Physical Basis of Reflection Seismology
Figure 2.1 illustrates the general principle of the seismic reflection method. We start with some acoustic pulse (a "bang") that generates an expanding wavefront. The bang is located at some elevation "A" (ground surface, water surface, etc). At any given point along the expanding wavefront, we can imagine a raypath that is perpendicular to the wavefront. The wavefront will expand until it reaches some interface, here located at depth "B", that causes some of the energy to be reflected back to the surface where it can be recorded. What is physically measured by the recording instruments (located back at the "surface") are: a) the strength of the reflected energy, and b) the time it takes for the energy to travel from the surface down to the reflecting horizon, then back up to the surface again. This time is referred to as the two-way traveltime or "TWT". In principle, if we measure the TWT at many points along an interface, we can get a picture of the relief on that interface - echosounders are a good example of this process.
In reality, we have some other things to worry about. First, we are generally interested in many interfaces, not just one. Although some of the energy from the bang will be reflected, some will be transmitted through each interface as well. Furthermore, we need to understand what dimensions of features can physically be imaged with seismic methods. We must understand something about the acoustic pulses that are used to illuminate the