Using seismology for groundwater management

New research showcases a pilot application using seismometers to monitor groundwater aquifers in California. As climate change increases the number of extreme weather events, groundwater management is key for a sustainable water supply. Current groundwater monitoring tools are either costly or insufficient for deeper aquifers. It limits our ability to monitor and practice sustainable management in populated areas.

Now, a new paper published in Nature Communications bridges 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 of and complementary to traditional observations,” says Shujuan Mao PhD ’21, lead author of the paper. “It provides a new way to dictate groundwater management and evaluate the impact of human activity on shaping underground hydrologic systems.”

Currently a Thompson Postdoctoral Fellow in the Geophysics department at Stanford University, Mao conducted most of the research during her PhD in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). Other contributors to the paper include the EAPS department chair and Schlumberger Professor of Earth and Planetary Sciences Robert van der Hilst, as well as Michel Campillo and Albanne Lecointre from the Institut des Sciences de la Terre in France.

While a few different methods are currently used for measuring groundwater, they all have notable drawbacks. Hydraulic heads, which drill through the ground and into the aquifers, are expensive and can only give limited information about their location. Noninvasive techniques based on satellite- or airborne sensing lack the sensitivity and resolution needed to observe deeper depths.

How the research helps groundwater management

Mao proposes using seismometers to measure ground vibrations, such as the waves produced by earthquakes. They can measure seismic velocity and the propagation speed 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.

Using seismic velocity to characterise property changes in rocks has long been used in laboratory-scale analysis. Still, scientists have only recently been able to measure it continuously in realistic-scale geological settings. For aquifer monitoring, Mao and her team associate the seismic velocity with the hydraulic property, or the water content, in the rocks.

Seismic velocity measurements use ambient seismic fields, or background noise, recorded by seismometers. “The Earth’s surface is always vibrating, whether due to ocean waves, winds, or human activities,” she explains. “Most of the time, those vibrations are small and considered ‘noise’ by traditional seismologists. But in recent years, scientists have shown that the continuous noise records contain a wealth of information about the properties and structures of the Earth’s interior.”

To extract useful information from the noise records, Mao and her team used seismic interferometry. They analyse wave interference to calculate the seismic velocity of the medium the waves pass through. For their pilot application, Mao and her team applied this analysis to basins in the Metropolitan Los Angeles region, an area suffering from worsening drought and a growing population.

By doing this, Mao and her team could see how the aquifers changed physically at a high resolution over time. Their seismic velocity measurements verified measurements taken by hydraulic heads over the last 20 years. The images matched very well with satellite data. They could also see differences in how the storage areas changed between counties in the area that used different water pumping practices. This knowledge is important for developing water management protocol.

Seismometers support groundwater management

Mao also calls using the seismometers a “buy-one-get-one-free” deal. Seismometers are already in use for earthquake and tectonic studies across California and worldwide. They could help “avoid the expensive cost of drilling and maintaining dedicated groundwater monitoring wells,” she says.

Mao emphasises that this study is just the beginning of exploring 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. Mao is currently applying it to oil and gas fields. But in places like California, currently experiencing megadroughts and that rely on groundwater for a large portion of their water needs, this kind of information is key for sustainable water management.

“It’s really important, especially now, to characterise these changes in groundwater storage so that we can promote data-informed 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 Thompson Fellowship at Stanford University.