76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

We ELI1 02Evaluating Ocean-bottom Seismic Acquisition inthe North Sea - A Phased Survey Design CaseStudyM. Branston* (Schlumberger Geosolutions), R. Campbell (SchlumbergerGeosolutions), M. Rowlands (TOTAL E&P UK), B. Duquet (TOTAL E&P UK) &E. Palmer (Schlumberger Geosolutions)

SUMMARYOcean-bottom seismic (OBS) is experiencing a resurgence of popularity in the North Sea. This is due inpart to recent advances in acquisition equipment and operational efficiency as well as advances ingeometry design and processing algorithms. Using two recent case examples, we review the key stages ofthe evaluation process and share the derived conclusions. The primary goal of these studies was toinvestigate and validate the image improvement associated with an OBS survey. The studies helpedestablish the optimal OBS geometry design by benchmarking it against legacy data as well as a number ofalternative towed streamer and ocean-bottom acquisition solutions. The secondary goal of these studieswas to increase the understanding of OBS acquisition options that is, ocean-bottom node (OBN)acquisition and ocean-bottom cable (OBC) acquisition. Using a multiphase feasibility study, whichintegrates geometry design, illumination analysis, and finite-difference modelling, we were able tosuccessfully evaluate the suitability and value of OBS for a number of seismic acquisitions in the NorthSea. By investigating the natural sampling, illumination characteristics, and processing considerations ofeach geometry, we were able to design and optimise an OBS geometry that met the imaging andoperational challenges of each area.

76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

Introduction Ocean-bottom seismic (OBS) is experiencing a resurgence of popularity in the North Sea. This is due in part to recent advances in acquisition equipment and operational efficiency as well as advances in geometry design and processing algorithms. Using two recent case examples, we review the key stages of the evaluation process and share the derived conclusions. The primary goal of these studies was to investigate and validate the image improvement associated with an OBS survey. The studies helped establish the optimal OBS geometry design by benchmarking it against legacy data as well as a number of alternative towed streamer and ocean-bottom acquisition solutions. The ultimate selection of the acquisition solution was based on the quality of structural imaging over the area of interest. The secondary goal of these studies was to increase the understanding of OBS acquisition options that is, ocean-bottom node (OBN) acquisition and ocean-bottom cable (OBC) acquisition (both parallel and orthogonal acquisition). The merits and limitations of each option were investigated and documented in the optimisation process. Methodology A three-phase evaluation strategy was adopted to design and validate an ocean-bottom survey that would provide optimal illumination and imaging of the targets. A phased approach to the modelling allows for the complementary use of the available software tools. Once we have designed our geometries, we use ray-based illumination analysis to rank those geometries, as the method is quick and efficient. However, ray-based modelling requires a comparatively smooth model and does not ordinarily include all wave types (e.g., multiples and diffractions). To compensate for this, we use finite-difference modelling to model those geometries that perform best in the illumination analysis. A successful application of this approach is demonstrated by Christian et al. (2012). Phase 1: Geometry Design and Evaluation. The design process included assessing the offset and spatial sampling required to obtain optimal resolution to image steep dips as well as guard against aliasing in the various processing domains. Fold of coverage (including trace density and unique offset/azimuth coverage) and duration and relative cost of operation were established for the acquisition strategies such that the proposed solutions could be benchmarked against each other. In addition, frequency content (i.e., bandwidth), migration aperture, signal-to-noise ratio, and multiple suppression were factors considered. Determining the maximum usable offset at the target is an import requirement for the survey design. The maximum offset for each area was estimated in several ways: with a normal moveout (NMO) stretch limit of 1.2, simple ray tracing, and acoustic modelling of a synthetic gather (Figure 1). As advanced imaging processes now call for extra-long offsets, the maximum usable offset is an important consideration in designing an OBS survey. During subsequent processing of a new survey, the data will pass through transforms into several domains. In the design of OBS surveys, it is important to consider the aliasing criteria for such transforms and how it will impact processing. We also consider the aliasing criteria to preserve coherent noise. Following review of the preceding outlined analysis, several geometries were passed through to Phase 2 in which the illumination characteristics of each design would be reviewed. A key consideration during the evaluation of each proposed geometry is, of course, the time required for acquisition. To estimate this accurately, items such as water depth, tidal consideration, migration halo, and surface and seabed obstructions were taken into account. Reviewing the acquisition durations in such

76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

practical detail allows a fair comparison of the geometries and their ability to illuminate the target.

Figure 1 The maximum offset was estimated using acoustic modelling of a synthetic gather. The input model was based upon the P-wave Velocity (Vp) model supplied for the feasibility study. Phase 2: Ray-Based Modelling. The modelling for the 3D illumination analysis used a 3D Earth model to simulate acquisition of the proposed seismic surveys using advanced 3D ray tracing. The objective was to ascertain how the legacy seismic survey illuminated the target horizon and determine the impact that a variety of ocean-bottom seismic acquisition solutions would have on improving the target illumination. To achieve this, we generated illumination hit and amplitude maps (Figure 2). We also normalised the amplitudes to remove the acquisition effort (fold) from the results, enabling us to determine if a particular geometry had an inherently better ability to illuminate the target. Local imaging analysis was also conducted to ascertain the imaging and resolution characteristics of each survey. This workflow builds upon that proposed by Zuhlsdorff et al. (2010). To broaden the analysis, maps were generated to determine the hits and amplitudes upon the target horizon within certain ranges of incident angle. This was conducted for both the legacy geometry and the preferred OBS solutions, enabling further evaluation of the proposed acquisition and comparison with the legacy geometry. To optimise the extent of the survey, additional ray-based analysis was undertaken to establish the minimum survey area required to adequately illuminate the key areas of interest. This involved establishing which shots and receivers contributed to the target illumination within the usable offset ranges. Any improvement in illumination was benchmarked against the associated acquisition cost (Figure 3).

76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

Figure 2 Illumination analysis was undertaken to evaluate each of the proposed acquisition geometries against one another. Plotted here are the results of simulating the migration amplitude across the target horizon. The plot is an XY orientation and the same colour scale is used throughout; blue is low amplitude, red is high amplitude.

Figure 3 Illumination analysis across a key, steeply dipping horizon. The plot in the middle shows the illumination across that horizon for the existing survey area. The plot on the left and right show which shots and receivers (respectively) contributed to the illumination of the target. Phase 3: Finite-Difference Modelling. In the final phase of the evaluation, 3D finite-difference modelling was used to generate accurate synthetic shot gathers for OBS geometry and the legacy, narrow-azimuth geometry to quantify the benefits of the new acquisition program via high-fidelity imaging. To establish the potential success of future time-lapse analysis, the production simulations were integrated into the models to facilitate a 4D signal. Combining the image comparison with time-lapse binning analysis helped establish the impact on future 4D analysis.

76th EAGE Conference & Exhibition 2014 Amsterdam RAI, The Netherlands, 16-19 June 2014

Discussion of Ocean-Bottom Seismic Options OBS acquisition can be in the form of a cable (OBC) system or a node (OBN) system. Both systems have common advantages over conventional towed streamer systems, namely

removal of receiver ghost from signature improved demultiple through PZ summation.

In addition, the acquisition geometry and source/receiver spacing are somewhat different; consequently, each approach has a set of advantages and disadvantages inherent to their design. Considering these in addition to the illumination characteristics of each solution is an important aspect of this evaluation. OBC surveys are typically acquired with one of two generic geometry types - parallel or orthogonal geometry. OBN surveys use a sparse grid of receivers on the seabed together with a carpet of shots. It is noticeable that the OBN survey has a combination of advantages and disadvantages from both the OBC parallel and orthogonal techniques. Conclusions Using a multiphase feasibility study, which integrates geometry design, illumination analysis, and finite-difference modelling, we were able to successfully evaluate the suitability and value of OBS for a number of seismic acquisitions in the North Sea. By investigating the natural sampling, illumination characteristics, and processing considerations of each geometry, we were able to design and optimise an OBS geometry that met the imaging and operational challenges of each area. Acknowledgments The authors would like to thank TOTAL E&P UK and Schlumberger for permission to publish this work. Special thanks go to Pablo Alejandro, James Gara, and the Schlumberger Geosolutions processing team (which is located within TOTAL E&P UK) for the work they did to process the finite-difference results. References Christian, P., Pringle, T., Zuhlsdorff, L., Drottning, A., Brown, G. and Webb, B. [2012] A survey design case history using complimentary ray tracing and wavefield extrapolation techniques. 74th EAGE Conference and Exhibition, Extended Abstracts, Z016. Zuhlsdorff, Z., Gjoystdal, H., Branston, M., Drottning, A., Bergfjord, E. and Rasmussen, T. [2010] An improved survey evaluation and design workflow. 72nd EAGE Conference and Exhibition, Extended Abstracts, P294.