Reflection tomography is often run with beam migration for iterative velocity refinement. Both are ray-based algorithms: beam migration shoots rays down from the surface; while tomography shoots rays up from RMO picking points to the surface. The former ray tracing is straightforward with initial directions defined by a fan of preset angles; however, the latter is difficult, similar to a two-point ray tracing problem. Historically ray tracing in migration and tomography are decoupled. But in this study, we will establish a relationship between them, so that ray paths computed in beam migration provide ray take-off directions for reflection tomography. This coupling concept dramatically boosts the efficiency and accuracy of standard ray-based reflection tomography.
Using a combination of 3-D synthetic and field datasets, we motivate and demonstrate the effectiveness of a combined waveform inversion workflow using both single-frequency and broadband or time-domain inversion. We initially use single-frequency waveform inversion to isolate and invert the lowest temporal frequencies in the data, thereby updating the model with the longest spatial wavelengths first. We then follow these inversions with either broadband or time-domain waveform inversion to add high-frequency detail to the model. The advantage of this workflow is that by inverting for the lowest possible frequencies first, prior to the addition of higher frequencies, the chances of cycle-skipping are minimized. We motivate our workflow by examining gradient computations for single-frequency inversion, broadband discrete-frequency inversion, and time-domain inversions of synthetic data. We then present a data example illustrating the effectiveness of the workflow on field data from offshore Trinidad.
The Southern Trend area of Trinidad is characterized by gas chimneys and shallow gas distortions that are difficult to image due to the poor data quality from conventionally acquired data. The goal of seismic tomography is to estimate a velocity model, so that migration produces an optimum image. In this case the post- migration reflection tomography does a good job updating the velocities outboard of the gas prone areas, but not in areas where significant gas is present. In order to improve imaging in these areas, full-waveform inversion was applied to improve the velocity model, with the primary goal of improving the migrated image. Both isotropic and anisotropic inversions, including updating of anisotropic parameters were run, and the results will be shown in this paper.
We review our one-way and two-way wave-equation based illumination weighting workflows and discuss their impact on synthetic well tie estimation. We begin by comparing our wave-equation illumination weighting workflows, explain the differences between the one-way and two-way methods and describe the well tie procedure. We then demonstrate the impact of our illumination weighting workflow on improving the well tie estimation at the Thunder Horse subsalt development in the Gulf of Mexico