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[China Instrument Network Instrument Development] Secret intelligence agencies may be happy because researchers have just proposed a new method for tracking air targets using sensors for recording underground earthquakes.
Iceland geophysicists noticed some strange things when they were studying the volcanic vibrations. Usually magma moves underground and produces some rapid mini earthquakes. The researchers analyzed the Hekla volcano data and found that there was a vibration not caused by the volcano but by a helicopter.
Due to the rotation of the rotor blades, the helicopter produced a series of intensive pressure pulses and drew underground from the air. These pulses appear to have formed a shock and were captured by a seismic detector 40 kilometers away under the influence of terrain and wind.
The researchers said that seismic vibrations caused by floods, mudslides, and glaciers have been recorded, but this is the first time that an aircraft has detected a shock. Related papers were recently published in the International Journal of Geophysics.
In order to further explore the relevant signals and ensure that helicopter signals are not misidentified volcanic signals, the researchers recorded the vibrations of a helicopter flying over a network of seven seismic detectors. By measuring the frequency change of the signal caused by the helicopter flying over each detector, the researchers determined the approximate azimuth, altitude, speed, and direction of flight of the aircraft.
Although most helicopters cannot change the rotor speed, researchers are still able to use vibration frequencies to infer the number and speed of rotor blades. As long as they have enough information on different helicopter specifications, researchers may determine the model of the helicopter.
Although the study only focused on helicopters, theoretically this new method can track any rotating or propeller aircraft, including drones.
(Original Title: Seismic Detection Equipment Can Track Helicopters)