Published:January 01, 1997
The goal of seismic data acquisition and processing is to provide a realistic image of the subsurface structure of the Earth. Since that structure is three-dimensional, one might suspect that 2-D data acquisition and processing can be inadequate. That is indeed the case: 2-D seismic data can provide a misleading image of the earth’s subsurface, even for the simplest 3-D structures. Figure 161, for example, shows a reflector that has a dip direction perpendicular to the shooting direction of a 2-D seismic line. When the data are processed, the cross dip causes two problems in the image. First, the part of the reflector being imaged is not the part that actually lies vertically under the 2-D line, as an interpreter unaware of the cross dip would assume. Second, the reflector’s depth is incorrect. Both of these problems occur because the actual reflection points do not lie in the vertical plane below the line.
The solution to these problems is to record 3-D seismic data and to process them using 3-D rather than 2-D imaging algorithms. In 3-D acquisition, the data are collected over a 2-D surface area instead of along 1-D lines. After 3-D processing, reflection events appear as 2-D surfaces rather than as 1-D events. A dipping, reflecting plane such as that in Figure 161 is then correctly imaged.
French (1974) illustrated the benefits of 3-D versus 2-D seismic methods in a classical modeling study performed at Gulf Research Laboratories. One of French’s models (unpublished) had a normal fault
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.