The seismic reflection method
Let a ship be on the waves and it is all right. But let the waves be in the ship and down she goes. —Author unknown
In petroleum exploration, the geophysicist’s task is to look beneath the earth’s surface in the search for new deposits of oil and natural gas. Subsurface geologic structures of interest can be several miles deep. Geophysicists use the seismic reflection method in their search for oil and natural gas. The exploration geophysicist illuminates the earth’s subsurface by means of an energy source that generates seismic waves. The subsurface rock layers transmit and reflect those seismic waves. A seismic survey consists of collecting seismic reflection data over a selected geographic area, called the prospect. The seismic reflection method, when considered as an instrument for remote detection, has much in common with other disciplines that are based on noninvasive techniques for finding the structure of an inaccessible body, such as medical imaging and nondestructive testing.
What is seismic acquisition? The essential features of seismic data acquisition are as follows: (1) At a fixed point on the surface of the earth, a source of energy — such as an array of dynamite charges or air guns or swept-frequency vibrators (as in vibroseis) — is activated. Such an activated source is called a shot. (2) Seismic waves from the shot propagate downward from the source point and go deep into the earth (Bois, 1968,). (3) Eventually, the waves are reflected from geologic interfaces and propagate upward from those interfaces. A primary reflection is a reflection that travels directly down to the interface and then directly back up to the surface. A multiple reflection is a reflection that has bounced back and forth among various interfaces as it proceeds on its trip. (4) The reflected waves, both primaries and multiples, are detected on the surface by receivers. The receiver points are located at various horizontal distances from the source point. (5) The digitized signal recorded at each receiver point for a given source point is called a trace. (6) After one shot is completed, the source point and the corresponding receiver points are moved so that another shot can take place. This acquisition method is repeated again and again until the entire prospect is covered. It is not economically feasible to make a nearly continuous survey. Instead, within the confines of a given exploration budget, a fixed number of source and receiver points must be used. The points are chosen to obtain the best possible representation of the prospect. Such a procedure represents a sampling in the space domain.
What is seismic processing? Seismic traces, as recorded by the receivers, constitute the raw data that then are fed into computers for processing. The purpose of digital seismic processing is to transform raw data into computer-generated images of subsurface geologic structures. The high dynamic range of the receivers available today enables us to use precise signal-processing methods that give excellent image quality.
The earth’s subsurface structure and stratigraphy represent an unknown. The known information is in the form of the received (raw) seismic traces. These traces are a record of the seismic wavefield (the motion of seismic waves) at the surface of the earth. The geophysicist is faced with an inverse problem in which the received wavefield must be converted into a picture of the subsurface. The purpose of seismic processing is to solve that inverse problem to reveal the shape and properties of the geologic bodies that produced the recorded traces. Velocity analysis, deconvolution, and migration are among the powerful tools that geophysicists use in that task.
The final result of seismic processing is an image of the earth’s underground structure as well as some details of its stratigraphy. In a 3D survey, the final image is a picture of the subterranean volume of the earth. The horizontal dimensions are north and east, and the vertical dimension is time or depth, as the case may be. Whereas the recorded (raw) seismic data represent a wavefield, the processed seismic data should represent a geologic image, or picture, of the subsurface. This picture stands by itself, apart from its wave-motion origin. The many-faceted features of the picture are intended to reveal not only the geologic structure but also important clues about the stratigraphy.
At this point, a word about terminology is in order. A geologic image, within the computer, is in the form of an array of data points. The array is two-dimensional for a 2D image and three-dimensional for a 3D image. However, for historical reasons, the old terminology of trace (which originally was used only for wave motion) still is used in geophysics as a descriptive term in the makeup of an image. Thus, in current terminology, a geologic image consists of individual traces extending in depth (or traveltime), with one trace for each surface point. Traces occurring in a geologic image represent subsurface geology and not the wave motion from which they were derived. For a 2D image, the surface points for the individual traces would lie along a horizontal line, whereas for a 3D image, the surface points would lie in a horizontal plane.
- Bois, P., 1968, Determination de l’impulsion sismique: Geophysical Prospecting, 16, 4–20.
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Also in this chapter
- Seismic interpretation
- Absorption loss and transmission loss
- The wave equation
- Wave velocity
- Velocity analysis
- Seismic tomography
- Appendix C: Exercises