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Seismic data processing involves the compilation, organization, and conversion of wave signals into a visual map of the areas below the surface of the earth. The technique requires plotting points and eliminating interference. At one time, seismic processing required sending information to a distant computer lab for analysis. Currently, laptop computers equipped with seismic software allow geophysicists to enter and manipulate data on-site.
Blasts from explosives or vibrating machines that occur during hydrocarbon exploration or petroleum geology studies produce waves that travel through the ground and may cause it to move. Marine geology studies employ air guns that create pressure waves. Surrounding these devices is an array of geophones or hydrophones, which receive the waves reflected off the subsurface, convert them into an electrical signal, and record the receiving time. A specific area might receive hundreds or thousands of blasts over a predetermined period of time.
Processing the raw seismic data obtained from the geophones requires the software to make calculations based on distance, time, and velocity. As a computer performs seismic data processing, points are plotted on two and three-dimensional graphs. These coordinates often depict the distance from a sound production device to the geophones. Other points represent the travel time of the wave from its point of origin to the geophones. The display also illustrates the depth the waves reach before reflecting back to the surface.
After collecting the raw data and making the required calculations, the seismic data processing software may generate a two-dimensional reflection graph. By performing geometric calculations based on depth and time, the program can create a three-dimensional representation of the area. Geologists might also use colors to indicate various depths or to distinguish between layers. Often, these images require fine-tuning.
Deconvolution, in seismic data processing, shortens reflection waves and reduces the ghost effect that can might occur because of instrumentation, reverberations, or multiple reflections. This function generally shows more clearly defined layers. The mute function eliminates areas comprised of mainly noise or possibly refractions overlapping reflections. Velocity analysis filters clean the image by differentiating between an actual wave signal and noise, based on the frequency and velocity of the wavelet.
Using the travel time, wave velocity, and the number of waves reflected, geophysicists can determine the density, porosity, and fluid saturation of the substrate. The denser the rock formation, the faster the waves travel, and porous rock slows wave travel. Likewise, waves pass through water-filled areas quickly, but through air or gas pockets slowly.