John Mathewson

Data-driven tomographic velocity analysis in tilted transversely isotropic media: A 3D case history from the Canadian Foothills

We evaluated how velocity and anisotropy model-building strategies affect seismic imaging in the Canadian Foothills Thrust Belt by comparing the results of a model-driven approach with those of a data-driven approach. Two independently run Kirchhoff prestack depth-imaging projects were initiated using different static corrections for near-surface weathering layers and using different velocity and anisotropy model-building strategies. We observed that an isotropic data-driven reflection tomography velocity model-building approach resulted in a significantly better stack image than did a highly interpretive anisotropic model-driven velocity model-building approach. By carefully introducing anisotropy into the former, data-driven approach, we achieved significant improvements in positioning, including more accurate depth ties between the seismic image and well tops and better definition of structural geometries. The differences in the imaging observed at the various stages of this case history illustrate the...

The Case For Depth Imaging All 3D Data

Structural complexity in time-imaged seismic data is often due, not to geology, but to lateral-velocity variations that have not been properly addressed. Numerous data examples show the improved imaging and phase stability that depth migration provides when compared with time migration. Gridded cell tomography is the enabling technology that allows the development of geologically-consistent velocity models suitable for use with prestack depth migration. The incorporation of VTI or TTI parameterization in the imaging process can also provide benefits, especially if well calibration is invoked, since this will improve the structural response and form “true-depth” 3-D seismic volumes.

Prediction Of 3-D seismic footprint from existing 2-D data

Conventional methods of 3-D survey design concentrate on properties of acquisition geometry such as fold, offset distributions, and azimuth distributions and combinations of these properties such as fold within given offset and/or azimuth ranges. This, in a rough and ready way, enables one to make comparative statements about the relative merits of one design over another but without saying whether either will be good enough for a given target in a given area. This is because the approach neglects the “seismogram component”—i.e., the local earth response. In particular it ignores the characteristics of shot-generated noise that leak through the imperfect stack which is the consequence of most 3-D survey designs. This leakage gives rise to a so-called “footprint” on stacked data and further results such as migrated volumes derived from the stacked data.

A case study for azimuthally anisotropic prestack depth imaging of an onshore Alaska prospect

In a study of the Sterling-Triangle area of Alaska, U.S.A., we initiated prestack depth migration (PSDM) to improve imaging on a prospect initially identified on a prestack time-migrated (PSTM) volume. Under the isotropic media assumption, the first few iterations of the reflection tomography had difficulty in converging to the proper velocity model. Upon further investigation, a very-high-velocity conglomerate layer was identified in the middle of the section across the whole survey area. We adopted the salt-flood practice, routine in depth-imaging salt provinces such as the Gulf of Mexico. The strategy was to focus on the shallow section above the conglomerate first, followed by a constant-velocity flood for picking the conglomerate base. The finalisotropic PSDM result showed that significant residual moveout differences existed on gathers along different azimuths. The net anisotropic effect on the isotropic PSDM was a degraded final PSDM volume. In the subsequent anisotropic PSDM work, azimuthally vari...

Automated and Semi-Automated Depth Registration of PP and PS Images

The registration of PP to PS images is an essential step of iterative model building of multicomponent data. Depending on the quality of the data and the accuracy of the initial model of the earth’s interior, this operation may or may not require identifying corresponding geological boundaries. We developed a method for PP to PS registration that operates directly on images defined in the depth domain. This method can use horizons (either manually or automatically interpreted) or perform this operation in a data driven (automated mode) without interpreter input using only PP and PS seismic images. The application to synthetic and real data demonstrates the value of this method.

Advanced PP and PS Model Building – An Offshore Mexico Example

Model building and depth imaging with PS converted-wave data is more complicated than with P-wave data alone. We must determine appropriate compressional and shear velocities such that PP and PS sections match, and accuracy of anisotropic parameters is crucial to minimize residual moveout on both data types. Fortunately, recent technological developments, notably joint PP-PS tomography, allow us to produce excellent PS depth-imaged results with reasonable turnaround time. In this case study we describe the application of PP-PS model building and depth imaging to an OBC seismic survey. The area is characterized by complex structure combined with large velocity contrasts, both of which cause problems for PS model building. A state-of-the-art workflow was applied that included joint PP-PS tomography, maximum use of well data, and minimal interpretation. High-quality PP and PS results were produced in a reasonable timeframe.

Detection of Channels in Seismic Images Using the Steerable Pyramid

Channels are important geologic features in the exploration for oil and gas, because channel sands make excellent reservoirs for oil and gas. With 3D seismic images channels can often be mapped easily on time or depth slices, but in some situations they

Detection of channels in seismic images using the steerable pyramid

Channels have always been important geologic features in the exploration for oil and gas. With 3-D seismic data they can often be mapped easily on time or depth slices. In other situations they can be difficult to detect, due to structural complexity or other factors. There are a number of image-processing algorithms that can be used to enhance linear features such as channels in 3-D seismic volumes. One way involves the use of steerable pyramid filters to partition a seismic image in terms of scale and orientation. Features can then be characterized according to dimensionality and direction using the partitioned image. Here, we explain our implementation of the steerable pyramid in 2-D and 3-D, and show how it can be used to enhance image features. Examples of channel enhancement on synthetic seismic images demonstrate the efficacy of this processing.