Worldwide traffic/activiity reduction yields a measurably lower seismic background noise. https://www.nationalgeographic.com/science/2020/04/coronavirus-is-quieting-the-world-seismic-data-shows/ Could this relative "quiet" be useful in detecting underground noise sources?    Similarly, the power outages in California last year may have enhanced the detection of underground electrical activity. Blowers, generators, light rail...