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Field Procedures
 Seismic field acquisition involves three basic elements:
 a source of acoustic energy
seismic receivers, or geophones
a seismograph, or recorder, to collect and store the data
The choice of seismic source depends on the needs of the particular survey. For deeper work, a powerful source, such as an elatic wave generator (EWG), trailer-mounted accelerated weight drops, would be used. Shallow, high-resolution work, not requiring the power of an EWG, can be done by striking a steel plate with a sledgehammer or using the "Betsy" downhole shotgun. Geophones are also selected according to the needs of the survey: higher-frequency phones for high-resolution work, lower-frequency for deeper targets.
The arrangement of source and geophones depends on the nature of the survey. Typically, the geophones are placed along a line at equal intervals (1 to 5 feet for high-resolution, 10 to 20 feet for deeper work). For seismic reflection, the relative source and geophone positions are usually held constant with the entire 48-geophone array being moved along with the shot. (The logistical difficulties of this are eased by using a "roll switch", which selects 48 geophones from an overall spread of 96.) Refraction work requires shots at opposite ends of the spread, with additional shot locations depending on the particular needs of the job.
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