Dictionary:Static corrections, statics
Corrections applied to seismic data to compensate for the effects of variations in elevation, near-surface low-velocity-layer (weathering) thickness, weathering velocity, and/or reference to a datum. The objective is to determine the reflection arrival times which would have been observed if all measurements had been made on a (usually) flat plane with no weathering or low-velocity material present. These corrections are based on uphole data, refraction first-breaks (see Figure S-22), event smoothing, and sometimes other geophysical methods. The most common convention is that a negative static correction reduces the reflection time. (a) Uphole-based statics involve the direct measurement of vertical traveltimes from a buried source; see uphole shooting. This is usually the best static-correction method where feasible. (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. See Sheriff and Geldart (1995, 261–268, 303–307, 474) and Cox (1999).
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.