hydrogeologic conditions and to estimate specific yield of unconfined aquifers. Good examples of gravity and water-level relations and what they reveal are available from long-term monitoring across Arizona. Sites where water levels change but gravity does not (little storage change) are indicative of a confined aquifer (fig. 1). This site was expected to show a confined aquifer response on the basis of an assumed conceptual understanding of the local system. The combination of gravity and water-level monitoring verified this conceptual understanding. Relations at another nearby well, however, produce gravity and water-level variations that are semi-correlated (fig. 2), which suggests that storage changes are occurring in multiple unconfined aquifers and not only within the aquifer monitored by the water level in the well. The occurrence of multiple aquifers is verified by water cascading into the casing of nearby wells. Gravity and water levels are better correlated at a third nearby well (fig. 3a), which indicates an unconfined aquifer. The slope of a linear correlation of gravity and water level for an unconfined aquifer is a measure of aquifer specific yield (fig. 3b).

Monitoring at a network of stationsGravity change among a network of stations can be integrated to calculate total storage change in the network region as shown in figure 4. This estimate of storage change can then be used to improve groundwater budgets by supplying an estimate of a water budget component that is normally unknown or difficult to obtain through other methods. In fact, gravity monitoring is the only method of estimating large-scale groundwater-storage change.

  • Examples of monitoring a network of gravity stations include networks designed to monitor recharge near ephemeral channels (fig. 4; Pool and Schmidt, 1997; Goodrich and others, 2004; Pool, 2005; and Hoffmann and others, 2007) and basinwide networks designed to monitor storage change for entire aquifers (Pool and Anderson, 2006; Ferré and others, 2007).
  • A limited network of four stations was used to estimate ephemeral channel recharge along a profile across Walnut Gulch in the Walnut Gulch Experimental Watershed near Tombstone, Arizona, during 1999 and 2000. The small number of stations constrained the two-dimensional recharge volume very well. The two-dimensional volume was converted to a three-dimensional volume on the basis of the assumption that the two-dimensional distribution of gravity change also applies to the stream reach within 1 mile above the site. The resulting three-dimensional recharge volume was similar to the average of six other methods of estimating recharge (Goodrich and others, 2004).
  • A network of gravity stations in the Tucson, Arizona, area was used to estimate total storage change in the region for the period from 1998 through 2002 (fig. 5). After accounting for known water withdrawals and inputs to the aquifer, the total annual recharge for the area was estimated using water-budget methods as a residual. Results indicated that nearly all of the recharge occurred during the first year of the monitoring effort, with much smaller rates of recharge occurring thereafter.

Instruments Owned by the AzWSC

(Use of trade names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey)

♦ A10 Absolute Gravity Meter—The A10 absolute gravity meter is a portable device, designed for field operation, which determines the acceleration due to gravity directly by measuring the position of a falling mass at different time intervals. It can be operated either from vehicle or battery power, in all field conditions. Each measurement is independent and the survey loops required for relative gravity meter studies are not needed.

♦ Relative Gravity Meters—These meters measure the relative difference in gravity between two stations. Field procedures and processing are more involved than the A10 absolute gravity meter, but they provide similar accuracy at reduced cost. They are most suitable for stations located relatively close together (up to a few miles) with good access on paved roads.

Other Instruments Operated by the AzWSC

(Use of trade names is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey)

Superconducting Gravity Meters—These meters provide the most precise gravity measurements possible. The AzWSC has previously deployed one at its Tucson office to measure gravity change associated with pumping from a nearby well from January 2010 to March 2011. Two of the latest generation superconducting gravimeters, the iGrav™, are deployed at an artificial recharge facility from February 2012 to October 2012.

♦ gPhone Relative Gravity Meters—These meters are a temperature stabilized version of the LaCoste and Romberg relative meters that feature nearly linear drift. Over periods of months or longer, this drift can be removed to provide accuracy approaching 1 microGal. Three of these meters are to be deployed at an artificial recharge facility from March 2012 to October 2012.

♦ FG5 Absolute Gravity Meter—This instrument is the most precise absolute gravity meter available. Unlike the A10, it is primarily intended to operate indoors or in climate controlled conditions. The AzWSC had partnered with the National Geodetic Survey to collect FG5 data at a number of sites in Arizona.