Correcciones estáticas, estáticas
Correcciones aplicadas a los datos sísmicos para compensar los efectos de variaciones en la elevación, espesor de la capa superficial de baja velocidad (meteorización), velocidad sísmica de la capa meteorizada, y/o referencia del datum. El objetivo es determinar los tiempos de llegada de reflexión que se habrían observado si todas las medidas hubieran sido realizadas (usualmente) en un plano horizontal sin la presencia de capa meteorizada o de baja velocidad. Estas correcciones son basadas en datos de pozos verticales, modelos basados en primeros arribos de refracciones desde la base de la capa de baja velocidad (Ver la figura S-22), suavizado de modelos y algunas veces otros métodos geofísicos.
El acuerdo más común es que una corrección estática negativa reduce el tiempo de reflexión.
(a) Estática basada en tiempos verticales de pozo implica la medición directa de los tiempos de viaje verticales desde una fuente enterrada; ver disparos en pozos verticales. Este suele ser el mejor método de corrección estática cuando es posible.
(b) First-break statics are the most common method of making field (or first-estimate) static corrections, especially when using surface sources. The ABC method (q.v.) and variations for more complex assumptions are used for this determination; see refraction statics.
(c) Data-smoothing statics methods assume that patterns of irregularity which events have in common result from near-surface variations and hence static-correction trace shifts should minimize such irregularities. Most automatic statics-determination programs employ statistical methods to achieve the minimization. Data-smoothing methods are generally applied to remove small residual errors after first applying methods (a) or (b). Second-order statics corrections are often called trim statics. [1] [2]
Underlying the concept of conventional static corrections is the assumption that a simple time shift of an entire seismic trace will yield the seismic record that would have been observed (1) if the geophones had been displaced vertically downward (or upward) to the reference datum, an assumption not strictly true, especially if the surface-to-datum distance is large, and (2) that the subdatum velocity does not change horizontally. Conventional static correction methods are most apt to fail where there are (1) large rapid changes in the topography or base of weathering, (2) horizontal velocity changes below the weathering, thus violating the assumption that the subdatum velocity does not vary significantly, (3) large elevation differences between the datum and the base of the weathering, or (4) inadequate controls on long-wavelength statics (q.v.). Large sea-floor relief is apt to be associated with horizontal velocity changes that cannot be compensated with static corrections. S-waves statics are often appreciably different and much larger than P-wave statics.
References
- ↑ Sheriff, R. E; Geldart, L. P (August 1995). Exploration Seismology, 2nd Ed. Cambridge Univ. Press. p. 261–268, 303–307, 474. ISBN 9780521468268.
- ↑ Cox, Mike (1999). Static Corrections for Seismic Reflection Surveys. Society of Exploration Geophysicists. http://library.seg.org/doi/book/10.1190/1.9781560801818/.
See also
Refraction statics corrections
External links
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