SWAMI | Schlumberger

SWAMI

Surface wave analysis, modeling, and inversion

Characterization of heterogeneous near-surface geology

A key challenge in reservoir characterization with onshore seismic data is accurately compensating for the extreme horizontal and vertical velocity variations and the complex surface wave noise contamination that result from the highly heterogeneous near-surface geology.

Conventional processing philosophy has regarded these surface waves as unwanted coherent noise and removed them at the earliest opportunity. These attempts have met with variable and typically little success in the presence of highly spatially aliased and complicated multimode scattered ground roll.

SWAMI surface wave analysis, modeling, and inversion enables extraction of the phase velocity characteristics per frequency from the surface waves to facilitate both velocity model building and advanced coherent noise attenuation.

Surface Waves Characterize the Near Surface
Shot gather analysis of the ground roll provides a surface wave dispersion curve (top) used during the inversion to build near-surface velocity models. More accurate near-surface characterization provides the information between the upholes and a better static solution (bottom).

Near-surface characterization

SWAMI inversion enables converting measurements from the analysis of the surface waves into a near-surface velocity model. This model can be added to geological information and geophysical measurements to provide a more accurate representation of the near-surface structure and initiate tomographic analysis as part of the prestack depth migration process.

Prestack depth migration of land data is challenging because of velocity variations in the near surface. Generating a high-resolution velocity model addresses these velocity variations. The near-surface model is also used to calculate surface-consistent statics. Depending on the near-surface geology and additional ancillary information, the surface wave inversion can be used for

  • full elastic characterization—direct Vs/Vp depth model derivation from multimode inversion
  • Vs to Vp calibration using upholes in the absence of guided waves
  • Vs and Vp estimation using simultaneous joint inversion.

Surface wave noise attenuation

Using the same frequency versus phase velocity properties of the analyzed surface waves, SWAMI inversion also enables estimation and removal of the

  • direct aliased and nonaliased ground roll for multiple surface wave modes
  • first-order scattered ground roll for multiple surface wave modes.

These types of complex and coherent noise signals that are effectively attenuated using SWAMI surface wave analysis, modeling, and inversion cannot be handled with conventional velocity discrimination techniques, such as f-k filtering.

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