Noisy Oceans E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Geophysical Monograph Series
• Sprache: Englisch

Seitenzahl: 854

Veröffentlichungsjahr: 2023

Beschreibung

A comprehensive review of the sources and impacts of different types of marine noise Measuring devices such as ocean bottom seismometers and hydrophones designed to detect earthquakes pick up many other signals. These were previously ignored as background noise from unknown sources, but advanced technology now allows insights into the noise created from icebergs, ships, hydrothermal vents, whales, rain, marine engineering, and more. Noisy Oceans: Monitoring Seismic and Acoustic Signals in the Marine Environment is a comprehensive guide to non-tectonic marine noise originating from different environmental, biological, and anthropogenic sources. Volume highlights include: * Overview of marine soundscapes and their sources * Existing and new methods for studying acoustic signals * Case studies from around the world * Spans disciplines from geology and geophysicists to biology * Explores the impacts and implications of marine noise The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

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Geophysical Monograph 284

Noisy Oceans

Monitoring Seismic and Acoustic Signals in the Marine Environment

Gaye Bayrakci

Frauke Klingelhoefer

Editors

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List of Contributors

Chastity AikenIFREMERCentre BretagnePlouzané, France

Megan BakerDepartments of Earth Sciences, and GeographyUniversity of DurhamDurham, UK

David BarclayDepartment of OceanographyDalhousie UniversityHalifax, Nova Scotia, Canada

Gaye BayrakciNational Oceanography CentreSouthampton, UK

Mohammed BelalNational Oceanography CentreSouthampton, UK

Jonathan M. BullSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Emmy T.Y. ChangInstitute of OceanographyNational Taiwan UniversityTaipei, Taiwan (R.O.C.)

Mark ChapmanSchool of GeosciencesUniversity of EdinburghEdinburgh, UK

Chien‐Chih ChenDepartment of Earth SciencesNational Central UniversityTaoyuan City, Taiwan (R.O.C.)

Ying‐Nien ChenDepartment of Earth and Environmental SciencesNational Chung Cheng UniversityChiayi, Taiwan (R.O.C.)

Michael A. ClareNational Oceanography CentreSouthampton, UK

David DellongDépartement Acoustique Sous‐marineService Hydrographique et Oceanographiquede la MarineBrest, France

Jessica FisherSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Ángel F. García‐FernándezDepartment of Electrical Engineering and ElectronicsUniversity of LiverpoolLiverpool, UK

Yung‐Cheng GungDepartment of GeosciencesNational Taiwan UniversityTaipei, Taiwan (R.O.C.)

Timothy J. HenstockSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Finnigan Illsley‐KempSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UKandSchool of Geography, Environment and Earth SciencesVictoria University of WellingtonWellington, New Zealand

Bazile G. KindaDépartement Acoustique Sous‐marineService Hydrographique et Oceanographiquede la MarineBrest, France

Frauke KlingelhoeferIFREMERCentre BretagnePlouzané, France

Aude LavayssiereSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UKandInstitut de Physique du Globe de ParisParis, France

Florent Le CourtoisDépartement Acoustique Sous‐marineService Hydrographique et Oceanographiquede la MarineBrest, France

Timothy G. LeightonInstitute of Sound and Vibration ResearchUniversity of SouthamptonSouthampton, UK

Jing‐Yi LinDepartment of Earth Sciences, andCenter for Environmental StudiesNational Central UniversityTaoyuan City, Taiwan (R.O.C.)

Gwyn LinternNatural Resources CanadaGeological Survey of CanadaSidney, British Columbia, Canada

Calum MacdonaldSchool of GeosciencesUniversity of EdinburghEdinburgh, UK

Timothy A. MinshullSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Chung‐Hsiang MuInstitute of Earth SciencesAcademia SinicaTaipei, Taiwan (R.O.C.)

Edward PopeDepartment of Earth Sciences, andDepartment of GeographyUniversity of DurhamDurham, UK

Adam H. RobinsonSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Ben RocheSchool of Ocean and Earth ScienceUniversity of SouthamptonSouthampton, UK

Sean RuffellDepartment of Earth Sciences, andDepartment of GeographyUniversity of DurhamDurham, UK

Vera SchlindweinSection GeophysicsAlfred Wegener Institute Helmholtz Centrefor Polar and Marine ResearchBremerhaven, GermanyandFaculty of GeosciencesUniversity of BremenBremen, Germany

Steven SimmonsEarth and Environment InstituteHull, UK

Brendan SmithDepartment of OceanographyDalhousie UniversityHalifax, Nova Scotia, Canada

Peter J. TallingDepartment of Earth Sciences, andDepartment of GeographyUniversity of DurhamDurham, UK

Jean Baptiste TaryDepartment of GeosciencesUniversity of the AndesBogotá, Colombia

Chuen‐Teyr TerngCenter Weather BureauTaipei City, Taiwan (R.O.C.)

Bai TianInstitute of Sound and Vibration ResearchUniversity of SouthamptonSouthampton, UK

Victoria L.G. ToddOcean Science Consulting Ltd.Dunbar, UK

Morelia UrlaubGEOMAR Helmholtz Centre for Ocean ResearchKiel, Germany

Paul R. WhiteInstitute of Sound and Vibration ResearchUniversity of SouthamptonSouthampton, UK

Wei YiSchool of Information and CommunicationEngineeringUniversity of Electronic Science and Technologyof ChinaChengdu, China

Boxiang ZhangSchool of Information and CommunicationEngineeringUniversity of Electronic Science and Technologyof ChinaChengdu, China

Mohammad Zaki ZulkifliNational Oceanography CentreSouthampton, UK

Preface

The oceans are far from a “silent world”—they harbor rich and variable acoustic signals. The ocean soundscape includes natural sound sources with biological origins, such as fish and marine mammals, and environmental sound sources from earthquakes, volcanoes, and rain. In recent times, anthropogenic sound sources, such as marine engineering and construction, seismic experiments, and marine traffic, have been introduced into the oceans. The marine soundscape affects life in the oceans as well as human societies.

Many past studies using marine seismometers and hydrophones have studied earthquakes and the Earth’s deep interior. Similarly, biological noise, such as marine mammal calls, has been studied by acousticians to gather information about the behaviors of varied species. Although the ocean soundscape was recorded, these marine signals were often discarded, as it was impossible to identify the source of the different signals.

With the growing demand for marine resources and recent technological developments such as increased computational power and data storage, marine scientists have begun to show interest in the marine soundscape—previously discarded as noise—as a tool to study environmental processes and the human effect on them. Although there is a clear increase in nonconventional seismology exploring unknown sound sources, no work has been conducted to compile a set of signals frequently encountered in marine seismometer or hydrophone data.

This book attempts to fill this gap and compile a comprehensive set of nontectonic marine noise commonly observed by ocean‐bottom seismometers or hydrophones, providing a literature review of different signals and state‐of‐the‐art examples of research in which these signals are studied via existing methods or new methods developed for this purpose.

After an introductory chapter about marine soundscapes and their sources, this book presents 13 chapters discussing different types of noise: of environmental origins, such as from volcanoes, icebergs, landslides, and wind; of biological origins, such as from whales; and anthropogenic sounds, such as marine traffic. In the concluding chapter, we put the book into its broader context, show its possible impact and implications, and propose future actions.

This book includes review chapters and case studies from specific geographic locations, such as the North Sea and Taiwan. The authors, most of whom are internationally recognized scientists from academia and government institutions, work in subjects ranging from geology and geophysics to biology and risk assessment.

This work aims to deepen the understanding of marine soundscapes and the identification of signal sources. It is relevant to a broad community of students, teachers, and researchers studying the marine environment, including geologists, geophysicists, volcanologists, and biologists. It will also be useful for those who develop laws and regulations concerning the impact of sound on marine life.

We would like to thank all the contributing authors for their valuable chapters. We would also like to thank the reviewers of all the chapters for their work, which helped immensely to improve the book. We also thank the publishing teams at the American Geophysical Union and John Wiley & Sons, Inc. for their guidance throughout the editing of this book, including Noel McGlinchey, Jenny Lunn, Lesley Fenske, Layla Harden, and Keerthana Govindarajan. Warmest thanks to Rituparna Bose for inviting us on this great adventure of editing a book on submarine noise and Lihini Aluwihare from the Scripps Oceanographic Institute and AGU Books Editorial Board for editorial support.

Gaye Bayrakci

National Oceanography Centre, UK

Frauke Klingelhoefer

IFREMER (French National Institute for Ocean Science), France

1An Introduction to the Ocean Soundscape

Gaye Bayrakci1 and Frauke Klingelhoefer2

1National Oceanography Centre, Southampton, UK

2IFREMER, Centre Bretagne, Plouzané, France

ABSTRACT

The ocean soundscape is composed of sound from natural origins related to geological processes (geophony) and marine life (biophony) and of manmade origin (anthrophony). Early seismologic studies focused on earthquakes and classified most other signals as “noise.” Recent studies have shown that geological processes originating from the seafloor and subseafloor (submarine volcanoes, landslides, etc.) and the water column (e.g., microseisms and icebergs) are also recorded by seismoacoustic instruments and provide important information about how our planet works. Sound is the primary way that many marine species gather information and understand their environment, and it is a valuable tool for us to study their behavior. There has been a substantial increase in anthropogenic noise in the oceans since the Industrial Revolution, and assessing the human impact on the oceans is an emerging topic aiming to leave a healthy ocean for future generations. In this chapter, we briefly present the types of seismic waves and noise sources in the oceans. We explain the seismoacoustic tools used to record noise and give an overview of standard data‐processing methods to familiarize the reader with the concepts discussed in this book. We then summarize the following chapters and discuss future directions for seismoacoustic noise research.

1.1 INTRODUCTION

Early seismoacoustic submarine recording involved instruments towed behind a ship or installed on the seafloor, primarily to record earthquakes or seismic shots. Other sounds were classified as undesired “noise” and either cut out or filtered from the data. An earthquake is the shaking of the Earth due to movement along a fault or discontinuity within the Earth’s subsurface (Aki, 1972). The sudden displacement along the fault generates seismic waves corresponding to elastic waves. These waves are studied extensively by earthquake seismologists because of their impacts on human life and because they provide insight into the properties of the media in which they travel (Agnew et al., 2002). The Oxford English Dictionary defines noise as a loud or unpleasant sound, or irregular fluctuations that accompany a transmitted signal but are not part of it and tend to obscure it. Since seismology is the branch of science concerned with earthquakes and related phenomena, signals of biological or anthropogenic origin are considered noise in earthquake seismology.

Studies of ocean noise began in the 1960s with the advent of more affordable data storage and better instrument performance. They showed that the oceans are far from silent and are filled with noises from different origins: human, biological, and tectonic. In this book, we present many of these nonearthquake‐related ocean noises and explain various methods to interpret them.

The ambient sound field in the oceans is composed of sounds from natural and manmade processes. Natural noise sources in the oceans are of abiotic/geological (geophony) and biological (biophony) origin. Examples of geophony include microseisms related to the interactions of wind with oceans, volcanic events, landslides, icebergs, hydrothermal noise, rain, breaking waves, and gas bubbles, whereas biophony includes marine mammal vocalizations and noise from other marine species (e.g., crabs and shrimp). Ocean noise also includes manmade noise (anthrophony), which results from resource extraction (e.g., underwater mining), coastal or marine construction (e.g., pipeline and wind turbines), explosions, coastal and marine traffic, seismic exploration, navigation tools, etc.

Human audible sound is limited to 20 Hz to 20 kHz frequencies, but the term sound refers to more than sounds audible to humans. It means an oscillation in pressure corresponding to a particle displacement: that is, back‐and‐forth movement of particles caused by a passing wave. Within the oceans, as elsewhere, this expands over a wider range of frequencies than human audible sound.

Often, the origin of repeatedly recorded signals in the ocean remains unknown because there is no visual proof of their origin. For example, short‐duration events (SDEs) with durations of a few seconds have been observed on seismic records since the early 1980s; however, their origin remains unknown (see Chapter 8 of this book). At first, these events were explained as fish colliding with the instruments due to an observed decrease in the number of observations with increasing instrument depth (Buskirk et al., 1981). Later studies proposed different origins related to instruments settling into the sediment (Ostrovsky, 1989), microearthquakes (Sohn et al., 1995), oscillating clouds of methane bubbles in the water column (Pontoise & Hello, 2002), and release of gas from subsurface sediments (Bayrakci et al., 2014; Diaz et al., 2007; Sultan et al., 2011). In an attempt to define the origin of SDEs and other seismoacoustic signals, Batsi et al. (2019) deployed ocean‐bottom seismometers in front of a submarine observatory, monitoring the environment with an underwater camera. They found that marine species such as crabs and octopuses frequently interact with the instruments and leave signals in the seismic records (Fig. 1.1); however, no fish were observed colliding with the ocean‐bottom seismometer (OBS) spheres. During a different experiment in a fish tank filled with sediments and water, Batsi et al. (2019) also recorded gas bubbles leaving the sediments. They concluded that SDEs are signals resulting from gas expulsions from the subsurface. Although there is growing evidence for the relationship between SDEs and seafloor gas expulsion, the exact mechanism that generates these events (collapse of seafloor gas migration conduits, vibration of the conduit walls, migration pathways opening via hydraulic fracturing, etc.) is still debated, illustrating the challenge of clearly associating each signal with its origin.

This book aims to provide a comprehensive list of nontectonic seismic signals recorded in the ocean (Fig. 1.2). It includes review and case‐study chapters describing the characteristics of different signals, explaining the methods used for identifying and interpreting these signals and their wider significance. For each type of signal, peer‐reviewed publications by domain experts can be found in the literature. Since human impact on the oceans is a subject of growing importance, recent review papers on this topic (Duarte et al., 2021; Williams et al., 2015) are also available. However, to our knowledge, no book introduces all or most known noise sources in the oceans. Policy frameworks (e.g., the United Nation’s Convention on Biological Diversity or the European Commission’s Marine and Coastal Environment Policy) with recommendations and goals to reduce human impact on the ocean also introduce noise sources encountered in the oceans, but they usually lack the seismoacoustic methodology related to noise identification and analysis as this is not part of their communication goals. Because some ocean noises originate within the water column and are studied by acousticians, and some originate from the subseafloor and are studied by seismologists, different terminology can act as a barrier to knowledge transfer between two very close branches of science. Here, our goal has been to produce a homogenized scientific and educative document that summarizes various types of signals for the curious reader, policymaker, student, or researcher.

In this introductory chapter, we briefly present the types of seismic waves and different noise sources within the oceans. We then explain the tools for recording ocean noise and give an overview (nonexhaustive) of common data processing methods to familiarize the reader with the concepts discussed in the following chapters. We also offer short summaries of each chapter and finish with a subsection on the future directions of seismoacoustic noise research.

1.2 SEISMIC WAVES

“A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion” (https://www.usgs.gov/media/images/what-was-richter-scale). Seismic waves can travel within the Earth’s subsurface, where they are called body waves, or along Earth’s surfaces, where they are called surface waves.

1.2.1 Body Waves

Body waves include primary waves (P‐waves) and secondary waves (S‐waves). P‐waves are compressional waves that travel and displace particles longitudinally, parallel to the direction of the propagating wave. They travel faster than other waves and can propagate in all types of material, including liquids. P‐waves are also called acoustic waves because they travel as pressure fluctuations in fluids. S‐waves, also called shear waves, travel and displace particles transversely, perpendicular to the direction of propagation. Their speed is related to the medium's shear modulus; therefore, they do not travel in liquids, as liquids do not have any shear strength. In an anisotropic medium where, for example, the mineral crystals have varying properties in different directions (i.e., lattice‐preferred orientation) or the medium is made of thin layers of contrasting properties (i.e., shape‐preferred orientation), S‐wave splitting (or S‐wave birefringence) occurs. Here, S‐waves split into two phases: the first phase is polarized parallel to the preferred alignment (e.g., the longer mineral axis or parallel to the layering) and travels faster, and the second phase is polarized perpendicular to the alignment and propagates more slowly (Crampin, 1985).

Figure 1.1 (a, d) A crab and an octopus interacting with the geophones of an ocean‐bottom seismometer. (b, e) The associated waveforms. (c, f) The spectrograms. Batsi et al. (2019) / with permission from John Wiley & Sons.

Figure 1.2 Cartoon illustrating the ocean noise sources.

In addition to P‐ and S‐waves, Biot’s theory (Biot, 1956), which describes wave propagation in a porous medium fully saturated with a fluid (e.g., a solid frame and the fluid), predicts the presence of a slow compressional wave due to a 180‐degree out‐of‐phase movement of the fluid and the solid phase. The presence of these slow compressional waves is shown theoretically, but they have not been observed on seismograms.

In data from seismometers located on islands or coastal locations or moored hydrophones in the ocean, a third seismic phase called the T‐ (tertiary) phase is observed when the seismic energy from earlier phases is converted to acoustic energy. T‐phases can propagate long distances within the oceans in a horizontal layer called the SOFAR (sound fixing and ranging) channel. It acts as a waveguide because its sound speed is lower than that of the waters above and below. T‐phases may also convert back to elastic waves when they encounter a continental slope (Buehler & Shearer, 2015).

1.2.2 Surface Waves

Surface waves are the waves trapped at a surface marked by a change in density and/or elastic properties, such as the surface of the Earth, an interface within the subsurface, or the walls of a fluid‐filled subsurface crack (Takeuchi & Saito, 1972). Surface waves have larger amplitudes than body waves and travel more slowly. They are dispersive; longer wavelengths penetrate deeper and travel with higher velocities than shorter wavelengths. Types of surface waves include Love, Rayleigh, Scholte, Stoneley, Lamb, and Krauklis waves.

The particle motion (i.e., the back‐and‐forth movement of the particle due to the passage of the wave) of Love waves (Sheriff & Geldart, 1995) is parallel to the Earth’s surface and perpendicular to the direction of wave propagation (transverse). That of Rayleigh waves (Rayleigh, 1885) is elliptical, with vertical and longitudinal components (Vidale, 1986). Love and Rayleigh waves travel along a vacuum–solid interface like the Earth’s surface. Scholte waves and Stoneley waves (Stoneley, 1924) exhibit the same particle motion as Rayleigh waves but travel along fluid–solid and solid–solid interfaces, respectively. Lamb waves (Lamb, 1881, 1917) exhibit the same particle motion and propagate within thin plates (i.e., isolated elastic plates) surrounded by a contrasting material, in a symmetric (the particle motions at both surfaces of the thin plate are in phase) or asymmetric mode (Korneev et al., 2012). Similarly, Krauklis waves (Korneev, 2011; Korneev et al., 2012) propagate around a fluid layer bounded by two elastic half spaces, and they are polarized along the interfaces (e.g., the walls of a fluid‐filled conduit).

Gravity waves are a type of surface wave produced by the restoring force of gravity on the distorted ocean surface. They can also occur within a body of water where a density contrast exists between two superposed fluids, where they are called internal waves (Garrett & Munk, 1979).

1.3 NOISE SOURCES IN THE OCEANS

1.3.1 Noise from Geological Origins (Geophony)

Microseisms are seismic noise arising from the vertical oscillations of solid earth due to gravity waves (Ardhuin et al., 2015). They form the second‐lowest‐frequency noises recorded on seismometers, after the tides resulting from the gravitational forces of the Moon and the Sun on the oceans and the Earth, which have a semidiurnal period (two low and two high tides in 24.50 hours).

Microseismic noise usually displays two period peaks. The weaker peak is produced in shallow coastal environments by gravity waves with periods of up to ~20 seconds and amplitudes of up to several meters because of pressure fluctuations caused by breaking waves. These are called primary microseisms (or single‐frequency microseisms) and are observed at the period of the wave initiating them (0.05–0.1 Hz). The stronger peak displaying shorter periods is also caused by gravity waves. These are called secondary microseisms (or double‐frequency microseisms), resulting from ocean waves (gravity waves) with the same frequency traveling in opposite directions. Their period is usually half the period of the wave pair generating them (0.1–0.5 Hz). Ocean infragravity waves (a subcategory of gravity waves with periods >30 seconds; Webb et al., 1991) also create secondary microseisms with periods up to 50 seconds; they are referred to as “Earth’s hums” (Bertin et al., 2018).

Microseisms can be recorded anywhere on Earth and are a useful monitoring tool for climate change, as they allow the observation of ocean storm characteristics such as storm frequency or intensity. In this case, increasing microseismic noise indicates increased ocean storminess, likely due to climate change (Bromirski, 2009).

Another source of low‐frequency seismic noise is volcanic very‐long‐period (VLP) events of 0.01–0.5 Hz frequencies, considered to be the manifestation of magma migration and resonant fluid excitation in the volcanic system (Arciniega‐Ceballos et al., 1999; Chouet, 2022). When relocated, they reveal the locations of cracks, dikes, and sills in the volcanic system. Volcanos also produce monochromatic long‐period (LP) signals, usually with frequencies <5 Hz and durations of ~10 seconds. When LP energy is sustained over minutes to days, the signals are called volcanic tremors and are attributed to the dynamics of magmatic and hydrothermal fluids (Chouet & Matoza, 2013). Tremors can have dominant energy in the 1–9 Hz band and can be harmonic when related to conduits that are filling or emptying. In addition to nontectonic events, volcano‐tectonic (VT) events originating from shear failure‐like earthquakes occur in volcanic systems and act as a gauge that maps the stress conditions around the magma conduit (Chouet & Matoza, 2013). Note that the terminology for volcano‐seismologic signals is still evolving due to the ongoing discovery of new signals.

An interesting example of a developing marine volcano is the Mayotte volcano. The region became seismically active in 2018, with intense seismic swarms. In November 2018, a very low‐frequency tremor was recorded worldwide, confirming that the seismic crisis was likely of magmatic origin. Other nonearthquake signals, such as hydroacoustics probably associated with the volcano, were discovered during the monitoring of the volcano (Foix et al., 2021; Saurel et al., 2021).

Submarine landslides commonly occur at continental slopes, submarine canyons, volcanic islands, and ridges. They produce both a long‐period noise (20–50 seconds), related to the elastic rebound after the mass failure, and a large‐amplitude noise with frequencies from 1 to 10 Hz (1–0.1 second period) due to the impact of the mass on the base of the slope. They are associated with tsunamigenic hazards, and their hydroacoustic detection is suggested to be a powerful tsunami monitoring and modeling tool (Caplan‐Auerbach et al., 2001).

Noise from ice is the dominant noise source in regions covered by ice sheets. Iceberg bursts attributed to disintegration processes produce signals of uniform spectral power between 1 and 8 Hz lasting a few minutes. Monochromatic signals of <10 Hz that do not show multiple harmonic overtones (only one or two) are called iceberg tremors and are thought to be related to the resonance of fluid‐filled cracks within the ice. Iceberg harmonic tremors of a few Hz fundamental frequency with overtones occupying the frequency band below 200 Hz are also commonly recorded in locations covered by ice sheets, and they are attributed to the stick slip at the contact between icebergs or an iceberg and the ground (see Chapter 6 of this book).

Seabed fluid flow is a widespread natural process with consequences for seabed geological features, marine biology, and the compositions of the oceans (Judd & Hovland, 2009), which also contributes to the ocean soundscape. Compaction, petroleum, fresh water, and magma‐controlled systems differentiate the mechanisms through which fluids migrate in permeable sediments (Berndt, 2005). Faults, seismic pipes, and chimneys are sedimentary vertical fluid migration pathways; pockmarks and mud volcanoes are the seafloor expression of fluid emission in cooler environments; and hydrothermal vents driven by the presence of a heat source are mostly observed in regions with magmatic or geochemical control.

Hydrothermal vents occur when heated subsurface fluid is discharged at the seafloor. They are found near seafloor spreading centers and hot spots, often in water depths between 2,000 and 4,000 m. Usually the heat source is magma‐related, but it can also be sourced from exothermic metamorphosis, such as serpentinization, which is the alteration of the mantle peridotite by water (Früh‐Green et al., 2003). Hydrothermal vents generate seismic noise due to the interaction of turbulent flow with the vent structure and cavities (Fig. 1.3), the jet discharge into the cold ocean, and phase transitions of seawater, driven by rapidly changing temperature and pressure (see Chapter 7 of this book). In general, noise from hydrothermal activities exhibits frequencies less than 500 Hz. Passive soundscape monitoring of hydrothermal vents is currently new. Since sound can travel long distances within the ocean, it has been suggested that passive monitoring may allow the remote detection of vent sites and vent characterization. It has also been suggested that some biological organisms use vent noise as an acoustic cue to aid settlement (Crone et al., 2006). Since hydrothermal vents are proposed to be the catalysts for life on Earth (Martin et al., 2008), acoustic monitoring of hydrothermal vents may be a valuable tool in the search for extraterrestrial life exploration (Dziak et al., 2020).

Figure 1.3 A short‐duration event recorded by an ocean bottom node around the SEM1 seafloor massive sulfide deposit located on top of the Semyenov Oceanic Core Complex (OCC) at 13°N along the Mid‐Atlantic Ridge. (a) Waveform, (b) spectrogram, and (c) frequency spectrum of the signal. Note the monochromatic and harmonic characteristics of the signal.

Courtesy of Bramley Murton.

Like hydrothermal vents, active fluid discharge in cooler sedimentary environments generates seismoacoustic noise. Monochromatic (4–30 Hz frequencies and harmonics) SDEs (Fig. 1.3) a few seconds in length are recorded worldwide within sedimentary basins (see Chapter 8 of this book; Tary et al., 2012). They are best observed on geophones as only large‐amplitude events are detected by hydrophones, suggesting a seafloor origin for these events. The frequency range and waveforms of SDEs can be similar to VLP events, and they may be related to the resonance of sedimentary fluid conduits. Besides the seismic noise within the sediments, fluid emission from the sediments into the water column also creates bubbles and bubble clouds (with lower‐frequency oscillations), contributing to the acoustic noise.

A gas bubble is a volume of gas surrounded by a liquid (Leighton, 1994). Bubbles can be found throughout the water column and are produced in many ways, from volcanic emissions to breaking waves. The sound of a single bubble is insignificant compared to other marine noise sources; however, natural processes (rainfall, gas seeps, breaking waves) usually generate millions of them. At the seafloor, a single bubble forms when a small volume of gas from a larger reservoir intrudes into the water, and the two volumes of gas are connected by a thin neck (see Chapter 10 of this book). When the neck eventually breaks and releases the bubble into the water column, the snapping of the neck results in a jet of water being propelled into the bubble, triggering an oscillation at the natural frequency of the bubble that creates its acoustic signature. This phenomenon is called Minnaert resonance (Minnaert, 1933); the frequency of oscillation of the bubble is a function of the bubble radius, the ambient pressure, and the density of the liquid and spans several orders of magnitude (~0.1–100 kHz). Bubble clouds, groups of independent bubbles, also oscillate with lower oscillation frequencies than each individual bubble, comparable to frequencies of single, larger bubbles (Leighton, 2012).

Similarly, when a rain droplet falls on the sea surface, it forms an impact crater and may entrain a bubble into the water. An initial sound (a compressional wave) results from the impact of the droplet on the sea surface, followed by the simple harmonic motion of the bubble, which is again the Minnaert resonance phenomenon. The acoustic signature of rainfall is caused by this entrainment of bubbles, so the frequency spectrum of rainfall depends on the sizes of bubbles entrained by the rain droplets (e.g., 13–25 kHz for drizzle of 1.2–2 mm droplets; Ma & Nystuen, 2005).

Figure 1.4 Whale calls recorded during the Galicia 3D seismic survey at the Deep Galicia rifted margin offshore Spain and Portugal. The signal's waveform is shown in the top panel and the spectrogram in the bottom panel. Whale calls are the short‐duration signals observed after 35 minutes with a frequency range of 18–24 Hz. Tim Minshull (2022) / PANGAEA / CC‐BY‐4.0.

The acoustic signature of breaking waves also results from bubble‐cloud generation. The largest sound intensity occurs during the cavity collapse in the wave’s life cycle (see Chapter 10 of this book). The frequency of the acoustic noise is also related to the size of the created bubbles (e.g., 300 Hz for a bubble size distribution of 2 mm to >10 mm radius; Deane & Stokes, 2002).

1.3.2 Noise from Biological Origins (Biophony)

Sound is a highly efficient means of communication underwater and is the primary way that many marine species gather and understand information about their environment (Fig. 1.4). Cetaceans vocalize for social and echolocation purposes (Au & Hastings, 2008). Vocalizations can be periodic or nonperiodic and are a mixture of frequency‐ and amplitude‐modulated signals (Zimmer, 2011). The 7 Hz to 180 kHz frequency band is thought to contain all frequencies within cetacean vocalizations and hearing ranges (Barker & Lepper, 2012; Bittle & Duncan, 2013). Generally, larger species generate lower‐frequency sounds, and the frequency range of a given species is dictated by its anatomy. As the sound attenuates through geometric spreading, lower‐frequency songs of larger species propagate over longer distances. Long‐period recording of these calls is useful for gathering information about the species’ migration patterns.

1.3.3 Noise from Anthropogenic Origins (Anthrophony)

There has been a substantial increase in anthropogenic noise in both terrestrial and marine environments since the Industrial Revolution. Given the rapid pace of change in the ocean soundscape, there is an urgent need to assess the impact of anthropogenic noise on marine life and mitigate further disturbances for a healthy ocean in the future (Duarte et al., 2021). The anthropogenic soundscape in the oceans includes noise from fishing, mining, drilling (Fig. 1.5), resource harvesting and related underwater construction, geophysical exploration such as seismic surveys (10–500 Hz), and other active echosounders producing higher‐frequency noise to map the seafloor, among other purposes. Vessel noise with frequencies ranging from 10 Hz to tenths of kilohertz encompasses the frequency ranges of other anthropogenic noises, and it is by far the most studied. The International Maritime Organization adopted regulations for commercial shipping to reduce onboard noise to protect the ship personnel, along with a set of guidelines for reducing underwater noise.

Figure 1.5 Seafloor drilling noise recorded by an ocean bottom node mounted on the RD2 (British Geologic Survey) seafloor drill. (a) Waveform and (b) spectrogram of the signal. Note that drilling starts at the fortieth minute of the recording. (c, d) Two 10‐minute windows from the data shown in (a) with a silent and noisy (drilling) period, respectively. (e) Frequency spectrum of the silent signal in black and the noisy signal in grey. Note that the drilling produces low‐frequency noise centered around 30 Hz.

Courtesy of Bramley Murton.

1.4 TOOLS FOR RECORDING MARINE NOISE

Hydrophones, geophones, and accelerometers usually detect seismic waves traveling within the ocean or the Earth’s subsurface. Other instruments, such as cameras filming bubbles emitted from the seafloor or current‐meters measuring the velocity of water currents (acoustic Doppler current profiler, ADCP), are used to infer the origin of noise sources.

A hydrophone (Fig. 1.6) records pressure (absolute or differential) in the water using a piezoelectric transducer that generates an electric potential when subjected to a pressure change. It is therefore very similar to a microphone, which measures pressure variations in the air; the difference is the increased acoustic resistance of water, which is 3,750 times that of air. A hydrophone or microphone senses P‐waves, which are pressure transients. At low frequencies, the pressure variations are omnidirectional, and a hydrophone with a single cylindrical ceramic transducer is used to achieve omnidirectional reception. More sophisticated directional hydrophones containing focused transducers or transducer arrays can enhance the sensitivity of the signal coming from a given direction.

A modern geophone consists of a coil of wire wrapped around a mass suspended by a spring over a magnet. As the mass moves, the magnet moves the electrons through the wire coil, producing an electrical voltage. The geophone thus converts the ground velocity caused by a seismic wave into voltage. An accelerometer is similar to a geophone, but it uses a strain gauge or a capacitance sensor and a feedback mechanism that outputs the force required to make the mass follow the case. Since the force is the product of the mass and the acceleration, the output is proportional to the acceleration. The latest technology used in ocean‐bottom accelerometers is MEMS (micro‐electro‐mechanical system), which provides recordings of the signal uninfluenced by parameters such as the water temperature or the tilt of the instrument.

Figure 1.6 RS Aqua single‐channel Porpoise acoustic recorder/real‐time streaming device with a hydrophone.

Photo Credit: RS Aqua Ltd.

Three orthogonally mounted geophones are used in a seismometer to record the particle motion created by the Earth's vibrations. The first seismoscope, the ancestor of the seismometer, was created in China during the second century (Sleeswyk & Sivin, 1983). Seismoscopes detected motion of the Earth and indicated the direction of motion. Only in the nineteenth century were pendulum seismographs invented: they detect and record the Earth’s movements using a pencil attached to an inertial mass that moves with respect to a fixed case, and thus record the horizontal particle motions. Initially, the word seismograph referred to a device detecting and recording the Earth’s movements simultaneously, whereas a seismometer only detected the movement without recording it. Today the words seismometer and seismograph are used interchangeably.

Modern seismometers use electronic sensors, amplifiers, and recorders (data loggers). Seismic sensors (geophones and accelerometers) are defined by their bandwidth, sensitivity, self‐noise, resolution, and dynamic range. The frequency of natural seismic sources spans a wide range from 10‐5