9 December 2011

Divining water using gravity

Posted by kramsayer

The Gravity and Hydrology in Africa project measures gravitational anomalies using FG5 gravimeters, like the one pictured above. (Credit: NOAA)

Researchers are using technology a tad more sophisticated and scientific than dowsing rods to detect underground water from afar: sensors that measure minuscule changes in gravity.

Traditional probes used to detect groundwater measure water pressure, hydrogen atoms, and humidity at a specific point. Because they sense only the immediate surroundings, the measurements don’t accurately represent water content if it varies with depth underground.

One proposal to solve this problem involves measuring minuscule gravitational changes caused by water amassed underground. To test the idea, scientists measured the absolute gravity for three years at a site in northern Benin in West Africa that frequently gains and loses water as part of the Gravity and Hydrology in Africa project.

Basile Hector of the Institut de Physique du Globe de Strasbourg in Strasbourg, France, presented the Benin gravity measurements at a hydrology session Wednesday at the American Geophysical Union’s Fall Meeting.

After correcting the super sensitive measurements for other external contributions to gravitational changes, like ocean tides, atmospheric pressure, and polar motion, the tiny changes in gravity (about one hundred millionth the pull of Earth’s gravity) correlated with changes in water levels, Hector said.

“The residual is directly linked with the hydrological cycle,” he said. “It’s in phase with the hydrological cycle, which is pretty good.”

To get more precise estimates of water levels, researchers measured the water content at the site’s two main sources: the vadozone (the upper layer where the water is in the form of moisture) and the underlying water table (where water pools and saturates the soil).

For the vadozone, researchers buried probes 6 meters deep to measure hydrogen atom density and, by extension, water molecule density. For the water table, researchers buried instruments at a depth of 10 meters that measured water pressure, which helps to calculate water table depth. Then they compared that data with the gravity data.

“It showed a really good fit,” Hector said. “It’s quite promising for gravity and hydrogravimetric (research).”

However, Hector cautions the findings are preliminary.

“There will be a need for validating, which we are currently doing,” he said.

-Stephen Tung is a science communication graduate student at UC Santa Cruz.