Amazing stuff! Except it depends on "pre-existing fiber-optic cables"!
"... Scientists have figured out a way to measure soil moisture by detecting the vibrations of traffic noise travelling through the ground.
The technique is called distributed acoustic sensing (DAS) and it is normally used to measure seismic waves during earthquakes.
Seismological instruments have never been used to measure soil moisture at such a large scale for such an extended and continuous timespan before.
Researchers in the US repurposed a DAS system to measure moisture content in a part of the Earth known as the vadose zone. This is the shallow region between the underground water table (where the soil is fully saturated) and the surface. ...
In DAS, laser pulses are sent along unused, underground fibre-optic cables. This light bends and refracts as vibrations, such as seismic waves, pass through the cable. Measuring this tells researchers important information about the passing wave.
DAS can also be used to pick up the vibrations of ambient noise caused by humans. The more moisture there is in the vadose zone, the slower these vibrations move through it. ..."
From the abstract:
"Vadose zone soil moisture is often considered a pivotal intermediary water reservoir between surface and groundwater in semi-arid regions. Understanding its dynamics in response to changes in meteorologic forcing patterns is essential to enhance the climate resiliency of our ecological and agricultural system. However, the inability to observe high-resolution vadose zone soil moisture dynamics over large spatiotemporal scales hinders quantitative characterization. Here, utilizing pre-existing fiber-optic cables as seismic sensors, we demonstrate a fiber-optic seismic sensing principle to robustly capture vadose zone soil moisture dynamics. Our observations in Ridgecrest, California reveal sub-seasonal precipitation replenishments and a prolonged drought in the vadose zone, consistent with a zero-dimensional hydrological model. Our results suggest a significant water loss of 0.25 m/year through evapotranspiration at our field side, validated by nearby eddy-covariance based measurements. Yet, detailed discrepancies between our observations and modeling highlight the necessity for complementary in-situ validations. 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."
Seismic Detectors Measure Soil Moisture Using Traffic Noise "Caltech researchers have developed a new method to measure soil moisture in the shallow subterranean 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 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. Additionally, moisture within the vadose zone changes rapidly—a thunderstorm can saturate a region that dries out a few days later."
Fig. 1: Conceptual model for the vadose zone water dynamics and time-lapse seismology example on Ridgecrest DAS array.
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