How would an explosion be dampened

Underwater explosion tests

Recording of underwater pressure waves during a blast.

With air bubbles against World War II mines: Blast tests in the Baltic Sea make high demands
Joachim Hachmeister (for LTT Labortechnik Tasler GmbH)

After 1945 many thousands of tons of ammunition were also sunk in coastal areas of the Baltic Sea and still pose a threat to shipping today. Salvaging these huge amounts of explosives is often too risky, so that only a controlled detonation is possible. However, the detonation of several hundred kilograms of explosives under water creates shock waves that can be life-threatening for humans and animals. Marine mammals in particular are extremely endangered here. Against this background, Kiel researchers [1] have now investigated whether and to what extent the underwater pressure waves triggered by the explosions can be dampened by artificially created air bubble curtains. The “Defense Technology Service for Ships and Naval Weapons, Maritime Technology and Research” (WTD 71 for short) in Eckernförde on the Baltic Sea is one of ten defense technology and defense science services in the Bundeswehr. Today it covers the entire range of maritime defense technology in all phases of development and testing. In the research area for water-borne noise and geophysics, which is located in Kiel, this also includes investigations into the dampening effect of a bubble curtain on explosion shock waves, with the aim of removing old ammunition from the Second World War in the most environmentally friendly way possible. This work is carried out in cooperation with the Office for Disaster Protection of the State of Schleswig-Holstein, which is responsible for the disposal of ordnance.

For this purpose, experts from WTD 71, with the practical support of the company Hydrotechnik (Lübeck), have undertaken blasting tests in which a bubble curtain is created around the site of the explosion with the help of circular, perforated tubes on the sea floor.

While the first experiments (pictures 1 and 2) were still experimenting with three concentric tubes, they are now limited to a single tube from which an air flow of a total of 40 cubic meters of air per minute emerges. A ship anchored at a safe distance serves as the measuring platform. From there, an on-board crane is used to position six hydrophones at four different depths to measure the sound pressure. In the measuring system on board, the sound signals are first pre-amplified and pass through a 50 Hz high-pass filter before they are recorded by a transient recorder and then evaluated.

For these special tests, the research team has modern measuring systems at their disposal with the transient recorders of the LTT-184 (Fig. 3) and LTT-186 series, which meet the high demands on safety and robustness. These front-ends, developed and produced by LTT Labortechnik Tasler, a specialist in ultra-fast measurement technology from Würzburg, expand the range of conventional PC measurement technology to previously unattainable dimensions. The maximum sampling rate per channel is between 2.5 MHz for 16 bits and up to 20 MHz for 12 bits, depending on the desired resolution. A single device offers up to 16 differential inputs. Since the devices can be cascaded, synchronized acquisition is also possible with many more parallel channels.

Separate A / D converters and amplifiers for each input offer simultaneous sampling of all channels and channel-specific amplification with input ranges between * 1 volt and * 50 volt (optional: ± 10 to ± 200V). Each input has an adaptive anti-aliasing filter. To further simplify measurements in the future, LTT now also offers the universal measuring amplifier LTT-500. This can supply up to 8 sonar sensors with 20 volts (or 30mA). In combination with the transient recorder LTT-184/186, the sensor signals are amplified up to 1660 times with a bandwidth of 1MHz. The transient recorder is connected to the PC via SCSI, USB or Ethernet - if necessary for longer distances via fiber optic cables. When a PC is connected, the signals can also be displayed and checked remotely online. With the LTTview software, the acquisition, reproduction and analysis of the measurement data is considerably simplified. The extensive trigger function defines the start of data acquisition. The online math enables an initial assessment of the measurement while it is being recorded. The direct storage of the measurement data in file formats such as Famos, Diadem or National Instruments TDM allows direct further processing of the measurement data.

In order to be absolutely sure, the recorded measurement data are first temporarily stored in the device - either in a high-speed RAM of up to 512 megabytes or on an integrated, shock-resistant hard disk with a memory depth of up to 40 gigabytes. This ensures that the measurement data is reliably recorded even under the toughest operating conditions and also in the "worst case" - in the event of a possible loss of the connection to the PC.

Figure 4 shows a typical pressure curve for the detonation of a 300 kg mine, measured at a distance of about 800 meters. Very high sampling rates of up to 2.5 MegaSamples per second and channel are required to ensure sufficient signal bandwidth. This is necessary on the one hand to evaluate the short pressure peaks typical of explosions, and on the other hand to be able to analyze the damping effect up to the range of 100 kHz.

The harbor porpoises, which are threatened in the population in the southern Baltic Sea, react very sensitively to this frequency range. They use - like the dolphins related to them - a kind of "ultrasonic location system" that works similar to the sonar known from submarines. Figure 5 shows the third octave spectra of an undamped explosion and two explosions with an air bubble curtain. These first results show that no significant attenuation can be found in the lower frequency range up to 1 kHz, while attenuation averaging about 4 dB can be achieved above this. The damping effects are initially less than expected on the basis of other experiments. This is attributed to the fact that with such large explosive charges, considerable amounts of water are simply displaced by explosive gases, which of course affects the spread of pressure. Further experiments are to follow, with the primary aim of varying the diameter of the bubble curtain.

[1] E. Schmidtke, B. Usefulel and S. Ludwig: "Risk mitigation for sea mammals - The use of air bubbles against shock waves", Proceedings of the International Conference on Acoustics "NAG / DAGA 2009", Rotterdam, 2009, pp. 269-270 [2] Pictures 1 and 2 with the kind permission of CompAir Air Technology GmbH, Simmern.