Subsalt Reflectors Research Articles

Full waveform inversion of simultaneous long-offset data

SummaryThe standard workflow for velocity model building (VMB) in complex regimes is an interpretive process that requires time-consuming manual intervention, and remains an error-prone process that can produce suboptimal results. We discuss an application of Full Waveform Inversion (FWI) to automate the refinement of legacy velocity models generated by conventional workflows. We demonstrate our solution on a full-azimuth (FAZ) survey acquired in the Gulf of Mexico using dual-sensor streamers and blended sources in the form of simultaneous long-offsets (SLO). The dual-sensor acquisition provides low-frequency data, while the SLO configuration enables the recording of long offsets in excess of 16 km; thereby providing a unique dataset to meet the traditional data dependencies for robust FWI stability. The long offsets and low frequencies were key to using both refractions and reflections to update the deeper parts of the velocity model. Our solution directly inverts the simultaneous data as acquired in the field, and is particularly relevant to the industry growth in blended seismic data acquisition. We also employed an FWI velocity gradient that eliminates the migration isochrones; thereby removing the reflectivity imprint from the model updates. The FWI application to the field survey successfully refined the geometry of the salt bodies, including the base salt and the intra-salt enclosures. It also improved the RTM image; particularly the salt flanks and the subsalt reflectors.

Improving the greater K2 area subsalt imaging with advanced seismic acquisition, model building, and imaging technologies — A Gulf of Mexico case study

Subsalt imaging has proven challenging in part because of the complex geometries of the salt canopies, which can yield highly variable illumination on subsalt reflectors. The resulting complexity of the seismic wavefields has also challenged traditional velocity-model-building tools and migration algorithms. A case study for the K2 field illustrates how improved seismic data in the form of long-offset, full-azimuth acquisition and more advanced model building and imaging tools have substantially enhanced the imaging quality of this major subsalt field. The higher fold and more complete azimuthal coverage afforded by the full-azimuth coil data improved the effectiveness of the combined tomographic and full-waveform inversion approach for velocity-model building. Utilization of TTI RTM migration algorithms provided further benefits not only for better delineation of the salt geometry during model building but also in delivering superior final image volumes relative to that which could be provided by less robust algorithms, such as Kirchhoff, beam, and one-way wave equation.

Geophysics bright spots

The importance of low-frequency seismic for exploration can be appreciated every time we see the light of day. Thanks to atmospheric gases and other particles that scatter the blues and greens (high frequencies) toward us we see the blue sky instead of the darkness of space as we would on the moon. We see a reddish sky during sunrise and sunset again because blue and green colors are scattered away while the red and orange (low frequencies) stay on course along the atmospheric journey to reach us. The same concept applies to the illumination of subsalt reflectors, only now the propagation path is two-way. The low frequencies survive the torturous scattering by the rugosity of salt surfaces and other shallower inhomogeneities, carrying information about deep reflectors to the surface.

Subsalt velocity estimation by target-oriented wave-equation migration velocity analysis: A 3D field-data example

We apply target-oriented wave-equation migration velocity analysis to a 3D field data set acquired from the Gulf of Mexico. Instead of using the original surface-recorded data set, we use a new data set synthesized specifically for velocity analysis to update subsalt velocities. The new data set is generated based on an initial unfocused target image and by a novel application of 3D generalized Born wavefield modeling, which correctly preserves velocity kinematics by modeling zero and nonzero subsurface-offset-domain images. The target-oriented inversion strategy drastically reduces the data size and the computation domain for 3D wave-equation migration velocity analysis, greatly improving its efficiency and flexibility. We apply differential semblance optimization (DSO) using the synthesized new data set to optimize subsalt velocities. The updated velocity model significantly improves the continuity of subsalt reflectors and yields flattened angle-domain common-image gathers.

Local vertical seismic profiling (VSP) elastic reverse-time migration and migration resolution: Salt-flank imaging with transmitted P-to-S waves

To avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model, we introduce a vertical seismic profiling (VSP) local elastic reverse-time-migration (RTM) method for salt-flank imaging by transmitted P-to-S waves. This method back-projects the transmitted PS waves using a local velocity model around the well until they are in phase with the back-projected PP waves at the salt boundaries. The merits of this method are that it does not require the complex overburden and salt-body velocities and it automatically accounts for source-side statics. In addition, the method accounts for kinematic and dynamic effects, including anisotropy, absorption, and all other unknown rock effects outside of this lo-cal subsalt velocity model. Numerical tests on an elastic salt model and offset 2D VSP data in the Gulf of Mexico, using a finite-difference time-domain staggered-grid RTM scheme, partly demonstrate the effectiveness of this method over interferometry PS-PP transmission migration and local acoustic RTM. Our method separates elastic wavefields to vector P- and S-wave velocity components at the trial image point and achieves better resolution than local acoustic RTM and interferometric transmission migration. The analytical formulas of migration resolution for local acoustic and elastic RTM show that the migration illumination is limited by data frequency and receiver aperture, and the spatial resolution is lower than standard poststack and prestack migration. This new method can image salt flanks as well as subsalt reflectors.

Velocity modelling and prestack depth imaging below complex salt structures: a case history from on‐shore Germany

ABSTRACTIn recent years, the advances in velocity model building and depth imaging have provided a better understanding of complex subsalt plays. The tomographic approach to subsurface velocity modelling, using interpretive processes, has led to significant progress in solving subsalt imaging problems, which were once considered to be impenetrable barriers. We show how gravity data, as an alternative data source, can be integrated into iterative velocity–depth model building to constrain the overburden velocity model and delineate the shape of the salt body above the target reflector. In this way, a structurally accurate image of subsalt reflectors is achieved.

Sub-salt imaging using 3D pre-stack depth migration in the UK Southern North Sea - a case history

In the UK Southern North Sea the presence of salt structures can cause considerable distortion of the sub-salt seismic image. This distortion is due to strong vertical and lateral velocity variations, which lead to the refraction of seismic waves. Pre-stack depth imaging is considered the ultimate tool for accurate delineation of sub-salt reflectors in this geological environment. To achieve the best seismic image an optimal interval velocity/depth model has to be built. This in itself presents a substantial challenge when working in areas with complicated geological structures. In this paper, an iterative process of velocity model building is presented, which closely integrates seismic data with geological information, to derive a subsurface velocity distribution that enables production of the best seismic image.

Early Mesozoic rift stage half graben formation beneath the DeSoto Canyon salt basin, northeastern Gulf of Mexico

Multifold seismic reflection data provide the basis for recognition of an offshore Late Triassic‐Early Jurassic half graben complex beneath the DeSoto Canyon salt basin (DSCSB) along the northeastern Gulf of Mexico margin. The base of salt or equivalent (BSE) surface is a prominent unconformity recognized throughout the DSCSB that is mostly overlain and onlapped by extensive Middle Jurassic (Callovian age?) premarine evaporites (Louann salt and equivalents) and younger sedimentary rocks. Widespread dipping subsalt reflectors, truncated by the BSE, are the seismic expression of an interpreted thick section of synrift strata deposited within a half graben. The half graben probably overlies older prerift Paleozoic (and Precambrian?) sedimentary, igneous, and metamorphic rocks and widens basinward about a NE‐SW axis. Stratigraphic onlap relationships of subsalt reflectors close to the BSE surface suggest a lacustrine sequence may be present in the uppermost section of the rift fill; similar deposits occur within synrift strata below Aptian salt along the rifted West African margin. A NE‐ENE striking normal slip boundary fault beneath the Mississippi‐Alabama‐Rorida (MAFLA) shelf area and the inferred NW‐SE oriented Florida‐Bahamas transfer fault along the eastern margin of the DSCSB approximate the updip limit of rift fill. Trends of these structural lineaments and overall half graben morphology are similar to those of the South Georgia basin, a buried onshore early Mesozoic graben complex in northern Florida and southern Georgia, and the trend of exposed Triassic‐Jurassic continental rift basins along eastern North America. Structural architecture of the DSCSB half graben is consistent with NW‐SE rift phase extension during the early Mesozoic opening of the Gulf of Mexico.

Basin Architecture, Salt Tectonics, and Upper Jurassic Structural Styles, Desoto Canyon Salt Basin, Northeastern Gulf of Mexico

Despite awareness of the importance of continental extension during rifting, there are few quantitative studies that show the influence of crustal extension on basin architecture, the distribution of salt, and Late Jurassic sedimentation in the DeSoto Canyon Salt basin, northeastern Gulf of Mexico. Application of simplified isostatic principles using a lithospheric buoyancy model allow quantification of total tectonic subsidence, crust thickness, crustal extension, and crust type. Interpretation of over 4800 km of migrated multifold seismic reflection profiles and well data, integrated with computed isostatic relations, provide the basis to characterize Middle Jurassic (Callovian-age) salt halokinetic processes and to describe the structural development of overlying Upper Jurassic strata. An average crustal thickness of 25 km and crustal extension s values between 1.4 and 1.8 suggest the sedimentary succession is underlain by moderately stretched and attenuated continental crust. The widespread distribution and geometry of dipping subsalt reflectors, particularly in the shelfal areas, provide evidence for a Late Triassic-Early Jurassic phase of rifting prior to deposition of Middle Jurassic salt. The distribution of autochthonous salt and the overlying Upper Jurassic sediments reflect the presalt structural imprint and suggest that the basic architecture of the basin was established by the Middle Jurassic following significant attenuation of the crust. Although deposition occurred in a slowly subsiding, stable marginal setting, salt movement and associated growth faulting are the most significant tectonic elements affecting the stratigraphic and structural development of the overlying strata. Faults related to growth of salt structures root at the base of salt in what appears to be a common detachment or decollement for salt movement. The original distribution of salt is widespread and sheetlike with an estimated minimum thickness of 760 m. Progressive basinward subsidence of the margin and differential sediment loading by the overburden are the primary mechanisms initiating mobilization of salt and subsequent deformation of sediments during the Late Jurassic and from the Late Cretaceous through the early Cenozoic.