November 12, 2019
SIGMA-V experiment moves 750 feet up to learn more about hot rocks 600 miles away.
Researcher with the EGS Collab SIGMA-V experiment works on the 4100 Level at Sanford Lab. Photo by Nick Hubbard
Interstate-80 cuts across the undulating landscape of Utah and Nevada. Along the roadway, mountain ranges with alpine vegetation give way to low desert regions flecked with hunched sagebrush, only to ascend into another isolated mountain range.
The rippling rise and fall of the interstate is caused by fault valleys that run north and south through the Great Basin region. These geologic features, called horsts and grabens (hills and valleys), are the result of crustal extension.
“The whole area has extended, widening hundreds of kilometers over millions of years,” explained Patrick Dobson, geothermal systems program lead and staff scientist at Lawrence Berkeley National Laboratory (Berkeley Lab). With each stretch and tear, the Earth’s crust thins, allowing heat to seep from the Earth’s fiery mantle into the rocks nearer the surface.
Beneath the city of Milford, Utah, the Earth’s crust is especially thin. Just two kilometers below the surface lies hot granite at elevated temperatures of 175 °C. These hot rocks are good news for researchers like Dobson, who are developing technologies to advance geothermal energy systems.
“Geothermal systems are really just trying to mine heat,” Dobson said. “From a geothermal standpoint, higher temperatures at shallower depths are a good thing. We don’t have to drill as deep to get the temperatures that we need.”
Subsurface temperatures in the United States at a depth of 20,000 feet. Graphic courtesy University of Utah.
A site just northeast of Milford is a proposed location for the Department of Energy’s FORGE project (Frontier Observatory for Research in Geothermal Energy). If the project is successful in extracting natural heat, it could act as an enormous, domestic, clean energy resource.
There’s just one problem: the rock near Milford isn’t naturally permeable.
“Geothermal extraction requires three things: hot rock, permeable pathways through the rock and fluid to extract the heat,” explained Tim Kneafsey, principal investigator for the Enhanced Geothermal Systems (EGS) Collab Project and a staff scientist with Berkeley Lab. “Hot rock is an abundant resource in the US, but it is often missing open pathways that allow you to extract the heat.”
The EGS Collab is working to find better ways to extract heat from the earth’s hot rocks. Under the leadership of Berkeley Lab and Sandia National Laboratories, researchers are creating models that can predict the behavior of geothermal hot spots, before full-scale site research begins at the FORGE laboratory in Utah.
“The challenge is to safely and inexpensively create pathways in rocks that will allow us to circulate fluids and extract heat,” Dobson said. “Is it economically viable? Environmentally safe? Technologically feasible?”
These questions brought the EGS Collab to Sanford Underground Research Facility (Sanford Lab).
Taking research up a level
The rocks here aren’t hot, and that’s a good thing.
“It’s only about 32 degrees Celsius (90 F) in its native state at this depth,” Dobson said as he placed his hand demonstratively against the rock on the 4100 Level of Sanford Lab. In South Dakota, the Earth’s crust is thick, meaning researchers can access relevant depths and take measurements in situ—research that would be cost-prohibitive at another site.
Researchers have outfitted a drift on the 4100 Level for the next phase of the EGS Collab SIGMA-V experiment. Photo by Nick Hubbard
“At FORGE, they’ll be drilling from the surface down several thousand feet deep, with limited direct access to the rock,” Kneafsey said. “Here, we can stand right next to the rock and collect data at depth.”
The EGS Collab has used the underground drifts of Sanford Lab as a research and development testbed since 2015. The SIGMA-V experiment has probed the Poorman rock formation on the 4850 Level for years, collecting immense amounts of data.
Now, the EGS Collab is moving their equipment to the 4100 Level, where the rock is slightly different.
“This rock on the 4100 Level is amphibolite, a metavolcanic rock much more similar to the basement rocks that we expect in the Great Basin,” Dobson said.
“At Sanford Lab, physics experiments and laboratories are concentrated on the 4850 Level, but a large portion of our research community accesses other levels: the biologists, geologists and engineering groups,” said Jaret Heise, science director at Sanford Lab. “EGS Collab’s move to the 4100 Level takes advantage of the tremendous potential our facility offers.”
The testbed on the 4100 Level will focus on hydraulic shearing: opening natural, preexisting fractures in the rock.
“On the 4100 Level, we are trying to reopen existing natural fractures that have been sealed with mineralization,” Kneafsey said. “By opening them and causing them to shift slightly, the roughness of the fractures keeps them propped open. This self-propping allows water to flow through and, in hot rock environments, transfer heat.”
Just as they did on the 4850 Level testbed, researchers will drill multiple boreholes, both to stimulate the rock and to monitor it with a dense array of sensors. Through these boreholes, they can study how fractures open and shift.
Mapping the pathways
To engineer safe and economical geothermal systems at sites like Milford, Utah, researchers have created intricate computer models of the earth’s subsurface.
“A lot of our effort is contributed by people you don’t see onsite at Sanford Lab,” Dobson said. “They are working to recreate the physics of the earth using computer models.”
These models predict everything from fluid flow and heat transfer to geomechanical displacement and geochemical reactions between fluids and rocks. The experiments done at Sanford Lab and other test sites help researchers test the accuracy of these models’ predictions.
“We have a world-class team of modelers looking at the data,” Kneafsey said. “Oftentimes when you get real data from real sites, it doesn’t exactly fit the preconceived notions of modeling. Every field site helps us validate our models.”
Researchers with EGS Collab hope that data collected by the SIGMA-V experiment will propel enhanced geothermal energy systems in the future.
Photo by Nick Hubbard.
“We hope that the knowledge and the understanding we generate are very useful for implementing EGS in the future,” Kneafsey said. “We’ve published over 900 pages of conference papers and journal publications. We are trying to run this project as openly as we can, collaborating and sharing information with other researchers and test sites.”
The EGS Collab includes researchers from nine national labs—LBNL, SNL, Lawrence Livermore National Laboratory, Pacific Northwest National Laboratory, Idaho National Laboratory, Los Alamos National Laboratory, National Renewable Energy Laboratory, National Energy Technology Laboratory, and Oak Ridge National Laboratory; and seven universities—South Dakota School of Mines & Technology, Stanford, University of Wisconsin, University of Oklahoma, Colorado School of Mines, Penn State, and Rice University.
This EGS Collab Project is supported by the U.S. Department of Energy, Geothermal Technologies Office; part of the Office of Energy Efficiency and Renewable Energy (EERE).
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The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.
The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.
Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.
In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.
In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.
The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.
LUX’s mission was to scour the universe for WIMPs, vetoing all other signatures. It would continue to do just that for another three years before it was decommissioned in 2016.
In the midst of the excitement over first results, the LUX collaboration was already casting its gaze forward. Planning for a next-generation dark matter experiment at Sanford Lab was already under way. Named LUX-ZEPLIN (LZ), the next-generation experiment would increase the sensitivity of LUX 100 times.
SLAC physicist Tom Shutt, a previous co-spokesperson for LUX, said one goal of the experiment was to figure out how to build an even larger detector.
“LZ will be a thousand times more sensitive than the LUX detector,” Shutt said. “It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter.”
We celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment.
Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.
The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge 0νββ search.
CASPAR is a low-energy particle accelerator that allows researchers to study processes that take place inside collapsing stars.
The scientists are using space in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to work on a project called the Compact Accelerator System for Performing Astrophysical Research (CASPAR). CASPAR uses a low-energy particle accelerator that will allow researchers to mimic nuclear fusion reactions in stars. If successful, their findings could help complete our picture of how the elements in our universe are built. “Nuclear astrophysics is about what goes on inside the star, not outside of it,” said Dan Robertson, a Notre Dame assistant research professor of astrophysics working on CASPAR. “It is not observational, but experimental. The idea is to reproduce the stellar environment, to reproduce the reactions within a star.”