Changes

==Horizontal resolution==

==Horizontal resolution==

The waves giving rise to a reflection event are reflected from a fairly large, roughly circular area of the reflecting interface known as the first ''Fresnel zone''. Reflections from this zone arrive at a geophone so as to constructively interfere. The radius of this zone is often taken as the horizontal resolution for unmigrated seismic data. (A nomogram for Fresnel zone calculation is given by Sheriff.)<ref name=pt07r54>Sheriff, R. E., 1980, Nomogram for Fresnel-zone calculation: Geophysics, v. 45, p. 968–973, DOI: [http://library.seg.org/doi/abs/10.1190/1.1441101 10.1190/1.1441101].</ref> For the 60-Hz wavelet at 6000 ft/sec previously mentioned, this radius would be about [[length::500 ft]] at a depth of [[depth::5000 ft]]. As with the wavelength, the Fresnel zone size also increases rapidly with depth.

The waves giving rise to a reflection event are reflected from a fairly large, roughly circular area of the reflecting interface known as the first ''Fresnel zone''. Reflections from this zone arrive at a geophone so as to constructively interfere. The radius of this zone is often taken as the horizontal resolution for unmigrated [[seismic data]]. (A nomogram for Fresnel zone calculation is given by Sheriff.)<ref name=pt07r54>Sheriff, R. E., 1980, Nomogram for Fresnel-zone calculation: Geophysics, v. 45, p. 968–973, DOI: [http://library.seg.org/doi/abs/10.1190/1.1441101 10.1190/1.1441101].</ref> For the 60-Hz wavelet at 6000 ft/sec previously mentioned, this radius would be about [[length::500 ft]] at a depth of [[depth::5000 ft]]. As with the wavelength, the Fresnel zone size also increases rapidly with depth.

[[Seismic migration]] effectively collapses the in-line aspect of the Fresnel zone, so this measure of resolution is not appropriate with the migrated seismic data that are usually used for interpretation. In principle, after migration, horizontal resolution is reduced to trace spacing. However, migration smears out noise, including contributions from events to the side of the line, and this often becomes the limiting factor. Other factors contributing to limit the horizontal resolution include uncertainty in the velocity and the mathematical completeness of the migration algorithm. One of the most important limitations results from the spatial sampling because of the geophone group interval. At least two samples per wavelength are required by sampling theory to prevent aliasing. An aliased signal appears in the seismic record at lower frequency and/or [[dip]] than is real. This yields an equation relating the geophone group interval ''s'' to the frequency ''f'' and the maximum reflection dip ''d'' (in milliseconds per unit of distance) that can be properly imaged:

[[Seismic migration]] effectively collapses the in-line aspect of the Fresnel zone, so this measure of resolution is not appropriate with the migrated seismic data that are usually used for interpretation. In principle, after migration, horizontal resolution is reduced to trace spacing. However, migration smears out noise, including contributions from events to the side of the line, and this often becomes the limiting factor. Other factors contributing to limit the horizontal resolution include uncertainty in the velocity and the mathematical completeness of the migration algorithm. One of the most important limitations results from the spatial sampling because of the geophone group interval. At least two samples per wavelength are required by sampling theory to prevent aliasing. An aliased signal appears in the seismic record at lower frequency and/or [[dip]] than is real. This yields an equation relating the geophone group interval ''s'' to the frequency ''f'' and the maximum reflection dip ''d'' (in milliseconds per unit of distance) that can be properly imaged: