Subsurface Detection

The next time you go to a shopping mall on a bright afternoon, notice your reflection in the two sets of double doors at the entrance. You'll probably see yourself reflected twice, once from the outer set, and once from the inner set. Unlike opaque mirrors, glass doors permit some light to go through them, while at the same time they reflect some light back. Rock layers behave in a similar manner with respect to seismic waves. An explosion detonated at the surface will reflect off of many layers at depth. Detectors set up in an array can time the arrival of each of these reflected waves. Just as bats and dolphins can use the echoes of sound waves to locate food, geophysicists can use the echoes of seismic waves to locate reflecting boundaries at depth. These reflecting boundaries occur where there is an abrupt contrast in some seismic wave transmission properties (usually velocities) of the material. Most often this is a result of the sedimentary layering. A seismic reflection profile, which actually shows reflection horizons, is usually interpreted as revealing the structure of the underlying layers. The oil bearing structures in many productive oil and gas fields were located using reflection seismic surveys, so this technique has been very important to the petroleum industry.

At first seismic reflection profiling was used only by oil companies. Because it is economical to recover oil only if it is relatively near the surface, such surveys did not seek out much information at great depths. Over the last fifteen years or so, however, scientists have adapted the technique to probe the very deepest parts of the crust. One remarkable discovery is a nearly horizontal fault underlying much of Georgia and the adjacent continental shelf.

Explosives are still used to generate seismic waves in some areas, particularly at sea, but in many places they have been supplanted by special trucks which vibrate the ground beneath them with hydraulic jacks. The signals generated by these vibrations reflect just like any other seismic waves, but because the energy is put into the ground over a period of time, instead of instantly, there is less damage to structures in the area.