Using Seismology for Groundwater Management | MIT News
As climate change increases the number of extreme weather events, such as mega-droughts, groundwater management is critical to maintaining water supply. But current groundwater monitoring tools are either expensive or insufficient for deeper aquifers, limiting our ability to monitor and practice sustainable management in populated areas.
Now, a new paper Posted in Nature Communication links seismology and hydrology with a pilot application that uses seismometers as a cost-effective way to monitor and map groundwater fluctuations.
“Our measurements are independent and complementary to traditional observations,” says Shujuan Mao PhD ’21, senior author of the article. “It provides a new way to dictate groundwater management and assess the impact of human activity on the formation of subsurface hydrological systems.”
Mao, currently a Thompson Postdoctoral Fellow in Stanford University’s Department of Geophysics, conducted most of the research during her PhD in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). Other contributors to the article include Robert van der Hilst, director of the EAPS department and Schlumberger Professor of Earth and Planetary Sciences, and Michel Campillo and Albanne Lecointre of the Institute of Earth Sciences in France.
Although there are a few different methods currently used to measure groundwater, they all have notable drawbacks. Hydraulic heads, which drill through the ground and into aquifers, are expensive and can only give limited information about the precise location where they are placed. Non-invasive techniques based on satellite or airborne detection lack the sensitivity and resolution needed to observe deeper depths.
Mao proposes using seismometers, which are instruments used to measure ground vibrations such as waves produced by earthquakes. They can measure seismic velocity, that is, the speed of propagation of seismic waves. Seismic velocity measurements are unique to the mechanical state of rocks, or how rocks respond to their physical environment, and can tell us a lot about them.
The idea of using seismic velocity to characterize property changes in rocks has long been used in laboratory-scale analysis, but only recently have scientists been able to measure it continuously in geological contexts on a realistic scale. For aquifer monitoring, Mao and his team relate seismic velocity to the hydraulic property, or water content, of rocks.
Seismic velocity measurements use ambient seismic fields, or background noise, recorded by seismometers. “The Earth’s surface is constantly vibrating, whether from ocean waves, winds or human activities,” she explains. “Most of the time, these vibrations are really weak and are considered “noise” by traditional seismologists. But in recent years, scientists have shown that continuous noise recordings actually contain a wealth of information about the properties and structures of the Earth’s interior.
To extract useful information from noise recordings, Mao and his team used a technique called seismic interferometry, which analyzes wave interference to calculate the seismic velocity of the medium through which the waves pass. For their pilot application, Mao and his team applied this analysis to basins in the Los Angeles metropolitan area, an area suffering from worsening drought and a growing population.
By doing this, Mao and his team were able to see how aquifers physically changed over time at high resolution. Their seismic velocity measurements verified the measurements taken by the hydraulic heads over the past 20 years, and the images matched the satellite data very well. They were also able to see differences in how storage areas changed between counties in the region that used different water pumping practices, which is important for developing a water management protocol. ‘water.
Mao also calls the use of seismometers a “buy one, get one free” offer, since seismometers are already being used for seismic and tectonic studies not just in California, but around the world, and could help “avoid the high cost of drilling and dedicated maintenance”. groundwater monitoring wells,” she says.
Mao points out that this study is only the beginning of exploring the possible applications of seismic noise interferometry in this way. It can be used to monitor other near-surface systems, such as geothermal or volcanic systems, and Mao is currently applying it to oil and gas fields. But in places like California that are currently experiencing mega-droughts and rely on groundwater for much of their water needs, this kind of information is essential for sustainable water management.
“It’s really important, especially now, to characterize these changes in groundwater storage so that we can promote data-driven policymaking to help them thrive under increasing water stress,” she says.
This study was funded, in part, by the European Research Council, with additional support from the Stanford University Thompson Fellowship.