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.
Back to the top
|
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.
