Caltech Researchers Tap into Traffic Noise to Measure Underground Water

In a breakthrough that could revolutionize water management in drought-prone areas, scientists at Caltech have developed a novel method to measure moisture in the soil beneath our feet. The technique, which repurposes existing technology used to detect earthquakes, could provide crucial data for farmers, city planners, and conservationists alike.

The research, published in the journal Nature Communications on Aug. 5, offers new insights into a region where moisture levels can change rapidly—a thunderstorm can saturate an area that dries out just days later.

Developed by a team led by hydrologist Xiaojing (Ruby) Fu and seismologist Zhongwen Zhan, the new technique repurposes seismic technology typically used to measure ground shaking during earthquakes. Their method detects vibrations from everyday human activity, such as traffic, to gauge soil moisture levels.

Traditional methods of measuring vadose zone moisture have relied on satellite imaging, which provides only low-resolution averages and cannot penetrate below the surface. The Caltech team’s approach offers a more detailed and dynamic picture of underground water content.

The research builds on a technique called distributed acoustic sensing (DAS), pioneered in Zhan’s lab. This method uses lasers pointed into unused underground fiber-optic cables. As vibrations pass through the cable, they cause the laser light to bend and refract. By measuring these changes, researchers can gather information about passing vibrations, effectively turning a 10-kilometer cable into the equivalent of thousands of conventional seismic sensors.

The team applied this technology in Ridgecrest, California, following a magnitude 7.1 earthquake in 2019. Over five years, they collected data and created models showing how moisture in the vadose zone changes over time. Their findings revealed significant water loss during California’s historic drought from 2019 to 2022.

According to their calculations, moisture in the vadose zone decreased at a rate of 0.25 meters per year, surpassing the mean average precipitation. When extrapolated to the entire Mojave Desert, the researchers estimate an annual water loss equivalent to the volume of the Hoover Dam.

This new method of measuring vadose zone moisture in real-time could prove crucial for managing water use and conservation efforts. The research team plans to deploy the technology in other regions, including central California, where farming operations and snowmelt from the Sierra Nevada mountains create complex hydrological processes.

The study was a collaborative effort, with contributions from co-authors Zhichao Shen, now of Woods Hole Oceanographic Institution, Kyra H. Adams of the Jet Propulsion Laboratory, and Caltech DAS scientist Ettore Biondi.

The project was made possible through funding from Caltech’s Resnick Sustainability Institute (RSI) and the National Science Foundation. Neil Fromer, Executive Director of Programs with the Resnick Sustainability Institute, highlighted the importance of this interdisciplinary work: “This is exactly the type of interdisciplinary, creative science that the Resnick Institute was designed to support, bringing together colleagues that otherwise wouldn’t have worked together, and in that collaboration develop new tools that can help measure and manage water availability more sustainably.”