Scientists from the Institut Langevin in Paris and TU Wien in Vienna have developed a new technique for detecting hidden objects, such as buried treasure or submarines, using complex acoustical physics to differentiate them from the obscuring material.
The researchers believe the acoustic detection technique could be applied to other remotely sensed characteristics of obscured or hidden objects, such as temperature and pressure, and could also be adapted to sensors that detect light wavelengths like those used in medical imaging instead of sound.
Although several acoustic-based technologies for detecting obscured objects using sound waves, such as sonar and ultrasound, already exist, they all suffer from the breakup of sound as it travels through a medium like water, sand, or human tissue. Called multiple scattering, this phenomenon describes how sound waves bouncing off the targeted object become repeatedly reflected off the material between the sensor and the object, until they become virtually indistinguishable from the resulting acoustic noise.
“Instead of the object, all you see is a diffuse fog – this is a fundamental problem of imaging techniques, from sonar in submarines to imaging techniques in medicine,” explained study co-author Lukas Rachbauer in a statement announcing the team’s research.
The TU Wein-led team, curious about whether they could differentiate hidden objects from the surrounding material and locate them with a high degree of accuracy if their acoustic signal was known ahead of time, conducted several experiments. First, the researchers selected several metal spheres about the size of marbles. Next, the spheres were bombarded with sound waves, and the return signal was measured to create a “fingerprint matrix” of the unique way this signal becomes scattered before reaching the acoustic sensor.
After recording this acoustic fingerprint of the metal spheres, they were buried in a bed of “sand” made from tiny glass beads. Once again, the team bombarded the sphere-sand mixture with sound waves. Because the spheres were hidden from view by the sand, which is notorious for its acoustic signal scattering properties, the research team collected what would typically be considered an essentially valueless return signal.
“When ultrasonic waves are sent into this sand, they are scattered by the sand, but some of the sound penetrates so far into the sand that it is also scattered by the buried object,” Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien explained. “We cannot see the object, but the backscattered ultrasonic wave that hits the microphones of the measuring device still carries information about the fact that it has come into contact with the object we are looking for in the sand.”
After examining the data collected by the acoustic signature with a series of algorithms, the team compared the data with the recorded fingerprint matrix of the metal spheres. As hoped, the team was able to effectively isolate the fingerprint of the metal balls from the surrounding beads, confirming they were buried. The team was also able to effectively approximate the position of both hidden objects with a high level of accuracy that had eluded previous methods.
“From the correlations between the measured reflected wave and the unaltered fingerprint matrix, it is possible to deduce where the object is most likely to be located, even if the object is buried,” Professor Rotter explained.
When discussing potential applications, the team noted that multiple scattering can significantly limit medical imaging, making it unusually difficult to isolate and diagnose injuries or pathology in soft tissue, such as brain matter or muscle fibers. However, the researchers note that if a radiologist or other specialist knows the acoustic fingerprint of healthy tissue and pathology in advance, this could significantly improve diagnostic accuracy by revealing hidden objects or damage that is otherwise obscured by healthy surrounding tissue.
“This would be particularly exciting, for example, in the measurement of the human brain, where waves have to penetrate the highly scattering skull,” the team explained.
The researchers also noted that knowing the acoustic signal of a submarine before trying to detect it from long range using sonar waves could improve accuracy in object identification and location. The same approach could help treasure hunters “correlate” the acoustic fingerprint matrix of desired objects to differentiate them from dirt or other undesired material.
In the study’s conclusion, the team noted that signal fingerprinting could be adapted for use on light-sensing platforms as well as sound-based sensing systems. For example, RADAR signals that struggle to detect hidden objects in clouds can use a targeted object’s fingerprint to identify and locate it with a high level of accuracy. They also proposed adapting the technique to other remotely detectable signals that suffer from scattering, like pressure and temperature.
“The concept of the fingerprint matrix is very generally applicable – not only for ultrasound, but also for detection with light,” the professor said. “It opens up important new possibilities in all areas of science where a reflection matrix can be measured.”
The study “Detection and characterization of targets in complex media using fingerprint matrices” was published in Nature Physics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.