{"id":332,"date":"2013-05-04T10:09:00","date_gmt":"2013-05-04T10:09:00","guid":{"rendered":"http:\/\/ahay.org\/blog\/?p=332"},"modified":"2015-09-01T01:33:59","modified_gmt":"2015-09-01T01:33:59","slug":"program-of-the-month-sfvscan","status":"publish","type":"post","link":"https:\/\/ahay.org\/blog\/2013\/05\/04\/program-of-the-month-sfvscan\/","title":{"rendered":"Program of the month: sfvscan"},"content":{"rendered":"

sfvscan<\/a> implements seismic velocity analysis by scanning stacking velocities. This transformation is also known as the velocity transform or the hyperbolic Radon transform. <\/p>\n

The following example from bei\/vela\/vscan<\/a> shows an example for transforming a CMP (common midpoint) gather to a velocity (actually slowness) scan. <\/p>\n

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By default, sfvscan<\/strong> uses velocity as the horizontal axis. To change it to slowness, use slowness=y<\/strong>. It is also possible to use velocity or slowness squared by specifying squared=y<\/strong>. The range of velocities or slownesses to scan is given by v0=<\/strong>, dv=<\/strong>, nv=<\/strong>. In addition, it is possible to scan heterogeneity parameters<\/em> in the shifted-hyperbola approximation using smax=<\/strong> and ns=<\/strong>. <\/p>\n

The offset range in the input file is specified similarly to how it is done in sfnmo<\/a>: The offset can be either regular (specified as the second axis in the input file) or irregular (specified in offset=<\/strong> file). By default, half-offset is used. To use full offset, specify half=n<\/strong>. Additionally, it is possible to specify a mask (with mask=<\/strong> file containing 1s and 0s) for skipping certain input traces during the scan. <\/p>\n

The NMO correction involved in velocity scanning is associated with the phenomenon of “NMO stretch”, a non-linear stretching of time at large offsets. The maximum allowed relative stretch is controlled by str=<\/strong> parameter. The part of the data that is stretched more than the allowed stretch gets muted. The width of the mute zone is controlled by mute=<\/strong> parameter. <\/p>\n

By default, sfvscan<\/strong> outputs simple stack. To output semblance instead, use semblance=y<\/strong> or type=semblance<\/strong>. It is also possible to output other kinds of semblance attributes: differential semblance (type=w<\/strong>), AB semblance (type=a<\/strong>), or weighted semblance (type=w<\/strong>). See <\/p>\n

Li, J., & Symes, W. W. (2007). Interval velocity estimation via NMO-based differential semblance. Geophysics<\/em>, 72(6), U75-U88. <\/p>\n

Sarkar, D., Castagna, J. P., & Lamb, W. J. (2001). AVO and velocity analysis. Geophysics<\/em>, 66(4), 1284-1293. <\/p>\n

Sarkar, D., Baumel, R. T., & Larner, K. L. (2002). Velocity analysis in the presence of amplitude variation. Geophysics<\/em>, 67(5), 1664-1672. <\/p>\n

Fomel, S. (2009). Velocity analysis using AB semblance.<\/a> Geophysical Prospecting<\/em>, 57(3), 311-321. <\/p>\n

Luo, S., & Hale, D. (2012). Velocity analysis using weighted semblance. Geophysics<\/em>, 77(2), U15-U22. <\/p>\n

The following example from jsg\/avo\/avo2<\/a> shows a comparison between velocity scans computed using the conventional semblance and AB semblance.<\/p>\n

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For robustness, semblance values are averaged in time. The length of the averaging window in samples is given by nb=<\/strong> (the default is 2 time samples). By default, the input samples are stacked along the hyperbola with an asymptotically pseudounitary weight equal to the absolute value of offset times velocity. To apply a uniform weight, use weight=n<\/strong>. For the justification of the pseudounitary weighting, see <\/p>\n

Claerbout, J. F., 1995, Basic Earth Imaging<\/a>: Stanford Exploration Project. <\/p>\n

S. Fomel, 2003, Asymptotic pseudounitary stacking operators<\/a>: Geophysics, v. 68, 1032-1042. <\/p>\n

For the operation inverse or adjoint to the velocity scan, use sfveltran<\/a>, sfvelmod<\/a> or sfvelinv<\/a>.<\/p>\n

For a faster implementation of the velocity scan (using the buttefly algorithm), use sfradon2<\/a>. <\/p>\n

From the output of the semblance scan computed with sfvscan<\/strong>, the velocity trend can be picked automatically with sfpick<\/a>. <\/p>\n

10 previous programs of the month<\/h3>\n