Helicopter with tourists flying over the lavafalls at Fimvorduhals, Iceland

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Earthquake detectors can pick up helicopters, too

Hush-hush intelligence agencies might want to rejoice, as researchers reveal a counterintuitive new way to track airborne objects—using sensors designed to record earthquakes underground. Geophysicists in Iceland were studying volcanic tremors—rapidly repeating mini-earthquakes thought to come from magma moving underground—when they noticed something odd. Looking at data from Hekla Volcano, they found one tremor that arose, not from the volcano, but from a passing helicopter. As their rotor blades spin, helicopters create a series of closely spaced pressure pulses that travel through the air to the ground below. These pulses appear merged as a single tremor, which can be picked up by seismometers up to 40 kilometers away, depending on topography and wind. Seismic tremors have already been recorded for other phenomena—including floods, mud flows, and icebergs—but this is the first time they have been detected for an aircraft, the researchers report in Geophysical Journal International. To explore the signals further—and ensure helicopters aren’t mistaken for volcanoes—the team recorded tremors from a helicopter flying through a network of seven seismometers. By measuring the changing frequency of the signals as the chopper passed each one, the researchers determined the approximate location, altitude, speed, and flight direction of the craft. As most choppers don’t vary rotor speed during flight, the researchers were even able to use tremor frequencies to deduce the number of rotor blades and their revolution speed. With enough information on different helicopter specs, it might be possible to determine the model of chopper, the researchers say. And though the study looked only at helicopters, in principle the new approach could also track any rotor- or propeller-based craft, including drones.