Residual statics corrections
The synthetic data set in Figure 3.3-38 is the same as that in Figure 3.3-19, with the addition of residual moveout shifts. Residual moveout was introduced into the data by inverse NMO correcting the CMP gathers associated with the stack in Figure 3.3-19 using a velocity function v1(t), then by NMO correcting using a velocity function v2(t) ≠ v1(t). The solution (Figure 3.3-39) implies some traveltime distortion at the edges of the stacked section caused by low fold of coverage. (Compare this with the solution in Figure 3.3-22). Otherwise, stack response seems to be satisfactory. As long as the residual moveout variations are not large within the correlation window, the computed residual statics solution should be adequate. Use of more than one small correlation window during the picking phase may help minimize the time-dependent effect of residual moveout.
Figure 3.3-19 CMP-stacked section associated with a synthetic data set. Shot and receiver statics were applied on moveout-corrected gathers in a surface-consistent manner, while the structure term was applied in a subsurface-consistent manner. These terms are plotted above the stacked section. Random noise was added to prestack data with a spatially varying signal-to-noise ratio.
Figure 3.3-39 CMP-stacked section associated with the synthetic data set as in Figure 3.3-38 after the application of residual statics corrections. Compare the results with those in Figure 3.3-22.
Figure 3.3-22 CMP-stacked section associated with the synthetic data set as in Figure 3.3-19 with residual statics corrections. The derived shot, receiver, and structure terms are plotted at the top. Compare these estimates with the actual values in Figure 3.3-19. The maximum allowable shift is 80 ms. Also compare the resulting stacked section with that shown in Figure 3.3-20.
In some areas, the signal-to-noise ratio is so poor that a second pass of residual statics corrections must be done. The idea is that the first pass of residual statics corrections improves the signal to such a degree that a second pass should remove the residuals remaining from the first pass. For the second pass, the steps in Figure 3.3-12 must be repeated, such that the input are CMP gathers that already were corrected for residual statics. Velocity estimates must be revised between passes. Figures 3.3-40 and 3.3-41 show two different segments of a section before and after residual statics corrections that were done in two passes. Diagnostic plots of the shot and receiver statics shown in Figures 3.3-42 and 3.3-43 indicate that the first pass has taken out a significant part of the static shifts in the first segment. On the other hand, the second pass was most effective in the second segment, where the signal-to-noise ratio is relatively poorer. Repeated estimation and application of the residual statics and velocity estimation is common in some processing systems. Use of a large number of trace correlations and multiple correlation peaks in a statics program tends to minimize the number of passes required.
Figure 3.3-40 First portion of a land line illustrating the improvement in CMP stacking as a result of residual statics corrections. Stack A (a) before residual statics corrections and using preliminary velocity picks and (b) after two passes of residual statics corrections and using final velocity picks.
Figure 3.3-41 Second portion of the land line shown in Figure 3.3-40 illustrating the improvement in CMP stacking as a result of residual statics corrections. Stack B (a) before residual statics corrections and using preliminary velocity picks and (b) after two passes of residual statics corrections and using final velocity picks.
Figure 3.3-42 Diagnostics for segment A from the residual statics corrections applied on the first portion of the land line in Figure 3.3-40.
Figure 3.3-43 Diagnostics for segment B from the residual statics corrections applied on the second portion of the land profile in Figure 3.3-41.
Dip-moveout correction in practice
Dip-moveout correction is most effective at shallow times where velocities usually are low. Figure 5.2-29 shows a shallow portion of a CMP stack with and without DMO correction. Note that DMO correction has preserved the diffractions associated with the fault blocks and the fault-plane reflections. As a result, migration then has better imaged the subtle faults in the subsurface.
Albeit dip-moveout correction becomes relatively less significant at late times — just the opposite situation with migration, it can still produce a better stack than conventional CMP stacking. Figure 5.2-30 shows a moderately deep portion of a CMP stack with and without DMO correction. DMO correction has visibly enhanced the deep reflections and diffractions. Migrated sections (Figure 5.2-31) exhibit comparable imaging of the dipping events and the unconformity which envelopes these events from below. On the other hand, the crispness of the image from DMO stack (Figure 5.2-31b) also is noticeable.
The response of DMO correction to random noise is examined in Figure 5.2-32. Typically, random noise is more prominent at late times. Since DMO correction becomes increasingly less effective at late times, it may be concluded that it has minimal impact on random noise. On the other hand, random noise at shallow times may seemingly be attenuated by DMO correction. This action of the DMO process can be attributed to the fact that it is a migration process involving spreading of amplitudes along elliptical trajectories. Nevertheless, DMO correction should not be viewed and used as a process to attenuate noise.
DMO correction becomes insignificant in a medium with high velocities. Figure 5.2-33 shows a CMP stack with and without DMO correction. Velocities vary from 4000 m/s at the surface to 6000 m/s at the bottom of the section. Note that, diffractions and nearly flat reflections with conflicting dips appear to stack equally, with or without DMO correction. A way to distinguish between the two sections is the relative attenuation of the shallow coherent linear noise by DMO correction.
DMO correction must always be considered within the framework of time migration. Specifically, DMO correction is not meant to solve the problem of nonhyperbolic moveout of reflections below complex overburden structures which often are accompanied with strong lateral velocity variations. While it is not expected to solve this problem, fortuitously, DMO correction may not harm such events. The base-salt event in Figure 5.2-28 exemplifies this observation. Note that, the two diffraction-like segments of the base-salt event below midpoint 1116 between 1.8-2 s does not appear to be influenced by DMO correction.
For land data, DMO correction can be applied before statics corrections. Specifically, land data processing sequence that includes DMO correction is as follows:
- Estimate a model for the near-surface layer using refracted arrivals. The model parameters include the shape of the refractor (base of the weathering layer) and the bedrock velocity.
- Assume a value for the weathering velocity and use the near-surface layer to apply the shot and receiver refraction statics (refraction statics corrections) so as to replace the near-surface layer with the bedrock and move the shots and receivers from the topographic surface to a floating datum, which is a smoothed version of the surface topography.
- Perform preliminary velocity analysis and apply moveout corrections.
- Apply datum corrections to move the shots and receivers from the floating datum to a flat datum to which the CMP stack is referenced.
- Apply DMO correction.
- Estimate surface-consistent shot and receiver residual static shifts using methods described in residual statics corrections.
- Apply residual statics corrections to CMP gathers.
- Apply the inverse of step (d) to move the shots and receivers from the flat reference datum back to the floating datum.
- Apply inverse moveout correction using velocities from step (c).
- 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 (d).
- Apply mute and stack the data. The stacked section is referenced to the flat datum level specified in step (d).
Can DMO be used for trace interpolation? Consider the constant-velocity synthetic data set associated with the earth model shown in Figure 5.1-3. Throw away every other trace on each of the common-offset sections and simulate a coarser trace spacing (Figure 5.2-34b). The selected CMP gathers shown in Figure 5.2-34a also exhibit the discarded traces replaced with zero traces. Apply DMO correction to all of the common-offset sections (Figure 5.2-34c) and sort back to CMP gathers (Figure 5.2-34d). Note that on the common-offset sections, the zero traces are filled in at large offsets by DMO correction, whereas they are not quite filled in at small offsets. So, the amplitude distribution after DMO correction will not be uniform from one common-offset section to another. Note also the aliased energy on the CMP gathers. Stack the DMO-corrected gathers and compare the resulting section with the desired zero-offset section (Figure 5.2-35). These data have not been subjected to any gain treatment after stack; hence, the relative amplitudes have been preserved. The amplitude imbalance in the DMO stack from the gathers with missing traces is quite apparent (Figure 5.2-35c).
Figure 5.2-28 A portion of a CMP stack, (a) without, and (b) with DMO correction. Note the attenuation of coherent linear noise by DMO correction.
Figure 5.2-31 Migrated sections, (a) without DMO correction, and (b) with DMO correction. Stacked sections input to migration are shown in Figure 5.2-30.
Figure 5.2-34 (a) Selected moveout-corrected CMP gathers associated with the earth model of point scatteres depicted in Figure 5.1-3, with amplitudes on every other trace zeroed out; (b) selected common-offset sections; (c) same common-offset sections after DMO correction; (d) selected gathers as in (a) after DMO correction.
Figure 5.2-35 (a) Zero-offset section associated with the earth model of point scatteres depicted in Figure 5.1-3; (b) the DMO stack same as in figure 5.1-7c from the gathers as in Figure 5.1-6d without missing alternate traces; (c) the DMO stack from the gathers with missing alternate traces as in Figure 5.2-34d.
3-D survey design and acquisition
Almost all of the field operational aspects of 2-D acquisition are applicable to 3-D surveys. For example, selection of navigation and recording equipment depends on field conditions. The operating environment also must be considered. In the marine environment, water depth, tides, currents, sea conditions, fishing and shipping activity, and obstacles such as drilling platforms, wrecks, reefs, and fish traps must be considered. Modern marine 3-D surveys are conducted by deploying up to 12 cables and multiple source arrays. The logistics of a multicable operation requires taking careful account of such circumstances and obstacles. On land, environmental restrictions, accessibility, topography, cultivation, and demographic restrictions are factors that can affect survey design and acquisition. Because of these restrictions, careful planning and some adjustment of nominal shooting geometry often are required to achieve acceptable fold and offset distribution. Accurate surveying is a necessity for 3-D surveys, since data are collected with such dense spatial sampling; statics resulting from line-to-line surveying errors can seriously degrade the image quality obtained from 3-D migration. In fact, some claim that positioning error, rather than economics, is the limiting factor for line spacing in marine 3-D surveys.
- Residual statics estimation by traveltime decomposition
- Residual statics estimation by stack-power maximization
- Traveltime decomposition in practice
- Maximum allowable shift
- Correlation window
- Stack-power maximization in practice
- Topics in moveout and statics corrections
- Salt flanks
- Fault planes
- DMO and multiples
- DMO and coherent linear noise
- Aspects of DMO correction — a summary
- Migration aperture
- Spatial sampling
- Marine acquisition geometry
- Cable feathering
- 3-D binning
- Crossline smearing
- Strike versus dip shooting
- Land acquisition geometry