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A city’s unique magnetic signature can reveal its state of health

Cities are noisy, busy places that burst with activity. And wherever that activity is associated with the use of electricity, the flow of current through electrical cables and wires induces magnetic fields. So the patterns of induced magnetism in a city reflect the characteristics of the power network, as well as the usage of electricity by the city’s industries, businesses and residents. 

In a recent study of the magnetic fields of two United States cities, researchers from the U.S. and Germany investigated the kinds of information that can be extracted from magnetic field sensors in various parts of a city. They collected magnetic field data continuously during a four-week period, using synchronized measurements with a network of sensitive magnetometers which were placed at locations such as elevators in high-rise apartment blocks, subways, bridges and sidewalks. 

“A city is viewed as a physical system akin to a distant astronomical object that can be studied using a variety of multispectral techniques,” said Vincent Dumont, from Lawrence Berkeley National Laboratory. “In short, our project was inspired by our desire to apply what we learned practicing fundamental physics research to the study of cities.”

The researchers compared the magnetic footprint of two very different U.S. cities, Berkeley, California, and the Brooklyn borough of New York City. They propose that examining a city’s magnetic footprint can be used to monitor the health of that city, specifically in terms of optimizing its efficient use of energy and as a possible early warning system for trouble with pollution. Magnetic field activity from various sources in the city can provide insight into what is going on during a 24-hour period.

In the results of the study, published today in the Journal of Applied Physics, the researchers found that Berkeley reaches an almost zero level of magnetic field activity during the night, while Brooklyn’s magnetic activity continues throughout the day and night. 

“Again, not too surprisingly, we discovered that ‘New York never sleeps,’ or more seriously, that there are indeed a number of magnetic signatures specific to each city,” said Dumont. 

“This work builds on our earlier experiments conducted around the city of Berkeley, in the San Francisco Bay Area,” he explained. “We identified the dominant sources of magnetic signals – which, not too surprisingly, turned out to be the trains of the Bay Area Rapid Transit (BART) system, and learned to glean weaker signals from this dominant background.”

In recent years, the approach of studying cities by their magnetic ‘noise,’ as measured by magnetic field sensors, has become more popular and has been pursued in particular by the Center for Urban Science and Progress in New York. The researchers hope their network magnetometry and smart data analysis combination can become a valuable tool for future multidisciplinary studies in urban science. 

“We hope this line of research will be picked up and further developed both by the members of our team as well as others, hopefully within cities around the world,” said Dumont.

Techniques using data from magnetic field sensors are particularly useful for fundamental physics research, such as the Laser Interferometer Gravitational Wave Observatory experiment for the detection of gravitational waves or the Global Network of Optical Magnetometers for Exotic Physical Searches collaboration for dark matter searches.

By Alison Bosman, Staff Writer

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