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 stations—Monitoring 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 monitored—Monitoring 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.
♦ Monitoring at a network of stations—Gravity change among a network of stations can be integrated to calculate total storage change in the network region. 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.
General Desciption of Types of Instruments Owned by the AzWSC
♦ Absolute Gravity Meters—Absolute gravity meters can be portable or primarily be intended to operate indoors or in climate controlled conditions. Portable devices are designed to be operated in all field conditions and determine the acceleration due to gravity directly by measuring the position of a falling mass at different time intervals.
♦ Relative Gravity Meters—These meters measure the relative difference in gravity between two stations. They are most suitable for stations located relatively close together (up to a few miles) with good access on paved roads.
♦ Superconducting Gravity Meters—These meters provide the most precise gravity measurements possible.
Below are multimedia items associated with this project.
- Overview
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 stations—Monitoring 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 monitored—Monitoring 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.
♦ Monitoring at a network of stations—Gravity change among a network of stations can be integrated to calculate total storage change in the network region. 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.
General Desciption of Types of Instruments Owned by the AzWSC
♦ Absolute Gravity Meters—Absolute gravity meters can be portable or primarily be intended to operate indoors or in climate controlled conditions. Portable devices are designed to be operated in all field conditions and determine the acceleration due to gravity directly by measuring the position of a falling mass at different time intervals.
♦ Relative Gravity Meters—These meters measure the relative difference in gravity between two stations. They are most suitable for stations located relatively close together (up to a few miles) with good access on paved roads.
♦ Superconducting Gravity Meters—These meters provide the most precise gravity measurements possible.
- Multimedia
Below are multimedia items associated with this project.
- Publications