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.
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). Deflagration can reduce the lethal radius for fish
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. The future of UXO disposal lies not in
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. high-frequency collapse of a detonation bubble