Geoscience Reference
In-Depth Information
Moveout
Moveout can refer in several contexts to the way that the arrival time of a signal (e.g. a reflection from a
particular horizon) changes systematically across a set of seismic traces. In the particular case where
increase in travel-time from the zero-offset case is called the normal moveout (NMO). Correcting for
NMO is essential before traces can be stacked together. This NMO correction will shift the individual
samples of a finite-offset trace upwards by an amount that decreases with increasing travel-time. In
consequence, far-offset traces will be stretched (NMO stretch) and therefore shifted towards lower
frequencies. At shallow depth and long offset, the effect may distort traces so badly that they are
unusable and have to be removed (
muted out
) before stack.
ms
Abbreviation for millisecond (1/1000 of a second).
Multiple (reflection)
A seismic signal that has undergone more than one reflection. For example, the signal from a marine
source might be reflected from the seabed, reflected again from the sea-surface, then travel down to
a deep interface where it would be reflected back to a surface receiver. It will obviously arrive later
than a signal that has gone straight from source to deep interface to receiver (the primary reflection).
Similar multiples can be created by bouncing the signal between any two interfaces overlying the
deep interface, and there could be two or more bounces between these shallower levels, not just one.
Deep interfaces can therefore easily become obscured by multiples of interfaces lying above them, if
the primary from the deep interface arrives at the same time as the multiple of the shallower interface.
For this reason, elimination of multiples is often a key objective of seismic processing.
Mute
Reflection traces recorded at long offset and short travel-time will be strongly contaminated by
various types of unwanted signal, such as refractions, and will be distorted by the application of NMO
correction which stretches the individual loops. They are usually removed before stack by setting to
zero all trace values for offsets beyond a specified offset-TWT curve (the mute). Sometimes an inner
trace mute is employed also; this sets to zero all trace values for offsets less than a specified offset-
TWT curve, usually to remove traces heavily contaminated by multiples, which are often poorly
handled at short offsets by demultiple procedures that rely on NMO differences between primaries
and multiples.
NMO
See
Moveout
.
Offset
Source to receiver distance.
Pay (zone)
Hydrocarbon-bearing reservoir; in geophysical discussion, usually irrespective of whether hydrocar-
bons are producible, economically or at all.
Phase spectrum
When a signal is described as the sum of a series of sine waves by the methods of Fourier analysis,
each component sine wave has an amplitude and phase. The amplitude spectrum defines the peak-
to-peak amplitude of each sine wave as a function of frequency. However, this is not enough to
define the signal. We need also to know what the time alignment of the various sine waves should
be. This might for example be seen in the time of the first maximum after time zero. The phase
