Didier Lecerf

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.

Multivariate Geostatistical filtering of time-lapse seismic data for an improved 4-D signature.

4-D technology is moving into an accelerated phase with several successful projects to date. Acquisition and processing imprints are considered negligible with standard 3-D processing but become critical when computing time-lapse seismic differences. We introduce a spatial co-filtering geostatistical technique into the 4-D processing sequence to remove acquisition imprints and other uncorrelated noise that increases the repeatability and optimises the 4-D signature. Examples from two fields from the North-Sea, Draugen (Norske Shell) and Forties (BP) are presented.

On the Use of Geostatistical Filtering Techniques In Seismic Processing

When redundancy of seismic data exists factorial cokriging enables the estimation of (1) a common part, based on the common spatial behavior, and (2) the differences relative to the common part of the input data. Coléou (2002) first introduced the automatic implementation of factorial co-kriging (AFACK) as a filtering technique for the time-lapse (4D) processing sequence. It was specifically designed to optimize the critical time-lapse information such as the repeatability and the 4D seismic signature. However, over the last two years we have been developing new applications of this technique in very different processing environments. For example, applications providing data reduction, such as stacking or AVO and EI analysis, have a direct interest in the common part of consecutive offset cubes. Furthermore, we have successfully applied AFACK to more specialized problems such as the merging of OBC and streamer data or the decomposition of wide-azimuth data for fracture characterization.

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.

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.

Common-angle Processing Using Reflection Angle Computed By Kinematic Pre-stack Time Demigration

Recent works have shown the benefit of seismic data processing in common angle domain especially for AVA studies and stack resolution enhancement. When wide angle datasets are exploited, difficulties and reservations come with the genuine signification and the validation of the reflection angle value. The workflow presented includes two original methods providing accurate angle CDP gather from time migrated offset CDP gather. The dependencies offset, angle and time are computed through a kinematic pre-stack time demigration process taking into account local dips, the effective velocity and anelipticity model. The kinematic demigration takes advantage of the existing relationships between the reflection angle and the differential stretch in time of the wavelet for different offsets. This approach gives a better angle estimation than conventional methods especially for angles larger than 40 and for dipping events. In addition to the mapping of offset, angle and time correspondences, a direct (and reverse) angle transform is defined in order to provide seismic traces regularly distributed in angle. The “angle regularization” is performed by an iterative Fourier reconstruction minimizing the spatial frequency leakage. Because the amount of stretch is nearly stationary in time (if there are no variations of dip in time) for a common angle time-migrated trace, a spectral harmonization along angles can be easily implemented by single spectral matching operators. This stretch compensation impacts on the stack quality and resolution.

Repeatability Measure for Broadband 4D seismic

Summary Future time-lapse broadband surveys should provide better reservoir monitoring resolution by extending the 4D signal bandwidth. In this paper, we will review the consequence of extending the signal bandwidth for the computation of 4D attributes, primarily the widely used repeatability measurement NRMS. The re-formulation of NRMS shows the sensitivity of the repeatability metric with regards to signal time-shift and signal bandwidth. Broadening the 4D signal bandwidth will result in an increase of the overall NRMS value for an equivalent seismic data with the same level on non-repeatable noise. To compare the quality of 4D seismic, regardless of bandwidth, we propose a new repeatability measure called CNRMS. The bandwidth Calibrated NRMS provides repeatability metric for any 4D seismic as it would be calculated with a reference signal bandwidth. In order to extend the 4D signal bandwidth without compromising the repeatability, we propose that up-going pressure wavefields extracted from dual-sensor streamer are used for base and monitor surveys. It ensures the HF repeatability and highest 4D resolution.

Computation of kinematic attributes for pre‐stack time migration

In many areas time imaging still represents the majority of seismic imaging activity in the industry. In this context pre-stack time migration remains a central process. We propose a new approach for estimating various kinematic attributes associated with pre-stack time migrated results. Our approach is based on a kinematic demigration of locally coherent events characterized by their position and dip in common offset time migrated images. The reflection angle, the instantaneous velocity or the geological dip can be estimated with the assumption that time imaging exactly positions laterally the events. In addition, the stretch factor and the “kinematic invariants” (the demigrated facet in the unmigrated domain!) can be recovered accurately independently of the positioning accuracy of time imaging. As opposed to many conventional approaches, the estimation is directly based on the travel time curves used in pre-stack time migration with no assumption regarding the structural dips or the lateral variations of the velocity model. The attributes may be used at various steps of the processing sequence: stretch factor to design de-stretching operators, reflection angle to design partial angle stacks for AVO/AVA studies and kinematic invariants to perform reflection tomography. Computation of kinematic attributes for Pre-Stack time migration

High Resolution Processing for Time-lapse Seismic

Time-lapse seismic technology is moving from measurement of qualitative variations to quantitative analysis of fluid saturation and pressure variations. Increasing the resolution of time-lapse seismic data relative to the reservoir body scale offers the possibility of even more accurate measurements. The resolution improvement is directly dependent on the capacity to recover the high frequency content of the seismic measurement. In this paper, we investigate a data processing methodology that provides a 4D seismic signature with an extended frequency bandwidth. We pay particular attention to the seismic data preconditioning in order to remove random noise and spatial correlated noise, such as the acquisition footprint, which can contaminate the seismic high frequency content. Using a geostatistical technique, we apply the concept of common seismic cube derived from two adjacent seismic angle stacks. The common seismic cube does not include the acquisition imprint and has an improved signal to noise ratio, especially for the high frequencies. The concept of common spectrum is used to design two symmetric matching operators in order to increase the repeatability of the high frequencies. Once the data have been preconditioned, we use an innovative technology exploiting the spatial coherence of the seismic information with predefined reflector constraints to solve a sparse reflectivity distribution. A North-Sea example shows the benefits of the new high-resolution time lapse processing workflow.