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Seismic refraction measures seismic waves’ travel time through the ground to map depths of less than 100 feet, rock quality, and strength. Geophones record waves, and velocities provide information about the material beneath the surface. Seismic reflection collects images of bedrock from reflected waves.
The geological profiling method known as seismic refraction measures the time it takes for seismic waves or rays to travel through the ground, strike bedrock, and return 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 provide information about rock quality and strength.
Seismic waves are a type of force energy that moves through the earth. These waves can be created naturally, such as earthquakes, or through man-made methods, such as explosions. Small seismic waves can be created by firing a shot at the ground or by dropping something heavy on the ground. If the ground shakes or vibrates, this is due to a seismic wave.
In seismic refraction, energy is fired into the earth from the surface usually by projectiles fired from a rifle, a dropped weight, a small explosive, or by striking a plate with a hammer. The waves travel through the ground and then are refracted laterally along the bedrock before bouncing back to the surface. A series of geophones arranged in a straight line along the area under examination record the waves when they reach the surface again.
Geophones are small metal cylinders that are roughly shaped like a battery, although they come in different sizes. Inside the cylinder is a coil that hangs from a spring. There are magnets on both sides of the coil. When seismic waves cause the ground to vibrate, the coil moves through the magnetic field and the energy of the wave can be measured. Computers are often used to analyze data received from geophones and graphs are created showing travel times as a function of distance to calculate speeds.
Velocities provide information about the type of material beneath the surface of the ground as a wave will move through different types of land 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 about the materials between the surface and the bedrock.
Seismic refraction should not be confused with seismic reflection. While seismic reflection uses many of the same test methods, it collects images of the bedrock from waves that simply bounce or reflect off the bedrock rather than scattering at different angles. Unlike seismic refraction, seismic reflection cannot provide information about the thickness of the material or the speed of the rays. It is best used in marine environments where seismic refraction is unreliable.
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