Underwater Acoustic Characterisation Of Unexploded Ordnance Disposal Using Deflagration Online

Acoustic characterisation further reveals a crucial spectral shift. While detonation deposits energy uniformly across a wide band (10 Hz to >100 kHz), deflagration concentrates its energy in the low-frequency regime, typically below 500 Hz. This frequency content is governed by the bubble pulse—the oscillation of the hot gas bubble created by the deflagration. Unlike the violent, high-frequency collapse of a detonation bubble, a deflagration bubble undergoes slower, larger-amplitude oscillations. For marine mammals, this low-frequency bias is a double-edged sword. Many baleen whales communicate in these low frequencies, meaning deflagration could potentially mask vocalisations over long distances. However, the lack of high-frequency energy is beneficial for smaller cetaceans and fish, which are often more sensitive to frequencies above 1 kHz. Moreover, the low frequencies attenuate more slowly in water, but because the absolute source level is lower, the overall radius of impact for physiological harm is dramatically reduced.

Deflagration, in contrast, is a subsonic combustion process. The ordnance casing is deliberately perforated or breached, allowing the energetic material to burn rapidly rather than explode. The reaction front moves slower than the speed of sound in the material, meaning no shockwave is formed. Instead, the rapid gas generation pressurises the interior until the casing fails, releasing energy over tens to hundreds of milliseconds. Consequently, the underwater acoustic signature is fundamentally different. Experimental characterisation—using hydrophone arrays at calibrated ranges—shows that deflagration produces a non-impulsive, quasi-continuous pressure pulse with a significantly longer rise time (milliseconds vs. microseconds). Peak sound pressure levels (SPLs) are typically 20–40 dB lower than an equivalent detonation, bringing them closer to the range of natural sounds or large ship noise rather than seismic events. Unlike the violent, high-frequency collapse of a detonation

In conclusion, the underwater acoustic characterisation of UXO disposal using deflagration paints a picture of a promising but nuanced technology. It successfully replaces a single, catastrophic shockwave with a longer, lower-frequency rumble. The shift from impulsive to non-impulsive sound, the dramatic reduction in peak pressure, and the confinement of most energy to low frequencies represent a major advance in marine environmental protection. Deflagration can reduce the lethal radius for fish barotrauma from kilometres to tens of metres. However, the characterisation also cautions against complacency: the low-frequency energy and longer duration may still cause behavioural disturbance in sensitive marine mammals over wide areas. Therefore, deflagration should not be seen as a silent panacea, but as a tool that, when combined with careful site-specific acoustic modelling and real-time passive acoustic monitoring, allows for the safer, more environmentally responsible remediation of our underwater UXO legacy. The future of UXO disposal lies not in brute force, but in controlled energy release, and acoustic characterisation is the key to unlocking that future safely. However, the lack of high-frequency energy is beneficial

Despite its advantages, the acoustic characterisation of deflagration reveals several operational challenges. First, the process is less predictable than detonation. Variations in casing thickness, age of the explosive, and venting geometry cause shot-to-shot variability in the acoustic output. This uncertainty complicates risk assessment for protected species. Second, the longer duration of the acoustic event means that mitigation measures (e.g., marine mammal observers, passive acoustic monitoring) must be maintained for a longer window. Third, while the peak pressure is lower, the low-frequency bubble pulse can travel long distances with little attenuation, potentially disturbing species like the North Atlantic right whale over many kilometres, albeit without causing direct injury. to validate deflagration as a viable

To understand the acoustic benefits of deflagration, one must first contrast it with the physics of detonation. A high-order detonation involves a supersonic reaction front that generates a discontinuous pressure wave—a shock. In water, which is nearly incompressible, this shock propagates with devastating efficiency. The key acoustic parameters of a detonation are extremely high peak pressure (often exceeding 200 dB re 1µPa at 1m), a very short rise time (microseconds), and a high-amplitude, broad-frequency spectrum extending into ultrasonic ranges. This impulsive sound is particularly harmful to marine life, causing barotrauma (tissue damage from pressure changes), temporary or permanent hearing loss, and behavioural disruption over vast areas (tens of kilometres).

The practical acoustic characterisation of deflagration involves not just measuring pressure, but also derived metrics relevant to environmental regulation. Key metrics include Sound Exposure Level (SEL), which integrates the total acoustic energy over time, and peak-to-peak pressure. For a detonation, the SEL is concentrated in a few milliseconds; for deflagration, the same or lower total energy is spread over a longer duration. This results in a lower instantaneous peak pressure but a potentially comparable cumulative SEL at close range. Therefore, a comprehensive characterisation must assess the risk of behavioural disturbance (e.g., avoidance of feeding grounds) versus physical injury. Studies using caged fish and acoustic tags have shown that while fish may startle at the onset of deflagration, they rarely exhibit the lethal barotrauma (swim bladder rupture) common after detonations.

The legacy of past conflicts and military training exercises is a hidden hazard lying silent on the seabed: unexploded ordnance (UXO). Millions of tons of shells, bombs, and mines contaminate marine environments worldwide, posing significant risks to human safety, offshore construction (e.g., wind farms, pipelines), and marine ecosystems. Traditional disposal methods, such as high-order detonation using donor charges, are effective but increasingly controversial. They generate intense shockwaves, devastating acoustic trauma to marine mammals, fish, and invertebrates. In response, the defence and environmental communities have turned to low-order deflagration—a rapid, controlled burning rather than a supersonic explosion. However, to validate deflagration as a viable, quieter alternative, a rigorous underwater acoustic characterisation is essential. This essay argues that the acoustic signature of deflagration is fundamentally distinct from that of detonation, characterised by lower peak pressures, a shift in energy to lower frequencies, and a longer rise time, making it a potentially transformative but still challenging technology for UXO disposal.