Caltech fiber optic seismic sensing helps water management

06 Aug 2024

Use of existing optical cables reveals details of soil dynamics.

A project at Caltech has developed a fiber optic-based method to measure soil moisture in the shallow subterranean region between the surface and underground aquifers.

Reported in Nature Communications, the technique offers new details of the soil region called the vadose zone, where the groundwater is at atmospheric pressure and which is crucial for plants and crops seeking water for their roots.

Measuring how this underground moisture fluctuates over time and between geographical regions has traditionally relied on satellite imaging, which only gives low-resolution averages and cannot penetrate below the surface according to Caltech. Weather changes such as thunderstorms can also rapidly alter the local picture.

The Caltech method relies upon distributed acoustic sensing (DAS), in which fiber optic cables provide distributed strain sensing data. When physical forces and disturbances act on the fiber, the optical path length is altered, with consequences for the reflected intensity and travel time of laser pulses traveling through the fiber.

DAS has already proven valuable for monitoring of undersea oil and gas pipework, and as a security system scanning overhead power lines for failure or criminal damage.

Caltech's new project is built upon the way that the same DAS principle can detect how the ground shakes during earthquakes or respond to the vibrations of human activity such as traffic.

As these vibrations pass through the ground, they are slowed down by the presence of water: the more moisture, the slower the vibration moves. So the new study measures the water content in the vadose zone "through seismic rumblings from everyday traffic," noted Caltech.

Monitoring the risk from climate change

The project installed its DAS monitoring system around Ridgecrest, north of the Mojave Desert, following a 7.2 magnitude earthquake there in 2019. The area has suffered severe draughts in recent years, and the impact on water stored in the subsurface has been unclear.

Using a pre-existing telecoms fiber optic cable along the US Route 395 Business highway allowed Caltech to create a 10-kilometer DAS array, capturing the vadose zone soil moisture dynamics over an extended period of time.

"From the top 20 meters of soil in the Ridgecrest region, we can extrapolate to the entire Mojave desert," said Yan Yang from Caltech Seismological Laboratory. "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."

Seismological instruments have never been used to measure soil moisture at such a large scale for such an extended and continuous time span according to Caltech, which plans to apply the same principle in regions where different conditions apply. Central California, for example, has farming operations that withdraw water but also receives snowmelt from the Sierra Nevada mountains.

"Given the escalated regional drought risk under climate change, our findings underscore the promise of fiber-optic seismic sensing to facilitate water resource management in semi-arid regions," commented Caltech.