Traditionally, most seismic work fell under the category of 2D imaging. Sources and receivers are placed on a 1D (horizontal) surface line called the x-axis. The time axis is a 1D (vertical) line called the t-axis. Each source would have many traces - one for each receiver position on the x-axis. A plot of traces from a particular source is called a seismic record. In other words, all the traces from a single source make up a seismic record.
The waves that make up each trace take a great variety of paths, each requiring a different time to travel from source to receiver. Some waves are refracted, and others are scattered. Some waves travel along the earth’s surface, and others have been reflected upward from various interfaces. To reiterate, a primary reflection is an event that represents a seismic wave’s passage from the source to the depth point and then its passage directly back to the receiver. A multiple reflection is an event that has undergone three or five or some other odd number of reflections in its travel path. In other words, a multiple takes a zigzag course with the same number of down and up legs.
Depending on their time delay from the primary events with which they are associated, multiple reflections are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or as long-path, where they appear as separate events. Usually, primary reflections are simply called reflections or primaries, whereas multiple reflections are simply called multiples. Multiple reflections (or reverberations) between the water surface (i.e., the interface between air and water) and the water bottom (i.e., the interface between water and solid) are common in marine seismic data.
Primary reflections (i.e., events that have undergone only one reflection) are needed for image formation. To use the primary reflected signals, we must distinguish them from the other types of signals. Ambient noise, such as wind noise, usually is minor and in most cases can be neglected. The source signal generates the primary reflections and many other kinds of signals such as multiples, reverberations, ghosts, diffractions, surface waves, and the like. All of these received signals, except the primary reflections, are unwanted for the purpose of image formation. Such unwanted signals are called signal-generated noise. Thus, we are faced with a problem of (reflected) signal enhancement and (signal-generated) noise suppression.
In the old days of analog processing, separating signal from noise was attempted by using analog filters, but the final arbiter was the interpreter’s eye. The interpreter used the analog seismic data in conjunction with vivid imagination and intensive ingenuity to form suitable maps of the subsurface structure of the earth. Even today, with the clarity provided by digital imaging, the human attributes of imagination and ingenuity are still essential elements in oil exploration.
Each seismic trace is associated with a source point and a receiver point. The source and receiver points lie in the surface (x,y) of the earth. Signal-generated noise conceals or distorts the desired primary reflections. Signal enhancement is done either by single-channel processing of each trace, one by one, or by multichannel processing of sets of traces. Signal enhancement involves preservation of the primary reflections and suppression of all other effects on the trace. As a result, not only the usual unwanted random noise is suppressed but also the signal-generated noise such as airwaves, ground roll, reverberations, multiple reflections, and diffractions. The desired results of signal enhancement are traces that have only primary reflections - that is, primaries-only traces.
Deconvolution is used along with ancillary signal-processing techniques. On the processed traces, primary reflections are preserved and signal-generated noise is suppressed as well as possible. Seismic signal enhancement typically includes analysis of velocities and frequencies, static corrections, and deconvolution. After signal enhancement, we are left with traces that ideally have only primary reflections. Such records are now ready for migration.
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Also in this chapter
- Reflection seismology
- Digital processing
- The unit tangent vector
- The gradient
- The directional derivative
- The principle of least time
- The eikonal equation
- Snell’s law
- Ray equation
- Ray equation for velocity linear with depth
- Raypath for velocity linear with depth
- Traveltime for velocity linear with depth
- Point of maximum depth
- Wavefront for velocity linear with depth
- Two orthogonal sets of circles
- Migration in the case of constant velocity
- Implementation of migration
- Appendix B: Exercises