Seismic reflection is a principle which is utilized in geology to gather information about what is going on underneath the surface of the Earth. Sound waves underground are subject to the same physical principles which govern the travel of energy above ground, and with these principles in mind, geologists can use the movement of sound waves underground to generate data about subsurface geological formations. A closely related concept is seismic refraction, which involves the study of the ways in which sound waves bend as they encounter obstacles underground.
For a seismic reflection survey, geologists need something to generate noise, such as a large vibrating device, a controlled explosion, or a heavy object which can be dropped to create a sound wave. They also need geophones, sensitive listening devices which can be placed on the surface of the Earth to listen for the sound waves as they return. A field team operates the devices, collects the data, and makes observations about the setting of the seismic reflection study, taking note of anything which could skew results.
As the soundwaves move underground, some are reflected back up to the surface of the Earth, where they are picked up by the geophones. Using geophone data, researchers can create a plot which reveals the outline of formations and objects in the ground. In a way, seismic reflection works sort of like ultrasound and radar, with people generating soundwaves and listening for their return to get information about something they cannot see.
For a seismic reflection study to work well, the area being surveyed generally needs to be deep. If it is too shallow, the reflected sound waves can bunch together, making it hard to differentiate between the waves and muddying the results. It also helps to have multiple geophones to listen from several different angles. Refraction studies, on the other hand, are capable of being used in shallower ground, but they have some limitations, such as difficulty when surveying involves materials of high density overlayed on materials of lower density.
The principles behind seismic reflection studies are also used in the study of earthquakes, except that because the researchers do not know where the source of the energy is located, they need to be able to use input from multiple seismographs to narrow in on the epicenter. This is done by plugging the data into an equation, using an understanding of how energy moves through the Earth to determine where the energy is coming from.