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3 Science Assessments The proposed DUSEL science program encapsulates an initial suite of physics experiments and diverse multidisciplinary research experiments in subsurface engi- neering, the geosciences, and the biosciences and has the capacity for more future experiments. This chapter undertakes to present the committee’s assessment of the main physics questions to be addressed by the proposed physics experiments and of the impact of the proposed facility on research in fields other than physics. The proposed physics experiments are one or more dark matter experiments; a long- baseline experiment for the study of neutrino oscillations and proton decay that is also capable of measurements in neutrino astrophysics; a neutrinoless double- beta decay experiment; and an accelerator-based nuclear astrophysics experiment. Accordingly, the chapter assesses, in no particular order, the physics questions of dark matter, of long-baseline neutrino oscillations and neutrinoless double-beta- decay in the larger context of neutrino physics and, together with proton decay, in the context of unified theories; of nuclear astrophysics, and of neutrino astro- physics. It also undertakes an assessment of the impact of the proposed labora- tory infrastructure on research in fields other than physics—namely, subsurface engineering and the geosciences and biosciences. To give an idea of the scale of the experiments needed to address the elements of the proposed DUSEL program, the construction cost ranges estimated by the DUSEL project during the preliminary design process were $80 million to $200 mil- lion for the dark matter experiment(s); $785 million to $1,065 million for the long-baseline neutrino and proton decay experiment, $250 million to $350 million for the neutrinoless double-beta decay experiment, $30 million to $50 million for 28
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science assessments 29 the nuclear astrophysics facility, and $60 million to $180 million total for multiple experiments in subsurface engineering and geoscience and bioscience. The esti- mated incremental costs associated with efforts to detect supernovas and proton decay are not significant. Budgetary considerations and further development of the experiments will, of course, change the actual costs of these experiments. Because both the DUSEL program and the designs for the experiments to address the critical physics questions are still evolving, the committee chose to focus its assessment on the scientific merits of the questions to be addressed rather than on the technical merits of the experiments as they are now designed. Accordingly, it did not assess the technical merits of each experiment being sited at DUSEL or the suitability of alternative sites. Similarly, the committee chose to focus its assessment on the general scientific merits of research in the fields other than physics that would be enabled by the availability of an underground research facility rather than on the specific scientific or technical merits of a particular suite of nonphysics underground experiments. In choosing to focus in this way, the committee intends its assessments to be of value to the future direction of underground research, independent of whether the DUSEL program, as presently conceived, is realized. Finally, the committee assessed the intellectual merit of the underground science of the proposed DUSEL program in the general context of frontier scientific research worldwide. It was not a purpose of this study to rank the different fields or subfields of science, or to prioritize across programs. Neither the individual science questions nor the overall scientific program were compared with those of any other particular projects or investments. PHYSICS PROGRAM Dark Matter Overview Astronomers are sure that what can be detected by telescopes represents only a small portion of the Universe; furthermore, only a small fraction (~4 percent) is made of normal matter of the type that we live with here on Earth and observe directly elsewhere. The remainder of the Universe is composed of dark matter (about 22 percent), which has mass but does not emit or absorb light, and dark energy (about 74 percent). While dark energy is best studied using astrophysical techniques, direct detection of dark matter in the laboratory is possible, and direct experimental detection of dark matter interactions would profoundly change our understanding of both the microscopic world of elementary particles and the mac- roscopic astrophysical world, thus bridging the very smallest and the very largest objects in the known Universe.
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an assessment science ProPosed dUsel 30 of the for the The first evidence for the existence of dark matter came from observations of the rate at which astronomical objects such as stars, gas clouds, and galaxies rotate. It was discovered that bodies far from the center of rotation move faster than would be predicted using the laws of gravity and the visible mass of known objects, sug- gesting that unseen bodies existed on a grand scale. Additional evidence for dark matter comes from cosmological observations such as the fluctuation patterns of the cosmic microwave background, and further corroboration is provided by observations of colliding galaxies where the dark matter has been imaged using gravitational lensing. Depictions of this phenomenon have captured the imagina- tion of the general public (see Figure 3.1). Many explanations of the composition of dark matter have been proposed and compared with experimental data. Some of the dark matter could come from unobserved dark bodies of ordinary matter, such as massive compact halo objects or molecular gas clouds. However, to understand cosmological data requires the existence of exotic dark matter, and there now is consensus that most of the dark matter consists of as-yet-undiscovered elementary particles whose nature has yet to be determined. One possibility motivated by theory is that the dark matter arises from a particle called the axion. Experimental searches for axions and indirect astrophysical detection of dark matter use techniques that do not operate under- ground and so will not be discussed here. A second theoretically attractive possibil- ity is that dark matter consists of weakly interacting massive particles (WIMPs). Such WIMPs could be directly detected in underground experiments and would be the focus of an underground dark matter search program. Scientific Landscape Theories of elementary particle physics provide natural candidates for WIMPs. For example, in many supersymmetric models, the lightest supersymmetric particle is stable, and many of these theories naturally provide particles with masses and interaction cross sections that are consistent with astronomical and cosmological bounds on WIMP properties. There are also nonsupersymmetric theories that postulate the existence of particles with the appropriate properties. Several of these particles are being searched for in accelerator-based programs such as the Large Hadron Collider (LHC) of the European Organization of Nuclear Research (CERN). However, only the direct detection of naturally occurring WIMPs would assure that these particles, whether discovered at an accelerator or not, are in fact the source of dark matter. Because they are elementary particles not found in the Standard Model, it is likely that, when discovered, dark matter particles will be a central ingredient in finding solutions to known problems with present particle theory. Knowledge of the mass, the interaction rate, and the number density of dark matter particles
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science assessments 31 FIGURE 3.1 Color-coded image of colliding galaxies, with familiar matter shown in red (from x-rays) and dark matter shown in blue (modeled from weak lensing measurements). The interactions of familiar matter slow the collision, while the weakly interacting dark matter associated with each galaxy is essentially transparent and so passes through. The cluster, known as MACS J0025.4-1222, is a composite of separate exposures from the Hubble Telescope and the Chandra observatory. Astrono - mers say the images may shed light on the behavior of dark matter. SOURCE: x-ray image, National Figure 3-1 Aeronautics and Space Administration/Chandra X-ray Center/Stanford University/S. Allen; optical R02033 lensing image, National Aeronautics and Space Administration/Space Telescope Science Institute/ fixed raster image, not editable University of California at Santa Barbara/M. Bradac.
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an assessment science ProPosed dUsel 32 of the for the independent of any theoretical framework would allow predictions of produc- tion and annihilation rates that could be tested in future experiments. These data would also affect cosmological calculations relevant for describing the evolution of the Universe. Experimental Aspects The direct detection of dark matter would involve the search for collisions between ordinary nuclei and WIMPs from the halo of our galaxy. Such observa- tions would be difficult, since WIMPs interact rarely and the signals of the collision would be very faint. Therefore, detectors having a good likelihood of measuring such collisions would need to be large and operate deep underground to reduce backgrounds of cosmic ray origin that can mimic the signals being sought. These searches are based on the hypothesis that dark matter consists of WIMPs with a mass of a few tens of proton masses or greater. When such a particle collides with a target it should produce a recoiling nucleus whose energy can be measured through scintillation light flashes, phonons, or ionization produced by the nucleus. Learning to address the challenges associated with these types of studies requires a series of experiments with ever-increasing target mass and improvements in methods for rejecting background signals. History teaches that each generation of detector corresponds to an increase of about an order of magnitude in target mass. In the 25 years since WIMPs were first proposed as a dark matter candidate, the sensitivity of nuclear recoil experiments has improved by a factor of more than 1 billion. Once irreducible backgrounds are encountered for a specific detector, further running in the same configuration improves sensitivity only very slowly. It is much more efficient to determine appropriate solutions to identify and account for backgrounds and then to incorporate these improvements while also increas- ing the target mass. Past experiments are referred to as generation zero (G0) and ongoing experi- ments as generation one (G1). G1 experiments typically operate with tens of kilograms of target mass and are reaching much better background reduction and sensitivity than G0 experiments. Experience with the targets and the handling of backgrounds have informed next-generation designs, and G2 experiments are cur- rently under development and installation. These experiments will have hundreds of kilograms of target mass, and the following generation, G3, will have even greater target masses, 0.5 to multiple tons. The experiments considered for DUSEL are in the G3 category. The dark matter experiments summarized in Table 3.1 illustrate the current and future generations of detector and techniques. U.S. scientists historically have been heavily involved in this research and are expected to continue their involve- ment. The compelling nature of the science, and the high discovery potential,
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science assessments 33 makes it important that they do so and that opportunities exist for discoveries to be made in the United States. The strategy for experimental background rejection depends on which of the three signatures currently used to observe the nuclear recoil is chosen: scintilla- tion, phonons, or ionization. Some experiments use a “single signature,” including the shape and localization of that signal. These include single-phase noble liquid (xenon, argon) scintillation experiments and experiments exploiting the bubble chamber concept, where ionization in a supersaturated liquid creates bubbles that can be detected visually or acoustically or both. Other experiments, including most of the leading large experiments, use com- binations of two signatures to reinforce background rejection: (1) light/ionization together with phonons or heat in crystals at millikelvin temperatures and (2) light/ionization in noble liquid detectors. Experiments of the first kind use ger- manium or scintillating crystals; those of the second are double-phase ionization/ scintillation xenon or argon experiments, so called because they operate under conditions where the gas and liquid phases coexist, enabling amplification of the weak ionization signal in the gas. Research and development are under way on direction-sensitive detectors using low-pressure-gas “time projection chambers.” Debates regularly surface in the dark matter community about whether certain experiments have properly excluded or included claims of positive signals. To address these uncertainties about signals, it is important that a single experiment be able to collect multiple complementary signals and that multiple experiments using different nuclear targets are conducted. The most recent results over the WIMP mass range of 10-1,000 GeV exclude cross sections approaching 10-44 cm2 per nucleon for the simplest models (see Figure 3.2). However, the DAMA/Libra experiment has a long-standing observa- tion with an annual modulation of the event rate that is consistent with a WIMP having a mass of less than 10 GeV. These results have persisted over 7 years of data taking. However, the cross section indicated by the DAMA/Libra experiment is not consistent with limits from the CDMS and XENON-100 experiments in most WIMP models. There are also measurements that may indicate an excess above backgrounds at very low WIMP masses, but this signal is not as well established as the DAMA/Libra observation. Finally, a number of cosmic ray experiments (PAMELA and ATIC) report excess electron or positron signals that could be from WIMP annihilation in our galaxy. It is, however, somewhat complicated to find dark matter models that reconcile these results with the charged cosmic ray data from the Fermi/LAT experiment. However, several research groups have pointed out that conventional astrophysical sources of positrons could account for the putative PAMELA/ATIC signal. The theoretical community has been very active in trying to explain some or all of these results and has developed new models leading to new signals to search for at the LHC, in B-factory data and in electron-scattering
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an assessment science ProPosed dUsel 34 of the for the TABLE 3.1 Plans of WIMP Search Collaborations Using Nondirectional Detectors Around the World Current Generation (G1) Generation 2 (G2) Generation 3 (G3) Country/ Region Gross Mass Current Status Gross Mass Current Status Gross Mass Current Status United States LUX Assembly LZS Design LZD S4, R&D 350 kg Xe 2011 Install 1.5-3 tons Xe Same water 20 tons Xe 2017 Sanford Lab Sanford Lab tank as LUX DUSEL U.K./ ZEPLIN III Running Portugal/ 10 kg Xe (2009-2010) Russia Boulby, U.K. United States Darkside-50 Design 1 ton Design MAX S4, R&D 50 kg Ar DAr under Same 6 tons Xe 2017 Install LNGS procurement shield as 20 tons DAr 2011-2012 DarkSide-50 DUSEL United States/ XENON100 Running XENON1T Design Europe/ 80 kg 2.4 tons Xe 2012 Install China Gran Sasso United States/ SCDMS Construction SCDMS R&D GEODM S4, R&D Canada 10 kg Ge 2011 Install 100 kg Ge 2014 Install 1.5 tons 2018 Soudan SNOLAB DUSEL United States COUPP Construction 500 kg 2011 Design 16 ton scale S4 60 kg CF3 NUMI test 2013 Install R&D SNOLAB 2010 Canada PICASSO Running PICASSO II 2010/11 PICASSO III 2012/13 Install 2.6 kg 25 kg Install >500 kg SNOLAB United States/ MiniCLEAN Construction DEAP-3600 Funded CLEAN Planning Canada 500 kg Ar 2011 Install 3.6 tons 2012 Install 50 tons Ar/Ne R&D Europe Edelweiss Running EURECA Active R&D EURECA Planning Now 3 kg → 24 kg 100 kg Ge 2013 Install 1 ton Ge/ 2016 24 kg Ge funding interleaved Scintillator, 2011 Modane secured Ge/scintillator, LS Modane Modane extension Europe CRESST Running extension, 5 kg of CaWO4 Merging of Gran Sasso CRESST and Edelweiss Europe ArDM Construction 800 kg Ar 2011 Install Canfranc Europe/ WARP Running United States 140 kg Ar Gran Sasso
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science assessments 35 TABLE 3.1 Continued Current Generation (G1) Generation 2 (G2) Generation 3 (G3) Country/ Region Gross Mass Current Status Gross Mass Current Status Gross Mass Current Status Japan XMASS Installation XMASS II R&D XMASS III Planning 800 kg Xe Running 5 tons 2014 Install 10 tons 2016 Kamioka 2010 China JinPing lab Planning 100 kg 2015 R&D >1 ton 2020 Ge and/or Xe NOTE: All masses are the active masses of the central detectors. DAr, Ar depleted in 39Ar; LUX, Large Under- ground Xenon experiment; LZS, 1,500 to 3,000 kg liquid xenon detector; LZD, 20-ton liquid xenon detector; S4, NSF Solicitation 4; ZEPLIN III, two-phase Xe detector; Darkside, Depleted Argon [K]ryogenic Scintillation and Ionization Detector; MAX, Multiton Argon and Xenon detector; LNGS, Laboratori Nazionali del Gran Sasso; XENON100, Xenon 100-kg dark matter experiment; XENON1T, Xenon 1 ton dark matter experiment; SCDMS, Soudan Cryogenic Dark Matter Search; GEODM, Germanium Observatory for Dark Matter; COUPP, Chicago - land Observatory for Underground Particle Physics; MiniCLEAN, Mini-Cryogenic Low Energy Astrophysics with Noble liquids experiment; DEAP-3600, Dark matter Experiment using Argon Pulse-shape discrimination; CLEAN, Cryogenic Low Energy Astrophysics with Noble liquids experiment; EURECA, European Underground Rare Event Calorimeter Array; CRESST, Cryogenic Rare Event Search with Superconducting Thermometers; ArDM, Argon Dark Matter experiment; WARP, Wimp Argon Program experiment; XMASS, Xenon Dark Matter Search Experiment. SOURCE: Adapted from B. Sadoulet, University of California at Berkeley, “Dark Matter at DUSEL,” Presentation to the committee on December 14, 2010. experiments. At some level, dark matter imposes itself on every branch of particle physics. To keep all these communities from confusion as claims of discovery are made, definitive conclusions must be reached, and this will necessitate more than one detector that uses more than one technique. In the next 4 to 6 years, with the deployment of the G2 experiments such as MiniCLEAN, DEAP-3600, and LUX, and as new results from XENON-100 become available, sensitivities can be expected to increase by another order of magnitude. Two of the approaches under consideration for G3 detectors are 1-ton phonon- mediated low-temperature detectors and 1-ton or multiton noble liquids. U.S. scientists are playing leadership roles using both techniques, and contacts between this country and European groups are well developed. G3 experiments will push the cross section sensitivity below 10-47 cm2 per nucleon. Sensitivity near 10-48 cm2 per nucleon approaches a new background regime at which solar neutrino coher- ent scattering becomes important. This solar neutrino background is irreducible, and to progress past the regime, statistical background subtraction or directional detection become necessary, both of which represent a quantum step in difficulty. Thus, supporting G3 experiments are a natural goal for the next decade. On a lon- ger timescale, large directional detectors may be required. Underground access for detector development is essential because background signals at the surface make it impossible to accurately assess performance aboveground. Post-G3 and large directional detectors would likely require large caverns at great depth.
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an assessment science ProPosed dUsel 36 of the for the FIGURE 3.2 WIMP current limits (solid top curves) and sensitivity and goals (numbered) for the next 3 years; Generation 1 (G1), with results in 2013; Generation 2 (G2), with results in about 2016; and Generation 3 (G3), with results in about 2020. The shaded regions represent the expectation of several minimum supersymmetry models. SOURCE: Courtesy of Bernard Sadoulet, University of California at Berkeley, and Richard Gaitskell, Brown University. Figure 3-2 Once a definitive dark matter signal is established, the next goal would be to R02033 observe the annual signal modulation asraster, velocity relative to the dark matter bitmapped Earth’s not editable halo changes owing to Earth’s motion around the Sun. Such velocity effects, largest at the threshold energy of the detector, would take several years of operation to convincingly establish. An annual modulation signal would be compelling evidence and within the scope of a G2 or G3 experiment. Beyond annual modulation, a detector with directional sensitivity could poten- tially observe a daily modulation of the direction of dark matter at all energies due
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science assessments 37 to the finite rotational velocity at the surface of the Earth, thereby opening the door to dark matter astronomy. Directional detection would give information about the velocity distribution of WIMPs and would begin to discriminate between models of the dark matter halo. Directional detectors would rely on detecting the nuclear recoil in low-pressure gas and so represent a new technology. They would require large caverns and are not expected to be deployed before 2024. Summary The predominant mass in the Universe is dark matter. Demonstrating that dark matter consists of elementary particles would be a major discovery. Understand- ing the nature and composition of these particles is a major scientific challenge for our time.1 The direct detection of dark matter would provide a crucial experimental connection between particle physics and cosmology. To be definitive, their signa- ture signals would need to be significantly above the background and would need to come from different experiments. Concurrence between experiments will be essential: Several experiments have already claimed dark matter signals, but these have not been confirmed by other experiments. The program in dark matter detec- tion will by necessity involve a number of G2 experiments that will coalesce into a smaller number of highly sophisticated and massive G3 detectors. Based on the history of leadership by U.S. physicists on experiments using all detection modes, it is expected that there will be U.S. involvement in more than one G3 experiment, and given the importance of this science and the discovery potential, it would be desirable for the United States to be a leader in at least one. Once dark matter has been observed, a major program for understanding the properties of the new particles will be required. Conclusion: The direct detection dark matter underground experiment is of paramount scientific importance and will address a crucial question upon whose answer the tenets of our understanding of the Universe depend. This experiment would not only provide an exceptional opportunity to address a scientific question of paramount importance, it would also have a significant positive impact upon the stewardship of the particle physics and nuclear physics research communities and would have the United States assume a visible leadership role in the expanding field of underground science. In light of the leading roles played by U.S. scientists in the study of dark matter, together with the need to build two or more large experiments for this area, 1 NRC. 2006. Revealing the Hidden Nature of Space and Time. Washington, D.C.: The National Academies Press, p. 13.
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an assessment science ProPosed dUsel 38 of the for the U.S. particle and nuclear physicists are well positioned to assume leadership roles in the development of one direct detection dark matter experiment on the ton- to multiton scale. While installation of such a U.S.-developed experiment in an appropriate foreign facility would significantly benefit scientific progress and the research communities, there would be substantial advantages to the communities if this experiment could be installed within the United States, possibly at the same site as the long-baseline neutrino experiment. Tests of Grand Unification Theories The three other major physics experiments proposed for DUSEL—neutrino oscillations, neutrinoless double-beta decay, and proton decay—are among the most promising tests of theories that seek to provide a unified description of the forces.2 After providing a general overview of the nature of grand unification theo- ries, these three experiments, and the roles they might play in resolving outstanding questions, are described. We are able to observe the Universe because it contains important ingredients that are the stuff of ordinary matter: protons and neutrons, which are composites of quarks, and electrons. Whatever the history of the Universe, these particles were left behind and are stable enough to account for what is visible to us. Most of the properties and interactions of the visible matter made up of these particles can be accounted for by current particle theories. However, significant inconsistencies within existing theories and gaps in our knowledge remain. The remaining major physics experiments proposed for DUSEL should help fill those gaps and address those inconsistencies. What is now called the Standard Model evolved throughout the twentieth century and aimed to describe the physics of these elementary particles and how they interact. In the Standard Model there are two fundamental fermion-type particles, as shown on the left side of Figure 3.3. They divide into six flavors of quarks and six types of leptons, three with a charge—the electron, the muon, and the tau—and their associated neutrinos. The quarks are strongly interacting funda - mental particles that combine to make up the baryons (protons and neutrons); the leptons do not strongly interact. Each particle type is associated with a conserved quantum number. Quarks carry the baryon number, and baryon number conserva- tion guarantees the stability of the proton and many nuclei. However, quarks also come in different flavors, and weak nuclear interactions can change one flavor of quark into another. The leptons carry lepton number L, and until the 1990s and 2 The dark matter experiment discussed in the preceding section also has implications for tests of grand unified theories by way of the information it might provide on supersymmetry.
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science assessments 77 faults are invisible to indirect geophysical investigation.24 The widespread distri- bution of fracture arrays and the small size of individual fractures mean that vital characteristics25 of most subsurface fractures are little known. Well bores that do intersect fractures or faults may not be optimally located within the structure to provide insight into important processes. Because the geoscience data needed for breakthrough insights is inherently three-dimensional over a wide range of scales, small samples at a single point are bound to be inadequate, and they may provide no meaningful data or even misleading data. Operating mines provide access to the underground but do not usually allow for long-term studies, impeding our understanding of fluid flow and its associated physical and chemical processes. An alternative to subsurface studies is the investi- gation of rocks that have been buried and then uplifted to the surface. These rocks may preserve evidence of faults and fracture arrays and the by-products of chemical reactions that existed at depth, but key features may be obscured or overprinted during uplift. Moreover, these fossilized records lack the essential dynamic context of tectonic, burial, and thermal loading, fluid flow, and chemical reactions. Faults are important features that cross a wide spectrum of the geosciences and have important societal impacts beyond earthquakes. Understanding the nucle- ation and rupture of earthquakes on faults is a central theme of seismology and rock mechanics, and unraveling the history of slip is a central research area for structural geologists. The dynamic aspects of rock at depth have profound impli- cations for engineering operations that perturb the subsurface, including drilling, hydraulic fracturing, and fluid storage. Further, mass transport and mineral depo- sition along faults is an important source of metal ores, and understanding these processes is a significant challenge to geochemists and economic geologists. Faults affect preferential pathways for fluids at a wide range of lengths and timescales. An important fraction of Earth’s heat flow is carried by hydrothermal circulation through faults, and the circulation of cooler water through faults is a hydrogeologic process. Faults can also be a locus for microbial life. Despite their significance, the study of stresses and strain deep in Earth’s subsurface and their interaction with preexisting or growing fractures; moving or static fluids; and chemical or biochemical reactions is necessarily restricted to sparse point measurements in deep boreholes and deep mines26 that rarely include measurements over time27 and are seldom located in the most informative places 24 The Leading Edge, v. 26, no. 9, September 2007. 25 Such as length, height, and aperture distributions, connectivity, orientations, and patterns of mineral deposits, and variation of these attributes with position and rock type. 26 T. Engelder. 1993. Stress Regimes in the Lithosphere. Princeton, N.J.: Princeton University Press, at 451. 27 NRC. 1996. Rock Fractures and Fluid Flow: Contemporary Understanding and Applications. Washington, D.C.: National Academy Press.
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an assessment science ProPosed dUsel 78 of the for the or collected at the most interesting times. An example of a potentially interesting data set that is lacking and that illustrates the last two points would be measure- ments of all key parameters near the nucleation zone of an earthquake prior to, during, and after the event. The most desirable subsurface experimental setting would therefore enable observations over large volumes (hundreds of cubic meters to cubic kilometers) and for long periods of time (years to decades), providing researchers with the opportunity to target and perhaps even deliberately perturb28 specific key, instrumented areas within a given volume. Such a setting would allow systematic investigations of important interactions and the feedback on them that are suspected to exist among loading, fracture growth, closure or sealing, altered permeability and porosity and structure of the rock and fractures, altered composi- tion of fluids, altered stress, and pressures, directions, and rates of fluid movement. For example, fluid pressure changes can alter a rock’s elastic response to deforming forces, which could influence earthquake frequency and magnitude. As with perme- ability, variation in rock strain and stress as a function of measurement scale and sample position and size is not well understood because sufficiently large volumes of rock at depth have not been adequately measured or characterized. Many fracture and fault attributes and their behavior with respect to processes covered by the disciplines of geomechanics, geohydrology, geochemistry, and geo- physics could be addressed effectively in an underground laboratory. Such facilities would permit measurement of rock structure, fracture attributes, and their vari- ability with size, depth, and distance across the excavation. The scale of observa- tion has to be large enough to allow for the collection of meaningful evidence for coupled mechanical, geochemical, and microbiological processes occurring within the subsurface environment. These processes can play a vital role in how effectively fluids are stored in or transmitted through rock and how faults and opening-mode fractures behave over time spans of hours to decades to millennia and, thus, how they may respond to human intervention. The ability to investigate the rock volume after tracer tests or imaging may lead to improved techniques that can be applied elsewhere. Access to the large rock volume would permit testing the hypothesis that Earth’s crust is critically stressed and that some part of Earth is always close to failure by fracture. Significant rock permeability at depth may occur along critically stressed fractures. Mapping fractures, stress, and fluid flow within the subsurface will help geoscientists to confirm or extend theories about the mechanics of Earth deformation. Any disturbance of the subsurface, be it “natural”—for instance by volcanic or seismic activity—or as a result of engineering, will change the preexisting 28 Active experiments, such as placing heaters in the rock mass, might improve our understanding of how coupled mechanical, chemical, and fluid-flow behavior responds to environmental changes.
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science assessments 79 equilibrium, sometimes dramatically, as in the case of surface tremors induced by fluid injection at depth. The process of coring to obtain rock specimens from these subsurface environ- ments can change their properties to an unknown extent. In some cases, the behav- ior of cores, the primary basis for much of university laboratory rock mechanics research to date, may not be representative of rock’s behavior in situ. Bioscience Challenges Microorganisms have inhabited Earth for 3.5 billion years and hence have had a much longer time for adapting to life in a mineral world than some more recent microorganisms have had to adapt to life with higher organisms. During that long time some evolved mechanisms to capture energy from virtually every energy-yielding chemical redox couple. The more common inorganic reductants supporting microbial growth are Fe(II), S−2, H2, and NH4+, while Fe(III), NO3−, SO4−2 as well as O2 are common oxidants. Other minerals that are involved include but are not limited to Se, As, P, Mn, Cr, Co, U, and Zn. These minerals can also serve as electron donors and/or acceptors, supporting some microbial growth. Also, because of their long history, microbes are widely dispersed and serve as inocula in fissures within rocky materials, becoming available as life-sustaining niches. Besides their diversity in capturing energy, these microbes have also evolved adaptations to extreme conditions, such as long-term starvation, high and low temperatures, acidity and alkalinity, high pressures, and desiccation, to name the more relevant. In summary, most mineral environments with moisture and temperatures below 120°C can be expected to contain some microbial life. A number of recent high-profile studies from deep ocean drilling programs have expanded our knowledge of the physiological types, extent, activities, and diversity of the bacteria and Archaea that growth at depth.29 This has enhanced our knowledge of their biogeochemical role and the extent of the biosphere. Some information on the terrestrial microbes at depth has come from microbial studies in deep mines and oil drilling wells. The former have confirmed microbes living at depths; in one case, the genome of a novel bacterium from a 2.8 km deep rock fracture was sequenced.30 The studies of microbes in oil wells have focused on the microbial role in well corrosion and oil field “souring.” All such studies establish substantial and diverse microbial life at depth, but detailed information on the 29 B.B. Jorgensen and S. D’Hondt. 2006. A starving majority deep beneath the seafloor. Science 314: 932-934; J.S. Lipp, Y. Morono, F. Inagaki, and K.U. Hinrichs. 2008. Significant contributions of Archaea to extant biomass in marine subsurface sediments. Nature 454: 991-994. 30 D. Chivian, E.L. Brodie, E. J. Alm, et al. 2008. Environmental genomics reveals a single-species ecosystem deep within Earth. Science 322: 275-278.
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an assessment science ProPosed dUsel 80 of the for the indigenous microbes and their biogeochemical roles in rock environments are comparatively sparse. One consistently high-profile area for biological advance is the discovery of new microbes that expand our knowledge of the strategies and limits of life,31 such as microbes that harvest new sources of energy, live at even higher temperatures or pressures, or exhibit new biochemical reactions, some of which may have biotech- nological or pharmaceutical value. The discovery of these organisms often occurs in samples from unusual habitats where unique biology may have evolved. A facility for the described physics experiments would necessarily access subsurface material that could reasonably harbor unique biology, and the samples made available to the biological research community should be free from external chemical and microbial contamination. Important questions about the energy sources and energy efficiency of these organisms and about the evolution of small populations and horizontal gene exchange as well as mechanisms of mineral weathering could be addressed using the access enabled by the DUSEL physics facility. Other subsurface research facilities being put to use for the studies of microbes are the Ice Core Lab (http://nicl.usgs. gov) and the Integrated Ocean Drilling Program (http://www.iodp.org). Sites dedicated to cross-disciplinary research in the biological and geosciences would be valuable. For example, phenomena where faults play a role are closely interconnected, but the disciplines that address them are in many cases not closely interconnected nor do they enjoy much professional interaction. Fluid transport and chemical reactions contribute to microbial life, and the microbes probably facilitate chemical reactions. Chemical reactions alter permeability and affect fluid pressures, which in turn may influence fluid flow and mechanical stability. Damage and flow conduits formed during an earthquake rupture can be healed, and result- ing changes in permeability can be sealed by chemical reactions, thereby influenc- ing subsequent fault slip. The rupture process itself may release hydrogen, carbon, or other compounds that go on to take part in chemical and biochemical reactions. Limitations All existing and proposed underground facilities have important limitations, especially for subsurface engineering and geoscience research. Many of the most interesting processes occur at depths and temperatures deeper and hotter than any of the proposed underground facilities. Moreover, all of the processes and interac- tions described earlier are sensitive to characteristics such as rock type, tectonic and structural setting, and rock history. DUSEL is in a specific geological setting, 31 See H.N. Schulz, T. Brinkhoff, T.G. Ferdelman, H. Hernandez-Marine, A. Teske, and B.B. Jorgensen. 1999. Dense populations of a giant sulfur bacterium in Namibian shelf sediments. Science 284: 493-495.
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science assessments 81 principally metamorphic rock in a low tectonic environment. It is, however, sedi- mentary rock (carbonates, sandstones, shales, etc.) that is the focus of a great deal of research because of its importance to oil and gas discovery and extraction, as well as the potential benefits associated with CO2 sequestration. Moreover, although many generic experiments can be conducted at Homestake, engineering applica- tions may need to be demonstrated in specific rock formations. Yet, developing the tools to overcome scale and sampling challenges at an underground facility would have widespread impact. This limitation applies to any single underground research site. Thus, research in subsurface engineering, geosciences, and biosciences (EGB) would benefit from international cooperation and a strategy of several subsurface sites. Owing to important variations in rock types, the investigation of loading conditions, tem- perature, and fluid regime at many sites is likely to yield the most valuable insights. Some of these sites need not be extensive long-term underground laboratories, since much information can be gained from targeted drilling. Experimental Details Several broad classes of EGB experiments have been described to date. All of these are intended to be accomplished over the first decade of DUSEL operations (i.e., 2014-2024): 1. Scale effects and coupled thermohydromechanical processes; 2. Subsurface imaging (“transparent Earth”); 3. Modeling the mechanics of induced fracturing and fault slip; and 4. Biosciences Scale Effects and Coupled Processes Much of the research intended in this category was stimulated by the proposal to construct the large water Cherenkov cavity (~60 m span) at a depth of 1.5 km (4,850 ft). Such a cavity at this depth is unprecedented and would provide a unique opportunity for engineering research on the effects of (1) scale (both size and time) on rock deformation and (2) the preconditioning of rock mass (by blasting) to facilitate excavation and minimize damage to the final rock periphery. This experiment will require “halo” tunnels around the large cavity for instrumentation and monitoring. The dynamic response of various support systems installed in the halo tunnels could also be monitored during blasts as part of excavating the large cavern. Observation and characterization of fracture systems in the rock mass will be carried out in drifts (including in the halo tunnels) developed in preparation for the large cavern excavation.
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an assessment science ProPosed dUsel 82 of the for the The complex coupled nature of thermal-hydraulic-mechanical-chemical ( THMC) effects in subsurface systems is illustrated in Figure 3.10. The DUSEL experiment proposes to study the role of biological effects in such coupled pro - cesses, which may be significant in certain underground environments. Among the tests under consideration is a heated block test for studying THMC plus bio- logical processes. The study proposes to heat a 50 m × 40 m × 40 m block of rock by an array of electrical heaters to a maximum temperature of between 150°C and 300°C. The block will be delineated by two parallel drifts approximately 45 m (center to center) and a cross drift. Instrumentation will be deployed along the three drifts. Researchers will then study links between microbial activities, nutrient supply, biochemical reactions, and temperature. It is anticipated that this project will require approximately a decade to complete the heating and cooling phases. Convective Fluid Flow Thermal Hydrologic Energy Transfer Thermal Expansion (Rock and Fluids) Frictional/Deformational Heating Mass Transfer Permeability Alteration Modified Reactive Surface Area Mechanical Chemical Healing/Weathering/Dissolution FIGURE 3.10 Coupled THMC (thermal-hydraulic-mechanical-chemical) effects in subsurface fluid flow systems. SOURCE: Adapted and reprinted from J.L. Yow and J.R. Hunt, Coupled processes in rock mass performance with emphasis on nuclear waste isolation, International Journal of Rock Mechanics and Mining Sciences, 39 (2): 143-150, copyright 2002, with permission from Elsevier. Figure 3-10 R02033 vector editable
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science assessments 83 Subsurface Imaging (Transparent Earth) The opacity of rock is a major impediment in subsurface engineering. The problems range from the inability to “see” a few tens of meters ahead of a tunnel boring machine to the precise location of “producing horizons” at depths of several kilometers and occur in petroleum extraction and in the search for ore deposits in mineral exploration. Experiments to explore the potential of a variety of geo- physical techniques to make the rock more “transparent” are planned at DUSEL. Faculty from several universities are involved as a collaborative team, led by Steve Glaser at the University of California at Berkeley. Experiments include broadband and long-wavelength seismic arrays, passive electrical arrays, and electromechani- cal passive imaging. A rock block between two drifts 50 to 75 m apart is envisaged. One problem limiting the wider applicability of imaging tests in hard-rock underground sites such as Homestake is that the geology is either highly complex (folded and faulted metamorphic rocks at Homestake) or markedly different from that in areas generally of interest to geoscientists (homogeneous granitic rocks versus sedimentary rocks). This might lessen the usefulness of imaging experi- ment results in these facilities for clarifying questions of widespread interest in the geosciences. Verification test results could be ambiguous or techniques developed at Homestake might not work elsewhere. Mechanics of Induced Fracturing and Fault-Slip Modeling Induced fracturing is a major element of much of subsurface engineering. Perhaps the most common example is massive hydraulic fracturing that is used extensively in the oil and gas industry. Recent applications in the United States to stimulate the extraction of geothermal energy and natural gas by fracturing have led, in some instances, to seismic tremors and proposed legislation to prohibit the use of fracturing. Other important methods of inducing fracturing include use of explosives and rock-cutting tools in tunnel boring machines, all in an effort to increase drilling rates in deep borehole drilling.32 A study has been proposed to conduct hydraulic fracturing tests in a rock block similar in dimension to the heated block test discussed in the preceding section. Instrumentation would be installed to detect microseismic activity and velocity changes during fracture propagation. When a fault can no longer sustain the forces applied to it, dynamic slip may take place, resulting in earthquakes. A slip can occur from an increase in tectonic 32 Therate of drilling (including rock removal) and the time spent in reaching the producing horizon directly affect the economics of offshore drilling. Energy costs for drilling are a small component of the overall costs of maintaining an offshore rig.
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an assessment science ProPosed dUsel 84 of the for the loading, a decrease in fault slip resistance owing to hydrological, chemical, thermal, or other changes along the fault surface and in nearby rock, or to some combination of all of these factors. The spatial and temporal distribution of rock deformation leading to fault slip (and earthquakes) is inadequately known as are the processes that lead to and accompany progressive rupture. Many preexisting faults are believed to be active in today’s stress field (critically stressed faults).33 If forces on faults are in a state of critical equilibrium the implications for engineering operations that disturb this equilibrium are profound.34 A deep underground laboratory could allow measurements of rock strain as a function of time and position near faults and in the rock mass. These data would help explain the influence of geology and human activity on strain and stress distribution in rock, allow observation of how deforma- tion accumulates near faults and fractures, and provide insights into how laboratory and underground laboratory measurements of fault slip processes can be scaled to larger events. The understanding gained from this research could be a step toward reliable understanding of earthquake rupture processes and precursory phenomena. At least two potential issues with these fault slip experiments should be noted. First, if experiments succeed in activating an instrumented fault, the ramifications for nearby physics experiments (and physicists) would need to be considered.35 This possibility might necessitate conducting the geoscience experiments during site construction, although this would result in the experiments operating on shorter than optimal timescales. However, since all of the proposed sites are in relatively tectonically quiescent areas,36 the second potential problem is that the experimen- tal perturbations will be insufficient to cause an interesting response (i.e., there will be no earthquake). Selection of a site in a more seismically prone area or the application of an unfeasibly large perturbation might be needed before sufficient slippage will take place. Biosciences The proposed biology experiments fall into two categories: (1) those that seek to define and quantify the microbiological role in the rock weathering processes, 33 C.A. Barton, M.D. Zoback, and D. Moos. 1995. Fluid flow along potentially active faults in crystalline rock. Geology 23(8): 683. 34 For example, large-scale fluid injection for CO sequestration or hydraulic fracture water disposal 2 could lead to widespread seismicity in otherwise tectonically quiescent areas. 35 Efforts to prepare the Homestake mine for the physics experiments have included testing the structural capability of the surrounding rock and stabilizing and rehabilitating the space where needed. K.T. Lesko, Lawrence Berkeley National Laboratory, “Deep Underground Science and Engineering Laboratory (DUSEL) Project Overview,” Presentation to the committee on December 14, 2010, p.11. 36 However, in an otherwise tectonically quiescent area, numerous earthquakes occur near deep South African mines. Available at http://earthquakes.ou.edu/. Last accessed on September 29, 2011.
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science assessments 85 including their contribution to the coupled THMC, and (2) those of a discovery nature that explore unknown aspects of biology provided by access to a unique habitat. While the general capacities of microbes in rock weathering are known, their activities under field conditions—such as their natural rates, environmental con- trollers of those rates, biochemical mechanisms, and often the types of microbes themselves—are unknown. This information is important in quantifying the pro- cesses, their accurate modeling, scaling, and their integration into coupled pro- cesses. This gap in information is due largely to the lack of field laboratories at depth that would allow in situ studies under natural or nearly natural conditions. The reproduction of these natural conditions in a distant laboratory is currently impossible. While it may be possible to obtain the rock material, it is not possible to reproduce the natural water chemistry, including its natural redox state and flow conditions, or the indigenous microbial populations. Furthermore, the contamina- tion of samples with external microorganisms during drilling and sample process- ing becomes a much greater problem in off-site studies. An additional advantage of field laboratory studies is that the site hydrology and geochemistry information can be directly integrated with the biological information. Defining and quantify- ing the microbial role in the coupled processes is the science area where important new biological knowledge should be reliably obtained in a DUSEL-like facility. The experiments planned for this area are well integrated with the nonbiological components and would greatly benefit from the data synergy that would occur. Only one other underground microbiology laboratory exists in the world, the ASPO lab in Sweden. While it has proven the feasibility and value of such a lab at depth, it is small, fully used, only 400 m deep, and embedded in homogeneous granite, which offers only limited conditions for microbial study. The second category of experiments would expand our knowledge of biology by (1) defining the depth of the biosphere and (2) determining whether some unique biology exists in terms of energy sources, physiology, and evolutionary outcomes, including life as we do not know it. In the first effort, the proposal is to drill deeper into the crust to determine where life ceases to exist—perhaps at the 120°C isotherm? The drilling cost would be reduced substantially since drilling could start from the existing excavations at 7,400 ft, the deepest directly accessible level in North America. This experiment would better define Earth’s biosphere and biogeochemical inventory. However, while it would help fill gaps in our knowledge of the terrestrial biosphere, it would be costly relative to its potential science value. It is, after all, limited to a single location and the microbial densities are likely to be low, both of which are limitations compared to the proven value of ocean sedi- ment studies. The experiments for detecting novel biology are both intriguing and risky. The environment should select for novel energy specialists—“dark life,” different
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an assessment science ProPosed dUsel 86 of the for the evolutionary outcomes, and isolation from horizontal gene exchange with surface organisms—to name a few potential high-profile outcomes. But, the study is risky because the microbial density would probably be very low in this geologic mate- rial, conditions for microbial isolation might be difficult to determine, and the microbes might not have been isolated from the surface life for long enough to exhibit population or genetic differences. In evaluating the merits of this experi- ment, one must examine the extra cost for the biological objective in relation to the probable value of its results. The committee judges that undertaken alone, the experiment seems too costly. However, if the field lab and the THMC experiments are undertaken as well, then the extra cost of obtaining some biological samples is significantly decreased and at least some sample collecting would be warranted for these risky but potentially high-payoff experiments. Potential Future Lines of Inquiry The experiments proposed in the DUSEL program are only a fraction of the possible nonphysics studies that might take advantage of the existence of an underground research facility. Here, the committee presents a sampling of other promising lines of experimental inquiry. However, as noted in the preceding sec- tion, the value of the underground space for these experiments might depend on the types of rock present. Fracture Network Engineering The development and control of fracture networks at depth by remote stimula- tion of a rock mass is central to many aspects of subsurface engineering. Currently, although hydraulic fracturing is a major component of oil and natural resource development, it is still in some respects more art than science. It is a technology that is being applied increasingly to the development of other resources. In enhanced geothermal systems, for example, a fracture network is created at depth on the order of 6 km or more, where the rock temperature is approximately 300°C or higher. Water is circulated through the fracture system to extract heat. Cooling causes the rock to contract; fracture apertures change and hence also the pattern of circula- tion. Downhole-microseismic networks can monitor fracture development. It has been proposed to develop a model of the preexisting fracture network and develop a model of a stimulation plan, including predicted seismicity. The predicted activity can be compared with that observed and the stimulation procedure modified to improve the overall “heat exchange system.” Such ambitious schemes will need to be tested, modified, and made robust before they can be applied successfully. FNE research experiments could be an excellent development of or supplement to the THMCB experiment proposed above.
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science assessments 87 Other Potential Future Experiments • Large-scale rock mechanics experiments, including induced brittle failure on new or preexisting natural faults through controlled stress relaxation (e.g., with slow release of hydraulic support structures) or other means. • Seismic experiments to detect and monitor hydraulic fracture propagation and fault rupture with closely spaced monitoring devices and subsequent intense sampling or mining. An advantage of a dedicated site for such tests is that the dedicated site would not have the noisy active mining operations or nearby tunnels that are in use (traffic, water flow). • Hydrogeologic experiments, including effects of microbes on flow proper- ties. Such tests could include controlled flooding of deeper mine sections. • Experiments relevant to nuclear and chemical waste disposal—for example, radioactive tracer studies. • The underground access provided by DUSEL is an opportunity for deter- mining some “ground truths” and improving three-dimensional seismic and other surface-based geophysical exploration techniques by comparing the geophysical predictions with actual observations at depth. Finally, the increasing variety of engineering applications of the underground—for example, for nuclear and hazardous waste isolation, including CO2 seques- tration for the development of domestic natural gas resources, and for geo- thermal energy37—will stimulate a variety of engineering studies for which DUSEL will be well suited. Conclusion: The ability to perform long-term experiments in the regulated environment of an underground research facility could enable a paradigm shift in research in subsurface engineering and would allow other valuable experiments in the geosciences and biosciences. 37 The events in Japan resulting from the devastating earthquake in 2011 have reopened discussion of underground location of nuclear reactors to avoid the possibility of releases of dangerous concen- trations of radionuclides into the atmosphere.