Feature

André Revil Goes Beyond Sound Science
Geophysics professor uses subtle electrical measurements to gain new angles on subsurface imaging
The baristas at Cafe 13 in downtown Golden lit up when André Revil arrived.

"The usual, right? And André, the blueberry muffins are right out of the oven," one said, as she wheeled to prepare his double-espresso. The Mines geophysicist thanked her. He then admitted, quietly, "I'm a bit of a caffeine addict."

Revil, 39, with dark, tousled hair flecked by strands of gray and heavy circles under his eyes, fits the profile of a Frenchman entering his preferred habitat—the cafe. But Revil is equally at home on volcanoes, at industrial sites, in the lab or at his desk, touching up yet another academic paper.

Caffeine alone falls well short of explaining what fuels Revil, an associate professor of geophysics, whose interests span theory, laboratory and field work across a wide expanse of geophysical topics.

"He's in water, he's in environment, he's in volcanology, he's in earthquake seismology—he's just got a tremendous breadth of intellectual activity," said Terry Young, head of the Department of Geophysics. "He is incredibly prolific as a researcher."

The theme that runs throughout his work is a relatively novel geophysical technique for subsurface imaging: Revil is among the leading global experts at using electrical disturbances to image the underworld.

Rather than trying to map geological formations the traditional way—by interpreting the behavior of sound waves propagated at the surface by an explosion or a massive hydraulic hammer blow—these geoelectrical methods detect minute electrical signals, measured in thousandths of a volt, using either passive or active means. With passive geoelectric approaches, scientists use a web of electrodes, either on the surface or in boreholes, to sniff for subtle electrical charges generated as fluids pass through different kinds of rocks, clays or sands. Inferences can be made based on these electrical properties. Active approaches create an electrical charge through similar electrode arrays, then infer the makeup of geological strata based on how they conduct or resist the electrical current.

The method relies on the relationships between conductivity and mineral composition, porosity, saturation and the conductivity of the water saturating these rocks, which in turn depends on such factors as acidity and the liquid’s concentration of dissolved solids.

Both seismic and geoelectric techniques aim to characterize the netherworld, creating maps engineers can use to do their jobs. Seismic tools can go deeper—the bottom of Earth’s crust, Revil says—whereas geoelectric techniques penetrate about 500 meters of earth. But geoelectrical methods have the advantage of being able to differentiate various porous media as well as the liquids flowing through them: Seismic can detect fluids, but geoelectric can “taste” the difference between oil and water, a capability energy companies and hydrologists are most interested in.

Born in Annecy in the French Alps, Revil came from a family of French restaurateurs. He had visions of becoming a petroleum engineer, but got hooked on the idea of being able to do “things in the lab and things in the field and work on numerical modeling and inverse modeling”—the conversion of sensor data into comprehensible imagery—in addition to consulting with industry. He earned his PhD from Ecole de Physique du Globe de Strasbourg, studying the electrical conductivity of porous media as gathered by downhole measurements, and then went to work as a researcher for French national research giant CNRS. His work there spanned tracking groundwater contamination plumes, hunting for sinkholes, characterizing the underbellies of active volcanoes, and, for the French National Agency for Nuclear Waste Management, understanding how nuclear contaminants might migrate through clays.

Since his arrival at Mines in 2007, Revil has maintained a dizzying pace of research and publication. Working with various members of his team of 16 students (among them seven PhDs, four still finishing up in France), as well as international collaborators, Revil published nine peer-reviewed papers in 2008 and another six so far in 2009, which puts him on the roughly 20-a-year pace he maintained before his transition to the U.S. (Three or four peer-reviewed papers a year is more typical.)

Revil says his team of students is largely to thank for such productivity. They chase their own curiosities in the lab or out in the field—volcanoes, landslides, clays, biogeophysical studies—although he ensures there’s at least tangential relevance to the work of others in the group. And when they publish, students are generally the lead authors.

However, Revil’s intellectual dynamism is clearly an inspiration for the group. Over the recent winter break, Revil took advantage of his wife and daughter’s trip to France and spent 10 days of marathon theorizing with Mines postdoc Abdel Jardani, which resulted in two academic papers. Revil also collaborates with a network of researchers around the world. One, a Brazilian co-author he began correspondence with after refereeing a paper, he has never met. “I have no idea what he looks like,” Revil said.

Perhaps because Revil’s first college degree was in engineering, he enjoys hands-on field and laboratory work. One project he’s currently involved in with a former PhD student in France looks at how the hair-like appendages of bacteria that swarm through plumes of contaminants underground can act like microscopic power lines, helping to electrify an entire contaminant plume—a finding that could make remediation of contaminants easier in the future.

He’s also leading a new $800,000 Department of Energy-funded study in small-scale (5-10 megawatt) geothermal energy at the Mines geophysics field camp in the Mount Princeton Hot Springs area. Working with Professor of Geophysics Mike Batzle, the team will use different techniques to understand heat sources and groundwater flow, in effect “looking at the plumbing” of the field, Revil said. The goal is to take that knowledge to identify potential geothermal-energy hot spots throughout the Arkansas River Valley.

In the lab, some of Revil’s most interesting work revolves around an insight gained on a 2002 CNRS ski trip in the French Alps. He was trading stories with brain researchers from Marseilles who happened to work in electroencephalography (EEG), which tracks brain activity based on electrical signals picked up through the scalp. Revil realized how similar his geoelectrical work was, and with time acquired his own EEG setup, which is located in a basement laboratory in the Green Center.

Revil and master’s degree student Allan Haas are using this equipment in “sandbox experiments” involving large Petco fish tanks filled with, appropriately, wet sand. The sensitivity of the equipment, Haas said, picks up small changes in electrical behavior in real time. For example, using a Tupperware tub with a hole in it to simulate a leaking tank of contaminants, the EEG equipment will detect changing electrical behavior as the foreign substances seep through the moist sand.

If this technology can be scaled up, it could help solve some practical engineering problems, such as following the progress of fluid through rock. “It could enable real-time monitoring and imaging,” Haas said. “That’s never been done before.” It may also help engineers locate corroded steel embedded in concrete, thereby identifying dangerous structural weaknesses that are otherwise invisible.

Looking ahead to where his research is leading, Revil has some ambitious goals. “The big game in geophysics is to combine the information from different techniques—to do data fusion,” he said. One approach could involve seismoelectric methods—a hybrid approach that combines traditional seismic techniques with geoelectrical imaging. It involves a “boom,” but rather than just recording sound wave data, systems are also in place to sniff for the electromagnetic energy triggered by the shock. Russians first came up with the technique a half century ago, and it has the potential to combine the seismic method’s high resolution with the geoelectrical method’s sensitivity to fluids, Revil said. If successfully implemented, it could significantly improve geophysicists’ ability to image water tables, and oil and gas reservoirs. Revil recently convinced Shell and Exxon to share their seismoelectric data for use in further refining theories and imaging techniques, and the team has already submitted two papers on this method.

Even further ahead, Revil has his sights on a unified theory of electrical properties of porous rocks. Its math would combine the perspectives of various geophysical techniques—electrical as well as seismic—that paint different parts of the greater geological picture, Revil said. But he admits that this is an ambitious project, and given his many professional interests, it may be many espressos away.