Peak sound pressure and sound exposure level from underwater explosions in shallow water

In-situ comparison of high-order detonations and low-order deflagration methodologies for underwater unexploded ordnance (UXO) disposal

The unexploded ordnance (UXO) on the seabed off Northwest Europe poses a hazard to offshore developments such as windfarms. The traditional removal method is through high-order detonation of a donor explosive charge placed adjacent to the UXO, which poses a risk of injury or death to marine mammals and other fauna from the high sound levels produced and is destructive to the seabed. This paper describes a sea-trial in the Danish Great Belt to compare the sound produced by high-order detonations with that produced by deflagration, a low-order disposal method that offers reduced environmental impact from noise. The results demonstrate a substantial reduction over high-order detonation, with the peak sound pressure level and sound exposure level being around 20 dB lower for the deflagration. The damage to the seabed was also considerably reduced for deflagration, although there was some evidence for residues of explosives related chemicals in sediments.

The time-frequency analysis of the acoustic signal produced in underwater discharges based on Variational Mode Decomposition and Hilbert-Huang Transform

The experiments of underwater discharges in an anechoic pool were carried out and analysis of the time-frequency characteristics of the acoustic signals was conducted based on Variational Mode Decomposition and Hilbert-Huang Transform (VMD-HHT). We propose a relative center frequency difference method to determine the decomposition numbers K which has to be given before the application of VMD and the result is satisfying. The HHT spectrum and marginal spectrum are obtained, then, some valuable conclusions are drawn. The high-frequency components of the acoustic signal are mainly attributed to the shock wave, and the low-frequency components mostly result from the bubble pulse. The frequency range of the acoustic signal is basically from 0 to 90kHz, and the ratio of energy in the low-frequency band(0-4kHz) to that of the total acoustic signal is up to 55.56%. Furthermore, this ratio versus gaps is also explored and it has the minimum at the gap of 1.5 mm which is the optimal gap for the peak pressure and radiated energy of the acoustic signal. Therefore, we can not obtain the maximum energy of the acoustic signal and the maximum ratio in the low-frequency band simultaneously.

Acoustic characterisation of unexploded ordnance disposal in the North Sea using high order detonations

Results are presented of acoustic measurements made during the disposal of 54 items of unexploded ordnance (UXO) in the North Sea during the pre-construction phase of two offshore windfarms. The disposals were conducted using high-order controlled detonation of donor charges placed on the seabed adjacent to the UXOs. The total charge masses ranged from 2.5 kg to 295 kg TNT equivalent, and acoustic measurements were made at ranges of 1.5 km to 58 km from the UXO. High-order detonations can present a risk of injury or death to marine mammals and other fauna from the high sound levels produced, and these results represent the largest data set of acoustic measurements ever assembled for publication. Acoustic measurements were also made on small scare charges, used as mitigation. The sound pressure pulses are presented with their spectra, and the levels of peak sound pressure and sound exposure are presented as a function of range from the source. Measured levels are compared to data from a shallow-water propagation model, and to widely-adopted exposure level thresholds used for marine mammals, illustrating the potential for injury at distances of several kilometres.

Physical effects of sound exposure from underwater explosions on Pacific mackerel (Scomber japonicus): Effects on non-auditory tissues

Underwater explosions from activities such as construction, demolition, and military activities can damage non-auditory tissues in fishes. To better understand these effects, Pacific mackerel (Scomber japonicus) were placed in mid-depth cages with water depth of approximately 19.5 m and exposed at distances of 21 to 807 m to a single mid-depth detonation of C4 explosive (6.2 kg net explosive weight). Following exposure, potential correlations between blast acoustics and observed physical effects were examined. Primary effects were damage to the swim bladder and kidney that exceeded control levels at ≤333 m from the explosion [peak sound pressure level 226 dB re 1 μPa, sound exposure level (SEL) 196 dB re 1 μPa² s, pressure impulse 98 Pa s]. A proportion of fish were dead upon retrieval at 26-40 min post exposure in 6 of 12 cages located ≤157 m from the explosion. All fish that died within this period suffered severe injuries, especially swim bladder and kidney rupture. Logistic regression models demonstrated that fish size or mass was not important in determining susceptibility to injury and that peak pressure and SEL were better predictors of injury than was pressure impulse.

Noise of underwater explosions in the North Sea. A comparison of experimental data and model predictions

An analysis is presented of sound measurements performed near two detonations of unexploded ordnance (UXO) in the North Sea, at distances ranging from 1.5 to 12 km. The charge masses of the detonations were 325 and 140 kg TNT equivalent. The objective of the measurements was to improve the accuracy of model predictions of the area where UXO detonations affect harbour porpoises in the North Sea. For the predictions, an explosion emission model is combined with a shallow-water propagation model. The prediction model was previously validated for distances up to 2 km. The measurements reported here allowed validation up to a distance of 12 km. The measured levels and spectra are well explained by the model calculations. The model results depend strongly on the sea sediment layering. The propagation of high-frequency components appears to be affected primarily by the silty top layer, while low-frequency components are affected also by deeper sandy layers. Measured and calculated noise levels are used to determine permanent-threshold-shift effect distances for harbour porpoises (Phocoena phocoena). Values ranging from 2 to 6 km are found for the two detonations.

Underwater acoustic characterisation of unexploded ordnance disposal using deflagration

The seabed off North West Europe contains much unexploded ordnance (UXO), posing a hazard to offshore developments such as windfarms. The typical removal method is through high-order detonation of a donor charge placed adjacent to the UXO. This method poses a risk of injury or death to marine mammals and other fauna from the high sound levels produced. This paper describes a controlled field experiment to compare the sound produced by high-order detonations with a low-order disposal method called deflagration, which uses a shaped charge of modest size, is less energetic, and offers reduced environmental impact from lower acoustic output. The results demonstrate a substantial reduction over high order detonation, with the peak sound pressure level and sound exposure level being more than 20 dB lower for the deflagration, and with the acoustic output depending only on the size of the shaped charge (rather than the size of the UXO).

Physical effects of sound exposure from underwater explosions on Pacific sardines (Sardinops sagax)

Explosions from activities such as construction, demolition, and military activities are increasingly encountered in the underwater soundscape. However, there are few scientifically rigorous data on the effects of underwater explosions on aquatic animals, including fishes. Thus, there is a need for data on potential effects on fishes collected simultaneously with data on the received signal characteristics that result in those effects. To better understand potential physical effects on fishes, Pacific sardines (Sardinops sagax) were placed in cages at mid-depth at distances of 18 to 246 m from a single mid-depth detonation of C4 explosive (4.66 kg net explosive weight). The experimental site was located in the coastal ocean with a consistent depth of approximately 19.5 m. Following exposure, potential correlations between blast acoustics and observed physical effects were examined. Acoustic metrics were calculated as a function of range, including peak pressure, sound exposure level, and integrated pressure over time. Primary effects related to exposure were damage to the swim bladder and kidney. Interestingly, the relative frequency of these two injuries displayed a non-monotonic dependence with range from the explosion in relatively shallow water. A plausible explanation connecting swim bladder expansion with negative pressure as influenced by bottom reflection is proposed.

Underwater noise level predictions of ammunition explosions in the shallow area of Lithuanian Baltic Sea

Among the noisiest man-made activities in the seas, emitting very high acoustic energy are the underwater explosions of various objects and ship shock trials. Sound energy emitted by high explosives can be predicted or measured at sea. Sometimes, it can be convenient to apply empirical formulas and scaling laws to approximate the energy of underwater explosions. In addition, at some instances the determination of the spectral properties of the explosions is useful, i.e. when possible animal exposure to impulsive noise has to be evaluated. This paper presents an example of an application of freely available scaling laws and equations for prediction of noise levels of underwater explosions of historical ordnance in the shallow sea environments. Main findings of the study: An available scaling laws applied to model underwater explosion properties; spatial extent of explosion mapped; arising issues of modelling of underwater explosions in the shallow marine areas discussed.

Acoustic monitoring of coastal dolphins and their response to naval mine neutralization exercises

To investigate the potential impacts of naval mine neutralization exercises (MINEX) on odontocete cetaceans, a long-term passive acoustic monitoring study was conducted at a US Navy training range near Virginia Beach, USA. Bottom-moored acoustic recorders were deployed in 2012-2016 near the epicentre of MINEX training activity and were refurbished every 2-4 months. Recordings were analysed for the daily presence/absence of dolphins, and dolphin acoustic activity was quantified in detail for the hours and days before and after 31 MINEX training events. Dolphins occurred in the area year-round, but there was clear seasonal variability, with lower presence during winter months. Dolphins exhibited a behavioural response to underwater detonations. Dolphin acoustic activity near the training location was lower during the hours and days following detonations, suggesting that animals left the area and/or reduced their signalling. Concurrent acoustic monitoring farther away from the training area suggested that the radius of response was between 3 and 6 km. A generalized additive model indicated that the predictors that explained the greatest amount of deviance in the data were the day relative to the training event, the hour of the day and circumstances specific to each training event.

Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed

Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1 m from the source as a function of TNT-equivalent charge weight.