Processing sequence for statics corrections
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It is important that we revisit the processing sequence in Figure 3.3-12 and the near-surface model depicted in Figure 3.4-10 for a rigorous description of moveout and statics corrections. Starting with unprocessed field records, a detailed version of the processing sequence in Figure 3.3-12 is described below:
Figure 3.3-12 Processing flowchart with residual statics corrections.
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.
- Pick and edit first breaks from unprocessed field records.
- Assume or derive from uphole information a value for weathering velocity.
- For a downhole source, apply the uphole correction.
- Compute the bedrock velocity and intercept times at all shot and receiver stations using a refraction statics method, such as the generalized reciprocal or the least-squares technique.
- By using the weathering velocity, bedrock velocity and intercept times, compute the depth to bedrock at shot-receiver stations (equations 53a, 53b).
- Apply the shot and receiver statics to replace the weathering layer with the bedrock while placing the shot and receivers on a floating datum that corresponds to a smoothed form of the topographic surface. The static time shift Δτij to apply for a given source-receiver pair is (Figure 3.4-10)
where zj and zi are the thickness of the weathering layer at shot and receiver stations, ETj and ETi are the true shot and receiver elevations referenced to the topography, and EFDj and EFDi are the shot and receiver elevations referenced to the floating datum, respectively. The reason for moving the shots and receivers to a floating datum close to the surface topography, rather than to a flat datum, is to be able to preserve the hyperbolicity of reflection times while placing the shot and receiver pairs associated with a CMP gather over the local datum level that is nearly flat within the spread length.
- Apply geometric spreading correction and deconvolution to shot records and sort to CMP gathers.
- Perform preliminary velocity analysis and apply moveout corrections.
- Apply datum corrections to move the shots and receivers from the floating datum as specified in step (f) to a flat datum ED to which the CMP stack is referenced. Refer to Figure 3.4-10 and note that the datum correction Δτij for a source-receiver pair is given by
where EFDj and EFDi are the shot and receiver elevations with respect to the floating datum specified in step (f).
- Estimate surface-consistent shot and residual static shifts using methods described in Residual statics corrections.
- Apply residual statics corrections to CMP gathers from step (i).
- Apply the inverse of step (i) to move the shots and receivers from the flat reference datum back to the floating datum.
- Apply inverse moveout correction using velocities from step (h).
- Perform velocity analysis and apply moveout correction.
- Apply datum corrections to move the shots and receivers from the floating datum to the reference flat datum as in step (i).
- Apply mute and stack the data. The stacked section is referenced to the flat datum level ED specified in step (i).
- First breaks
- Field statics corrections
- Flat refractor
- Dipping refractor
- The plus-minus method
- The generalized reciprocal method
- The least-squares method
- Model experiments
- Field data examples
- Topics in moveout and statics corrections