Seismic detectors measure soil moisture using traffic noise

MADRID (EUROPA PRESS) – Caltech researchers have taken advantage of traffic vibrations to measure soil moisture in the shallow underground region between the surface and underground aquifers.

This region, called the vadose zone, is crucial for plants and crops to obtain water through their roots. However, measuring how this groundwater moisture fluctuates over time and between geographic regions has traditionally relied on satellite imagery, which only provides low-resolution averages and cannot penetrate below the surface. What’s more, moisture within the vadose zone changes rapidly: a thunderstorm can saturate a region that dries out a few days later.

The new method, based on seismic technology that studies earthquakes, can detect vibrations from human activity, such as traffic. As these vibrations pass through the ground, they are slowed by the presence of water: the more moisture, the slower the vibration. The new study measures the water content in the vadose zone through seismic noises from everyday traffic.

The research is a collaboration between the labs of hydrologist Xiaojing (Ruby) Fu, an assistant professor of mechanical and civil engineering, and seismologist Zhongwen Zhan, a professor of geophysics. A paper describing the work appears in the journal Nature Communications.

The new method is based on a technique pioneered in Zhan’s lab called distributed acoustic sensing (DAS). With this technique, lasers are pointed at unused underground fiber-optic cables (like the kind that provide the Internet). As a seismic wave—or any kind of vibration—passes through the cable, the laser light bends and refracts. Measuring the undulations of this laser light gives researchers information about the passing wave, making the 10-kilometer cable equivalent to a line of thousands of conventional seismic sensors.

In the wake of the 2019 magnitude 7.2 earthquake in Ridgecrest, California, Zhan installed a DAS array on a nearby cable to measure aftershocks. Working with Fu, the team quickly realized that the array could also be used to measure how everyday underground vibrations change based on soil water content. For five years, the team collected data and created models to illustrate how moisture in the vadose zone varies over time. They found that during California’s historic drought from 2019 to 2022, moisture in the vadose zone decreased significantly at a rate of 0.25 meters per year, outpacing the average mean rainfall.

“From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave Desert,” says Yan Yang, a graduate student in geophysics and co-lead author of the study. “Our rough estimate is that every year, the Mojave vadose zone loses an amount of water equivalent to the Hoover Dam. During the drought years from 2019 to 2022, the vadose zone has been increasingly dry.”

The ability to measure vadose zone moisture in real time is crucial for managing water use and conservation efforts. The team aims to deploy the technology in regions other than the desert.

“We know this method works very well for this particular site,” Fu says. “Many other interesting regions with the same climate might have different hydrologic processes, such as central California, where agricultural operations draw water, but the region also receives snowmelt from the Sierra Nevada mountains.”

Seismological instruments have never been used to measure soil moisture on such a large scale over such a long and continuous period of time.