One of the most important applications of seismology involves the probing of Earth's internal structure by examining energy reflected at steep incidence angles from subsurface layers. This technique may loosely be termed reflection seismology and has been used extensively by the mining and petroleum industries to study the shallow crust, generally using portable instruments and artificial sources. However, similar methods can be applied to the deeper Earth using recordings of earthquakes or large explosions. Because reflected seismic waves are sensitive to sharp changes in velocity or density, reflection seismology can often provide much greater lateral and vertical resolution than can be obtained from study of direct seismic phases such as P and S (analyses of these arrivals may be termed refraction seismology). However, mapping of reflected phases into reflector depths requires knowledge of the average background seismic velocity structure, to which typical reflection seismic data are only weakly sensitive. Thus refraction experiments are a useful complement to reflection experiments when independent constraints on the velocity structure (e.g., from borehole logs) are unavailable.

Reflection seismic experiments are typically characterized by large numbers of sources and receivers at closely spaced and regular intervals. Because the data volume generally makes formal inversions too costly for routine processing, more practical approximate methods have been widely developed to analyze the results. Simple time versus distance plots of the data can produce crude images of the subsurface reflectors; these images become increasingly accurate as additional processing steps are applied to the data.