In this case, the researchers from Scripps Institution of Oceanography got access to a network of 5,300 geophones (instruments that pick up vibrations in the earth) located in Long Beach, California. The geophones were originally installed as part of an oil and gas survey in the area, but the scientists figured out that in addition to being able to map out likely locations of resources, the network was also able to pick up on the vibrations caused by human transportation.
The authors write that "by using mostly standard signal processing, we can follow a metro schedule, count aircraft and their acceleration on a runway, and even see larger vehicles on a 10-lane highway." Future uses for this technology could include monitoring traffic, or, from a pure research standpoint, getting another level of data on how cities work by mapping out the vibrations in a city.
source: popsci.com By Mary Beth Griggs