David Walraven

Wave equation GSP prestack depth migration and illumination

A heterogeneous medium with strong velocity contrast and complex subsurface structures presents a great challenge to seismic imaging. Prestack depth migrations based on the wave equation are accurate but very expensive, thereby limiting their use. However, recent advances in efficient 3D wavefield extrapolators and the continued increase in computing performance are making application of full 3D prestack wave equation depth imaging more affordable. In addition, since PreSDM was first employed for subsalt and carbonate imaging, there have been major advances in efficient and accurate 3D wavefield extrapolation technologies. The generalized screen propagator (GSP) is a one-way wave-equation-based wide-angle propagator that can provide high-resolution and high-fidelity subsurface images. GSP neglects up/down reverberations between heterogeneities and correctly handles forward multiple-scattering phenomena including focusing/defocusing, diffraction, and wave-interference effects. The algorithm alternates between the space-domain and wavenumber-domain via the fast Fourier transform (FFT). The operations within each domain are self-adaptive to the complexity of the medium, making this method robust in the presence of strong velocity contrasts. GSP can be directly applied for prestack shot-record depth migration. The forward downgoing wave from the source side and backward extrapolation of the upgoing wave from the receiver side independently use the same propagator. At each depth step, application of a correlation or deconvolution imaging condition determines a depth image. Figure 1 shows a prestack shot-record migration on the Sigsbee2 data set provided by the Smaart JV Project. Figure 1b shows that the top of salt is well imaged by the GSP migration even for the very steep-sided synclinal feature in the top salt surface. The amplitude and phase of the base salt are imaged well and have good continuity, and the diffractors beneath the salt are clearly reconstructed. The image quality in the finite-difference migration (Figure 1c) is generally comparable to the GSP result, but the …

Illumination amplitude correction with beamlet migration

Subsalt imaging can pose significant imaging challenges for prestack depth migration algorithms. Large local velocity gradients are usually present. Velocity differences between subsalt sediments and salt can exceed 5500 ft/s (1670 m/s). Additionally, the 3D complexity of the salt can introduce rapid spatial variations in subsalt illumination. A methodology that provides a robust and elegant solution to these issues is beamlet migration which involves decomposing source and receiver wavefields into beamlets during the wavefield extrapolation process. Each beamlet has positional information associated with it due to its wavelet transform basis. Also, each beamlet propagates with a local reference velocity in which the local velocity perturbation is small, resulting in accurate wave propagation. Increased accuracy in wavefield propagation permits improved imaging of sediment reflectivity proximate to salt—a critical issue in achieving exploration/development success.

Elastic converted-wave path migration for subsalt imaging

Summary Using a simple salt layer elastic model with both flat and steep faults at the subsalt area, we demonstrated the concepts and method of selected C-path (converted-wave path) imaging to improve the subsalt illumination and reduce migration artifacts. First we investigate the survey efficiency of different C-paths through energy budget evaluation for different incident and scattering paths. Then we test the image quality for different migration C-paths. By using C-paths with at least one S-segment inside the salt body, the blind area of pure P-wave imaging for steep subsalt faults having dip larger than the critical angle (37 0 ) was eliminated. Using different C-path combinations can also substantially reduce the elastic migration artifacts caused by path-mismatches between the data and migration. Further study for eliminating the conversion mismatch artifacts is needed.

Predicting Seal Risk And Charge Capacity Using Chimney Processing: Three Gulf Of Mexico Case Histories

Summary This paper describes how seismically derived 3D chimney volumes or “cubes” can be used to assess seal risk in exploration wells in the shelf and upper slope of the Gulf of Mexico. In this study, four examples of hydrocarbon accumulations with effective seals, one example of a breached seal, and one untested prospect were evaluated. From these examples, criteria were developed to quantify the seal risk and charge capacity based on chimney character, distribution and other information. Such criteria can then be applied to predict seal integrity on un-drilled prospects. The emphasis will be on how chimney cube interpretation can be used in an integrated workflow to constrain uncertainty on both seal and charge for hydrocarbon exploration. For the intact seals, three of the examples studied had minor chimneys above the accumulation and clear evidence of chimneys below the reservoir interval. These traps were also characterized by relatively low relief and inferred low strain rates. One of the intact seals was adjacent to a zone of vertical chimneys related to salt movement. However the reservoirs themselves were outside this disturbed zone and were characterized by moderate to low relief and low strain. In contrast, the trap which represented a breached accumulation was adjacent to a major chimney which vented hydrocarbons to the surface. It was also characterized by high structural relief and was within the disturbed zone, inferring higher strain rates. The Prospect evaluated was also adjacent to a zone of vertical chimney development. However, the reservoir objectives for this Prospect were outside the disturbed zone inferring lower strain rates. The trap also had moderate relief and is interpreted as a moderate to low risk for seal failure.

3-D Subsalt Wave-Equation Depth Imaging: A Case Study From The Hickory Field

Summary Subsalt exploration in the Gulf of Mexico has been made feasible in large part by advances in depth imaging technology, leading to several commercial discoveries. These successes, and a new generation of high-powered massively parallel computers, are now leading to the next wave of depth imaging technologies based on full wavefield prestack depth migration. This paper presents the first published case study of 3-D wave-equation imaging in a subsalt setting, based on the Hickory Field in the Gulf of Mexico. The results demonstrate that the technique is now practical and illustrates the potential advantages that can be gained with a shot domain acoustic wave-equation imaging solution.

Application of Beamlet Propagator to Migration Amplitude Correction

Beamlet migration provides robust imaging in the presence of strong velocity contrasts by combining local perturbation theory with wavelet transforms. Source and receiver wavefields are decomposed into beamlets during the wavefield extrapolation. Each beamlet propagates with a local reference velocity in which the local velocity perturbation is small, resulting in accurate and efficient wave propagation. It simultaneously extracts local information in both space and angle. Such information can be further applied to the computation of imaging amplitude corrections using directional illumination in the local angle domain. Synthetic examples show the local angle imaging properties of beamlets and the improved amplitude balance of the depth image after acquisition aperture correction.

Application of GSP wave equation migration to the Hickory Field dataset

The generalized screen propagator (GSP) is a one-way wave-equation based, wide-angle wavefield extrapolation operator that has been successfully applied to 3-D pre-stack depth migration for imaging complex structures. The dual domain implementation of GSP provides a fast and accurate 3-D wavefield extrapolator and can produce highresolution and high-fidelity subsurface images. This migration technology was applied to the Diamond/PGS dataset covering the Hickory Field, Grand Isle South Addition, Gulf of Mexico. The objectives were two fold: (I) compute a prestack depth migrated volume encompassing the field and (II) obtain an improved understanding of subsalt illumination issues. Image results from 2-D prestack, 3-D prestack with restricted aperture, 3-D prestack with full aperture and 3-D poststack depth migration were computed and compared. An illumination study, including the computation of total and directional illumination intensity, was carried out to better understand the origins of subsalt amplitude variations. Significant illumination variations in the subsalt target areas due to specific features of the salt geometry were found. The Sigsbee2 model provides insight into the subsalt illumination differences encountered at Hickory Field.

Integrated seismic illumination and gravity modeling in refinement of a subsalt interpretation

Summary Top salt geometry exerts a strong effect on the illumination of both the base salt and on underlying subsalt reflectors. A 3D seismic illumination study can be useful for the interpretation of these events. It may also distinguish subsalt amplitude anomalies due to rock property changes (hydrocarbon saturation for example) from amplitude anomalies due to illumination differences. A 3D illumination study was carried out to assess the significance of an apparent bright spot on a key subsalt mapping horizon. The study indicated that the anomaly in question was probably not due to focusing effects. The illumination results also provided evidence for the existence of a salt root coinciding with the sudden disappearance of the base salt reflection event. The presence of the salt root is further supported by the results of a gravity modeling study.

Interpretation of Sub-Salt Converted Waves

Summary Despite improvements in 3-D imaging capabilities through the use of Pre-stack Depth Migration (PreSDM), interpretation of the resultant images in the areas of complex salt bodies is confused by additional events that are not primary PP reflections. This paper analyzes the results of seismic imaging underneath steeply dipping salt flanks using real and synthetic datasets. Based on real data problems, imaging of acoustic and elastic synthetic data sets show that some of the sub-salt sedimentary section may include converted waves. A practical way to correctly image these sub-salt converted waves is introduced.