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Why do we use inner mute to mute a seismic trace, and why do we mute using incidence angle?
Question Date: 2013-09-18
Answer 1:

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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 ocean currents.

Mutes are almost always tapered, to avoid introducing "edge effect" issues when subsequent processing is applied."


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