The constant roaring, teeth-gnashing traffic on Highway 99 might actually be good for something.

Researchers at Caltech have found a way to use the vibrations of passing cars to determine how much water is just beneath the Earth’s surface.

In a new study, researchers used seismic waves from traffic noise combined with unused fiber optic cables buried underground to create a new method to determine how much moisture is present in the top 20 meters of soil, a level of detail not achieved by other underground detection methods, the researchers said.

Researchers can use unused fiber optic cables and convert them into a dense array of seismic sensors to obtain detailed moisture content data.

The seismic vibrations respond to moisture in shallow soil and slow down when they encounter water.

Xiaojing Fu about SJV Water

The level of detail is significantly higher than with conventional methods, said Yan Yang, co-author of the study. This is because the researchers were able to use the fiber optic cable as a proxy sensor every eight meters. Conventional sensors are usually hundreds of meters apart, said Yang.

Due to the distance over which conventional sensors take measurements, moisture changes can only be detected in much deeper layers, where the groundwater is located.

“Because they are very close to each other with this technique, they can capture much flatter areas, which I think was very difficult to do before,” said Xiaojing (Ruby) Fu, who participated in the study.

The area of ​​soil between the surface and the groundwater is called the “unsaturated zone” and controls the movement of water to the aquifers and to the surface.

The research also has potential for practical applications, the researchers said.

The seismic method could enable the monitoring of water management cycles, from snowmelt to large-scale evaporation to agricultural irrigation.

“Monitoring this water balance would be really interesting because no one has been able to do it so far,” said Fu.

Linking this data with well measurements could also be helpful in enabling more precise water management planning, says Fu.

This method has some geographic limitations because it requires pre-existing fiber optic cables, but because California is densely cabled, there are many regions where this method can work, Fu said.

Other studies have used satellites to measure surface subsidence and uplift and electromagnetic measurements to determine the composition of the Earth’s deep layers, but no method has yet been able to measure the unsaturated zone with such precision, according to the Caltech team.

Many water managers in the valley are calling for more research and more technology to better understand what is happening beneath our feet.

“In our work to ensure sustainable groundwater supplies in the Central Valley, we are trying to measure and manage something we cannot see: water beneath the earth’s surface,” Aaron Fukuda, general manager of the Tulare Irrigation District, wrote in an email. “As we gain more information and data on how water moves and is stored in our soils and aquifers, we can make better decisions about management and projects.”

SJV Water is a nonprofit, independent online news publication covering water issues in the San Joaquin Valley. Lois Henry is CEO/Editor of SJV Water. She can be reached at [email protected]. The website is www.sjvwater.org