Published:January 01, 1997
Accurate positioning of a seismic line is as crucial as having the best possible data quality. Positioning is important for three reasons: (1) many data processing steps require accurate relative source and receiver positions; (2) tying several seismic lines together requires knowledge of where they are relative to one another; and (3) when drilling sites are selected from seismic data they have to be referenced back to an actual location on the Earth’s surface. Of these reasons, perhaps the last is most important: No exploration company wants to spend millions of dollars drilling, only to miss the target because the seismic data were mispositioned.
Accurate positioning is not a trivial task, especially for marine surveys. Once a seismic vessel has sailed along an intended line, no permanent evidence remains behind to show where the ship actually sailed. Furthermore, at sea, intended shot and receiver positions cannot be identified by markers prior to shooting. Finally, during shooting, both the ship and the trailing equipment are somewhat at the mercy of the wind, currents, and wave action; the position of the shots and receivers cannot, therefore, be controlled accurately. For these reasons, positioning in marine surveys is a so-called real-time activity; that is, position measurements have to be made, recorded, and processed as a line is shot.
For land seismic surveys, positioning does not have the real-time urgency that it does in marine surveys. The shot and receiver positions can be marked on the ground either before or during the shooting of
Figures & Tables
A Handbook for Seismic Data Acquisition in Exploration
The science of seismology began with the study of naturally occurring earthquakes. Seismologists at first were motivated by the desire to undetand the destructive nature of large earthquakes. They soon learned, however, that the seismic waves produced by an earthquake contained valuable information about the large-scale structure of the Earth’s interior.
Today, much of our understanding of the Eart’s mantle, crust, and core is based on the analysis of the seismic waves produced by earthquakes. Thus, seismology became an important branch of geophysics, the physics of the Earth.
Seismologists and geologists also discovered that similar, but much weaker, man-made seismic waves had a more practical use: They could probe the very shallow structure of the Earth to help locate its mineral, water, and hydrocarbon resources. Thus, the seismic exploration industry was born, and the seismologists working in that industry came to be called exploration geo-physicists. Today seismic exploration encompasses more than just the search for resources. Seismic technology is used in the search for waste-disposal sites, in determining the stability of the ground under proposed industrial facilities, and even in archaeological investigations. Nevertheless, since hydrocarbon exploration is still the reason for the existence of the seismic exploration industry, the methods and terminology explained in this book are those commonly used in the oil and natural gas exploration industry.
The underlying concept of seismic exploration is simple. Man-made seismic waves are just sound waves (also called acoustic waves) with frequencies typically ranging from about 5 Hz to just over 100 Hz. (The lowest sound frequency audible to the human ear is about 30 Hz.) As these sound waves leave the seismic source and travel downward into the Earth, they encounter changes in the Earth’s geological layering, which cause echoes (or reflections) to travel upward to the surface. Electromechanical transducers (geophones or hydrophones) detect the echoes arriving at the surface and convert them into electrical signals, which are then amplified, filtered, digitized, and recorded. The recorded seismic data usually undergo elaborate processing by digital computers to produce images of the earth’s shallow structure. An experienced geologist or geophysicist can interpret those images to determine what type of rocks they represent and whether those rocks might contain valuable resources.