Raypath Traveltime Tomography: As discussed in Chapter 1, geophysical traveltime tomography is the procedure for inverting traveltimes of specified body waves for the velocity distribution of the earth. It has been the primary means for earthquake imaging of the Earth’s interior from the early 1970s. Until recently, it was implemented mostly using the high-frequency approximation of ray tracing, but there is a growing tendency to use finite-frequency tomography, in which traveltime residuals are back projected along wavepaths rather than raypaths. Finite-frequency wave-equation tomography, also known as wave-equation tomography or wave-equation traveltime tomography, can be more accurate than raypath tomography. However, it is at least an order of magnitude more costly because the wave equation must be solved numerically for each source. Traveltime tomography also is used in a variety of other fields, such as medical imaging, nondestructive evaluation, and meteorology.