At the start of the SEAM project, a general consensus was that a finite-difference (FD) method or related method would be the most efficient to create a synthetic seismic data set from the SEAM geologic model. This was not a requirement, and SEAM was open to alternative simulation approaches. As with all methods, FD has its specific limitations and costs associated with a particular simulation. The SEAM organization tried to anticipate many of these when choosing the geologic model size and survey specification so that the simulation could be performed in a reasonable amount of time and cost.
The main limitation is that the model is sampled at discrete points along a regular grid. Thus, it is not possible to capture jumps in material properties at arbitrary points. This leads to time shifts in output data when comparing results of simulations that use finer or coarser spatial sampling. These time shifts are a major cause of differences as discussed in “Benchmark Comparisons” and related references.
The discretization of a continuous model also has implications for the solution. In short, the spatial wavelength has to be adequately sampled (cf. the Nyquist criterion). Inadequate spatial sampling leads to what is called numerical dispersion, i.e., different frequencies propagate at different velocities. The numerical dispersion is different for FD schemes of different accuracy order. A higher-order scheme in general is less affected by numerical dispersion, which allows the use of a larger grid size for a given amount of dispersion; however, higher-order schemes require.
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SEAM Phase I: Challenges of Subsalt Imaging in Tertiary Basins, with Emphasis on Deepwater Gulf of Mexico
“SEAM is a collaborative industrial research effort dedicated to largescale, geophysical numerical simulation projects. The projects are designed to provide the geosciences exploration community with earth models and simulated data that represent significant geophysical challenges of high business value to the petroleum resource industry. The Phase I project produced a deepwater subsalt earth model designed to capture as much physics and realism as possible in a 3D model that was relevant to oil and gas exploration. The 3D model covers a 40 õ 35 õ 15 km area and includes a complex salt intrusive in a folded Tertiary basin. The primary deliverable was the seismic data set of variable density acoustic simulations consisting of 200 TB of uncompressed traces for over 60,000 shots. Also delivered to the participants were several smaller compressed subsets of these data (“classic” data sets) intended for easier handling, simpler distribution to third parties, and easier comparison of imaging tests results. This report covers how the prime objectives of Phase I were met. Details are outlined in chapters on Model Development, Numerical Design and Vendor Qualification, Acquisition Design, Production Simulations, Quality Control, and Data Storage and Distribution.”