The method of geological profiling known as seismic refraction measures the time it takes seismic waves or rays to move through the ground, hit the bedrock, and be rebounded back to the surface. Used in geophysics, this method is most accurate when mapping depths of less than 100 feet. In addition to measuring the depth of bedrock, seismic refraction can give information on rock quality and strength.
Seismic waves are a type of force energy that moves through the earth. These waves can be created naturally, as in the case of earthquakes, or through artificial methods, like explosions. Small seismic waves can be created by firing a shot at the ground or by dropping something heavy on to the ground. If the ground shakes or vibrates, that is due to a seismic wave.
In seismic refraction, energy is shot into the earth from the surface usually by blanks fired from a shot gun, a weight dropped on the ground, a small explosive, or hitting a plate with a hammer. The waves move into the ground and then are refracted laterally along the bedrock before rebounding back up to the surface. A series of geophones arranged in a straight line along the area being tested record the waves when they again reach the surface.
Geophones are small metal cylinders that are roughly battery shaped, though they come in different sizes. Inside the cylinder is a coil which hangs from a spring. On either side of the coil are magnets. When the seismic waves cause the ground vibration, the coil moves through the magnetic field, and the energy of the wave can be measured. Computers are often used to analyze the data received from geophones, and graphs plotting travel times versus distance are created to calculate velocities.
Velocities give information about the type of material under the surface of the ground since a wave will move through different types of earth at different speeds. For example, a seismic wave will move through clay at a different speed than it would move through sand. This is why geophysicists can gather information on the materials between the surface and the bedrock.
Seismic refraction should not be confused with seismic reflection. Although seismic reflection uses many of the same testing methods, it gathers images of the bedrock by the waves simply rebounding, or being reflected, off the bedrock rather than dispersing at different angles. Unlike seismic refraction, seismic reflection cannot provide information about the thickness of the material or the velocity of the rays. It is best used in marine environments where seismic refraction is unreliable.