Underwater blasting tests

Recording underwater pressure waves during a blast.

For these special experiments, the research team has modern measuring systems available in the form of the LTT24 series transient recorders (Fig. 3), which meet the high demands for reliability and robustness. Developed and produced by Labortechnik Tasler, the Würzburg-based specialist in ultrafast and high-resolution measurement technology, these measuring instruments extend the bandwidth of conventional PC measurement technology to previously unattainable dimensions. Depending on the desired resolution, the maximum sampling rate per channel ranges from 2.5 MHz at 16 bits to up to 20 MHz at 12 bits. 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.

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

After 1945, many thousands of tons of munitions were also dumped in areas near the coast of the Baltic Sea and still pose a threat today, especially to the environment and maritime sping. Salvaging these huge quantities of explosives is often too risky, so that only controlled detonation is possible. However, the detonation of several 100 kilograms of explosives underwater generates shock waves that can be life-threatening for humans and animals. Marine mammals in particular are extremely endangered here. Against this background, researchers from Kiel [1] have now investigated whether and to what extent the underwater pressure waves triggered by the explosions can be attenuated by artificially generated air bubble curtains. The "Defense Technology Center 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 or defense science centers of the German armed forces. Today, it covers the entire range of maritime defense technology in all phases of development and testing. In the research area for waterborne sound and geophysics, which is based in Kiel, this includes studies on the damping effect of a bubble curtain on explosion shock waves, with the aim of disposing of munitions waste from World War II in the most environmentally friendly way possible.

This work is being carried out in cooperation with the Office for Disaster Control of the State of Schleswig-Holstein, which is responsible for explosive ordnance disposal.

 To this end, experts from WTD 71, with practical support from the company Hydrotechnik (Lübeck), have undertaken blasting tests in which a bubble curtain is created around the explosion site using circular, perforated tubes on the seabed.

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

Separate A/D converters and amplifiers for each input provide simultaneous sampling of all channels and channel-specific gain with input ranges between * 1 volt and * 50 volts (optional: ± 10 to ± 200V).  Each input features an adaptive anti-aliasing filter. To further simplify measurements in the future, LTT now also offers the LTT-500 universal measurement amplifier.This can supply up to 8 sonar sensors with 20 volts (or 30mA).In combination with the transient recorders 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 required for longer distances also via fiber optic cable. With the PC connected, the signals can be displayed and monitored online, even remotely. The LTTview software considerably simplifies the acquisition, playback and analysis of measurement data. The extensive trigger function defines the start of data acquisition. The online mathematics allows an initial evaluation of the measurement while it is still being recorded. Direct saving of the measurement data in file formats such as Famos, Diadem or National Instruments TDM allows direct further processing of the measurement data.

To be absolutely sure, the recorded measurement data is 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 up to 40 gigabytes of storage depth.
This ensures that the measurement data is reliably recorded even under the toughest operating conditions and also in the "worst case" - in the event that the connection to the PC is lost.

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.On the one hand, this is necessary in order to evaluate the short pressure peaks typical of detonations, and on the other hand, in order to be able to analyze the attenuation effect up to the range of 100 kHz.

Harbor porpoises, which are endangered in the southern Baltic Sea, react very sensitively to this frequency range.
Like the dolphins, which are related to them, harbor porpoises use a kind of "ultrasonic localization system" that functions similarly to the sonar known from submarines. Figure 5 shows the third-octave spectra of an undamped blast and two blasts with an air bubble curtain.These first results show that in the lower frequency range up to 1 kHz no significant attenuation can be detected, while above that attenuations of about 4 dB on average can be achieved.The attenuation effects are initially smaller 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 pressure propagation. Further experiments are to follow, mainly varying the diameter of the bubble curtain.

Source references:
[1] E. Schmidtke, B. Nützel 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] Figures 1 and 2 courtesy of CompAir Drucklufttechnik GmbH, Simmern.