The constant, rumbling, teeth-gritting traffic on Highway 99 might actually be good for something. Researchers at Caltech have figured out a way to use vibrations from passing cars to see how much water sits directly beneath the ground’s surface. In a new paper, researchers used seismic waves from traffic noise coupled with unused fiber optic cables underground in a new method to determine how much moisture there is in the top 20 meters of soil.
It’s a level of detail that hasn’t been achieved by other methods of underground detection, according to the researchers. Researchers are able to make use of unused fiber optic cable fibers and convert it into a dense array of seismic sensors to get detailed data on moisture content. The seismic vibrations react to moisture in the shallow ground, slowing down when they hit water.
The level of detail is significantly greater than conventional methods, said Yan Yang, co-author of the study. That’s because researchers can use the fiber optic cable as proxy sensors every eight meters. Traditional sensors are usually spaced hundreds of meters apart, said Yang.
Because of the distance of traditional sensing, changes in moisture can only be detected at much deeper levels, where groundwater lies. “In this technique, because they’re very close, they can sense much shallower regions, which I would say, was very difficult to sense before,” said Xiaojing (Ruby) Fu, contributor to the study. The area of soil between the surface and groundwater is called the “vadose zone,” and it controls the movement of water to aquifers and to the surface.
The research has potential for practical uses too, said the researchers. The seismic method could allow for monitoring hydrological cycles, from snowmelt, large scale evaporation and agricultural irrigation. “Monitoring this water balance would be really interesting because no one has been able to really do it,” said Fu.
Coupling this data with well readings could also be useful to come up with much more accurate water budgeting, said Fu. There are some geographic limitations to this method since it requires already present fiber optic cables. But California is dense with cables so there are many regions where this method can work, said Fu.
Other studies have used satellites to read sinking and uplifting of the surface and electromagnetic surveying to see deep underground compositions. But nothing so far has been able to measure the vadose zone in this level of detail, according to the Caltech team. Many water managers in the valley have been outspoken advocates for more research and tech to better understand what’s going on underneath our feet.
“As we work to achieve groundwater sustainability in the Central Valley, we are trying to measure and manage something we cannot see, water beneath the ground’s surface,” wrote Aaron Fukuda, general manager of Tulare Irrigation District, wrote in an email. “As more information and data are learned about how water moves and is stored in our soils and aquifers, our decisions about management and projects will be better.”.
