TGS-NOPEC Geophysical Company Seismic Imaging: How we use waves to see the interior of the earth Apache Symposium on Imaging November 6, 2008 Y. C. Kim, TGS-NOPEC

2 Outline Reflection of waves Seismic imaging using wave equation Challenges in seismic imaging Summary

3 Reflection of Waves or

4 Seismic Data Acquisition Known: Unknown:

5 One-Way Wave Equation Reflectivity using OWWE: Downward continuation: One-way wave equation: Wave equation:

6 Reverse Time Migration Reflectivity: Wave equation:

7 RTM Subsurface Image

8 Compute resources Compute cycle Memory requirements (~10 GB) Data storage (> 100 TB) Earth model building Velocities Anisotropic parameters Challenges in Seismic Imaging Parameter scan

9 Earth is Anisotropic! Isotropic ModelAnisotropic Model

10 Velocity scan Brute force method Get what you see Computationally high cost Imaging condition scan Scan for focusing Get better focused image, but mispositioned Computationally low cost Parameter Scan

11 Image Before WEM Scan Depth (km)

12 Image After WEM Scan Depth (km)

13 Imaging Condition Scan By scanning either ∆ x or ∆t, one can compensate for misfocusing due to velocity errors

14 Conventional WEM Image Depth (km)

Depth (km) WEM Image with Imaging Time Scan

16 Seismic imaging uses reflections for imaging the earth’s interior Wavefields in the subsurface are computed using wave equation Use scanning techniques to mitigate errors in model parameters Summary