Keith talks with Benjamin Drenth, research geophysicist with the United States Geological Survey. Drenth studies passive geophysics. Seismic, or active, geophysics involves using outside energy sources needed to create sonic waves through the earth. Whereas passive geophysics is a more subtle measuring of the earth's gravitational or magnetic fields. https://profile.usgs.gov/bdrenth
Aired April 19, 2015.