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An 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...