🔭 astrophysics

A Search for Hydroacoustic Signals from Bolides

This study surveys CTBTO hydrophone data to detect hydroacoustic signals from 30 fireballs, finding no unambiguous detections and concluding that such signals are extremely rare with a coupling efficiency estimated at approximately $10^{-10}$ .

Original authors: P. Brown, L. McFadden, D. McCormack, M. Adams, D. Vida

Published 2026-04-15

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CC BY 4.0

Original authors: P. Brown, L. McFadden, D. McCormack, M. Adams, D. Vida

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Listening for Space Rocks in the Ocean

Imagine a giant rock from space (a meteoroid) smashing into Earth's atmosphere. It burns up, creating a massive fireball and a huge shockwave, like a sonic boom from a supersonic jet, but much bigger.

Scientists have been listening for these "space booms" in the air (using microphones) and on land (using earthquake sensors) for years. But this paper asks a new question: Can we hear them underwater?

The team of researchers decided to turn the world's oceans into a giant listening post to see if the shockwaves from these space rocks could travel through the water and be picked up by underwater microphones (hydrophones).

The Setup: The Ocean's "Super-Highway"

To understand how they did this, you need to know about the SOFAR channel.

Think of the ocean like a layered cake. Usually, sound gets muffled and dies out quickly in water. But there is a specific layer deep underwater (about 1 kilometer down) where the water temperature and pressure create a "sound tunnel" or a super-highway.

The Analogy: Imagine shouting in a long, empty hallway. The sound bounces off the walls and travels much further than it would in an open field. The SOFAR channel is that hallway for the ocean. Sound waves get trapped there and can travel thousands of miles without losing much energy.

The researchers used the CTBTO, a global network of sensors originally built to listen for secret nuclear bomb tests. These sensors are sitting right in that "sound tunnel," ready to hear anything loud that happens in the ocean.

The Hunt: What They Were Looking For

The team looked at 30 big fireballs that happened over the ocean between 1996 and 2025. They were looking for two things:

The Shockwave: When the fireball explodes in the sky, does the "boom" punch through the water's surface and travel down the highway?
The Meteorite Splash: Sometimes, pieces of the rock survive the fire and splash into the ocean. Does that splash make a loud enough sound to be heard?

They treated the ocean like a giant crime scene, checking the audio recordings for every single fireball to see if a matching "sound signature" appeared at the right time and from the right direction.

The Results: The Great Silence

Here is the disappointing (but scientifically important) news: They didn't find a single clear match.

Out of 30 fireballs and 53 different listening stations, they found zero confirmed underwater sounds from space rocks.

The "False Alarm": They did find one weird sound that happened at the right time and from the right direction (off the coast of Alaska in 2003). It looked promising! But when they did the math, they realized there was a 27% chance it was just random ocean noise (like a whale, a ship, or a storm) that happened to look like a fireball. It was a "maybe," but not a "yes."

Why Was It So Hard to Hear?

You might wonder, "If the explosion is so big, why couldn't we hear it?"

The paper explains that the ocean is a very bad translator for air sounds.

The Analogy: Imagine you are shouting at a glass wall. Most of your voice bounces right back at you; very little gets through to the other side.
The Physics: Air and water are very different. When a shockwave hits the water from the air, about 99.9% of the energy bounces back into the sky. Only a tiny, tiny fraction (less than 0.1%) actually enters the water.
The Attenuation: Even that tiny fraction has to travel down through the water to reach the "sound tunnel." By the time it gets there, it's so quiet that the background noise of the ocean (waves, ships, biology) drowns it out.

The "Airplane Crash" Comparison

To prove that their microphones could hear something, the researchers looked at a real-world example: a high-speed F-35 fighter jet crashing into the ocean in 2019.

The Result: The jet crash was loud enough to be heard clearly by the underwater microphones.
The Lesson: A meteorite hitting the water is like a pebble dropping in a bucket, while a jet crashing is like a cannonball. Even though the fireballs were huge explosions in the sky, the energy that actually made it into the water was too weak to compete with the jet crash.

The Conclusion: Look Up, Not Down

The main takeaway of this paper is simple: Trying to hear space rocks underwater is like trying to hear a whisper in a hurricane.

The energy coupling (the transfer of sound from air to water) is just too inefficient.

What works: Listening for fireballs in the air (infrasound) or on the ground (seismometers) is much better.
What doesn't work: Listening underwater for the shockwaves of airbursts.

The researchers set a new "limit" on how efficient this process is, essentially saying, "If you want to hear a space rock underwater, it would have to be a million times louder than what we usually see."

In short: The ocean is a great place to listen for submarines and earthquakes, but for space rocks exploding in the sky, it's too quiet to hear them. We need to keep our ears (and eyes) on the sky, not the sea.

1. Problem Statement

While fireballs (bolides) are frequently detected via optical, radar, infrasound, and seismo-acoustic methods, confirmed hydroacoustic detections of fireballs are non-existent in the scientific literature.

The Challenge: Detecting atmospheric explosions underwater is difficult due to the high acoustic impedance mismatch between air and water, which causes most shock wave energy to reflect rather than transmit. Furthermore, the ocean background noise is generally higher than atmospheric noise.
The Hypothesis: Despite low coupling efficiency, sufficiently energetic fireballs (kiloton-class) occurring over the ocean might generate detectable hydroacoustic signals that propagate through the SOFAR channel (Sound Fixing and Ranging channel), a deep-ocean waveguide that allows low-frequency sound to travel thousands of kilometers with minimal attenuation.
Objective: To conduct a systematic survey to detect hydroacoustic signals associated with bright fireballs and establish upper limits on the efficiency of energy coupling from the atmosphere to the SOFAR channel.

2. Methodology

The study utilized data from the International Monitoring System (IMS) operated by the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), specifically focusing on its six hydrophone arrays.

Target Selection: The authors identified 30 candidate fireballs from the CNEOS (Center for Near Earth Object Studies) database that met specific criteria:
1. High Energy: Bolides with energy $E \ge 5$ kT.
2. Proximity: Events occurring within 500 km of a hydrophone station.
3. Favorable Coupling: Events occurring over regions where the SOFAR channel depth is shallow (< 500 m), potentially improving transmission efficiency.
Signal Processing:
- Data was analyzed using the DTK-GPMCC (Graphical Progressive Multi-Channel Correlation) software.
- Parameters included 10 linearly spaced filter bands (1–40 Hz), 15-second windows with 90% overlap, and a consistency threshold of 0.05.
- Search Windows: Arrival times were calculated based on the fireball's peak brightness height, assuming a shock travel speed of 0.31 km/s to the ocean surface and a hydroacoustic celerity of 1.42–1.55 km/s. A 1-hour search window centered on the expected arrival time was used to establish background noise rates.
- Association Criteria: A signal was considered a candidate if it fell within the time window and had a backazimuth within $\pm 5^\circ$ of the expected great-circle path from the fireball.
Theoretical Modeling:
- Shock Coupling: Modeled as a spherical point explosion (1 kT TNT equivalent) at 25 km altitude. Calculated the transmission coefficient across the air-water interface ( $\approx 10^{-3}$ ) and attenuation to the SOFAR channel.
- Meteorite Impact: Evaluated the potential for surviving meteorite fragments impacting the ocean at terminal velocity. Compared this to a well-documented F-35 aircraft crash (2019) off Japan, which provided an empirical benchmark for high-speed surface impacts.

3. Key Results

A. Detection Statistics

Total Survey: 53 station-fireball pairs were analyzed.
Candidates: 15 hydroacoustic signal families were found within the time windows.
Valid Associations: Only one signal met both the time and azimuth criteria.
- Event: A 1 kT fireball off the coast of Alaska on September 2, 2003.
- Detection: Recorded at station H03N (Juan Fernandez Archipelago) at a range of 12,300 km.
- Signal Characteristics: Impulsive signal, 26s duration, center frequency ~7 Hz, peak pressure 0.04 Pa.
- Statistical Validity: The authors conclude this is a weak association. Given the random background rate of signals and the single-station detection (no corroborating data from other stations), the probability of this being a false positive (random coincidence) is estimated at 27%.

B. Coupling Efficiency Limits

Using the non-detections in the other 52+ station-fireball pairs, the authors established strict upper limits on energy transfer:

Fireball Shock Coupling ( $\eta$ ): The study sets a conditional upper limit for the efficiency of fireball shock waves coupling into the SOFAR channel at $\eta \le 10^{-10}$ .
- This is significantly lower than seismic coupling efficiencies ( $10^{-2}$ to $10^{-7}$ ) and theoretical estimates for near-surface nuclear bursts ( $\sim 10^{-7}$ ).
Meteorite Impact Coupling: Based on the F-35 crash data (900 MJ impact energy), the empirical coupling efficiency for surface impacts into the SOFAR channel is estimated at $\sim 10^{-4}$ . However, typical stony meteorite fragments from a 1 kT fireball possess insufficient energy to be detected at long ranges.

C. Reflection Analysis

The authors investigated whether signals might be arriving via underwater reflections (bathymetric features) rather than direct paths.

For high-energy events (e.g., Oct 7, 2004), signals were found that matched reflection geometries.
However, waveform analysis suggested these were likely T-phases from local earthquakes rather than fireball reflections, as the waveforms resembled seismic T-phases (lens-shaped coda) rather than impulsive explosive sources.

4. Key Contributions

First Systematic Survey: This is the first comprehensive search specifically targeting hydroacoustic signatures of bolides using IMS data.
Empirical Constraints: Provides the first observational upper limits on the hydroacoustic coupling efficiency of atmospheric explosions ( $\le 10^{-10}$ ), confirming that direct detection is extremely difficult.
Benchmarking: Uses the F-35 crash to provide a real-world empirical estimate for surface impact coupling ( $10^{-4}$ ), distinguishing between shock-wave coupling and impactor coupling.
Methodological Framework: Establishes a rigorous pipeline for correlating CNEOS fireball data with CTBTO hydroacoustic data, including handling of time windows, azimuthal constraints, and background noise estimation.

5. Significance and Conclusion

Detection Feasibility: The study concludes that direct hydroacoustic detection of fireballs in the SOFAR channel is very rare and likely impossible for standard kiloton-class events using current technology and processing methods. The energy loss at the air-water interface and subsequent attenuation prevents the signal from rising above the oceanic noise floor at long ranges.
Implications for Planetary Defense: Since hydroacoustic detection is inefficient, infrasound and seismic technologies remain the primary tools for characterizing oceanic airbursts. The lack of hydroacoustic detections does not imply a lack of events, but rather a fundamental physical limitation in energy transfer.
Future Work: The authors suggest that future detection might require:
- Events occurring at much lower altitudes (closer to the surface).
- Extremely energetic events (multi-kiloton to megaton).
- Advanced modeling of wave propagation through complex bathymetry and surface wave effects, which are currently simplified in this study.

In summary, the paper demonstrates that while the theoretical possibility exists, the practical reality is that the ocean acts as a highly effective shield against atmospheric shock waves, making hydroacoustic monitoring of bolides a low-yield strategy compared to infrasound.