Geoscience Reference
In-Depth Information
Appendix 2
Glossary
Accommodation space
The space available for potential sediment accumulation, controlled by processes such as changes in
sea-level, tectonic movements, compaction of pre-existing sediment and subsidence.
Acoustic impedance
A property of a rock, defined as the product of density and seismic velocity. The impedances on either
side of an interface determine the reflection coefficient for seismic waves
(section 3.1
)
.
Aeolian
An aeolian depositional process is one where the dominant agent is wind. A typical modern example
is a hot arid desert such as the Sahara. There may be areas covered with sand dunes, and other areas
of bare exposed bedrock. Large areas of sand dunes form sand seas or
ergs
. Aeolian sandstones can
be important hydrocarbon reservoirs, as for example in the Permian of the North Sea.
AGC
Automatic Gain Control is the process of varying the gain of a seismic trace display with TWT, so as
to maintain the average absolute level constant within a time window. If a short time window is used,
the process has the effect of destroying information about lateral and vertical changes in reflection
strength, which is highly undesirable if any attempt is being made to recognise effects due to fluid
content or lateral change in lithology. If a long window (e.g. 1 s) is used, however, the process makes
it easier to view seismic displays across the full TWT range, and can even be beneficial to amplitude
studies by removing effects of variation in near-surface absorption, for example.
Aliasing
Seismic traces are not continuous measurements in time; instead, the amplitude is recorded digitally
at a certain sample interval in time (usually 2 or 4 ms). Nor do we record traces at every possible
(
x
,
y
) location; rather, we shall have traces on a regular grid, perhaps 25 m
25 m. All our data
processing and interpretation assumes that this is an adequate approach, in the sense that a smooth
curve through the sampled points is a true picture of reality. This would obviously not be true if there
were strong high-frequency variations present in the data, so that in reality there were high-amplitude
oscillations happening between the sample points. It can be shown that signals can be recovered
from a regularly sampled representation provided that they do not contain frequencies higher than
half of the sampling frequency. Frequencies higher than this cannot be recovered, and are said to be
aliassed; they will mimic the behaviour of a lower-frequency signal. Care is therefore needed not
to alias signals when they are recorded; if there is a danger of this, the sampling frequency must be
increased or the bandwidth of the incoming signal decreased before it is sampled for turning into digital
form.
×
196
