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The word "seismic" in Seismic Surveys refers to vibration of the earth. Seismic images are produced by generating, recording and analyzing sound waves that travel through the earth (also called seismic waves). Explosives or vibrating plates generate the waves. A line or grid of geophones (devices that convert ground movement into voltage readings---see top photo to the left, where a crew is laying geophones) records them. The offshore version of a geophone is called a hydrophone. Lines of hydrophones, called streamers, are towed behind seismic vessels to acquire seismic wave data offshore.

Density changes between rock or soil layers reflect the waves back to the surface. How quickly and strongly the waves reflect back indicates what lies below. To get a really "deep" picture (thousands of feet below the surface), as is needed for oil and gas exploration, dynamite charges or vehicle-mounted vibrator plates (called vibroseis trucks and buggies) are used to generate waves from multiple source points.

Dynamite charges are usually buried in 50-to-100-foot-deep holes (called shot holes---see middle photo to the left). Relatively small amounts of explosives are used in shot holes. Charges are small and deeply buried for seismic surveys, so a person standing near the site would feel only a small pulse up to a few hundred feet away, and beyond that, her or she is not likely to feel anything at all. 

Vibroseis trucks (see bottom photo to the left) and buggies come in various designs and sizes. All of them  release less energy than is generated from shot holes. Each has a large pad that is lowered to the surface and then vibrated to generate seismic waves. In urban areas, vibroseis-generated waves are less noticeable than background noise generated by buses, trucks, and trains.

Once the seismic data are acquired, they are "processed" to improve the signal-to-noise ratio and re-arrange the data into a graphic display. After processing, the data, in the form of patterned, recorded seismic signals, are translated into a geological cross-section. These cross-sections are then interpreted to identify potential reservoirs of oil or natural gas.