Compressional and shear waves
Motion characteristics. Motion characteristics.
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Direction of propagation —» * Direction of propagation
Compressional or P wave Rotational or Shear wave »
Roylelgh wave (Ground roll)
DIRECTION Of PROPAGATION
surface
0 0 00 0 0
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Particle motion is elliptical and retrograde, that is the motion at the top of the ellipse is toward the source. The magnitude of the motion decreases with depth.
At boundaries between media of different characteristics, a more complex type of propagation occurs. One type of behavior is the surface wave, which largely confines itself to the boundary region. One kind of surface wave, Rayleigh wave, exhibits a retrograde elliptical particle motion. It can be recorded on the seismic section and it is known as ground roll.
Reflection – when an incident compressional wave strikes a boundary between two media having different velocities of wave propagation, part of the energy is reflected from the boundary.
Refraction the portion of the incident energy that is not reflected is transmitted through the boundary and into the second layer. The transmitted ray travels through the second layer with changed direction of propagation.
| wave front | фронт сейсмической волн |
| snapshot | моментальный снимок |
| ray | луч |
| wave theory | |
| compressional wave (p-wave) | продольных сейсмических волны |
| propagation | распространие |
| incident | падающая волна |
| shear wave | поперечная волна |
| surface wave | поверхностная волна |
| Rayleigh wave | волна Pэлея |
| reflection | отраженная волна |
| refraction | преломленная волна |
| wave equation | уравнение волны (Pэлея) |
Acquisition of seismic data
Some parameters of a seismic acquisition program are:
- Maximum offset : distance from the source to most remote receiver;
- Minimum offset: distance from the source to nearest receiver;
- Group interval: distance between geophone arrays. Constant for a survey;
- Shot intervals: distance between holes;
- Fold coverage: number of times a subsurface point is surveyed by different sources and detectors;
- Sample interval: the time interval between digital samples of the signal, which varies from less than 1ms to 4ms. This sample rate is chosen not to limit the vertical resolution and to record the desired maximum frequencies;
- Choice of source and geophone arrays;
- Number of recording channels.
Acquisition of parameters and noise
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The fig. below is a schematic seismic section showing the major signal and noise features. It has been corrected for normal movement (NMO) which is the time correction applied to each trace to account for its offset. The first arrivals at the top of the record are labeled P-waves; these are typically refracted waves from near-surface formations. These are followed by two coherent noise patterns. The first one, characterized by low frequency and a velocity that varies from 3500 to 5500ft\sec., is the surface wave (ground roll). The second one is the air wave, which is energy traveling from the source to the detectors through the air having a high-frequency component and low velocity of 1100ft\sec. The third type of noise is the multiple, which is a repeat reflection from the same interface. It may be either a simple multiple from one o the shallow reflectors or an intrabed multiple bounced between two reflectors and back to the surface or one of several other types in which the reflected ray reverses direction at some point.
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