High explosives detonated underwater have been used since the Second World War as a powerful compact impulsive source for echo ranging, oceanographic research, and for military activity. In the early days of seismic surveying they were used as a sound source; they have now been completely superceded by airguns in this application. Their use in both commercial and military applications is generally becoming less common due to both environmental considerations and safety.
TNT and other nitroglycerine based "high" explosives explode by a detonation process, where a violent chemical reaction following in the wake of the shock front propagating through the explosive turns the solid of the explosive into a gas at extremely high pressure. The velocity of detonation of high explosives is about 5,000 to 10,000 ms-1, and a shock wave that propagates in all directions is produced in the medium. The main features of the shock wave from TNT fired under water are:
As a result of military interests, the pressure wave from unconfined TNT detonated underwater is very well documented. Barrett quotes the formula below. "Impulse" may be defined as the average pressure level of the wave multiplied by its duration. Its use arises from the fact that for many consequences of the wave (such as injury) both the level of the wave and its duration are significant, and hence impulse is a better indication of its severity than peak pressure.
The Source Level can be calculated from the above as:
Thus for a 1 kg charge, the Source Level is 274 dB re 1 µPa.
In propellants such as black powder, the explosion process is one of deflagration, or burning, rather than detonation and occurs at a much lower velocity than for high explosives, of about 5 ms-1. The deflagrating material gives rise to a relatively low, broad pressure peak. Although the pressure from propellants is much lower than for high explosives, the pressure wave is also of much greater duration than that for high explosives.
Increasing use is being made of propellants for underwater engineering. Needham reports on measurements of the pressure wave from bolt guns. Bolt guns operate on the principle of detonating a small explosive charge in the end of a barrel; this forces a bolt out of the barrel and into a target at high speed. In addition to achieving their objective of securing the bolts to a substrate that they are fired into, bolt guns may generate high levels of underwater impulsive noise. The pressure waves typically have a rapid initial rise to a peak pressure of about 10 kPa (equivalent to a Source Level of 194 dB re 1 µPa at 1 metre) followed by a well-damped low frequency oscillation. The spectra are dominated by a broad peak in level over a low frequency band from about 10 Hz to 100 Hz. A typical example of the pressure impulse from a bolt gun is shown in the figure below; in this case the Source Level is rather higher at about 200 dB re 1 µPa at 1 metre.
Pressure waves are substantially modified when underwater blasting involves the use of charges buried in boreholes to fragment rock prior to dredging. As a consequence of laboratory measurements, Nedwell notes that peak pressure is reduced substantially, to 5% or so of the value for freely suspended charges, and impulse to about 30%. No bubble pulses occur. However, the duration of the shock wave is increased tenfold, typically to 1-2 ms. The rise time of the wave is also greatly extended to the order of a millisecond.
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