New Embedded Hydrophone System Hears Underwater Explosions Hundreds of Kilometers Away

When an explosive charge detonates underwater, the blast does not stay quiet for long. The initial shock wave races outward, and the hot gas bubble left behind expands and contracts in a slow, rhythmic pulsation that can carry acoustic energy across entire ocean basins. By the time these very-low-frequency signals have traveled hundreds or even thousands of kilometers, their dominant frequencies have dropped from tens of hertz down to just a few. Capturing such faint, slow oscillations has long been a technical headache, because most commercial underwater acoustic equipment is optimized for the much higher kilohertz-to-megahertz bands. A team of Chinese researchers now reports a purpose-built embedded acquisition device that recorded real underwater explosion signals from more than 500 kilometers away, and their results suggest a practical path toward long-term, autonomous VLF monitoring.

The study, published in Frontiers in Sensors on 24 April 2026, was led by Xuexu Li and colleagues at Nanjing University of Information Science and Technology together with the State Key Laboratory of Chemistry for NBC Hazards Protection in Beijing. The motivation is straightforward: dedicated off-the-shelf equipment for very-low-frequency underwater explosion monitoring is scarce, and existing buoys and monitoring platforms struggle to maintain sufficient sensitivity and dynamic range in the VLF band, where signals are small in amplitude and easily buried under sea-state disturbances and background noise. Underwater explosion signals also span an enormous dynamic range, from the millisecond-scale pressure spike of the initial shock wave to the gentle, low-frequency bubble pulsations that follow, so any acquisition system must handle both extremes without saturating or losing the weak components.

The researchers addressed these challenges with a modular architecture that physically separates the sensor from the controller. At the front end sits a self-developed spherical piezoelectric hydrophone designed for deep-water operation down to 3000 meters, with an operating bandwidth of 1 to 1000 hertz. The hydrophone’s weak current output passes through a low-noise signal-conditioning chain built around an AD8220 instrumentation amplifier configured at unity gain, which acts primarily as a high-input-impedance buffer. A selectable transimpedance resistor, switchable between 10 kilohms and 1 megohm, converts the sensor current into a voltage and lets operators trade sensitivity against headroom depending on the expected signal amplitude. The single-ended sensor output is then converted to a differential signal, a step that improves common-mode interference rejection, which is particularly important when trying to tease microvolt-level VLF signals out of a noisy underwater electrical environment.

Digitization is handled by a 32-bit ADS1285 analog-to-digital converter, a chip prized in precision measurement circles for its excellent noise performance and wide dynamic range. The ADS1285 integrates a programmable gain amplifier and configurable digital filters, and in this design it runs at a data output rate of 2000 samples per second with an FIR filter whose passband extends from DC up to roughly 750 hertz. The converter is clocked by a 4.096 megahertz source with frequency stability on the order of parts per million, keeping the sampling rate deviation at 2000 samples per second within a fraction of a hertz over long deployments. An STM32F429 microcontroller orchestrates the whole pipeline, using direct memory access to shuttle samples into RAM buffers without blocking the processor, so continuous acquisition never stalls behind storage operations.

Timing is where many distributed monitoring systems fall apart, and the team tackled it with a hybrid synchronization scheme. Each device carries a BeiDou-based GNSS timing module, the UM980, whose UTC time messages and one-pulse-per-second output calibrate the microcontroller’s real-time clock at the hardware level. The embedded software, written in C, runs a five-state machine that waits for GNSS lock before starting, falls back to the local clock if the satellite signal times out, performs an initial high-precision calibration, then periodically resynchronizes during the main acquisition phase to correct residual drift. This ensures that multiple nodes deployed across a monitoring network share a consistent time base, which is essential for triangulating explosion locations and correlating signals between stations.

Storage posed its own engineering puzzle. Conventional SD cards offer limited power-loss protection and write stability, which is risky for unattended long-duration recording. The researchers instead chose an industrial-grade eMMC chip connected over a 4-bit MMC bus for higher write bandwidth, managed through the FATFS file system with segmented files and a cyclic overwrite strategy to avoid exhausting storage space. A write transaction protection mechanism reduces the risk of data corruption from unexpected power loss, and a triple-buffer scheme, with active, write-pending, and idle buffers rotating in sequence, decouples real-time sampling from asynchronous disk writes. When it comes time to retrieve the data, the device plugs into a computer over USB Type-C and enumerates as a standard mass storage drive, eliminating the need to disassemble the housing or pull memory cards in the field.

To validate the design, the team ran lake trials in an inland water area with an explosive source at a depth of about 11.3 meters. Two acquisition units, fitted with sensors S1 and S2, were suspended at horizontal offsets of roughly 40 and 100 meters from the source. Both recordings clearly captured the initial shock-wave response followed by a multi-cycle bubble pulsation sequence. The researchers extracted the first bubble pulsation period from each recording and compared it against a shallow-water-corrected version of Cole’s classical bubble dynamics model, which predicts the period from the explosive yield and detonation depth. Measured periods ranged from 0.437 to 0.484 seconds against a theoretical value of about 0.4945 seconds, with relative deviations between 2.12 and 11.63 percent, confirming that the digitized data are physically meaningful rather than electronic artifacts.

A head-to-head comparison with a reference hydrophone system under the same conditions revealed both a limitation and a strength. The reference instrument recorded a peak shock pressure of about 2.57 megapascals, while the prototype registered only 0.64 megapascals, a gap the authors attribute to the 2000-hertz sampling rate, which cannot fully resolve the millisecond-scale pressure peak of the shock wave. Yet the overall waveform structures were strikingly similar, and the first bubble pulsation periods extracted from the two systems, roughly 0.50 seconds for the reference and 0.48 seconds for the prototype, both agreed with the theoretical prediction of about 0.49 seconds. In other words, the device trades peak-fidelity for the continuous, low-noise, long-duration capability that VLF monitoring actually demands.

The most dramatic results came from open-sea trials, where acquisition units were deployed in the far field at ranges of 518 to 523 kilometers from 1-kilogram TNT-equivalent explosion events detonated at depths of 100, 300, and 1000 meters. At distances exceeding 520 kilometers, the system still reliably captured VLF signals with peak amplitudes between 0.70 and 0.94 pascals and dominant frequencies concentrated in the 55 to 75 hertz band, well above the system’s estimated equivalent acoustic noise floor of about 0.53 pascals RMS after filtering. Spectrogram comparisons with the reference hydrophone showed consistent temporal structures and matching dominant spectral peaks across all three explosion depths, even though absolute amplitudes differed due to sensor sensitivity and installation differences. The recorded RMS pressures ranged from 0.09 to 0.11 pascals, and waveform correlation confirmed that the prototype reproduced the key temporal and spectral fingerprints of each event.

The researchers conclude that their embedded system delivers low-noise acquisition, adequate electrical dynamic range, reliable GNSS-based time synchronization, and long-term autonomous storage, meeting the engineering requirements of VLF underwater acoustic monitoring, explosion effect assessment, and underwater acoustic experiments. Beyond its immediate application to monitoring specialized underwater explosions, the architecture offers a template for any scenario where faint, slow ocean signals must be recorded continuously and autonomously, from hydroacoustic verification networks to studies of submarine volcanoes and long-range propagation environments. As demand grows for persistent, distributed ocean listening posts, this work demonstrates that the quietest sounds in the sea can now be captured with hardware compact enough to survive the deep.

Subject of Research: Embedded very-low-frequency underwater acoustic acquisition technology for monitoring underwater explosions

Article Title: Embedded very-low-frequency underwater acoustic acquisition technology and experimental study for underwater explosion monitoring

Article References: Li, X., Song, Z., Pang, X., Yang, F., Lu, Y., Chang, Y., Yin, H., & Liu, Y. (2026). Embedded very-low-frequency underwater acoustic acquisition technology and experimental study for underwater explosion monitoring. Frontiers in Sensors, 7, Article 1753123. https://doi.org/10.3389/fsens.2026.1753123

Image Credits: AI Generated

DOI: 10.3389/fsens.2026.1753123

Keywords: underwater acoustics, very-low-frequency signals, underwater explosion monitoring, hydrophone, embedded systems, GNSS synchronization, ocean monitoring, Embedded, very-low-frequency, underwater, acoustic, acquisition