There are a variety of methods through which seismic surveys can be performed. One method is vibroseis, in which a truck is periodically dropped on the ground to produce waves through the ground. Seismic tomography is a method using seismic waves from naturally occurring earthquakes, but this method requires many recorded earthquakes to define the subsurface of the region. Marine reflection surveys are used if the area of interest is underwater.
Receivers are placed at specific distances and record data when the wave reaches them. This serves as a record that can be used by a computer program to create a travel-time graph. For the vibroseis method, a truck is a necessary instrument needed to drive a short distance and then be dropped on the ground. For seismic tomography, seismometers are the main instruments. Air guns and the boats that carry them are the instruments used for the marine-reflection surveys. Of course, all that is needed is a wave source, so methods can use something as simple as dynamite to get the data needed.
A seismic reflection profile is the record of the wave paths through the rock. This is made by first using a computer program to process the data acquired by the receivers and make a travel-time graph. This travel-time graph can then be turned into a seismic-reflection profile. Seismic-reflection profiles show some of what is happening in the subsurface, but not all. They must still be interpreted using travel-time data based on rock properties and whether the wave bounced off of a layer of rock or penetrated it. This interpretation will give a better indication of the layers of rock through which the waves passed.
The interpretation of a seismic-reflection profile is a great aid in identifying the location of oil in the subsurface. Oil is often caught in geologic structures such as anticlines (upside down U-shaped folds), synclines (U-shaped folds) and salt diapirs (mushroom-shaped fluid salt formations). These structures are usually identifiable in seismic-reflection profiles. When a large mushroom-shaped form in the profile is pushing up rock layers above it, this is a good indication that it is a salt diapir. The rock properties determined from a survey, such as porosity, fluid saturation and composition, may also lend clues about the presence of oil. Seeing these structures and properties in a profile does not always indicate that oil is present, but it is a fairly reliable method until more research has been completed.