Electrochemical Sensing: Reliably Detecting Explosives in the Ocean
Examples of this are innovative electrochemical sensors developed by the Fraunhofer Institute for Chemical Technology ICT in Pfinztal near Karlsruhe. Sensor systems engineer Sebastian Geiger clarifies: “I like to compare our system to a bloodhound that can detect explosives underwater.” Any suspicious find could then be quickly investigated. The electrochemical nose is currently still mounted on a rover, a remotely operated underwater vehicle. However, it can also be mounted on autonomous underwater vehicles such as the manta ray to complement the existing sensor system. It must be as small as possible but also extremely robust in order to fit as much as possible onto a support system. “The underwater environment places similar demands on sensor technology as in space, perhaps even greater ones,” says Geiger. Saltwater is corrosive and contains suspended particles that can contaminate electrode surfaces and block lines. Sometimes there are strong currents, and there are significant fluctuations in pressure and temperature.
Underwater Sensors Put to the Test: Salt Water, Currents, and High Pressure
Even so, the sensor system developed by Fraunhofer ICT performed well in practical testing in the Bay of Kiel. Here, just two kilometers from the beach in the Baltic Sea resort area of Heidkate, lies one of the largest officially designated munitions dumping sites. After World War II, mines, grenades, bombs and small arms ammunition were disposed of en masse in the Kolberger Heide dumpsite. The intention was to render Germany’s stockpiles of weapons unusable as quickly and irreversibly as possible. Geiger reports: “The seabed is contaminated with munitions over an area of about 1,200 hectares.” The researchers systematically approached mines with their rover in this restricted area. It is important here, says Geiger, that the electrochemical sensor mounted at the front of the underwater vehicle approach the target against the current. “A bloodhound is best able to detect scents carried to it by the wind.”
Sniffing out TNT underwater
Geiger and his team use the cyclic voltammetry method for chemical analysis. “Many substances have their own electrochemical fingerprint,” explains Geiger. This can be measured by cyclic voltammetry.” The researchers do this by applying a constantly changing electrical voltage to the water sample that is automatically pumped into the sensor head, while simultaneously measuring the current with high precision. Electrical voltages can cause chemical substances to absorb or release. The more electrons migrate back and forth, the stronger the current. The measured current/voltage curve is characteristic for each material.
The electrochemical sensor can detect TNT and other known powerful military explosives such as RDX or HMX at concentrations down to few micrograms per liter. The unit can take a sample roughly every five minutes for over twelve hours without having to resurface. “This would enable surveying and monitoring of larger underwater areas at various measurement points,” says Geiger. The research team is currently working on further improving the sensitivity of the sensors.
Why Sonar Is Indispensable in the Search for Old Ammunition
However, the electrochemical bloodhound can only detect explosives if they are already leaking and are thus present in the water. Geiger explains: “It’s like the human body. We don’t use only our sense of smell but also other senses, such as hearing, i.e. sonar, to better perceive the situation.”
Fraunhofer IGD researcher Sascha Krohmann adds: “Sound waves from sonar travel very well through water. If they are powerful enough, they can even penetrate the seabed.” If they encounter objects along the way, the sound waves are reflected and picked up again by underwater microphones known as hydrophones. Depending on the frequency used, the strength and characteristics of the echo can be used to determine the location, material, size or surface structure of the object.
Krohmann and his team are currently collaborating with Danish partners to test equipment including an extremely powerful side-scan sonar, not in the UXO fields this time but off the Danish coast. It is incorporated in an innovative device equipped with numerous other sensors and towed across the seabed by a specialized ship. “This sonar gives us a high-resolution scan of the area around the ship for about 200 meters in every direction, twice as far as with conventional equipment,” says Krohmann. It is combined with a multibeam sonar the researchers use to map large areas of the seabed before each search operation and can already detect larger objects and mounds in the sediment. Finally, sub-bottom sonar provides a view beneath the seabed, as does a magnetometer that complements the sonar systems by detecting magnetic objects even behind the edges of inclines or in murky water where sonar images are often difficult to interpret.
Artificial Intelligence in Explosive Ordnance Disposal: Automatically Classifying Unexploded Ordnance
“We want to combine and analyze all the data we are collecting here and have already collected in the UXO gardens to narrow down the number of suspicious objects as much as possible,” says Krohmann. AI-supported analysis tools should help here. In the future, these tools could determine not only whether clearance is necessary but also the type of grenade or mine, how severely damaged it is and whether it is still capable of exploding. Krohmann and his team have already created 3D images of roughly 20 percent of the unexploded ordnance, with which they are training their AI models. The images also show how the ammunition changes as it corrodes or if parts are already missing. “The more data we have, the more accurate we become,” says Krohmann, “and the faster we can rid our oceans of these toxic and hazardous pollutants.”
Fraunhofer-Gesellschaft
