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Complex overburden geology often creates difficulties in imaging reservoirs situated below. We used a field dataset from the GOM to demonstrate the benefits of combining FWI and LSM. We were able to generate a high-resolution velocity model through FWI, then use LSM to further improve the reservoir amplitude fidelity by compensating for the overburden and acquisition illumination effects through a round-trip modelling/migration process

The North West Shelf, situated in Western Australia, is a world-class offshore hydrocarbon province. The presence of hard water bottom, near water bottom reflectors and shallow Tertiary carbonates not only generates strong multiples but also distorts the ray-paths for deeper reflectors. Seismic data quality is severely deteriorated due to residual multiples, limited bandwidth, and poor signal-to-noise ratio, impeding reservoir delineation and further AVO/QI analysis. With the recent advancement of seismic imaging technologies, we propose an integrated workflow including (1) comprehensive demultiple and (2) hybrid tomography and Time-lag Full-Waveform Inversion (TLFWI) to overcome these long-standing imaging challenges. The significant uplift of the reprocessed image provides deeper insights into the subsurface geology and improves confidence of prospect mapping for exploration.

In the North Sea, water bottom related surface multiples offer one of the largest challenges to a seismic processing project with a careful balance between multiple attenuation and primary preservation required when using a model and subtract approach. For Ocean Bottom Node (OBN) data Up/down deconvolution can be used to output the reflectivity where multiple and source signature will be removed, eliminating the need for an adaptive subtraction as part of the principle de-multiple process. Residual multiple can however still be present where the assumptions within the up/down deconvolution method do not adequately describe the reality of the data. Often, these residuals are only noticeable once migrated, particularly where a diffracted component is prevalent. In this paper, a method using the coherency of residual multiple evaluated on migrated and stacked data is used to drive pre-migration residual multiple attenuation through the use of a matched-subtraction.

Stratal geometries, reservoir and source facies are different in true lakes, in lakes with restricted access to the global ocean, and in “lakes” continuously connected to the global ocean but capped by large quantities of freshwater. We illustrate the importance for exploration and development of addressing these issues in the pre-salt sequences of the South Atlantic. We have achieved this through interpretation of high quality seismic images of the pre-salt section, in a geological context provided by analysis of well and rock data, prediction of palaeoclimate and palaeohydrology, and an understanding of relative plate motions and global oceanic sea levels. We conclude that undrilled reservoir facies are present on the flanks of the main structural highs in the hypersaline basins of south Brazil, and in the freshwater basin of south Gabon.

Small-scale heterogeneities create distortions in the wavefield that are usually resolved by Full Waveform Inversion (FWI). When FWI fails (for lack of diving waves or low frequencies), recent works promote the use of targeted stochastic approaches maximizing the stack power. We instead revisit here ray-based tomography and propose an innovative approach for a high definition tomography which can recover small-scale anomalies without any a priori information on their localization. While previously proposed tomographic approaches aimed at vertical resolution and structural conformity, our new approach focuses on enhancing lateral heterogeneities. The first key component is an accurate RMO picking with structural dip information honoring the variation of dip with offset. Second, is an efficient nonlinear slope tomography employing small grids and a minimum level of constraints. Our approach is demonstrated on a 2D synthetic dataset with small-scale lateral variations and confirmed on a 3D streamer dataset from the Barents Sea.

The quality of time-lapse analysis depends highly on the repeatability of the acquisition. However, in practice, it is almost impossible to perfectly mimic a base survey due to environmental conditions and inaccurate measurements. Lack of repeatability often results in 4D noise, which may compromise the 4D signal. In the presence of subsidence, caused by the depletion of the reservoir, 4D signal exists outside of the reservoir area, and its extraction from noisy 4D data can be challenging without a priori information. Water layer tomography has already been proposed to recover uncertain parameters from the acquisition in order to address the non-repeatability effects in the data but not with as many parameters as presented in this paper: water velocity, source position, top of the water layer and start of data time. Unlike most water layer tomography, our method not only relies on the inversion of the water-bottom primary but also of the first-order multiple travel times picked in the data. An application to a 3D deep-water survey offshore Angola is presented. The flow is applied independently to all vintages of a 4D project resulting in significant reduction of the 4D noise and a clear visibility of the subsidence.

Sub-basalt seismic imaging is very challenging due to large impedance contrasts at sediment-basalt interfaces. The impedance of basalt usually gives a strong reflection coefficient at the top of basalt, and thus generates strong multiples. Offshore western India, this issue is compounded by short-period seabed multiples generated by the shallow sea floor. Moreover, the presence of the basalt layer limits the angle of reflections from sub-basalt structures, making velocity modeling difficult. The combination of strong, complex multiples and the challenges of obtaining a reliable velocity model gives rise to poor imaging beneath and within the basalt. In this study, a comprehensive pre-migration demultiple flow was devised to tackle the strong surface and interbed multiples. For velocity model building, full-waveform inversion (FWI) was applied for the shallow velocity update and non-linear scanning tomography was then utilized to update the velocity within and beneath the basalt layer. Due to the poor initial velocity model, an enhanced dynamic-warping FWI approach was used to mitigate the cycle-skipping issue, and the maximum FWI frequency was extended to 20 Hz. With the benefits from the comprehensive demultiple process and advanced velocity model building, imaging of the complex sub-basalt structures in this area was improved.

The attenuation of surface related multiples is typically one of the most challenging steps in the processing of shallow water marine projects. Least-squares wave-equation deconvolution imaging is a powerful tool to address this challenge, but images derived from a deep target level may produce sub-optimal demultiple results for the shallower section. We introduce image domain sparseness weights to the least-squares problem, derived from a water-bottom depth estimate. This provides a reflectivity with a sharp contrast at the water-bottom, and the corresponding multiple prediction exhibits improved temporal resolution compared to least-squares wave-equation deconvolution imaging. We also illustrate how the multiple prediction from sparse wave-equation deconvolution imaging may be combined with source-side targeted multiple prediction to improve multiple attenuation for complex multiple generators. Data examples from the Central North Sea and the West of Shetland confirm the benefits of the proposed methods in attenuating residual multiples.

The Taranaki Basin is one of New Zealand’s largest basins. Initially forming as a Cretaceous rift basin, it has over 400 exploration and production wells. Early basin history is characterized by extensional fault blocks, and as basin evolution continued, thrusting and inversion associated with the convergent active margin set up trapping mechanisms for petroleum accumulations. Recent fault blocks within shallow Plio-Pleistocene sediments exhibit strong azimuthal anisotropy. Without considering this effect, seismic imaging in the area suffers structural discontinuity and fault misplacement. Below these fault blocks, it is challenging to image the thrust system and sub-thrust structures due to poor illumination from strong velocity variation around the thrust. To overcome these challenges, we focused on two major aspects. First, we built a tilted orthorhombic (TORT) velocity model to handle the strong azimuthal anisotropy in the overburden. At the time, we only had access to narrow-azimuth (NAZ) data, thus we derived the TORT parameters through scanning based on stack and gather responses. Second, we applied least-squares (LS) TORT Q-RTM to honor azimuthal anisotropy and compensate for poor illumination from the complex velocity. We observed significant imaging uplifts compared with the vintage data that subsequently provided an improved geological interpretation.