François Audebert

Imaging complex geologic structure with single-arrival Kirchhoff prestack depth migration

We compare various forms of single‐arrival Kirchhoff prestack depth migration to a full‐waveform, finite‐difference migration image, using synthetic seismic data generated from the structurally complex 2-D Marmousi velocity model. First‐arrival‐traveltime Kirchhoff migration produces severe artifacts and image contamination in regions of the depth model where significant reflection energy propagates as late or multiple arrivals in the total reflection wavefield. Kirchhoff migrations using maximum‐energy‐arrival traveltime trajectories significantly improve the image in the complex zone of the Marmousi model, but are not as coherent as the finite‐difference migration image. By carefully incorporating continuous phase estimates with the associated maximum‐energy arrival traveltimes, we obtain single‐arrival Kirchhoff images that are similar in quality to the finite‐difference migration image. Furthermore, maximum‐energy Greens function traveltime and phase values calculated within the seismic frequency ban...

Insights into migration in the angle domain.

Summary We examine some aspects of the Kirchhoff migration in the angle domains. The angle domains we refer to are the scattering angle domain (both opening and azimuth of scattering), which replaces the surface related offset and azimuth of acquisition, and the illumination dip angle domain (having two components in 3D). We examine the insight given to the Kirchhoff summation and to the stationary phase approximation, by virtue of the decomposition of the migration in the illumination dip angle domain. Additionally we point out some differences between angle domain and offset domain migration.

A 3D subsalt tomography based on wave-equation migration-perturbation scans

We have developed a simple but practical methodology for updating subsalt velocities using wave-equation, migration-perturbation scans. For the sake of economy and scalability (with respect to full source-receiver migration) and accuracy (with respect to common-azimuth migration), we use shot-profile, wave-equation migration. As input for subsalt-velocity analysis, we provide wave-equation migration scans with velocity scanning limited to the subsalt sediments. Throughout the migration-scan sections, we look for the best focusing or structural positioning of characteristic seismic events. The picking on the migration stacks selects the value of the best perturbation attribute (alpha-scaling factor) along with the corresponding position and local dip for the chosen seismic events. The associated, locally coherent events are then demigrated to the base of the salt horizon. Our key observation is that this process is theoretically equivalent to performing a datuming to a base of salt followed by subsalt migr...

Kirchhoff PreSDM interactive dip‐gather stacking and dip illumination panel generation

Kirchhoff Pre-Stack Depth Migration (PreSDM) is used to output image gathers in structure dip angle instead of the traditional offset attributes gathers in sub-salt areas that are hard to image. These new image gathers can help processors and/or interpreters to better define geological structures through interactively stacking the image gathers in dip within various dip angle ranges based on different interpretations. Illumination panels can also be obtained for structures with different dips. They can be used to determine if observed amplitude anomalies are caused by geology or seismic acquisition and imaging processes. We demonstrate using the Sigsbee 2A synthetic dataset that partial dip-angle gather stacking and illumination compensation can be used to effectively suppress sub-salt migration artifacts.

True amplitude migration in the angle domain by regularization of illumination

We describe an approach of true amplitude Kirchhoff migration in scattering angle, based on a multi-angle regularization of illumination. The scattering angle replaces the surface related offset and azimuth of acquisition. The regularization of illumination is done in the multidimensional domain of scattering angles and illumination dip angles. We show that the regularization of illumination is in principle equivalent to the classic application of the Beylkin Jacobian, but its different implementation makes it suitable to a wider range of real-life of irregular acquisitions. We illustrate with realistic examples the issues of amplitude regularization, elimination of artifacts and removal of footprint.

A Fast And Low Cost Alternative to Subsalt Wave Equation Migration Perturbation Scans

Subsalt velocity analysis using prestack wave equation migration scans through perturbed velocity models is an accurate but expensive approach. To reduce turnaround time and computation cost, we investigated an alternative approach for subsalt velocity analysis. In this approach, we first use the current best velocity model to perform one prestack migration to produce a subsalt image. We demigrate the subsalt image to the base of salt to produce demigrated zero-offset seismic data. Using this demigrated data as input at the base of salt datum, we perform a scan of post stack wave equation migrations through perturbed subsalt velocity models. We demonstrate the necessity of demigrating only to the base of salt in order to avoid significant image degradation and show the feasibility of this methodology using the 2D Sigsbee data set.

Migrated Focus Panels: Focusing Analysis Reconciled with Prestack Depth Migration

The theory of focusing analysis has been developed, assuming a horizontal reflector and a constant velocity; thus the building of focus panels assumes implicitly that zero-offset rays are vertical rays. All these assumptions are grossly violated in cases where pre-stack depth migration is usually needed. In this paper, we propose a method of building focus panels consistent with the use of pm-stack depth migration, i.e. without making any a priori assumptions about the reflectivity or velocity model. We call this method “migrated focusing analysis”, and it produces so-called “migrated focus panels”. According to this method, a multi-offset extrapolation is performed to a given extrapolation depth, from where the new zero-offset part of the data is extracted. There, instead of performing a simple vertical time to depth conversion of the zero-offset traces, we perform a full depth migration of the zerooffset data. The obtained depth-migrated traces constitute focus data where events are properly located in space according to the velocity model we used. This method allows us to build focus panels compatible with pre-stack depth migration, provided we use the same velocity model and the explosive reflector equivalent of the pre-stack migration algorithm. Finally we discuss the improvement made by this new method.

Sensitivity Analysis of Time-lapse Images Obtained By Differential Waveform Inversion With Respect to Reference Model

SUMMARYFor monitoring purposes, one of the promising techniques ded-icated to assess physical properties changes in target regionsis the differential waveform inversion, both in the acoustic andelastic cases. A central question of this technique regards thechoice of the reference model. One solution could be the useof the reconstructed baseline image provided through the stan-dard Full Waveform Inversion (FWI) procedure of initial data.However, how the accuracy of the baseline reconstructed imagewill affect the precision of further time-lapse images is of cru-cial importance. Here, we present a sensitivity analysis of time-lapse images obtained from differential inversion, with respectto various reference models. Density, P- and S-wave velocitychanges could be converted into fluid property changes thanksto an empirical downscaling relationship. For accurate estima-tion of fluid parameter changes, the construction of highly re-solved time-lapse images presenting acceptable errors is a keyissue for the downscaling procedure. We illustrate on a specificsynthetic example that the sensitivity analysis over the refer-ence model variation provides linear convergence towards thetime-lapse image obtained when using the exact baseline. Anaccurate baseline reconstruction is essential and could benefitfrom other data collected for monitoring purposes.INTRODUCTIONFWI is a data fitting procedure aiming to develop high resolu-tion quantitative images of the subsurface, through the extrac-tion of the full information content of the seismic data (Taran-tola, 1984). Beside the exploration application, the FWI methodcan be also used for monitoring applications, such as oil and gasreservoirs, steam injection, CO

3D Finite Angle Tomography Based On Focusing Analysis

We have developed a new 3D finite angle tomography based on the analysis of focusing errors. The seismic input to this new velocity analysis is a set of migration panels that are stacked images formed at various imaging conditions, zerotime or non zero-time. By comparing these different common focusing error panels the best focused events and their corresponding focusing errors are picked. The picked focusing errors and associated local reflector attributes (position and dip) are then fed into the existing 3D finite angle tomography to update the velocity model. This new technique complements the input to our existing 3D tomography based on either residual curvature analysis or migration scan, and is a cost effective alternative to the latter. It is particularly appropriate in subsalt areas where migrated seismic data have poor signal to noise ratio.

Subsalt velocity analysis by combining wave equation based redatuming and Kirchhoff based migration velocity analysis

Due to the geometrical complexity of the typical Gulf of Mexico (GOM) velocity models, with embedded salt bodies of any shapes, wave equation migration is used preferentially over Kirchhoff methods for subsalt velocity model building. This preference is based on the ability of wave-equation based migrations to overcome the need for tracing complex ray paths through the salt bodies and for a better handling of multi-path arrivals via wavefield reconstruction. Subsalt velocity analysis uses prestack wave equation migration scans that are created from perturbed velocity models: this is an accurate albeit expensive approach that requires multiple runs of prestack wave equation migration. To reduce the computation cost, we present in this paper a low-cost alternative to perform subsalt velocity analysis. For those cases where sediment velocity structure is relatively simple, we perform a single one-time redatuming to the base of salt (BOS), using existing prestack wave equation tools. By redatuming, we remove the complexity of the wavefield caused by the salt bodies. Once having obtained a simplified wavefield by stripping off the effects of the complex overburden, we can employ less expensive Kirchhoff imaging algorithms for performing subsalt velocity model building.