1998 UTAM ACCOMPLISHMENTS

This report summarizes the 1998 research accomplishments of the UTAM researchers. Fourteen companies joined for 1998: Advance Geophysical, Advanced Data Systems, Amerada Hess, Amoco, Arco, Chevron, Conoco, Exxon, Japan National Oil Co., INCO, Mobil, Noranda, Phillips, and RC2.

Several new directions were initiated this year and we expect substantial progress in the coming year.

• Migration deconvolution. Jianxing Hu and Paul Valasek demonstrated with 3-D marine data that migration deconvolution (MD) can noticeably improve the clarity and resolution of 3-D poststack migration images. Comparison with conventional post migration processing procedures shows a modest improvement in lateral resolution for MD. I believe that MD may become a standard 3D processing step in the near future. In the coming year, Jianxing will be working to show that prestack MD can suppress the acquisition footprint of 3-D prestack migration.

• Migration deconvolution. J. Hu also showed the effectiveness of migration deconvolution in correcting for streamer feathering and positioning errors in cables. He is seeking 3-D marine data with feathering problems to test the validity of his method.

• Wavepath migration. Kirchhoff migration is typically very time consuming for 3-D velocity analysis. As an alternative we developed the wavepath migration method, where the reflection amplitude is backprojected along wavepaths, rather than ellipses. The computational savings, in principle, should be significant compared to standard Kirchhoff migration. Tests on 2-D synthetic and field data confirm this computational efficiency, but at the cost of some degradation of image quality. The next step is to improve image quality and apply this method to 3-D data.

• Salt flank imaging. Salt flank imaging by migration of PS transmitted waves in VSP data. Sheley shows a crosswell data set where strong PS transmitted waves emanate from the boundary of an ore body. These arrivals are migrated to delineate the ore body boundary. We then show strong PS transmitted waves can be generated from the boundary of a salt flank, and propose that such arrivals recorded by a VSP experiment can be used to delineate salt flank boundaries. Tests with synthetic data verify this proposal. The next step is to obtain an offset VSP data set collected near a salt flank, and test the idea of imaging PS-transmission arrivals.

• Multiple removal. Yue Wang presents his preliminary results at removing multiples using the least squares migration filtering (LSMF) method. He extends the previous work of Liu to inhomogeneous media and also shows how LSMF can be used to interpolate aliased data so that it can be used for multiple removal. Trials with synthetic data and the Unocal marine data show that the LSMF method can be competitive with other multiple attenuation methods. However, there is much room for improvement as Yue will try to improve the effectiveness of LSMF this coming year.

• True amplitude remigration. Jing Chen proposes reflector-based remigration as an improved implementation of true amplitude migration. This method requires knowledge of the reflector's location and orientation, which can be obtained from data processing. Numerical tests show that this method can produce images with their amplitudes representing the true reflection coefficients.

• Wavepath reflection tomography. An algorithm is tested to reconstruct the velocity model by back-projecting reflection traveltime residuals along ray paths. Using the incidence angles of reflection traveltimes in both the CSG and CRG domains, the messy parameterization of the interface is avoided. Dynamic smoothing is used to stabilize the algorithm and produce a more accurate tomogram. This algorithm can be applied to CDP and crosswell seismic data. Preliminary results show that this algorithm is capable of partially reconstructing the velocity field from reflection traveltime data. In the coming year, Jianmeng will extend this method to 3-D, and apply it to field data.

• Diffraction slice theorem for traveltime tomography in an inhomogeneous medium. The diffraction-slice theorem is valid for waveform data associated with a homogeneous background medium. Under the Rytov approximation and using Beylkin's theory, J. Sheng extends the diffraction-slice theorem to the case of wavepath traveltime tomography in an inhomogeneous background and an arbitrary configuration of sources and receivers. As a special case, the result for a constant background velocity is obtained analytically. In the coming year J. Sheng will exploit this extended theorem for its computational advantages and also for its ability to predict resolution.

• Velocity continuation for arbitrary velocity. Schuster derives the governing equations of a migration image, and shows how these can be manipulated to get the general equations for remigration in an arbitrary velocity medium. The implication is that migration images can be remigrated with a different migration velocity by solving the wave equation using the migration image as a grid of source terms.

• Entropy migration. J. Yu uses an entropy regularization term with the least squares migration formalism. He shows that entropy migration can suppress high frequency noise in the least squares migration image, but at the cost of a small loss of resolution.
• VSP least squares migration. Hongchuan Sun and Sen Chen (Exxon) applied the least squares migration (LSM) method to synthetic VSP data. Their results show that LSM can improve the vertical and lateral depth resolution of the migrated image.
• JAVA SU. Hao Zhang develops a JAVA GUI interface to execute SU commands. This interface will facilitate the use of the SU codes in processing seismic data and will be available upon request.