Joseph P. Stefani

Turning-ray tomography

Turning-ray tomography is useful for estimating near-surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning-ray tomography in its capacity to estimate absolute near-surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities.The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite-difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard-rock overthrust structure with a low-velocity zone and pinchouts, and a contemporaneous near-shore marine trench filled with low-velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one-fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/ datuming applications can also use these velocities.The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers.Turning-ray tomography estimates near-surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.

Anisotropic Reverse-Time Migration Using Co-Processors

Co-processors offer attractive acceleration opportunities to waveform-based imaging and inversion applications in challenging exploration and production environments. Unlike seismic forward modeling, the large amount of data involved in seismic imaging and inversion can pose a significant challenge to scalable acceleration. We provide and compare several computational schemes to perform anisotropic reverse-time migration on two co-processor platforms: FPGAs and GPUs. Our ongoing experiments so far indicate that both platforms can potentially achieve high speedups using acceleration-friendly schemes which minimize interruptions to computation from data movement and storage.

2D Elastic Model For Wavefield Investigations Of Subsalt Objectives, Deep Water Gulf Of Mexico

A full elastic 2D synthetic model has been generated by the SMAART* Joint Venture Consortium to simulate the primary signal and noise characteristics of sub-salt objectives associated with the deep-water environment of the Gulf of Mexico. A very detailed geologic model has been derived based on real exploration data and has served as input to a finite difference shot computation process. The data has been rigorously computed with many offsets and can be decimated to mimic shot geometries of long offset streamer data. This dataset has been primarily used for benchmarking multiple attenuation algorithms. It is now being released to the geophysical community for further research in de-multiple and other wavefield processing techniques.

Ground Truth Results of 3D Pre-stack Subsalt Imaging For Structure And Stratigraphy

Chevron’s deepwater reservoir discovery, delineation and reserves analyses of the Gulf of Mexico subsalt play have been progressing over the last several years. A substantial factor of this effort is the continuous improvement in geophysical tools and procedures to create and evaluate images of subsalt reservoirs. A highly integrated approach for this work process enables improvements in quality and timelines of subsalt imaging, plus a consistently documentable understanding of subsalt reserves for business decisions (Stefani et. al. 1998)