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There are three major phases of seismic data: 1) Acquisition 2) Processing 3) Interpretation
Data acquisition involves sending energy into the ground and recording the energy as it returns to the surface after bouncing (or echoing) off of the underground rock type boundaries. The boundaries between rock types cause a change in what is called "acoustic impedance" which is a technical term that simply means that the energy changes when it hits that boundary. Some energy passes through the boundary and some energy bounces off of it and returns to the surface where it is recorded. Acquisition has two main components
1) sources 2) receivers
Sources and recievers are different for land and marine cases.
For land - the source is predominantly an array of vibratory source trucks that "excite" the ground and cause the energy to propagate through the earth. For marine - the source is an array of "air guns" The airguns build a tuned bubble of air in the water that causes a pressure wave to travel through the water which continues to propagate in the earth at the sea floor.
For land - the receiver is an array of geophones. At it's core, the geophone is a magnet and a coil. As the energy returns from the rock boundary interfaces it causes the earth to move at the surface and the geophone records that movement. The voltage values caused by the coil moving over the magnet is digitized into a digitally sampled seismic representation of how the surface of the earth moves and responds to the input stimulus from the vibrator trucks.
For marine - the receiver is a pressure sensor commonly referred to as a hydrophone. As the energy travels through the water the pressure changes are recorded and digitized into a digitally sampled seismic representation of how the water responds to the input stimulus from the air gun bubble pulse.
In both cases the source is generally referred to as a "point source" which means that all of the energy emanates from a single location, but the receivers are laid out in a big areal array. The array can be a single long line of receivers for the 2D seismic or in a variety of different 3D surface arrays for 3D seismic.
Other terms you can look up for the 3D cases might include Full Azimuth and Narrow Azimuth.
Recording at multiple locations generates seismic "traces" or digital representation of the energy returning to the surface at different locations at different directions and distances from the source. The traces are combined in different ways in processing to enhance the replication of reflected signal and remove the energy that is noise or unwanted energy.
There are many articles and web sites that you can look at for more information, for example WikipediA has a good write up