Field statics corrections
It is appropriate now to review various methods of field statics corrections. Consider the near-surface model depicted in Figure 3.4-10. If shots (denoted by S) are located below the weathering layer, then the total static correction to apply to the trace associated with midpoint M is tD = tS + tR, where tS and tR are the shot and receiver static corrections, respectively, down to a specified datum D. From the geometry of Figure 3.4-10, the field statics correction ΔτD can be computed by
where ED is the datum elevation and ES and ER are the surface elevations at the shot and receiver stations, respectively, DS is the depth of the shot hole beneath the shot station, and DR is the depth of the shot hole near the receiver station, tUH is the uphole time measured at the receiver location (the time associated with the distance DR in Figure 3.4-3). Finally, vb is the bedrock (subweathering) velocity that may be derived from a deep uphole survey (to a point well below the weathering layer) conducted in the area.
Figure 3.4-10 A near-surface model for statics corrections when shots are situated below the weathering layer. Here, S = shot, ES = elevation at the shot station on the ground, R = receiver, ER = elevation at the receiver station on the ground, T = surface topography, B = base of weathering, D = datum, ED = datum elevation, vw = weathering velocity, and vb = bedrock velocity.
Figure 3.4-3 A shot record with distinct first breaks.
An uphole survey involves placing shots down the hole at various depth levels, then recording the arrivals at the surface near the hole. Alternatively, shots and receivers can be reciprocated if there is a caving problem down the hole. The hole must be deep enough to reach below the weathering layer. This provides a plot of time versus depth from which the bedrock velocity is obtained.
In land surveys, shots are not always placed in the bedrock for economic reasons, especially in areas with a thick weathering layer. Also, impulsive sources are not always used. Instead, surface sources such as vibroseis often are used. When surface sources or sources in shallow holes are used, the refracted arrivals can, at least in theory, be used to compute the static correction ΔτD down to a specified datum.
- First breaks
- Flat refractor
- Dipping refractor
- The plus-minus method
- The generalized reciprocal method
- The least-squares method
- Processing sequence for statics corrections
- Model experiments
- Field data examples
- Topics in moveout and statics corrections