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"A mute is simply an area of data that is
zero'd; it might be based on a line (with data
above or below the line being muted), or even a
polygon, with data inside the polygon being muted.
There's a few ways in which mutes are used in a
seismic processing sequence.
The first is to remove any strong, coherent
noise that is generated by the shot, such as the
direct and refracted arrivals. The mute in this
case is usually an "outer trace" or "front mute",
where data *above* the mute time is zero'd. These
strong arrivals might also be attenuated by muting
them in another domain - for example they may be
more isolated from the data if you transform to
the Tau-P and apply a mute there. In the Tau-P
domain the mute is usually an "inner trace" or
"tail" mute, where the data is zero'd below the
mute line. Mutes in the FK domain can also be
effective, especially in the form of polygons.
The second thing a mute is used for is to remove
data that has been "over stretched" on common
midpoint (CMP or CDP) gathers when you have
applied an NMO (Normal Move out) correction; this
"flattens" the hyperbolic shape of a reflection
based on the offset and a determined velocity.
Where the correction is very large (in the shallow
part of the section and at longer offsets) the
correction applied at the top and bottom of a
given signal maybe so large as to distort the
event. This distortion is called "NMO stretch", as
the event is stretched out - when you stack data
with a lot of NMO stretch it creates low frequency
artefacts that obscure the real image. NMO stretch
is usually avoided by muting the stretched data,
either using a percentage stretch mute, or
manually picking a mute.
An inner trace mute on NMO corrected gathers can
also be used to attenuate multiples, as a
significant part of the multiple seen on a stack
comes from the inner traces where the Normal Move
out difference between primary and multiple signal
is small, so the multiple "stacks in"; SRME and
other model-based de-multiples are making the use
of inner trace mutes less common, but you may
still see it deployed on some datasets.
Finally, its usual to mute stacked marine data
above the seafloor, and remove "water column
noise" - this is sometimes called a "trim mute",
and is often stored in a trace header and
reapplied after processes like “migrational
filtering”, that can add noise into the water
column. It is worth noting that the "water column
noise" can be low amplitude reflections from
subtle density variations in the ocean, and I've
met a number of people who are studying this
particular type of "noise" and using it to map
Mutes are almost always tapered, to avoid
introducing "edge effect" issues when subsequent
processing is applied."
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