Bruno Kaelin

Least-squares attenuation of reverse-time-migration artifacts

Reverse-time-migration artifacts occur when diving waves, head waves or backscattered waves crosscorrelate. These events are particularly strong where high velocity contrasts exist. Simple filtering of the final image can lead to good results but might compromise the integrity of the signal of interest. We demonstrate that a better technique is to apply least-squares filtering with prediction-error filters, a method traditionally used for S/N separation.

Imaging condition for reverse time migration

In reverse time migration the imaging condition is estimated by cross-correlating the source wave-field with the receiver wave-field under the basic assumption that the source wave-field represents the down-going wave-field and the receiver wave-field the up-going wave-field. However, for large impedance contrasts and complex geological structures the wave-fields cannot be separated efficiently. In these cases the cross-correlation leads to low frequency artifacts. The imaging condition can be improved by dividing the cross-correlated image of each shot by the receiver illumination. This method is simple, it does not introduce phase shifts and it requires little additional computation, because the receiver illumination can be directly computed from the receiver wave-field.

Least-square Attenuation of Reverse Time Migration Artifacts

SUMMARY Reverse time migration artifacts occur when diving waves, headwaves or backscattered waves cross-correlate. These events are particularly strong where high velocity contrasts exists. Simpleminded ltering of the nal image can lead to good results, but might compromise the integrity of the signal of interest. A better technique consists in applying a least-square ltering with prediction-error lters, a method traditionally used for signal/noise

Eliminating imaging artifacts in RTM using pre-stack gathers

Thanks to advances in computer technology wave-based methods like shot profile migration and reverse time migration (RTM) (Baysal et al., 1984; Whitmore, 1983) have become established methods during the last decade. However, RTM is also known for producing low frequency artifacts, when sharp velocity contrasts are present. These artifacts are created by the unwanted cross-correlation of head waves, diving waves, and backscattered waves, which are not included in the imaging condition. There are different alternatives for attenuating these artifacts: 1. Wave-field propagation approaches where the wave equation is modified to attenuate reflections at the velocity contrasts (e.g., Baysal et al., 1984; Fletcher et al., 2005). 2. Imaging condition approaches in which only the energy created by reflections is kept in the final image (e.g., Yoon et al., 2004). 3. Wave-field decomposition approaches (e.g., Faqi et al., 2011) 4. Post-imaging condition approaches in which the artifacts are filtered after imaging on each shot or the stacked image (e.g., Guitton et al., 2007). The filtering can be done on subsurface offset or angle gathers (e.g., Sava and Fomel, 2003).

Using seismic crosswell surveys to determine the aperture of partially water-saturated fractures

An air injection experiment in a shallow fractured limestone at Conoco’s borehole test facility near Newkirk, Oklahoma, has shown large effects on the amplitude but small effects on the traveltime of the transmitted seismic waves. We have analyzed data from a seismic monitor survey in the kilohertz range performed during the experiment and have modeled the fracture zone as a single fracture. The large amplitude decrease during the experiment is mainly attributable to the impedance contrast between the small velocities of gas‐water mixtures inside the fracture and the formation. The intrinsic attenuation of the fracture fluid seems to be a second‐order effect for small apertures. During the experiment the seismic wavelengths inside the fracture become comparable to the aperture dimension, which allows an estimation of fracture apertures. We also have analyzed a crosswell survey acquired shortly after the experiment and computed aperture and gas concentration profiles. Our aperture estimates range from <1 m...

3D migration of a simulated wide‐azimuth towed‐streamer survey

A 3D synthetic wide-azimuth towed-streamer (WATS) dataset is migrated with different migration techniques. These migration techniques are suited for imaging complex overburden. First, a wave-equation method based on a oneway propagator is used. Second, a two-way method that utilizes one-way propagators for the wavefield extrapolation downward and upward is tested. Finally, a method based on the solution of the two-way acoustic wave equation, also known as Reverse Time Migration (RTM) is selected.

Imaging Methods in Complex Overburden

C023 Imaging Methods in Complex Overburden A. Guitton* (3DGeo Inc.) A. Valenciano (Stanford University) F. Ortigosa (Repsol YPF) & B. Kaelin (3DGeo Inc.) SUMMARY Three migration techniques suited for complex overburden are presented. The first one is a wave-equation method based on a one-way propagator. The second one is a two-way method that utilizes one-way propagators for the wavefield extrapolation downward and upward. The third one is based on the solution of two-way wave equation also know as Reverse Time Migration. The selection of a migration algorithm should be based on practical and geophysical considerations. The complexity of the subsurface