Seismic tomography is a relatively new technique that looks into the Earth’s interior.
This method is similar to medical ultrasound, in which doctors use high frequency sound through a mother’s belly to map out the various densities within the womb, using the changing velocities of the ultrasound waves. The image, in the form of a tomograph, is actually a cross section of the mother and child, with the various density readings interpreted as the baby’s bones, skin, and organs.
Seismic tomography, first invented by Adam Dziewonski of Harvard University in the 1970s, depends on a somewhat similar procedure. Instead of using ultrasound, this tomography looks at seismic waves generated by numerous earthquakes, with the travel time of the seismic waves compared to a reference model. By combining the data from many earthquakes, subtle changes in wave speed can be identified. (In general, if the waves move quickly, it usually indicates cooler or denser rock; slower waves indicate warmer or less-dense rock.) Scientists then use computers to construct three dimensional density images of the Earth’s interior, including upwelling and downwelling of molten rock in the mantle.