Yet these are but a sample of the sounds that come from the coast. Most of the acoustic energy from the surf is far too low in frequency for us to hear, traveling through the air as infrasound and through the ground as seismic waves.
Scientists at UC Santa Barbara have recently characterized these low-frequency signals to track breaking ocean waves. In a study published in Geophysical Journal International, they were able to identify the acoustic and seismic signatures of breaking waves and locate where along the coast the signals came from. The team hopes to develop this into a method for monitoring the sea conditions using acoustic and seismic data.
While pressure waves below 20 hertz (Hz) are still ordinary acoustic waves down to about 0.01 Hz, the frequency, or "pitch," is too low for humans to hear. "These hidden sounds of Earth's atmosphere are produced by numerous natural and anthropogenic sources," explained senior author Matoza, a geophysicist in UCSB's Department of Earth Science. These include volcanoes, earthquakes and landslides; ocean storms, hurricanes and tornadoes; even auroras and the wind flow over mountains. Understanding the type of signals generated by each phenomenon can provide a bounty of information about these events.
Working from UCSB's seaside campus, it was natural that Matoza eventually turned his attention toward the beach. He and his students were curious what their seismo-acoustic techniques could tell them about the surf breaking along the coast.
Francoeur deployed an array of sensors atop the headland at UCSB's Coal Oil Point Reserve, part of the UC Natural Reserve System, to record infrasound and seismic waves produced by the surf. He paired this data with video footage of the beach to identify what signals corresponded to a breaking wave.
While many phenomena produce infrasound, the signal from the surf was fairly clear in the data. It arrived at the sensors as repetitive pulses between 1 and 5 Hz.
It was also fairly loud. Well, sort of. "'Loudness' is a description of a human perception," Matoza explained, "so infrasound cannot have 'loudness.'" However, what we perceive as volume relates to the amplitude of the acoustic wave.
Most of the wave infrasound was around 0.1 to 0.5 pascals. This would be about the volume of busy traffic (74 to 88 decibels (dB) relative to a 20 uPa reference pressure), or about the volume of a busy restaurant, if it were shifted into the frequency range of human hearing. Particularly strong swells reached 1 to 2 Pa, or the din of a noisy factory (94 to 100 dB).
"The sound of the surf is pretty loud when you're out there on the beach," Francoeur said, "so it's interesting that the majority of energy is actually produced in the infrasound range."
The team was curious whether this signal would align with sea conditions. They found that the infrasound amplitude correlated with significant wave height, which is the height of swells on the open ocean. "But the correlation between what we were seeing with the video data compared to what we were seeing acoustically and seismically was a lot more complex than we initially imagined it to be," Matoza said.
Francoeur was also able to use the array to triangulate the signals' origin from small differences in arrival times, a technique called reverse-time-migration. "It was interesting to me that all of the directions seemed to align to the same region of the beach," he said: "the rock shelf at Coal Oil Point." The authors suspect that the point's bathymetry forces a large proportion of waves to crash simultaneously, producing those synchronized bubble oscillations.
Matoza will continue to investigate these questions with his lab, a task made simpler by the project's location merely 2.5 miles from his office. "Having this field site very close to campus was really a fantastic opportunity because it was a lot of trial and error trying to figure out good array geometries," he said. "The proximity meant that we could quickly deploy."
It's also a boon to his students' budding careers. "They get to take part in the whole geophysical workflow - from collecting data in the field, deploying the instruments, analyzing the data, hypothesis testing and writing the paper. And we can do that all within Goleta," Matoza said.
He hopes to ultimately develop a way to characterize surf conditions solely from infrasound and seismic signatures. This could complement video monitoring systems that may be limited by darkness and fog.
Research Report:Identification of transient seismo-acoustic signals from crashing ocean waves: template matching and location of discrete surf events
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