Seismographic tomography is a technique which is used to generate three dimensional images of the inside of the Earth. It is often compared to computed tomography (CT) scanning, a technique used in medicine to look inside the body. In fact, the two techniques work in very similar ways, with each method generating a series of “slices,” flat pictures in a particular plane which are created by reading the way in which energy travels through the area of interest, whether it's a body or the Earth.
Two different energy sources can be used for seismic tomography. One is earthquakes, which generate waves which can be picked up with receivers on the surface of the Earth. Using information from a group of receivers, geologists can create an image of the materials the waves passed through, because the waves will move at different rates through different types of rock. Seismic tomography can reveal the presence of various rock and soil formations, along with cavities filled with water.
Geologists can also generate waves and listen for their reflection. This technique can be used to collect data from a specific targeted area, or to supplement earthquake data to get a more complete picture of an area of interest. Waves can be generated with controlled explosions or devices which vibrate, creating a wave of energy which can be tracked by its reflections. Just as with a CT scan in the hospital, this type of tomography creates an image as the reflections of the energy bounce back to the surface.
People can use seismic tomography for all sorts of purposes. Many researchers use it when they study earthquakes to find the epicenter of the quake and to learn more about the damage caused by the quake. It can also be used to learn more about the nature of the inside of the Earth in general; since it is not possible to look deep into the Earth, seismic tomography is the only way to learn about certain topics of interest in geology. Many of the fascinating maps of subsurface formations and the inside of the Earth used in geology classes come courtesy of seismic tomography.
Data from seismographic equipment is readily available to researchers conducting seismographic tomography studies. A number of software programs are designed to do the complex math involved when interpreting seismic data. Some of these programs can work extremely quickly, which may be useful when researchers need information rapidly to use in disaster response or to keep local officials informed about ongoing geological activity.