Estimation of the overburden model

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

Subsalt imaging in the North Sea

Start the analysis by interpreting the time horizons from the unmigrated CMP stack that correspond to layer boundaries with significant velocity contrast. These horizons are denoted in Figure 10.1-3 — water bottom (H1), base Miocene Unconformity (H2), base Upper Tertiary (H3), base Lower Tertiary (H4), base Cretaceous Chalk (H5), base Upper Triassic (H6a), base Lower Triassic (H6b), and base Zechstein (top Rotliegendes) (H7). The time horizons are assumed to be equivalent to zero-offset reflection times that are used in coherency inversion (models with horizontal layers).

Horizon H6b is the top-salt boundary, which separates the overburden and substratum parts of the model. Although interpreted, horizon H7 — base-salt boundary, is not included in the analysis sequence for modeling the overburden. Instead, it is dealt with as part of modeling the substratum.

The depth horizon associated with the water bottom is obtained simply by normal-incidence depth conversion of the time horizon (H1 in Figure 10.1-3). Then, the following sequence was applied to horizons H2-H6b, one layer at a time, starting at the top. For the sake of the discussion here, assume that the velocity-depth model for the first n − 1 layers already have been determined, and that the nth layer is under consideration.

  1. Perform coherency inversion along the horizon under consideration, pick semblance maxima, and derive an interval velocity profile as a function of the midpoint location along the line. Available velocity gradient information is incorporated into the interval velocity estimation. As demonstrated in models with horizontal layers, the accuracy in velocity estimation degrades with shorter effective cable length, faster velocity and deeper horizon. Spurious peaks on semblance curves and rapid lateral variations in velocity should be avoided. Accordingly, smoothing is applied to the velocity profile as much as geologically plausable, but not excessively so as to retain lateral velocity variations that are realistic.
  2. Create a gridded velocity-depth model that consists of two parts — the known part on top, with the n − 1 layers already established, and the unknown part underneath, defined as a half-space with its velocity equal to the velocity of the nth layer derived in step (a).
  3. Perform poststack depth migration using the gridded velocity-depth model from step (b) down to a depth just below the layer of interest.
  4. Interpret the depth horizon associated with the base of the layer under consideration from the depth-migrated section.
  5. To verify the accuracy of velocity estimation from coherency inversion and, if needed, to update the layer velocity (model updating) derived in step (a), perform prestack depth migration to create image gathers at some interval along the line. The velocity-depth model used for prestack depth migration is the same as that used for poststack depth migration in step (d).

Repeat steps (a) through (e) for all the layers within the overburden down to top-Zechstein boundary (Horizon 6b), and thus establish a velocity-depth model for the overburden (Figure 10.1-4).

3-D structural inversion applied to seismic data from the Southern North Sea

Based on the volume of stacked data (Figure 10.6-2) and the well data from the area, we make the following characterization of the earth model in depth:

  1. We may consider the earth model in depth in two parts — the overburden above the salt diapir and the substratum that includes the salt mass and the underlying strata.
  2. Time migration may be acceptable within the overburden, but depth migration is imperative within the substratum.
  3. The top-salt boundary (the brown horizon in Figure 10.6-2), which also is the boundary between the overburden and the substratum, is where the most severe ray bending takes place.
  4. The overburden above the Zechstein formation comprises layers with significant velocity contrast — Tertiary, Cretaceous chalk, and Triassic units.
  5. As a result of extensional tectonics, the overburden is intensely faulted. Note the collapsed structures above the apex and to the left of the salt diapir.
  6. Vertical velocity gradients within the overburden layers are significant and vary spatially.
  7. The base-Zechstein reflection times (the red horizon in Figure 10.6-2) are severely distorted by the complex overburden.
Figure 10.6-2  Selected crosslines as in Figure 10.6-2 from the 3-D volume of unmigrated DMO-stacked data with the superimposed color-coded horizons that correspond to layer boundaries with significant velocity contrast.

Based on these characteristics, an appropriate procedure for estimating an earth model in depth involves layer-by-layer application of 3-D coherency inversion to estimate layer velocities and 3-D poststack depth migration to delineate reflector geometries within the overburden. This is then followed by 3-D prestack depth migration for estimating the velocity field within the substratum.

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