Seismic sensors used to identify types of aircraft flying over Alaska

An array of seismic sensors deployed to capture aftershocks from the 2018 magnitude 7.1 Anchorage earthquake also collected distinctive signals from hundreds of flights crossing over Alaska.

In their study published in The Seismic Record, Isabella Seppi and colleagues at the University of Alaska Fairbanks show that these signals can be used to identify the type of aircraft, along with details such as the closest time, distance and speed of each plane or helicopter as it flew above the seismic array.

Acoustic waves generated by flying aircraft vibrate the ground below, transforming sound energy into ground motion that can be detected by seismic sensors.

While previous studies have used seismometers to record aircraft, Seppi and colleagues took this research a step further by identifying specific source frequencies in the seismic data that were associated with different aircraft types.

The researchers could then distinguish frequencies generated by piston, turboprop, jet and helicopter aircraft.

“Even though we can see clear signatures of aircraft in the data, it is still amazing to realize that the acoustic waves emitted by planes from kilometers up in the sky can move the ground,” Seppi said. “And these data are good enough to determine the flight parameters and the aircraft source frequencies.”

“In one case, we could detect the subtle difference in RPM [revolutions per minute] for a propeller plane ascending on a flightseeing tour of Denali versus descending,” she added.

Todd Rust, owner of K2 Aviation in Talkeetna, Alaska, confirmed the regular changes in RPM during the particular flightseeing tour that Seppi and colleagues saw in their seismic data.

The researchers used data collected from 303 seismic sensors in February and March 2019. The sensors were placed along the Parks Highway in central Alaska, between Nenana and Talkeetna.

“We were aware that these sensors, like microphones, could pick up signals from aircraft. And we know that Alaska is an exceptionally quiet place with exceptionally interesting aircraft, so it seemed like a great realm to explore,” Seppi explained.

Seppi and colleagues estimated flight parameters for 1216 known flights from 48 aircraft types. They were able to check their estimates using flightradar 24’s ground-truth data set of flight paths in Alaska that crossed the seismic array during the two months.

Seppi said the sensors recorded with high sample rates, which allowed the researchers to capture the high frequency signals generated by the aircraft. “An optimal sensor to record an aircraft is probably one that is installed right at the ground surface. Ours were installed into frozen ground, many under the snowpack,” she noted.

Seppi and colleagues say their research could show seismologists one way to remove the contaminating data of aircraft signals from seismic data used for earthquake monitoring. Scientists could also use these data to help quantify the impacts of aircraft noise on wildlife and people in a specific region.

With a more permanent seismic network and analysis methods like machine learning, the seismic approach might be able to detect specific flight paths and aircraft type in poor weather or an acoustically “noisy” environment, the researchers write in their study.