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Multiple-domain, simultaneous joint inversion of geophysical data with application to subsalt imaging

We describe an effective method for joining the benefits of inversion of different kinds of measurements. We show the simultaneous joint inversion objective function, which allows users to link different inversion domains, like seismic with gravity or seismic with magnetotellurics. This function can be extended to any number of domains and does not require that they be sampled on the same grid. Our parallel implementation allowed us to scale well with large volumes of data and a large number of unknowns, and it has already been included in production workflows. Furthermore, it is generic and constitutes a framework where new inversion techniques can be easily plugged in. We also present two different ways of linking various inversion domains for establishing relationships between different model domains and how they can be chosen and used to achieve the best result. Applications of the algorithm using synthetic and field data produce model features generally not achievable with single-domain inversions. Specifically, we applied our technique to real data from the Walker Ridge area in the Gulf of Mexico, and we used the results to reinterpret and remigrate seismic data. The final migrated section clearly found improved quality with respect to previous efforts. Our results document the fundamental importance of integrating nonseismic methods with seismic techniques to increase the image quality of geologically complex areas.

Simultaneous joint inversion of refracted and surface waves

The shallow subsurface often exhibits large and rapid vertical and horizontal variations due to structural and stratigraphic reasons. The low compaction and cementation of shallow formations can generate extreme velocity changes, especially in arid areas without a shallow water table. From a geological and geophysical point of view, the near surface can be extremely complex, with multiple velocity inversion and sharp lateral velocity variations. It is well known that the impact of near-surface perturbations on seismic reflection data must be removed to obtain the correct geometrical image of deeper horizons and representative reservoir attributes. Additionally, the near surface is an important, but challenging, portion of velocity model building in depth imaging. The near-surface characterization is, therefore, an important part of seismic data processing, in particular for land data. Conventional approaches involve the use of refracted waves, or diving waves, for P-wave velocity model estimation. The use of Rayleigh waves can be an alternative when data are acquired properly. Refraction tomography (RT) and surface-wave inversion (SWI) are based on different physical principles, make use of different components of the wavefield, and have different limitations and strengths. Refraction tomography often provides deeper models and estimates directly the compressional wave velocity. But, with land data, refraction techniques can be challenging in areas with a complex near surface. The data quality can be critical; for instance, picking the near offset can be difficult. Often, only a limited offset range can be picked reliably. Moreover, velocity inversions and hidden layers can produce ambiguities, and they might not be resolved uniquely. Surface waves have a very high resolution in the shallow near surface and are very robust versus model complexity and data quality. However, even if the low-frequency penetration reaches hundreds of meters, the vertical resolution decreases with depth and the resolution at the final investigation target may be insufficient in some areas. Finally, conversion from shear to compressional velocities requires calibration with P-wave events. When RT and SWI are used, integration and reconciliation of the two models can be done in different ways. A robust framework for this task is simultaneous joint inversion (SJI). The two measurements are input into a single inverse problem, where a unique model with multiple properties is estimated, minimizing the data prediction error and a link between the two domains. The synergies between the two techniques can be exploited. Different portions of the model are resolved by the interacting contributions of the measurements.

Near Surface Characterization through Simultaneous Joint Inversion of Surface Waves and Refracted Waves

Near-surface characterization is an important part of seismic data processing, especially for land data. Conventional approaches rely on refracted waves and use of inversion techniques to estimate compression-wave velocity models from the first-break traveltimes. However, in a complex geologic setting in the shallow part of the subsurface, velocity inversions and hidden layers may reduce the reliability of the refraction solution, even with high-quality first breaks. Surface-wave inversion techniques provide shear-wave velocity models that can be integrated in the near-surface characterization workflow. The VP and VS models inferred from first-break traveltimes and Rayleigh waves have several synergies. Their integration can provide an effective solution for challenging geological situations. A robust way of integrating the two methods is simultaneous joint inversion: within this inversion scheme, the two measurements, together with some geological and/or petrophysical relationships linking the subsurface properties, are used in the computation of a unique cost function, which is to be minimized. The solution of the inverse problem results in a multiproperty model fitting data of both measurements and the linking relationship. Different portions of the model are resolved by the interacting contributions of complementary measurements. A field example from Kuwait is discussed to demonstrate the method.

Seismic and geometrical attributes from image gathers

We examine migrated angle domain gathers to extract seismic and geometrical attributes of reflectors. We illustrate the procedure on a real 2D dataset: the angle domains we refer to are the illumination dip angle and the aperture scattering angle. We use the illumination dip angle gather to track the main reflectors and to create a map of the dip values. This information and the scattering aperture angle gather enables to improve AVA analysis and to build maps of seismic attributes. The procedure is automatic.

Improving Subsalt Imaging by Incorporating MT Data in a 3D Earth Model Building Workflow - A Case Study in Gulf of Mexico

In this work we present the simultaneous joint inversion of seismic and EM data in WalkerRidge area, Gulf of Mexico. The deep-water Gulf of Mexico can be a difficult geologic and imaging environment due to the diverse and repeated salt tectonic episodes that have resulted in complex allochthonous salt formation. Accurate salt body characterization and careful earth model building are key objectives for properly assessing the exploration potential of the area. Seismic methods alone, cannot always properly image the subsalt reflectors. The inclusion of non-seismic data, such as magnetotelluric (MT) is very useful in complex salt provinces where the method can leverage the differences between salt and sediment resistivity as well as between seismic velocity.

Simultaneous Joint Inversion of Seismic, Gravity, and EM Data for Subsalt Depth Imaging in Gulf of Mexico

This paper illustrates an advanced process for integration: 3D simultaneous joint inversion (SJI) of seismic, electromagnetic, and gravity data to better define the base salt in the Green Canyon-Garden Banks-Keathley Canyon-Walker Ridge areas. SJI enhances the ambition of improving the existing velocity models for prestack depth migrations and the consistency of seismic and non-seismic representations of the subsurface in complex salt geometries. We produced a new structural framework with renewed interpretations of the allochthonous and autochthonous salt, providing new tools for interpretation of the complex salt, reducing inversion uncertainties, and most importantly, defining a new strategy for subsalt interpretation, thereby enhancing the role of non-seismic methods as supporting complex seismic depth imaging.

Gaussian Beam Migration: Prestack, Common-shot, TTI, True Amplitude In Angular Domain

Gaussian beam prestack depth migration (GB-PSDM) represents an excellent tool for the migration of seismic data in complex velocity fields, as it is an implicit multiarrival migration algorithm. With the turning waves feature, it is also appropriate for imaging of steep dips. Application in common-shot domain makes it suitable for irregular acquisition geometries. In addition, an anisotropic acoustic approximation of the raypath as the central ray of the beam can be easily parameterized for handling complex anisotropy (e.g., tilted transversely isotropic (TTI)). Implementation of the Gaussian beam migration in angular domain gives additional value for post-migration processing and true amplitude normalization by means of hit counter equalization.