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.