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It would be nice if scientists had some magical x-ray equipment that would let them look under the earth's surface to locate oil and gas reservoirs. Unfortunately this technology is not yet available and so scientists use other techniques to try to develop a picture of the underground rock layers and crude oil and natural gas deposits. Seismology
Energy is directed into the surface and passes through some layers and is reflected off of layers at different depths. This energy returns to the surface at different angles and with different energy levels. The path the energy takes will be different depending on the shape of the surface it hits and the density of the surface. The energy is in the form of sound waves. The concept is similar to dropping a ball on a surface. If the surface is flat the ball will bounce straight up. If the surface is curved the ball will return at an angle. If the ball is bounced on a flat concrete surface it will return to nearly the same height from which it was dropped. If the ball is dropped on the grass it may not return at all. In seismic surveys the energy is returned to surface with a specific set of properties. The first step in performing a seismic survey is to create energy directed into the surface. This energy is produced by small explosive charges, mechanical methods, or sound waves. In the past scientists placed small explosive charges at various locations on the surface to create sound waves. Today mechanical equipment is used to produce these sound waves. In one device a steel slab is dropped onto the surface to create the energy while another method generates low-frequency sound waves. The next step is to measure the energy that is returned from the earth's layers. Sensors on the earth's surface called geophones receive the reflected sound waves, translate them into electronic impulses, and send them to the seismograph. The seismograph amplifies and records the electrical signal and produces a picture or seismogram. The seismogram is a two-dimensional picture of the subsurface. It is generate a seismic section, which is a two-dimensional slice from the surface of the earth downward. This section is known as 2D seismic because it shows the width and depth. The information from a seismic survey indicates the types of rock, their relative depth, and whether a trap is present. The seismogram indicates the types of rock, relative depth, and the presence of structural traps. Scientists are now applying seismic technology to produce three-dimensional images of the earth's subsurface. Three-dimensional seismic or 3D seismic creates an image that includes length, width, and depth. Seismic crews obtain data on a 2 or 3 mile square of the earth's surface. Large, high-speed computers analyze the data and create a picture of the surface. Scientists can then take slices of this in a variety of directions to examine the formation. This technique is also be used at different time periods to study the movements of hydrocarbons. This produces a 4D seismic history of the formation with time as the fourth dimension. Although explorationists usually use geophones on the surface of the earth to gather seismic data, they may also use them in an existing well. In this method, scientists lower geophones into the well and attach them to the wall at intervals of 20 to 100 feet. This method is useful to obtain information about the geological structures close to the well. Marine Seismic Methods Offshore exploration uses similar equipment to land exploration except that the data is collected on a ship. Like land seismic analysis, an energy source directs sound waves through the water to formations below the seabed. These formations reflect the seismic waves to hydrophones. Hydrophones are usually towed behind the ship on steel cables. Sometimes the hydrophones are anchored in place in a specific pattern using cables. The ship creates sound waves around the array of hydrophones. The seismic data can be processed on the ship or sent via satellite hookups to land-based data processing centers.
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