Seismic & traffic vibes measure soil moisture

Measuring underground moisture usually relies on low-resolution satellite imaging averages. Remote sensing is also unable to penetrate beneath the surface. However, Caltech has found that seismic technology and traffic vibrations can gauge soil moisture.

The new method is able to measure water content in the unsaturated or vadose zone, the shallow region between the surface and groundwater table. In addition to monitoring ground shaking during earthquakes, researchers found that water reduced the speed at which traffic vibrations moved through the ground: the more moisture, the slower the vibrations.

The distributed acoustic sensing (DAS) technique directs laser lights into unused underground fiber-optic cables. The light refracts as vibrations pass through the cable. Researchers then measure the changes in light, transforming a 10-km cable into thousands of seismic sensors.

Seismologist Zhongwen Zhan, hydrologist Xiaojing Fu and their teams observed that the same DAS array could track changes in underground vibrations related to soil water content. They collected data and developed models over five years to show soil moisture variation in the vadose zone over time. From 2019 to 2022, during a California drought, they observed a significant decrease in vadose zone moisture: a rate of 0.25 m per year, surpassing the average annual precipitation.

Co-first author of the study, Yan Yang commented, “Our rough estimation is that every year, the Mojave vadose zone loses an amount of water equivalent to the Hoover Dam. Over the drought years of 2019 through 2022, the vadose zone has been drier and drier.”

Measuring soil moisture has far reaching implications. NASA recently launched missions to measure freshwater. The Signals of Opportunity P-Band Investigation (SNoOPI) utilizes radio signals from commercial satellites and a low-noise radio receiver to measure root-zone soil moisture. James Garrison, principal investigator for SNoOPI explained, “By monitoring the amount of water in the soil, we get a good understanding of crop growth. We can also more intelligently monitor irrigation.”

Real-time soil moisture measurements inform water management, conservation and forecasting. The innovative techniques developed by Caltech and NASA pave the way for more publicly available data and open-source models. In turn, water utilities, irrigation districts and flood forecasters can access additional data sources to help manage water resources in drying environments.

To help reduce the risk of drought and flooding, we support the safe monitoring of stream discharge and precipitation as well as advanced analysis of environmental data including soil health and water quality. In regions affected by wildfire, consultants and scientists conduct long-term soil health monitoring to better understand shifts in baseflow contributions, changes in water chemistry and their impacts on water contamination.