Jean-Luc Boelle

Comparison of different strategies for velocity model building and imaging of PP and PS real data

In 1999, a 2-D/4-C data set acquired with ocean-bottom cable on Mahog-any Field, Gulf of Mexico, was distributed at the SEG-EAGE Research Workshop to test the feasibility of using converted waves to image under a salt diapir. We show and discuss results obtained by several imaging methods—some in the time domain, others in the depth domain—applied to both PP and converted-wave fields ( PS , also referred to as the C -wave). After a review of the standard time-domain approach (DMO, common conversion point or CCP binning, NMO stack, and poststack time processing), we will consider more elaborate approaches. We start with methods that use observations in the unmigrated time domain, inversion of stacking velocities and inversion of picked prestack traveltime. We finish our review with methods that use observations in the migrated domain, migration velocity analysis using prestack time migration, and migration velocity analysis using prestack depth migration. The multicomponent 2-D Mahog-any data set is oriented E-W and was acquired with a 1.5 km 4-C cable at a quasiconstant water depth of 118 m (Figure 1). Maximum offset is 11.5 km. Record length is 10 s with a 2-ms sampling rate. The line was shot vertically above the receivers, and the line direction chosen to minimize 3-D effects. Raw shot gathers show that PP waves were mainly recorded on the P (pressure) and Z (vertical) components. PS waves were essentially recorded on the X (in-line horizontal) component. Some time processing steps specific to OBC acquisition were undertaken—surface-consistent compensation of coupling between the receivers and the sea bottom, P-Z summation for the PP waves, and medium wavelength surface-consistent receiver statics for the PS waves to account for the extremely low S -wave velocity in the near-sea bottom. The polarity of positive offsets was reversed to obtain radial polarity X -component gathers …

Ocean Bottom Nodes (OBN) repeatability and 4D

During a recent Ocean Bottom Nodes (OBN) campaign in offshore Angola, a repeatability test was carried with two repeated swaths of sources shot over a patch of twin receivers. This technique allowed decoupling independent node and source effects in order to appreciate their relative contribution to the overall pre-stack repeatability noise. Moreover, it also provided the bases for a quantitative, analytical 4D formalism accounting accurately for the observations made, and whose fundamentals will be introduced in this paper. From this framework, the importance of minimizing positioning differences to constrain “4D noise” is reviewed and quantified in an analytical form. With this objective in mind, an original technique to derive OBN coordinates in the sea-bottom with an accuracy one order of magnitude greater than the existing state-of-the-art is proposed. Although presented here in the particular context of OBN surveys, the underlying concepts of our work are general enough to be easily transposed to other 4D acquisitions and pave the way for predictive NRMS studies.

Robust Full Waveform Inversion of Surface Waves

We have investigated a local optimization procedure for seismic imaging by full waveform inversion of surface waves. These waves are highly energetic when considering near-surface imaging and present dispersive effects as well as significant forward scattering. These propagation behaviours have led us to consider alternative data domains where the data fitting should be performed. The frequency-slowness domain as well as the frequency-wavenumber domain have been considered. We have shown how to build the gradient of the misfit function in these two domains based on the adjoint approach. The adjoint source term has been estimated with a systematic procedure based on Lagrangian multiplicators, making the cost of the procedure identical to the one formulated in the standard time-offset domain. A simple 2D synthetic example shows that the reconstruction of the shear velocity is possible, although real applications could require a more extensive investigation including windowing and filtering, as a hierarchical strategy appears to be necessary when considering surface waves with high variations in amplitude. A linearised approach is therefore possible and quite efficient. This avoids the picking of dispersion curves and attempts to fill in the velocity spectrum, extending our search domain to models with lateral velocity variations as done previously using global model search.

High Resolution 3D parabolic Radon filtering

Summary 2D parabolic Radon filtering is a widely used method for multiple attenuation. However, for dense and wide-azimuth gathers that exhibit azimuthal variation effects, this approach can fail. Because of the variation of the curvature of the events with the azimuth, the bin gathers can not be processed in one go but must rather be split into sub-collections where the azimuth either has little variation or varies smoothly. We instead propose herein to take into account the azimuthal effects by incorporating an elliptical model for the variations of the curvature with the azimuth, and hence define a 3D parabolic Radon filtering. This is a more natural way of processing dense wide-azimuth gathers, by honoring their actual 3D geometry, which results in more consistent decompositions and in a better filtering.

Autonomous 4C Nodes used in infill areas to complement streamer data, deepwater case study.

Summary During the summer of 2004 an experimental OBS survey was acquired and processed by CGG over the Girassol field operated by Total Angola, offshore West Africa. Five ARMSS nodes (Autonomous Reservoir Monitoring Seismic System) were deployed in this area at a water depth of 1300 meters. The primary objective of this trial was to verify the operational sequence and performance of this new generation of 4C recording equipment. There were also a number of secondary objectives which were to benchmark the recorded data with a view to infilling the streamer acquisition in this difficult environment and to evaluate the added value of recording four components. In this paper, after a short description of the recording system and the acquisition layout, the results of the processing are compared with streamer data acquired previously in the same area. The 4D capabilities of the node technology are assessed. A comparison of the results proves that nodes could be used in infill areas to complement streamer data.

WAZ mirror imaging with nodes for reservoir monitoring, Dalia pilot test.

Repeated marine seismic recorded with towed streamer have been proved successful for imaging reservoir productions. Unfortunately major infrastructures (FPSO) constitute a “blind zone” for the reservoir illumination from the sea surface. Because undershooting surveys come with repeatability and HSE issues, nodes imaging appears to be a valuable solution. In 2009 a deep water nodes surveys was acquired by Total Angola. Since “Base” surveys are acquired usually with marine streamers, the first objective of this paper is to find out how to reconcile nodes and streamer data in order to provide comparable images. The second objective is to propose an azimuth compliant processing approach valid for an optimum node WAZ mirror imaging. Two original processing approaches are described: firstly, data cross-matching is done in angle domain in order to provide similar ray path and equivalent sea surface offset. Secondly, we show that the concept of offset vector binning using hexagonal tiles is applicable to the nodes acquisition geometry. Mirrored data migration in common offset vector domain provides CIGs with preserved offset and azimuth information. Post-migration processing like full azimuthal residual move-out and azimuthal illumination selection can then be applied for an optimal reconciliation between nodes and streamer data.

Deep Target Pre Stack Time Migration And Migration Velocity Analysis Using Converted PS Data

The low level of average PS reflectivity and the high sensitivity of the shear mode to scattering make the imaging with converted PS waves a challenging task for deep targets. After a short summary of the pre stack time migration technique, we describe the problem of estimating the γeff parameter through migration velocity analysis. Combined solutions inspired from the literature are proposed and tested on a 2D/4C acquisition. The results show that, even for deep target in a highly faulted environment, coherent PS signal is existing and can be focused to deliver images which can be compared to P section.

Can we correct for azimuthal variations of residual move-out in land WAZ context, using depth non-linear slope tomography? An imaging case history.

Summary High-density wide azimuth (WAZ) land surface acquisitions have demonstrated superior imaging capabilities. However, processing of such data exhibits several challenges related to the traditionally poor signal-to-noise ratio of land data and the necessity of reconciling the kinematics of the various azimuths. In this paper, we present an imaging case history involving WAZ non-linear slope tomography. Using the concept of kinematic invariants, velocity model update is performed both in depth and time based on the same picking. Our dense automated dip and residual move-out (RMO) picking is done on an initial pre-stack time migrated (PreSTM) dataset after application of a structurally consistent filtering that greatly improves the signal-to-noise ratio. Our case study demonstrates that non-linear slope tomography in the depth domain greatly improves the imaging of the structures when compared to the initial PreSTM result. We observe that even if tomography in the time domain significantly enhances imaging, it cannot successfully honour the kinematics of the various azimuths within the constraints of time imaging assumptions. On the contrary, WAZ nonlinear slope tomography in the depth domain offers an efficient way to reconcile these kinematics, thus promoting the use of depth imaging when processing high-density WAZ data, even in the context of mild geological complexity.

Sparse receiver and multi‐azimuthal simulations from a high fold OBC campaign in the UK North Sea

After a reminder of the possible superiority of OBC versus conventional streamer acquisitions due to a better water layer multiples elimination, the advantages of high fold and multi-azimuth illumination are pointed out. Many different acquisition designs are simulated: from very sparse receiver distributions to high fold and from narrow azimuthal to wide-azimuthal illumination. The results which have been obtained have important implications in both acquisition design and processing sequence definition.

Seismic Imaging Improvement Thanks to a True Wide-azimuth Pre-processing Sequence Applied to a 3D OBC Dataset

High density wide-azimuth Ocean Bottom Cable acquisitions have proven their efficiency to better image complex structures in the North Sea. Yet conventional pre-processing sequences have not taken into account the full potential offered by such acquisitions (Soudani et al. 2006). In this paper, we demonstrate the advantages of a true wide-azimuthal pre-processing regarding noise attenuation on data where the low signal to noise ratio is a main challenge. Following Soudani et al. (2006), a decomposition of the common receiver gathers into the 3D -px-py domain is used to filter out linear noise. A specific two-step sequence is proposed to attenuate the artefacts produced by the strongly aliased direct wave with acceptable run-time effort and data storage. Our results are compared, before and after pre-stack depth migration, with those derived from a more conventional processing approach.