WAVEFIELD PREDICTION OF WATER-LAYER-MULTIPLES Ruiqing He University of Utah Feb. 2004

Outline Theory Synthetic experiments Application to Unocal data Introduction Theory Synthetic experiments Application to Unocal data Conclusion

Introduction accurate wavefield prediction of multiples. Other works: Primary-preserving multiple removal demands accurate wavefield prediction of multiples. Other works: - Delft - Amundsen, Ikelle, Weglein, etc. Multiple prediction is the prerequisite for multiple subtraction. If the multiple prediction is more accurate, then the subtraction is more effective. Water layer multiples

Outline Theory Synthetic experiments Application to Unocal data Introduction Theory Synthetic experiments Application to Unocal data Conclusion

Berryhill and Wiggins’s Methods Kirchhoff Summation Forward extrapolate traces down to water bottom Off-shore seismic data Kirchhoff Summation Forward extrapolate bottom traces up to receivers  Emulated Muliples Multiple attenuation Filtered Subtraction The problem lies in the receivers recorder neither up-coming waves nor down-going waves’ pressure, the receivers recorder their superposition waves’ pressure. Water surface Receiver line Water bottom

The proposed method FD: Finite Difference Decomposition of receiver-side ghosts Wave forward extrapolation to the water bottom Off-shore seismic data FD FD FD Multiples with last round-trip in water layer Primary- preserving multiple removal Wave forward extrapolation to the receivers DS filtering FD: Finite Difference DS: Direct (simple) Subtraction Other multiple attenuation

Why Finite Difference? - speed - convenience Advantage - speed - convenience - capability: heterogeneous medium Because Kirchhoff method is based on homogenous assumption, it has difficulty to handle heterogeneous media. Disadvantage - dispersion?: reality, high-order FD

Types of Water-Layer-Multiples LWLM: Multiples that have the last round-trip in the water layer. Other WLM: other water-layer-multiples except LWLM. In very strict situations, any other WLM can have an identical LWLM that has the same traveltime and waveform in seismic traces. In most cases, LWLM and other WLM are similar (e.g. 102 and 201 in the previous talk).

Wavefield Extrapolation of RSG Mirror image of the Receiver line Water surface Receiver line U RSG

Decomposition of RSG RSG + DATA f U Mirror image of the Receiver line Water surface Here f is an inexplicit function that can be implemented by many means. f Receiver line U

Outline Theory Synthetic experiments Application to Unocal data Introduction Theory Synthetic experiments Application to Unocal data Conclusion

Synthetic Model water BSR Depth (m) Salt dome Sandstone 1500 3250 water BSR Depth (m) BSR: Boundary Simulating Reflectors. Salt dome Sandstone 1500 Offset (m) 3250

Synthetic seismic data 400 Time (ms) The water-bottom multiples are so strong that they blur the reflection from the salt dome. 2500 Offset (m) 3250

Decomposed RSG 400 Time (ms) 2500 Offset (m) 3250

Predicted LWLM 400 Time (ms) 2500 Offset (m) 3250

Waveform Comparison between Data & RSG Amplitude 2400 600 Time (ms)

Waveform Comparison between Data & LWLM Amplitude 2400 600 Time (ms)

Waveform Comparison between Data & RSG+LWLM Amplitude 2400 600 Time (ms)

Elimination of RSG & LWLM 400 Time (ms) This result is primary-preserving, but not all water-layer related peglegs are eliminated. As mentioned before, other WLM are not predicted, and remain there, but there are similar to the predicted LWLM, so they can be attenuated by additional subtraction algorithms. 2500 Offset (m) 3250

Further Multiple Attenuation 400 Time (ms) 2500 Offset (m) 3250

Outline Theory Synthetic experiments Application to Unocal data Introduction Theory Synthetic experiments Application to Unocal data Conclusion

Unocal field data 600 Time (ms) 2400 Offset (m) 3175

Inadequate RSG Decomposition 600 Time (ms) Because the real seismic waves are propagation in 3D, one single receiver line is not enough for exact RSG decomposition. I leave this challenging topic for future research. 2400 Offset (m) 3175

Emulated LWLM 600 Time (ms) 2400 3175 Offset (m) Emulate LWLM as Berryhill and Wiggins did, but with FD rather than Kirchhoff methods. 2400 Offset (m) 3175

Waveform comparison between Data & Emulated LWLM Amplitude 1400 Time (ms) 2400

Attenuation of WLM 600 Time (ms) 2400 Offset (m) 3175

Attenuation of WLM

Attenuation of WLM 600 Time (ms) 2400 Offset (m) 3175

Attenuation of WLM

Subtracted WLM 600 Time (ms) 2400 Offset (m) 3175

Outline Theory Synthetic experiments Application to Unocal data Introduction Theory Synthetic experiments Application to Unocal data Conclusion

Conclusion Theoretically revives Berryhill and Wiggins methods for primary-preserving removal of one kind of water-layer-multiples. Requirements are practically obtainable, and can be derived from seismic data. Applicable to field data with approximations. The differences between this research and Berryhill & Wiggins’s methods are: 1: It uses FD rather than Kirchhoff methods. 2: It decomposes Receiver-Side-Ghosts in synthetic data. 3: It makes distinctions among water-layer related multiples. 4. It results in primary-preserving multiple removal. Overcomes the Delft method by alleviating acquisition requirements and the need to know the source signature.

Future Work Ghost decomposition for field data. 3D to 2D seismic data conversion. Multiple subtraction.

Reference Berryhill J.R. and Kim Y.C., 1986, Deep-water pegleg and multiples: emulation and suppression: Geophysics Vol. 51, 2177-2184. Wang Y., 1998, Comparison of multiple attenuation methods with least-squares migration filtering: UTAM 1998 annual report, 311-342. Wiggins J.W., 1988, Attenuation of complex water-multiples by wave-equation-based prediction and subtraction: Geophysics Vol.53 No.12, 1527-1539.

Thanks 2003 members of UTAM for financial support.