Geoscience Reference
In-Depth Information
it would have for a zero-offset trace. This involves shifting the energy both laterally and vertically.
After DMO has been applied, traces can be stacked together at each CMP surface location without
smearing the image, because all the traces contain reflected energy from the same subsurface point.
Dip section
Seismic section shot parallel to the predominant geological dip direction, and so in the direction of
maximum horizontal gradient of reflecting horizons and often perpendicular to the principal faults.
Fold
As can be seen in
fig. 2.7
, seismic acquisition is normally laid out so that traces recorded at different
source-receiver offsets can be added together ('stacked') to enhance signal to noise ratio. (They will,
of course, require correction to be made for the more oblique ray-paths at the longer offsets.) The
number of individual source-receiver pairs that contribute to the stack is called the fold of the data.
Around the edge of a survey area, the fold decreases because there will be progressively fewer long
offsets available. The 'full fold' area is therefore surrounded by a zone where the fold progressively
tapers to 1. Because of the increased noise level, the data in the zone of reduced fold are of limited
use and are often largely excluded from the migration process.
Fresnel zone
If we think in terms of rays, then a reflection comes from a point on a reflecting surface. In terms of
wave theory, however, a reflection is made up of energy returning from a finite area of the reflector.
A Fresnel zone is the area from which reflected energy arriving at a receiver has a phase difference
of no more than half a cycle, and is therefore able to contribute constructively to the reflection. For
the case where source and receiver are coincident and a distance
h
above the reflector, then most
of the reflected energy comes from a circular zone of radius
r
given by
r
2
=
λ
h
/
2
,
where
λ
is the
wavelength of the seismic signal.
GPS positioning
The Global Positioning System depends on time-ranging to a set of satellites, which are distributed in
various orbits so that a user can receive signals from at least four of them at any point on the Earth's
surface at any time. Since the satellite positions in their orbits are known to high accuracy, four
range measurements are enough to calculate the user's latitude, longitude and height, plus the timing
offset between the user's clock and the satellite system clock. For the highest accuracy, differential
GPS (DGPS) is used, where a fixed station monitors its apparent GPS position and the deviations
from its known location are used to refine the apparent position of a user in the field. In this way it
is possible to correct for uncertainties in orbital parameters, atmospheric refraction, and deliberate
signal degradation by the system's owners, giving a positional accuracy of, for example, 2 m within
2000 km of the reference station.
Inline
A 3-D seismic survey consists of traces on a rectangular grid. One of the axes of this grid is called
the inline direction and the other is called the crossline. Lines in these two directions are numbered,
and then co-ordinates of traces within the survey can be specified by means of the (inline, crossline)
co-ordinate pair. The choice of which direction is called inline and which crossline is sometimes
arbitrary. The inline direction may refer to the original shooting direction for the survey, but a bin
grid may be used for processing that is not simply aligned to the original acquisition layout.
Invasion effects
When a borehole is drilled, the drilling fluid will penetrate into the formation being drilled. When
wireline logs are run to measure formation density and seismic velocity, for example, then they may
