The surf’s roar is more than just what we perceive with our ears. While the audible crashing of waves is familiar, much of the acoustic energy produced by breaking waves exists at frequencies below 20 hertz (Hz), known as infrasound. These low-frequency pressure waves, along with associated seismic vibrations passing through the ocean floor, provide a rich but hidden acoustic landscape. The research team at UCSB employed sophisticated arrays of infrasound sensors coupled with seismometers to probe these subtle signals. Their findings were recently published in Geophysical Journal International, detailing the unique acoustic and seismic footprints left by the surf and demonstrating the feasibility of pinpointing wave-breaking locations along the coastline through this approach.

The genesis of these inaudible waves lies in the physical mechanics of wave breaking. When waves collide with the rocky shore or seabed, air pockets get entrained, forming and collapsing bubbles that oscillate collectively due to pressure instabilities. Jeremy Francoeur, lead author and former UCSB graduate student, describes this phenomenon as a synchronized expansion and contraction of bubble clouds, generating persistent pressure oscillations. These oscillations translate into infrasound waves that travel upward through the atmosphere and downward as seismic waves through the Earth’s crust. Though these pressure variations lie beneath the human auditory range, their amplitude reaches levels comparable to everyday urban noise, making them significant natural acoustic sources.

Infrasound below 20 Hz includes ordinary acoustic waves, but their low pitch means that they go unnoticed by humans. Senior author and geophysicist Robin Matoza emphasizes that these “hidden sounds” originate from a broad spectrum of natural and anthropogenic activities worldwide. They include major geological and atmospheric events such as volcanic eruptions, earthquakes, landslides, hurricanes, and even atmospheric phenomena like auroras and wind flowing over mountainous terrain. Each source produces distinct low-frequency acoustic signatures, which scientists can harness to better understand and monitor Earth’s dynamic processes.

Motivated by UCSB’s coastal location, Matoza’s research group turned their attention to the acoustic mysteries of surf noise. Deploying an array of sensors at Coal Oil Point Reserve—a protected site within the UC Natural Reserve System—the researchers recorded infrasound and seismic data synchronized with high-definition video of wave activity. This multi-modal data set allowed them to correlate specific acoustic pulses with precise moments of wave breaking, greatly enhancing signal identification compared to previous single-sensor studies. By aligning sound signals with video “snapshots,” the team was able to discern a robust acoustic fingerprint unique to breaking waves.

The infrasound signals identified arrived in repetitive bursts between 1 and 5 Hz frequency, distinctly marking the crashing surf’s rhythmic energy. Though “loudness” is a subjective notion tied to human hearing perceptions, the acoustic wave amplitudes measured reached remarkable levels. Typical surf-generated infrasound ranged between 0.1 and 0.5 pascals, comparable to the sound pressure of busy traffic, while stronger swells produced waves as intense as 1 to 2 pascals—akin to the noise of a factory floor. This quantitative analysis reveals that the ocean’s low-frequency acoustic emissions rival common urban soundscapes in energy, a startling discovery since these sounds remain inaudible.

A key insight emerged from the team’s exploration of how these infrasound signals relate to actual sea conditions. The researchers found a correlation between infrasound amplitude and significant wave height, a critical parameter measuring the vertical scale of open ocean swells. However, they noted that the relationship between acoustic data and observed wave behavior was more complex than initially hypothesized. The interplay of factors such as tides, wind patterns, and bathymetry introduced nonlinearities that challenged simplistic models, underscoring the need for further investigation into environmental influences on surf-generated acoustics.

The array’s capability extended beyond mere detection; by measuring minuscule variations in arrival times of infrasound waves at multiple sensors, the team applied reverse-time migration techniques to triangulate the exact origins of breaking waves. Remarkably, the analysis localized the acoustic source consistently to the rock shelf at Coal Oil Point. This led to the hypothesis that specific underwater topography concentrates wave impact zones, triggering synchronized bubble oscillations that amplify the infrasound output. Understanding these spatial patterns opens exciting avenues for mapping coastal processes through sound.

Looking ahead, the researchers aim to explore whether individual beach segments universally serve as primary infrasound emitters or if such patterns vary with geography and environmental conditions. Questions linger about how wave infrasound signatures might differ between globally diverse shorelines like Santa Barbara and Tahiti, or how dynamic factors like changing tides and fluctuating wind fields modulate these acoustic emissions. Unraveling these complexities will extend the applicability of surf acoustic monitoring as a powerful natural observatory tool.

Matoza’s lab enjoys unique advantages owing to the proximity of Coal Oil Point Reserve, only 2.5 miles from UCSB’s main campus. This closeness enables rapid deployment and iterative refinement of sensor arrays, facilitating extensive field experimentation and hypothesis testing. Moreover, students actively engaged in this project gain hands-on experience across the entire scientific workflow, from data collection and instrument installation to advanced signal analysis and scholarly writing. This immersion cultivates the next generation of geophysicists skilled in cutting-edge Earth science methodologies.

The ultimate goal is the development of an autonomous system capable of characterizing nearshore surf conditions solely from infrasound and seismic data, independent of visual observations. Current video monitoring technologies suffer from limitations imposed by darkness, fog, and adverse weather, which reduce visibility and reliability. Acoustic and ground-motion sensing could thus become indispensable complements, offering continuous, all-weather surveillance possibilities vital for coastal management, hazard preparedness, and environmental research.

This pioneering research sets a precedent in marine geophysics, unveiling a novel sensory interface through which the Earth’s atmospheric and oceanic interactions reveal themselves. By “listening” below the human audible spectrum, scientists can access a previously hidden domain of natural signals rich with information on coastal wave dynamics. As this technology matures and integrates with existing oceanographic tools, it promises significant advances in our understanding of ocean processes, coastal environments, and their responses to climate and anthropogenic changes.

Subject of Research: Acoustic and seismic signatures of breaking ocean waves

Article Title: Researchers characterize infrasound and seismic signals from surf to monitor coastal wave dynamics

News Publication Date: Not specified

Web References: https://academic.oup.com/gji/advance-article/doi/10.1093/gji/ggaf317/8236357

References: Study published in Geophysical Journal International

Image Credits: Elena Zhukova

Keywords: Space sciences; Seismology; Oceanography; Coastal processes

Fibre optic cables are commonly known for their role in telecommunications, providing high-speed data transfer for internet, television, and phone services. Recent technological advances now suggest that these same cables can be transformed into a dense network of seismic sensors. As seismic waves travel through the Earth, they can now be detected by using these cables, which has the potential to revolutionize how we monitor and respond to seismic activity. This move towards integrating fibre optic technology with seismic monitoring is poised to not only improve the detection of earthquakes, but also to enhance monitoring in areas such as volcanic eruptions and glacier movements.

The innovative research led by Dr. Thomas Hudson at ETH Zurich highlights the compelling possibilities of this technology. By adapting an existing physics-based algorithm, the team has made it possible to include fibre optic data alongside conventional seismic measurements. This adaptability is crucial because it allows for a more comprehensive analysis of seismic activity that could be beneficial for earthquake early warning systems. The integration of these data streams could create a more holistic view of seismic phenomena compared to traditional monitoring methods alone.

One of the most significant challenges researchers face in this field is the complex geometries of real-world fibre optic networks. Unlike controlled experiment settings, the arrangement and framework of these cables in urban environments introduce noise that can hinder effective seismic detection. This inherent noise complicates the task of distinguishing earthquake signals from other vibrations, such as those created by traffic or industrial activities. The research addresses this issue directly, as the new algorithm effectively filters out noise, enabling clearer identification of seismic signals even in bustling environments.

Distributed Acoustic Sensing (DAS) has emerged as a prominent method for turning fibre optic cables into powerful seismic tools. By detecting subtle acoustic signals and vibrations, DAS technology can monitor a wide array of scenarios—from leaky pipelines to the structural integrity of buildings, and now earthquakes. The full potential of this technology could redefine our seismic monitoring capabilities, offering a connected and widespread sensor network that surpasses what is currently achievable with isolated traditional seismometer systems.

Real-time data processing remains a vital component of this innovative monitoring solution. The computational techniques employed must be robust enough to handle the voluminous data generated by fibre optic networks while simultaneously ensuring rapid analysis. The new algorithm suggests a promising approach to this issue, converting energy recorded over time at various points along the fibre optic cable into coherent seismic readings. This capacity for swift and precise analysis is critical in scenarios where seconds could mean the difference in an effective warning.

Moreover, the new algorithm is designed to be open-source, which presents an exciting opportunity for the broader scientific community. By making the methodology accessible, researchers worldwide have the prospect of adapting and improving upon the initial findings, facilitating collaboration and innovation. Such cooperative efforts could accelerate the development of even more effective seismic monitoring technologies, enhancing safety measures in earthquake-prone regions.

One of the groundbreaking aspects of this research is its focus on real-world application. While preliminary studies demonstrated the algorithm’s effectiveness, additional tests in diverse environments will be critical in validating its widespread utility. Researchers aim to evaluate the system’s performance across various geological settings to ensure reliable operation in different conditions. This practical focus underscores the team’s commitment to bringing theoretical advancements into functional applications that can directly benefit society.

Recognizing that most significant seismic events originate from sources that traditional networks may not access, this innovative approach places a premium on flexibility and adaptability. The ability to install fibre optic cables in various locations, including remote areas, expands monitoring efforts beyond established fault lines. As a result, the geographic reach of seismic detection efforts could be dramatically enhanced, providing early warnings in new places that previously lacked sufficient monitoring capabilities.

Notably, the impact of this technology extends beyond just earthquake detection. By also addressing volcanic activity and glacier monitoring, it presents an all-encompassing solution with vast environmental implications. As the effects of climate change and geological activity become increasingly relevant, these advancements can significantly improve our readiness for natural events that could threaten life and infrastructure.

In summary, the advent of a new algorithm that exploits global fibre optic networks for earthquake detection marks a paradigm shift toward more intelligent and resilient seismic monitoring systems. By effectively integrating the robustness of optical sensing with traditional seismometric techniques, researchers are paving the way for superior seismic detection and response capabilities. With ongoing research and collaboration, the potential for this technology could redefine how we approach earthquakes, ultimately saving lives and enhancing public safety.

Subject of Research: Earthquake Detection Using Fibre Optic Networks
Article Title: Towards a widely applicable earthquake detection algorithm for fibreoptic and hybrid fibreoptic-seismometer networks
News Publication Date: 3-Feb-2025
Web References: Geophysical Journal International
References: DOI: 10.1093/gji/ggae459
Image Credits: Credit: Dr Thomas Hudson

Keywords

Seismology, Fibre optics, Planet Earth, Internet, Glaciers