Time-frequency analysis of seismic reflection data

When seismic waves propagate through the subsurface, energy is reflected back toward the surface at acoustic impedance contrasts. The strength of this impedance contrast is called the reflectivity. The seismic cross section of Figure 8.1(b) is a representation of the (band-limited) reflectivity of the subsurface. If the seismic wave velocity is known, then the time axis can be converted to depth. In the image of Figure 8.1(b), a two-way (down and up) travel time of 1 sec corresponds roughly to a depth of 1 km below the surface. When a rapid change in acoustic impedance occurs at a certain depth, this generally implies a change in the composition of the rocks. As a result, it is possible to make an educated guess of the geologic structure of the subsurface, based on the seismic image.