2024-08-14 04:30:12
When summers are dry, farmers will want to know how much water is left in the soil. And, in the event of rain, the amount will be stored. An American team from the California Institute of Technology (Caltech) has just proposed an original method for obtaining these answers without placing special sensors on the ground and without resorting to satellites, which provide similar indications but for shallow depths.
Methods, presented in Nature communicationson August 5, uses road traffic and serves fiber optic communications in their test area, California, in the northern Mojave desert. “We have already used this 10 kilometer cable to monitor the damages after the violent Ridgecrest earthquake in 2019. But we understand that the data can also be used to get information on the water content of the buildings. It took us four years to show it”introduced Zhichao Shen, the first author of the study.
The use of optical fiber as a seismic sensor is not new. The oil industry uses it to control ground disturbances and scientists monitor volcanoes or large earthquake zones. The process takes advantage of subtle optical phenomena. The propagation of light in a thin glass channel is not perfect; Microscopic defects reflect part of the light, like mirrors. By sending short flashes of light, and comparing the correlations of the reflections on two “mirrors”, it is possible to know how the land, in which the sea is buried, has moved. “This turns the optical fiber into thousands of seismic sensors that measure deformation along the fiber”summarizes Destin Nziengui Bâ, who has just defended a thesis on the subject at Grenoble-Alpes University.
20 meters deep
In addition, the method here is “passive” because the “seismic” measured are those mainly due to road traffic. Thus, researchers know the propagation speeds of seismic waves in the ground up to 20 meters deep. Then they used a model – developed in 2021 mainly by Damien Jougnot, a CNRS researcher at the Sorbonne University – which linked this speed to the water content of the soil. As the water increases, the tides decrease.
Result: over two and a half years of observation, the desert has lost, through evapotranspiration, the equivalent of 25 centimeters of water height per year, when it only receives 5 cm thanks to rain. “These results are convincing and open new ways for the use of oceans in environmental studies, especially in relation to the water content of the so-called unsustainable environment”Damien Jougnot writes in his editorial report for the magazine. “It is a complementary approach to others”adds Michel Campillo, professor emeritus at Grenoble-Alpes University. It can be deployed quickly, at low cost.
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