# Snowmass 2021
Underground Facilities & Infrastructure
Topical Report on
Synergies in Research at Underground Facilities

C. Curceanu, D. Elsworth, J. Formaggio, J. Harms, D. Robertson, W. Roggenthen,
H. Wang
Additional contributors from the community: S. de Jong

Document compiled by J.L. Orrell

###### Contents

  1. 1 Introduction
  2. 2 Quantum Information Science
    1. 2.1 Science goals
    2. 2.2 Current underground-based research
    3. 2.3 Underground usage and needs
    4. 2.4 Overlap with the high energy physics community
    5. 2.5 Outlook
  3. 3 Accelerator-based nuclear astrophysics
    1. 3.1 Science goals
    2. 3.2 Current underground-based research
    3. 3.3 Underground usage and needs
    4. 3.4 Outlook
  4. 4 Experiments in fundamental symmetries
    1. 4.1 Science goals
    2. 4.2 Current underground-based research
    3. 4.3 Underground usage and needs
    4. 4.4 Outlook
  5. 5 Gravitational wave detection
    1. 5.1 Science goals
    2. 5.2 Current underground-based research
    3. 5.3 Underground usage and needs
    4. 5.4 Outlook
  6. 6 Geology and geophysics
    1. 6.1 Science goals
    2. 6.2 Current underground-based research
    3. 6.3 Underground usage and needs
    4. 6.4 Outlook

##  1 Introduction

A broad range of scientific and engineering research is possible in
underground laboratories, beyond the physics-focused activities described in
the other Underground Facilities and Infrastructure Topical Reports. These
areas of research include nuclear astrophysics, geology, geoengineering,
gravitational wave detection, biology, and perhaps soon quantum information
science. This UF Topical Report will survey those other scientific and
engineering research activities that share interest in research-orientated
Underground Facilities and Infrastructure. In most cases the breadth and depth
of research aims is too large to cover in completeness and references to
surveys or key documents for those fields are provided after introductory
summaries. Additional attention is then given to shared, similar, and unique
needs of each research area with respect to the broader underground research
community’s Underground Facilities and Infrastructure needs. Where potential
conflicts of usage type, site, or duration might arise, these are identified.

##  2 Quantum Information Science

Writer: Joe Formaggio (MIT)

###  2.1 Science goals

As both nuclear physics and particle physics involve the quantum interactions
of many sub-atomic particles, there has always existed a strong interplay
between these fields and the study of quantum physics and quantum information
systems (QIS). This interplay has accelerated in recent years, particularly
with the emergence of new, highly sensitive technologies, nascent access to
quantum computing environments at the O(10)-O(100)-bit scale, and the use of
coherence and entanglement to enhance sensitivity to novel and exotic
phenomena. One unusual area of interplay between the two disciplines that has
recently emerged is the role of background radiation and background mitigation
on highly sensitive systems such as qubits.

In order to make quantum computing a viable and usable technology, the
underlying units of computation –qubits– must exhibit high fidelity and long
coherence times. Over the past two decades, advances in device design,
fabrication, and materials have increased coherence times by almost six orders
of magnitude [56]. Nonetheless, to realize the full promise of quantum
computing, far longer coherence times will be needed to achieve the
operational fidelity required for fault-tolerant computation. Coherence for
superconducting qubits can be spoiled by an excess density of quasi-particles,
and quasi-particle densities far exceed what is naively expected from thermal
equilibrium. Recent measurements made by several groups [49, 53, 54, 55] have
shown that one creeping contribution to quasi-particle poisoning appears due
to ionizing radiation stemming from external gamma radiation, cosmic rays, and
radiogenic contamination of materials surrounding the qubit. This source of
quasi-particle poisoning is particularly worrisome for QIS applications, since
ionizing radiation appears to affect multiple qubits simultaneously [53, 54].
Since quantum error correction (QEC) schemes –necessary for scaling quantum
computing– rely on qubits to exhibit random, uncorrelated errors, correlated
error sources would render such schemes ineffective. Ionizing radiation has
been deemed ”catastrophic” because of its ability to potentially circumvent
traditional QEC algorithms.

###  2.2 Current underground-based research

Fortunately, a number of techniques are being devised to reduce the impact of
radiation. These include spatially distributed error correction schemes [46],
material engineering to promote phonon down-conversion [52, 47], and
developing a more comprehensive understanding of the underlying microscopic
physics that leads to quasi-particle poisoning [48, 50]. As cosmic ray
radiation is a non-negligible portion of the environmental radiation,
underground facilities may also provide a unique resource for studying quasi-
particle poisoning in radiation-quiet environments. Here “underground
facilities” is a catchall for the decades of experience in the nuclear and
particle physics communities for background mitigation and suppression.
Underground laboratories specifically may offer perhaps one of the only ways
to effectively reduce the cosmic ray flux impingent upon a multi-qubit system.
Although precise models of how cosmic rays impact multi-qubit systems are
still being studied, preliminary measurements on similar systems have shown
that significant overburden could improve qubit performance [51, 55].

###  2.3 Underground usage and needs

Underground facilities could be excellent locations for studying qubit systems
in low background environments, with relatively modest investments in
infrastructure and resources. Small facilities have already started to emerge
in both deep- and shallow sites, such as FERMILAB, Pacific Northwest National
Laboratory, and Gran Sasso. As qubit systems operate at cryogenic
temperatures, such facilities would need to sustain use of dilution
refrigerators underground. Such systems are already in place for several dark
matter experiments that operate at cryogenic temperatures (e.g. EDELWEISS
[57]).

###  2.4 Overlap with the high energy physics community

The quantum information science (QIS) community pursuing superconducting
qubits for quantum computing, has meaningful overlap with quantum sensor
research and development performed by high energy physics researchers. This
overlap is evident through the collaborations that have assembled under the
umbrellas of the DOE Office of High Energy Physics’ Quantum Information
Science Enabled Discovery (QuantISED) program, DOE’s National QIS Research
Centers, and the National Science Foundations’ Quantum Leap Challenge
Institutes. Many of the techniques used by the QIS community are now being
applied to challenges in high energy physics, such as particle theory quantum
computation and methods for direct detection of dark matter. Likewise, device
physics expertise from the high energy physics community is assisting the QIS
community with elucidating some of the underlying processes that affect the
performance of superconducting qubits. It is anticipated these research and
development overlaps will continue in the coming decade, and some fraction of
those efforts may take place or be demonstrated in underground facilities.

###  2.5 Outlook

As quantum computing systems advance in computational power and capability,
radioactivity is likely to play an ever-increasing role in their performance.
Underground facilities, coupled with decades of experience of running
sensitive, low-background experiments underground, offer a unique space to
study how such systems behave in radiation-quiet environments. These
laboratories can also be a catalyst for further collaboration between the
fields of particle physics, nuclear physics and QIS. Investment in this line
of research is strongly encouraged.

##  3 Accelerator-based nuclear astrophysics

Writer: Daniel Robertson (Notre Dame)

In the energy range of relevance to stellar burning the cross-sections of
reactions of interest are extremely small. The rate of reaction drops very
rapidly while moving lower into this energy range, reducing the measurable
event rate below background. The nature of accelerator-based nuclear
astrophysics experiments requires the active generation of radioactive decays
through the forced interaction of nuclei of interest, this is in-order to
detect the low-intensity signatures of thermonuclear reaction channels of
significance in stellar burning environments. This process appears somewhat
counter intuitive to being performed underground, where most other experiments
relocate to avoid such event generation. The requirements to perform these
interactions of interest however, show considerable overlap with numerous
other fields moving underground for the same reason of background suppression.

###  3.1 Science goals

At the epicenter of the field of nuclear astrophysics is the drive to
understand the synthesis of the elements in stellar environments [2].
Understanding the generation of elemental abundances requires wide ranging
information on stellar environments, particle interactions and energy
generation. The current galactic elemental abundance is the result of numerous
reaction paths all building on one another to create energy and elemental
production.

Initial production processes see the burning of hydrogen through either
proton-proton interactions or through the CNO-cycle [3] dependent on the mass
of the star involved and available material present. Significant reactions of
interest associated with the CNO cycle are the 12C(p,$\gamma$)13N and
14N(p,$\gamma$)15O, both of which are actively under investigation both above
and below ground. The 14N(p,$\gamma$) reaction is of further significance
through the first ddetection of 15O neutrinos by Borexino [4]. Continued
burning passes through a helium phase where the triple-alpha process tales
place, this key reaction of 12C($\alpha$,$\gamma$)16O determines the ratio of
carbon oxygen in our Universe and is considered the ”holy grail” of nuclear
astrophysics [5]. The complexity of this measurement and its obvious
importance make it a prime candidate for underground measurements.

Processes for the production of elements above the iron peak can be considered
as two groups Group one for the production of elements far from stability and
associated with multi-generational stars or explosive environments, r-process
[6], p-process [7], rp-process [8] and n-process [9]. Group two for the
production of elements in non-explosive burning scenarios, i-process [10] and
s-process [11]. Of specific interest for underground nuclear astrophysics is
the s-process, where the slow capture of neutrons onto a seed nucleus creates
almost half of all nuclei above mass 56 and along the valley of stability.
Understanding and quantifying the source of neutrons for this process is of
great interest to the community, the two main reactions thought to feed the
neutron abundance are 13C($\alpha$,n)16O and 22Ne($\alpha$,n)25Mg. Significant
efforts are underway to measure these reactions through direct and indirect
methods, capitalizing on above ground and underground capabilities. Competing
reactions which would reduce the abundance of 13C and 22Ne available for
neutron production and those reacting with neutrons before s-process nuclei,
known as neutron poisons, are also of strong importance.

Recent white papers outlining the future direction of nuclear astrophysics
[12, 13] have highlighted the current and expected future importance of
underground reaction measurements. It is expected that underground facilities
will grow even further into a significant tool for the measurement of key
reactions of interest, with strong support from the nuclear astrophysics
community.

###  3.2 Current underground-based research

Ongoing research underground is strongly supported by its connection to above
ground facilities performing higher energy measurements. These same facilities
are striving to push the range of their measurements to lower energy through
higher beam intensities, active shielding adaptations and more sophisticated
detection techniques. The gains made through these methods help to better
inform the requirements needed when inevitably the measurement must move
underground. Hybrid facilities are operating tens of meters underground such
as the Felsenkeller [14], where some background suppression from rock
overburden can be augmented with passive and active shielding, but this can
only push reaction measurements so far into the region of interest.

In the US, underground nuclear astrophysics is pioneered by the CASPAR
(Compact Accelerator System for Performing Astrophysical Research)
collaboration, operating the only US-based deep underground low energy
accelerator facility. Located at the 4850 level of the Sanford Underground
Research Facility (SURF), the system is centered on a 1 MV Van de Graff style
JN accelerator and has been fully operational since 2018 [15].

Current studies at CASPAR are focusing on multiple avenues of CNO cycle
physics and s-process neutron seeds. Due to the recent multi-disciplinary
interest [4] a CASPAR (p,$\gamma$) campaign has measured the
14N(p,$\gamma$)15O reaction [16] over a large energy range, generating a
significant overlap with measurements above ground and previous underground
investigations. The main focus however is in alpha capture reactions,
($\alpha$,$\gamma$) and ($\alpha$,n) with enhanced detection techniques of
both $\gamma$ and neutron radiation. The big questions surrounding s-process
neutron production and competing reactions are an on-going drive for future
campaigns with necessary improvements on the already compelling work out
there, for example 18O($\alpha$,$\gamma$)22Ne [17] and
22Ne($\alpha$,$\gamma$)26Mg [18].

Further work is needed to reach deeper into the burning regime of interest.
Specifically the upgrade of current equipment to cover larger energy ranges
with increased beam intensity to target, the inclusion of more sophisticated
detector techniques as a mechanism to cancel out natural background
contamination from both detector materials and ambient rock, and a renewed
effort by the community in target material production and the reduction of
background producing contaminates. A number of this R&D projects are actively
being pursued above ground with anticipated extensions to underground work in
the near future.

Underground nuclear astrophysics was first established in 1992 when the LUNA
(Laboratory for Underground Nuclear Astrophysics) collaboration installed
their first 50 kV accelerator and measured the 3He(3He,2p)4He reaction at
solar energies [19]. The group currently operates a 400 kV Singletron
accelerator [20] allowing for a wider energy range of measurements including
the ability to push into the energy range of interest for the
13C($\alpha$,$\gamma$)16O reaction for the first time [21]. Commissioning work
is underway to expand their capabilities with the addition of a new 3.5 MV
accelerator and new laboratory space.

Further international efforts in China are centered around the JUNA (Jinping
Underground Nuclear Astrophysics) facility located in the China Jinping
underground Laboratory (CJPL) [22]. The JUNA facility came online in 2021 and
uses a high intensity ECR ion source and 400 kV platform to combine the
background suppression available from the large rock overburden, with
increased beam intensity at the lower energies required. Recent work has led
to exciting results for the CNO cycle reactions with a direct measurement of
the important 19F(p,$\alpha$)16O reaction [23].

###  3.3 Underground usage and needs

  * •

Location: As with other underground experiments, nuclear astrophysics
laboratories make the move underground to suppress the high levels of cosmic
induced background. The three deep underground facilities currently operating
all make use of a rock overburden greater than 3500 m.w.e, however below
$\sim$ 2000 m.w.e, the further reduction of cosmic induced background must be
balanced against ambient backgrounds generated via uranium and thorium decay
chains naturally occurring in the surrounding rock. This trade off must be
studied per specific location, but may result in different levels of
underground labs not suitable for other experiments, being useful to nuclear
astrophysics. Secondary considerations stem from the general nature of the
active production of nuclear reactions to generate radioactive decays, whereas
the levels of production are intrinsically small, hence the need for
underground, the production of possible secondary reactions during testing and
R&D stages may be undesirable to neighbouring experiments. Such interactions
may be negated by either increased shielding at the accelerator facility or
the consideration of more remote locations for future lab spaces.

  * •

Space requirements: Required footprints for these underground accelerator
facilities must consider not only the equipment hall, but at minimum a
separated control room in close proximity. A compact system can be considered
in as little as 4000 - 6000 sq ft, if suitable workshop and preparation space
is available at nearby locations. Minimum height clearances for some high
voltage platforms can be above 10 - 14 ft, but enclosed acceleration systems
are modest if utilities can be located clear of working areas. Changing room
facilities are also required, these are necessary as a transition zone between
outer mine areas and lab spaces but do not have to meet the higher standards
of clean room levels.

  * •

Cleanliness levels & environment: Air quality and climate controls for these
accelerator based experiments are reliably uniform across different variations
of systems. The transport of beam and interaction of nuclei of interest is all
performed in vacuum, with only the detection system exposed to the ambient
environment. However once these systems are exposed to air for configuration
changes or maintenance, maintaining a clean, dry and dust free environment is
necessary. Basic level requirements are outlined here, for future open-air
high voltage platform installations it is felt that stricter controls maybe
required dependent on the stability of humidity and dust levels. Most
requirements overlap with those outlined by other groups.

    * –

Cleanroom classification levels are not required for lab spaces, normal
“office” air quality levels are sufficient with a need for any concrete floors
to be sealed to reduce dust. A strong reduction of mine dust is a necessity at
all entry and exit points to the main experimental halls and maintenance
areas.

    * –

Humidity levels are required between 30-50% to reduce moisture uptake of
vacuum systems and minimize condensation on water cooled equipment.

    * –

Vibration levels are important for acceleration systems and detector set-ups
and spaces should be considered at levels $<$ 100 micrometer/sec rms.

    * –

Radon mitigation is required at detector stations. Current systems utilize
localized radon reduction boxes with a flow of dry nitrogen as a purge gas. A
lab wide radon reduction infrastructure may be considered if beneficial to
multiple groups.

  * •

Utilities: Infrastructure requirements are inline with other groups outlined
in this section. Cooling water, uninterrupted power feeds, clean power at
specified locations, high capacity wired and wireless network connections and
liquid nitrogen supplies are all necessities.

  * •

Support and access: All accelerator based experiments are strongly interactive
and have significant access requirements. Whereas systems can be left to
operate with more passive controls in place, start-up, shutdown and target
changes need a physical presence. Access availability during campaigns must be
considered on a flexible basis as 24/7 availability is a strong requirement
for such systems.

###  3.4 Outlook

The outlook for the field of underground nuclear astrophysics experiments is
extremely encouraging. As international efforts and upgrades are coming online
new measurements continue to confirm the importance of key reactions and
associated pathways in stellar models. A key challenge in nuclear astrophysics
has always been the need to move away from the use of extrapolations from
higher energies into astrophysically relevant burning regimes, reducing the
uncertainties and thus refining stellar burning models for comparison and
evaluation of observational data. Continued discoveries and refined
measurements from these underground accelerator labs are used to guide
research both above and below ground and are a driving force behind
understanding the energy generation, elemental production and eventual life-
cycle of stellar objects.

Strong multi-disciplinary interaction and collaborations with current
underground efforts is a necessity moving forward in developing and
strengthening the filed of U.S. underground nuclear astrophysics. As
highlighted throughout this Underground Facilities section there are many
areas where requirements and capabilities can overlap and strengthen each
other.

##  4 Experiments in fundamental symmetries

Writer: Catalina Curceanu (LNF, INFN)

###  4.1 Science goals

Refined measurements of X- and gamma-rays in an underground laboratory,
represent an unique opportunity to investigate space-time related symmetries
and the main conundrum of quantum theory, i.e. the collapse of the wave
function. These investigations are being performed by extremely sensitive
searches of atomic and nuclear transitions violating the Pauli Exclusion
Principle (PEP), and as such, the spin-statistics connection, and of the
spontaneous radiation predicted by the dynamical collapse models which solve
the so-called “measurement problem” in quantum mechanics by adding non-linear
terms to the Schroedinger equation. Possible violations of the Pauli Exclusion
Principle are embedded in algebraic models, such as the quon model constrained
to the Messiah-Greenberg (MG) superselection rule [1-4], and in Non-
Commutative Quantum Gravity models (NCQG), common to both String Theory and
Loop Quantum Gravity [5-9]. On the other side, the collapse models, such as
the Continuous Spontaneous Localization and the gravity-related collapse
models, have as a specific signature the so-called spontaneous radiation, i.e.
a radiation with specific spectral features, depending on the collapse models
parameters and on the emitting material, continuously emitted by the charged
particles making up the matter (i.e. electrons and protons). The measurement
of such radiation is a trademark of the collapse models [10-12].

###  4.2 Current underground-based research

Presently, the violation of the PEP is being investigated by various
underground experiments, either by a dedicated search of the PEP violating
atomic transition, such as the VIP experiment at the LNGS-INFN, or in
experiments searching for other types of physics, such as DAMA/LIBRA or
BOREXINO [13], [14] which used their data to also constrain the PEP. The VIP
experiment singles out by having performed a dedicated search of PEP violation
[15-16] with a method which allows to constrain the probability of the PEP
violation by taking into account the Messiah-Greenberg superselection rule.
The searches for the spontaneous radiation from collapse models was pioneered
in the LNGS-INFN underground laboratory by the VIP collaboration, where the
simplest version of gravity-related collapse models was ruled out [12], and
tight constraints on CLS models were also set [11]. Presently, other
underground experiments, not directly dedicated to the search of spontaneous
radiation, are using their data to also try to constrain collapse models [17].

###  4.3 Underground usage and needs

Presently, searches of PEP violations are being performed by the VIP-2
collaboration at the LNGS in a dedicated experiment by circulating current
through a copper (silver in next generation) target and searching for atomic
transitions prohibited by PEP. Other underground experiments, not dedicated to
PEP violations studies, measuring X- and gamma-rays, analyze their data in the
view of searches for PEP violating nuclear and atomic transitions. The VIP-2
collaboration is also continuing to measure the spontaneous radiation
predicted by the collapse models in dedicated measurements with High Purity
Germanium detectors at the low-radioactivity facility at LNGS.

One can investigate the PEP violation and spontaneous radiation also in
experiments measuring radiation with a different primary goal (dark matter,
neutrino physics) as a synergistic activity; these experiments can analyze
their data also to search for signals related to PEP violation and/or collapse
models. However, there is not possible for these experiments to search for PEP
violation taking into account the Messiah-Greenberg superselection rule [3],
for which one needs to circulate a current into the target, so requiring a
dedicated setup. In this case, one needs to adapt the setup by implementing a
dedicated current source and target, as well as an additional shielding
structure.

In the future, the VIP Collaboration is preparing a new version of the
apparatus, VIP-3, to perform a more sensitive search, in the coming 3-5 years
in the dedicated box at LNGS. This will be paralleled with dedicated searches
of spontaneous radiation. The experimental needs are limited, both in terms of
occupied space (basically a box), time duration and financing. One can
envisage clever synergetic collaboration with other ongoing or planned
experiments measuring radiation with a different primary purpose, which can be
integrated/adapted to also perform searches for PEP violation and spontaneous
radiation in various underground laboratories around the world.

###  4.4 Outlook

The research performed by the VIP collaboration in searches of possible
violation of PEP and of signals from collapse models in an underground
laboratory, triggered theoretical research in the field which, in turn, is
producing more refined models to be tested in the future. On one side, the PEP
violation emerging from NGQG models requires a refined search of PEP violation
as function of energy (i.e. along the periodic table). Recent theoretical
studies are investigating the connection between PEP and CPT/Lorentz possible
violations [8], which will require additional refined studies of PEP violation
in dedicated experiments, eventually performed in various underground
laboratories embracing various continents in Northern and Southern
Hemispheres.

For the collapse models, under the pressure of recent published limits, new
models are being developed, which go beyond the simplest versions by embedding
dissipative and non-Markovian effects. All these models have specific spectral
features which need to be investigated in future experiments. There is a huge
window of opportunities for experiments in underground laboratories which
could discover or set extremely tight constraints on theories which are
impacting on our understanding of Nature, such as the spin-statistics
connection and quantum mechanics. It is an excellent time to perform searches
for PEP violation and collapse models in parallel with searches for dark
matter and neutrino physics. The synergies in these research fields will
enhance searches of radiation related to PEP violation and collapse models,
impacting on our basic concepts rooting the entire modern science.

##  5 Gravitational wave detection

Writer: Jan Harms (GSSI, L’Aquila)

###  5.1 Science goals

Gravitational waves (GWs) carry an enormous amount of information about their
sources and propagate virtually without loss through the Universe, which makes
them a unique astrophysical and cosmological probe. Observations of GWs from
mergers of binary neutron stars allow us to study extreme states of matter,
and through multi-messenger studies of these sources, we can understand the
formation of heavy elements and the engines of high-energy EM transients. The
emission of GWs from coalescing black-hole binaries provides a unique probe of
spacetime in the strong-curvature regime. It opens a new window into quantum-
gravity effects and for the detection of new particles like axions
accumulating near black-hole horizons. New sources of GWs might become
observable in the future like core-collapse supernovae or rotating neutron
stars. While some of the GW science still lies hidden in the noise of current
GW detectors, a new generation of detectors will be able to access the full
range of physics with GWs with high potential for breakthrough observations
and a revolution in our understanding of the Universe [41].

###  5.2 Current underground-based research

Up to now, all GW observations were done with the LIGO and Virgo detectors,
which are located on the surface. Surface environments are noisy in terms of
seismic, atmospheric, and electromagnetic phenomena, which pose a limit to the
sensitivity of low-frequency GW detections [42]. The KAGRA detector is located
in a quiet underground environment and has recently joined the global detector
network. It is projected to eventually exceed the low-frequency sensitivity of
surface detectors. To unlock the full potential of underground GW detection,
specific low-frequency instrument designs are required as planned for the
proposed Einstein Telescope in Europe [43]. Such designs foresee the
implementation of quantum back-action evading techniques, cryogenic
technologies for the test masses and their suspensions, and further reduction
of environmental noise using a new generation of isolation systems and
advanced noise cancellation. It is worth noting there is substantial benefit
to correlated and complementary observation from multiple GW
detectors—inclusively above and below ground—to verify detection, refine
direction, and observe details of the signal.

###  5.3 Underground usage and needs

The KAGRA detector will remain the only underground GW detector facility for
the next decade at least to operate together with the LIGO and Virgo detectors
[44]. It is providing crucial insight into the operation and maintenance of
underground GW detectors. For the breakthrough science promised with next-
generation facilities like Cosmic Explorer and the Einstein Telescope, it will
be necessary to construct detectors with greatly increased lengths of the
interferometer arms compared to current detectors. Cosmic Explorer is planned
as surface detector, but having its ends located inside mountains is still
under consideration to benefit from reduced environmental noise. The Einstein
Telescope is the only currently planned next-generation underground facility.
It is proposed with a 10 km arm length in the shape of an equilateral
triangle. In preparation of a site selection and to inform its final
underground infrastructure and detector designs, extensive studies of
underground environments are being carried out at its candidate sites, but
also elsewhere in the world including the KAGRA site. It is of great
importance to understand the impact of service plants on underground
infrastructure noise. In this regard, other types of underground research
facilities like the Sanford Underground Research Facility and the National
Laboratories of Gran Sasso can provide key insights, and analyses of data from
these facilities have begun.

###  5.4 Outlook

The quest to operate GW detectors underground is entirely connected to the
goal of extending the observation band of terrestrial detectors from currently
about 20 Hz to a few Hz in the future. Gravitational-wave observations in the
few Hz to 20 Hz band will enable the detection of mergers with intermediate-
mass black holes up to a few 1000 solar masses. They are thought to have
played a key role in the formation of the Universe’s large-scale structure as
seeds of supermassive black holes and in the dynamics of star clusters, but
their population is poorly understood today. Similarly, it will be possible to
observe black-hole binaries to much larger redshift and to identify a possible
primordial black-hole population. Even the analyses of less-massive sources
like neutron-star binaries will greatly profit from the low-frequency
sensitivity, which makes it possible to observe these sources for several
hours. The Einstein Telescope is predicted to detect a few 100,000 GW signals
per year up to redshifts of 100 with signal-to-noise ratios improved by more
than an order of magnitude compared to current detectors, which will lead to
the most extensive cosmological study of the Universe so far [45].

##  6 Geology and geophysics

Writers: Derek Elsworth, William Roggenthen, Herb Wang

###  6.1 Science goals

Underground research laboratories (URLs) offer opportunities for research that
either cannot be performed at the surface or that can be performed more
effectively in a URL. Goals include physics-based understanding of controls on
permeability, stress, temperature, and chemical and biological processes
across a variety of time scales ranging from fractions of a second to years
and through both passive- and active-experimentation at length-scales of the
order of 5-50m. Such experiments accommodate the effects of structure,
heterogeneity, and fracturing/faulting at all scales under environmental
conditions (e.g., stress, temperature) and with coupled processes relevant to
engineered and natural geophysical processes with the ability to monitor and
observe processes directly in situ.

#### Permeability

Permeability, both natural and induced, is critical in a wide variety of
applications. For example, the permeability of fault zones cannot be studied
easily in the laboratory, but fluid transmissivity of faults is important in
such diverse problems as the sealing of reservoirs, waste disposal strategies,
geothermal energy production, and subsurface fluids (e.g., carbon dioxide,
natural gas, and hydrogen) storage.

#### Stress

Stress is a fundamental parameter for understanding the behavior of rocks in
the subsurface. Development of improved measurement techniques is an important
goal, especially in the confirmation and improvement of modeling approaches
for the prediction of time dependent stresses.

#### Temperature, Stress, and Chemical Processes

Combined effects of temperature, stress, and chemical processes involves
changes in the in situ conditions that result in alteration of the behavior of
the rock to the passage of fluids, changes in the chemical composition of the
rock, and mechanical behavior that are important in nuclear waste disposal and
enhanced geothermal energy reservoirs.

#### Rock Mass Characterization

Geophysical techniques for rock mass characterization further the development
of rock mass characterization using techniques such as various tomography
methods, e.g. seismic and electrical resistivity, to allow prediction of
underground stability and longevity of excavations.

#### Faulting, Fracturing, and Seismicity

Rock failure occurs over many spatial and temporal scales both from natural
and engineered causes. Microearthquakes (MEQs) are a ubiquitous feature in the
subsurface, accompanying underground excavation, construction, fluid injection
or extraction, and other active experimentation. But large, induced
earthquakes can also occur along with MEQs. Inducing and observing benign MEQs
in URLs present useful analogs to understand modes of initiation and progress
in natural and triggered events.

###  6.2 Current underground-based research

Current research is diverse and includes areas such as carbon sequestration,
geothermal development, nuclear waste disposal issues, induced seismicity, and
advances in underground excavation. The large physics-related URLs typically
have aspects of geoscience and geoengineering involving research in rock
mechanics that are necessary to ensure rock stability of the excavations. URLs
such as the Sanford Underground Research Facility (SURF) are well-positioned
to support the wider interests of the geosciences and geoengineering community
and host geophysics, ground water, and geothermal experiments, as well. In
addition to opportunities presented by URLs developed for physics
investigations, research in many URLs is aimed toward work on issues relevant
to nuclear waste disposal, such as that conducted at the Grimsel Test Site and
Mont Terri Project in Switzerland and the WIPP Site in New Mexico, USA. The
status of active, non-active, and planned URLs with this focus by Tynan and
others (“A Global Survey of Deep Underground Facilities; Examples of
Geotechnical and Engineering Capabilities, Achievements, Challenges…”, 2018)
shows that thirteen of these types of URLs are currently active with more in
the planning stage. In many instances, these facilities also provide platforms
for needed advances in fluid flow through rocks and underground rock stability
although none appear to have the capability of supporting experiments with
extensive excavations at depths greater than 1 km.

###  6.3 Underground usage and needs

Typical installations consist of drilling of boreholes with associated
instrumentation. At any one URL, an estimate of three such installations for a
total of $\sim$750 m2 of habitable space would be required. Any facility would
desire borehole access to the adjacent geologic host—with characteristics
selected that are dependent on the investigation—viz. varied stresses,
pressures, and temperatures (typically controlled by depth), specifics of rock
structure, heterogeneity and fracturing and specifics of the stress field
(obliquity & orientation). These features are all key in aligning the
characteristics of the URL experiment with those of the natural or engineered
geological prototype. In addition to providing site access, necessary
facilities include access to local workshops, qualified onsite technical
personnel and management to support science, electrical power, water,
compressed air, and ethernet access.

###  6.4 Outlook

The outlook for mid-scale and highly constrained in-situ experiments in URLs
is strong and are especially pertinent to societal needs such as those
associated with energy and the environment. The Dept. of Energy has shown
strong support for research in geothermal energy at SURF and carbon
sequestration is of interest to both DOE and NSF. The initiatives are
consistent with the NSF grand challenges, such as Growing Convergence Research
and Mid-scale Research Infrastructure. The applied nature of the research is
of interest to a range of industries, as well.

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