Geotimes
Feature 
Physics and Earth Science Go Underground
Brian McPherson, Herb Wang, Steve Kesler, Tullis Onstott and William Roggenthen



Sidebar:
Where will the lab be built?



About every 10 years, the subject comes up: a request to construct an underground earth science laboratory. Now another opportunity has arisen that may provide hope for such a facility. The history of the efforts for a laboratory is long and distinguished.

First it was in the late 1970s. Following their seminal experiments at the Stripa Mine in Sweden, Paul Witherspoon and colleagues at the Lawrence Berkeley National Laboratory led an effort to establish an underground laboratory for earth science research. Although the initial motivation was for nuclear waste disposal, they quickly realized that many important and basic questions of geomechanics, geohydrology, geochemistry and geophysics could be addressed only in an in situ laboratory. The team went as far as to prepare technical reports that surveyed existing underground facilities.

Going below to find lofty answers: In 2002, physicist Raymond Davis Jr. of the University of Pennsylvania received the Nobel Prize for his work capturing neutrinos. He built a large tank to capture the cosmic particles deep below the surface in the Homestake gold mine in Lead, S.D. The physics community is working to establish an even deeper underground laboratory to improve neutrino detection, and earth scientists have joined the effort to insure that an earth science laboratory is part of the proposal. Photo courtesy of the Brookhaven National Laboratory Media and Communications Office.

In 1982, George Kolstad, the geosciences program director in the Department of Energy’s Office of Basic Energy Sciences, created a working group of national laboratories to develop a proposal for agency action on an underground laboratory. Despite the group’s creation of a white paper that went as far as to dub the proposed facility “Earthlab,” and despite additional proposals from the Lawrence Berkeley National Laboratory in 1986 and 1987, no budget initiative ever went forward, and the working group dissolved. Meanwhile, underground laboratories were active in Canada, Switzerland and Sweden.

Interest was stirred again in 1993 after Congress decided not to fund completion of the proposed Superconducting Supercollider in Texas after 22 kilometers of tunnel had already been excavated. One request had been to use a portion of the abandoned tunnel for earth science research. When Texas took title back for the site, federal interest in sponsoring research disappeared. Competing proposals for the abandoned tunnel included mushroom farms and a prison. Most recently, the site was being considered for purchase by a private company for a counter-terrorism training camp.

As earth scientists struggled with the establishment of an underground laboratory, astrophysicists in the United States, Canada and Europe were quantifying the solar neutrino flux with large neutrino detectors sited thousands of feet underground. In 2002, the physicist Raymond Davis Jr. of the University of Pennsylvania earned a share of the Nobel Prize for physics for his pioneering neutrino work and having constructed the first neutrino observatory in the 1960s. He built it 1.6 kilometers underground in the former Homestake Mine in Lead, S.D., to use Earth’s crust as a shield to block cosmic rays. Trillions upon trillions of neutrinos that stream from the sun and far-off stars and galaxies pass through Earth because of their extremely low mass, and yet their collective bulk probably outweighs all the stars and galaxies put together. In the well-known model of the Big Bang, neutrinos left over from the creation of the universe are the most abundant particles in the universe.

To detect these nearly massless particles, Davis used a tank filled with 378,000 liters of perchloroethylene (dry-cleaning fluid). When the neutrinos interacted with chlorine-37, argon-37 was produced, leaving a radioactive marker of how many neutrinos passed through the tank over certain time periods. The experiment worked, and it detected fewer solar neutrinos than current theories of stellar evolution held, so the physics community built more and deeper underground neutrino observatories in order to improve its ability to detect low energy neutrinos. Most recently an observatory in Canada has determined the mass of a neutrino. Because neutrinos travel in straight lines, linking several observatories will permit astronomers to determine their origin and help map now unknown regions of the universe.

Over the past few decades the mass sensitivity of neutrino detectors has improved by three orders of magnitude, but to capitalize on this greater sensitivity means finding a way to work even deeper underground. And so the discussion for an underground laboratory has started again. The physics community has proposed constructing a National Underground Science Laboratory as deep as 2.5 kilometers. At the request of the White House and Congress, the community is assembling proposals for where such a laboratory would be and how it might be constructed (see sidebar on page 21).

Showing strain: This bent doorframe, once straight, in the Homestake Gold Mine is a record of the underground deformation, or rock strain, ongoing beneath Earth's surface. Photo by Bill Pariseau, University of Utah.

Wick Haxton, a physicist from the University of Washington, along with other investigators collaborating to create a National Underground Science Laboratory, recognized that physicists would gain great advantage by teaming up with earth scientists in the subsurface work. In the early 1980s, when the Department of Energy was discussing an underground laboratory, Haxton became aware of the potential and had attended a 1982 physics workshop called “Workshop on Science Underground.” More recently, he and other members of the collaboration organized an October 2001 conference to bring the earth science and biology communities into the current effort. Although the initiative was started by the physics community, it now includes an earth science component called EarthLab, an underground earth science laboratory.

A year later, during a September 2002 conference called Neutrinos and Subterranean Science sponsored by the National Science Foundation, physicists and earth scientists were both present. In fact, scientists from a broad range of disciplines were present to discuss how such a new laboratory and observatory would facilitate observations and experiments in astronomy, biology, chemistry, geology, hydrology and nuclear physics that otherwise would not be possible.

In situ

EarthLab would be a permanent facility. Much in the way they use the research vessels of the Ocean Drilling Program to take turns accessing the ocean, scientists could spend weeks to months watching Earth processes below the surface. The facility could not only be used for specific experiments but also could provide a longer-term platform for instrumentation such as three-dimensional seismic arrays.

As earth scientists, we find the opportunity to share the proposed facility simply invaluable. For example, the typical hydrogeologist or sedimentary basin geologist works with borehole data. A popular analogy for borehole data is to imagine drilling a hole into a wall to see the inside of the wall, but the hole is only one-eighth of an inch thick and the diameter of a thin thread. The snapshot of the wall that can be made within that hole is about the relative size of the snapshot gained by one borehole within a basin. One would have to determine the type and amount of wiring and other properties within the entire wall using a few of these tiny holes.

If you could knock a hole in the wall with a hammer, you might have a better chance. EarthLab is analogous to such a hole, the proposed facility being kilometers deep and kilometers wide. Imagine being within Earth itself, with tens of kilometers of volume accessible, able to monitor directly where water, gas and bacteria flow and why. To realize this goal, it is essential for hydrogeologists to go underground.

Fluid flow and transport: EarthLab would revolutionize the field of hydrogeology by providing in situ temporal and spatial measurements of fluid flow and transport over vast volumes and at great depths. It would provide the opportunity to verify and confirm through direct observations the fundamental physics and chemistry of fluid flow at depth, to this day only understood in the simplest of terms. Geophysical imaging techniques used to monitor this fluid flow will both assist the direct observations and be improved during the process. Knowledge of fluid flow and transport is important for assessing drinking and irrigation water supplies, hazardous waste disposal sites and remediation of contaminated groundwater.

It is well known that fluid flow and transport are active at great depths in the subsurface. However, the nature of that flow, its range of rates, and the role deep flow and transport play in other processes are largely unknown because they are extremely difficult to measure. Direct observations and experiments in the subsurface are rare, and Earth’s surface and boreholes are typically the only venues available for study. Unfortunately, samples of deep rocks obtained from drill holes are usually small and have been disturbed a great deal by the drilling process, making them poor materials for testing factors that control fluid flow.

Hydrology research is currently going on at the proposed nuclear waste repository at Yucca Mountain in Nevada. This underground facility, however, is much shallower than what is proposed for EarthLab.

Using tracer and geophysical imaging tests over the entire volume of the laboratory, EarthLab scientists would characterize rock structure, fracture connectivity and transport properties, and their variability with scale, with depth and with distance across the excavated zone. The ability to investigate the rock package directly after imaging or performing tracer tests may provide tremendous improvements in the techniques that can be applied in other investigations, such as those by the oil and gas industry and geothermal industry.

Life at depth: Geomicrobiologists could use EarthLab to develop and test technologies for detecting life underground, a potentially useful tool for exploring other planets. The discovery in the past decade of what appears to be a subsurface biosphere has opened a new scientific frontier. Earth sciences, chemistry, physics and biology are now merging to provide insights into how microbial life on this planet, and possibly others, may have originated and evolved and how subsurface microorganisms dissolve and precipitate minerals, leaving behind trace fossils.

Imagine an artificial drilling mole, a tube filled with scientific instruments attached to a cable that could burrow hundreds of meters under the Martian landscape, searching for water, organic chemicals and indications of life. It will be impossible to develop such technologies without going underground to watch and test them directly.

EarthLab would advance our understanding of the origins, diversity, distribution, function and adaptation of microbial communities in deep, largely inaccessible, often extreme subsurface environments. This expansion of knowledge can only occur if the coupling between biological processes and those associated with heat, energy and fluid transport and rock deformation can be determined experimentally in situ.

One plan is to use EarthLab to evaluate what factors enable microbial communities to survive at great depths. A subsurface lab can make it easer to identify what minerals serve as nutrients and the ranges of physical conditions that enable growth.

Inside Stripa: During the 1970s, researchers with the Lawrence Berkeley National Laboratory used the abandoned Stripa iron mine in Stripa, Sweden, to investigate options for storing nuclear waste. They soon realized an underground laboratory would be valuable for earth scientists. From the Ernest Orlando Lawrence Berkeley National Laboratory image library.

In fact, the high sensitivity equipment used by the physicists to quantify low energy particle fluxes could also be used to measure the glacially slow pace of subsurface microbial life. Indeed, microbial life is probably one of the most complex processes to be studied at EarthLab requiring carefully planned, multidisciplinary interactions. For example, fluid velocities and diffusion control the flux of soluble nutrients to the microorganisms, and this flux is also controlled by the permeability and porosity of the rocks that host the microorganisms. In turn, microorganisms may precipitate minerals or generate gases that alter permeability and fluid flow. Fluid advection perturbs or controls microbial migration rates and influences temperatures. Typically, temperatures change slowly, but if the change is more rapid than the ability of the microorganisms to migrate, they must adapt or die. Fluid flow determines whether and where subsurface communities survive.

EarthLab would be the only facility in the world where these geomicrobiological and biogeochemical processes could be delineated as a function of temperature and pressure to depths of several kilometers.

Rock deformation: Knowledge of rock strain, or deformation, is particularly important in evaluating earthquake hazards. EarthLab would permit continuous, direct measurements of strain, and provide an opportunity to evaluate factors that control it.

Rock strain and earth stresses deep in the subsurface are not well characterized, except for a limited number of measurements in deep boreholes and deep mines. Active rock strain can only be estimated by surface-based methods such as satellite-generated Interferometric Synthetic Aperture Radar and Global Positioning System data and associated analysis. As with permeability, how rock strain and stress vary as a function of position and sample volume, or measurement scale, are not well understood because sufficiently large volumes of rock at depth have not been adequately measured or characterized yet.

Access to the large rock volume in EarthLab would permit testing the hypothesis that Earth’s crust is “critically stressed,” that some portion of the rock is always close to failure by fracture. Repeated shearing of such fractures can keep flow paths open that might otherwise close by mineral cementation. The most significant rock permeability at depth, therefore, occurs in areas of critically stressed fractures. Mapping fractures, stress and fluid flow within the subsurface will help earth scientists to confirm or extend theories about the mechanics of earth deformations. Finally, active experiments, such as emplacing a heater in the rock mass, can improve our understanding of how coupled mechanical, chemical, and fluid flow properties respond to environmental changes.

Mineral resources and environmental geochemistry: EarthLab will allow more complete direct testing of mineral-forming hydrothermal processes that operate in the upper crust and that form ore deposits. These processes have only been inferred from theory and from observations of minerals after they’ve formed. In other words, by studying minerals in situ, in an actively evolving system, it will be possible to verify directly what typically is inferred by indirect methods.

Specifically, many of the mineral resources on which society depends are formed or concentrated by fluid flow in the subsurface. These concentrations reflect both chemical and physical processes of the surrounding rock. Oil, natural gas and some brines are localized in the crust largely by their physical response to fluid flow. Most metals, such as iron, copper and gold, are localized by chemical processes involving dissolution and subsequent deposition of minerals containing these metals. In both cases, the fluid flow system that concentrates the mineral resource gathers material from a large volume of rock and concentrates it in much smaller volumes that we call mineral deposits. Studies in a deep laboratory would contribute to our understanding of mineral deposit formation both through more detailed study of ancient deposits, which are likely to be exposed in any deep lab, and through active experiments involving fluid flow under controlled conditions. Although considerable progress has been made in understanding the processes that form mineral deposits by observation of fossil systems, we badly need to study these processes in an active environment. In most cases, these active environments, such as geothermal areas, are too hot and deep for direct study.

Fluid flow through rocks is also critically important to the release and concentration of metals and organic compounds that are of environmental concern. Most such releases take place because, through the mining process, rock from deep in the crust is exposed to water and oxygen, causing minerals formed at depth to decompose. The most widely known of these processes is acid mine drainage, which results when pyrite is oxidized by near-surface waters. Most studies of acid mine drainage and related processes have been confined to the points at which these waters exit mines or other underground sources. EarthLab would allow more direct observations of the early stages of these processes at depth, which will lead to better methods to control or stop dispersal of elements and compounds.

The current set of data earth scientists use to model active or ancient hydrothermal or fluid and mineral processes is woefully inadequate to accurately model “real” systems. EarthLab would provide a unique experimental venue whereby laboratory measurements of these parameters can be directly validated. For the first time, geochemists will be able to calibrate directly the accuracy of their experimental extrapolations.

From remote to direct

Direct access for the study of deep subsurface processes is limited to the few deep mines in the world and the small number of deep boreholes that penetrate the crystalline basement. Work in deep mines is difficult because mines are primarily industrial operations and because accessing areas of most interest often constitutes serious engineering challenges.

Work with deep core samples is complicated by the small sample sizes and disturbance caused by drilling, and because it is often difficult to extend interpretations beyond the immediate vicinity of the borehole. Examining larger-scale subsurface processes is currently accomplished using methods such as measurements of seismic velocities and attenuation, satellite-based remote sensing, and gravity, magnetic and electrical methods. These techniques are used to infer Earth’s structure and processes, but all of them may be considered types of remote sensing. EarthLab would provide an opportunity to go well beyond point sampling and to verify remote sensing methods with direct observation and measurement.

Much as surgery permits a physician to examine internal bones and organs recognized on X-rays or CAT scans, EarthLab would be a fully instrumented, dedicated laboratory and observatory for scientists to examine Earth’s active interior.

Where will the lab be built?

Since the Homestake gold mine in Lead, S.D., closed in December of 2001, physicists have been working for the mine’s conversion an underground laboratory.

Although Homestake has many advantages, a decision is on hold. Legal liability issues about any environmental damage the mining caused are one of the significant factors. In the mean time, federal funding is keeping the mine usable (for example, regular pumping prevents the mine from flooding).

Other sites that have been suggested are:

A currently undeveloped site beneath Mount San Jacinto near Palm Springs, Calif.
The lab would burrow into the mountain, and its portal would be the existing station for the Palm Springs Aerial Tramway.

The Waste Isolation Pilot Plant (WIPP) near Carlsbad, N.M.
The world’s first deep geologic repository for radioactive waste, WIPP is run by the U.S. Department of Energy and has been operating since 1999. Waste is stored about 900 meters underground beneath salt beds.

The Soudan mine in Soudan, Minn.
The former iron mine is a state park that also houses a high-energy physics lab about half a mile deep that is run cooperatively by the University of Minnesota and the U.S. Department of Energy’s Fermi National Accelerator Laboratory. Two major physics experiments, one to attempt to quantify the mass of neutrinos, are using the Soudan mine.

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McPherson is associate professor of hydrology at the New Mexico Institute of Mining and Technology. His research focuses on deep subsurface flow, including multiphase and reactive transport processes. E-mail him at: brian@nmt.edu.

Wang is professor of geophysics at the University of Wisconsin-Madison, and has been involved in several underground laboratory initiatives.

Kesler is professor of economic geology and geochemistry at the University of Michigan. He studies the origin of mineral deposits and related geochemical anomalies, working in numerous underground mines around the world.

Onstott is professor of geosciences at Princeton University and has spent the past 10 years investigating microbial transport and survival in subsurface environments.

Roggenthen is professor of geology and geological engineering at the South Dakota School of Mines and Technology.


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