42 Oilfield Review

Developments in Full Azimuth Marine Seismic Imaging

The Coil Shooting technique, in which a single vessel acquires full azimuth 3D

seismic data by sailing in circles, delivers more-accurate and reliable subsurface

images than conventional 3D methods in areas of complex geology. Recently, a

multivessel implementation of the technique has been developed to address subsalt

imaging challenges in deepwater areas.

Tim BricePerth, Western Australia, Australia

Michele BuiaEni E&PMilan, Italy

Alex CookeRio de Janeiro, Brazil

David HillEd PalmerGatwick, England

Nizar KhaledSérgio TchikanhaEnrico ZamboniTotal E&P AngolaLuanda, Angola

Ed Kotochigov Oslo, Norway

Nick MoldoveanuHouston, Texas, USA

Oilfield Review Winter 2012/2013: 24, no. 4.Copyright © 2013 Schlumberger.For help in preparation of this article, thanks to Paul Bidmead, Gatwick, England; and Giuseppe Uncini, Eni Indonesia, Jakarta.3D GSMP, Coil Shooting, DSC, Dual Coil Shooting, ObliQ, Q-Fin and Q-Marine are marks of Schlumberger.

Traditionally, vessels acquire 3D marine seismic data by sailing in a series of straight, parallel lines over a survey area. This recording configura-tion suffers from an inherent problem: Although the source wavefront propagates in all directions, only a small proportion of the reflected wavefront is captured by the surface receiver spread, and the seismic raypaths are aligned predominantly in one direction, or azimuth.

In the presence of complex geology, ray bend-ing can leave portions of the subsurface untouched by seismic waves when only a narrow range of source-receiver azimuths is recorded (left). Attempts to solve this problem have led to the development of wide azimuth (WAZ), rich azi-muth (RAZ) and multiazimuth (MAZ) acquisition configurations (next page, top left). By “shining a light” on the formations from many directions, these methods deliver better illumination of the subsurface, higher signal-to-noise ratio (S/N) and improved seismic resolution in challenging imag-ing areas such as beneath complex salt bodies.1

Wide azimuth surveys are typically conducted using three or four vessels, each shooting in straight, parallel lines. As in conventional sur-veys, the time taken to turn vessels around between the end of one swath of straight lines and the start of the next has, to date, been accepted as inevitable nonproductive time (NPT) (next page, top right).

> Image distortion. Refraction of light through the irregular surface of a glass drinking mug (top) causes parts of a spoon to be invisible or distorted when viewed from different directions; the image changes depending on azimuth. Similarly, seismic images of a subsurface structure (bottom) offshore Angola differ depending on the source-receiver azimuth of the contributing data.

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In 2007, WesternGeco began testing the Coil Shooting technique, in which a vessel sails in overlapping circles in corkscrew fashion, recording continuously, to deliver full azimuth (FAZ) data (right).2 The method provides higher fold and better azimuthal coverage than other techniques.3 FAZ surveys may be more cost-effective because data are acquired using a sin-gle seismic vessel and are recorded continuously, minimizing NPT.

The ability to successfully perform Coil Shooting acquisition is made possible by the Q-Marine point-receiver marine seismic system. Calibrated

>Multivessel line changes. The four vessels in a typical Gulf of Mexico linear WAZ acquisition configuration follow a circuitous path between the end of one swath and the start of the next, which is necessary to align the vessels for the start of the new swath and avoid collisions during line turns. This vessel movement configuration results in nonproductive time.

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> A four-vessel wide azimuth (WAZ) configuration. This acquisition arrangement—two vessels towing streamers and sources, plus two additional source vessels—has been widely used in the Gulf of Mexico. The offset-azimuth plot (inset) indicates the offsets and azimuths acquired by this configuration—in this case a 60°

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range of azimuths. Azimuth corresponds to the angle clockwise from the top of the circle. Offset corresponds to the distance from the center of the circle. Colors range from purple for a low number of traces to green, yellow and red for a high number of traces.

1. Camara Alfaro J, Corcoran C, Davies K, Gonzalez Pineda F, Hampson G, Hill D, Howard M, Kapoor J, Moldoveanu N and Kragh E: “Reducing Exploration Risk,” Oilfield Review 19, no. 1 (Spring 2007): 26–43.

2. Buia M, Flores PE, Hill D, Palmer E, Ross R, Walker R, Houbiers M, Thompson M, Laura S, Menlikli C, Moldoveanu N and Snyder E: “Shooting Seismic Surveys in Circles,” Oilfield Review 20, no. 3 (Autumn 2008): 18–31.

3. Fold is a measure of the density of seismic measurements. It is usually computed as the number of different source-receiver pairs that record reflections from a particular target layer in each of the rectangular bins (typically 25 m × 25 m [82 ft × 82 ft]) of a 3D grid over the survey area. High fold usually improves S/N.

> Coil Shooting schematic. For Coil Shooting single- vessel full azimuth acquisition, the seismic vessel sails in overlapping circles in corkscrew fashion, recording continuously. The offset-azimuth plot (inset) indicates that this survey configuration acquires complete azimuthal and high offset coverage.

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single sensors enable noise attenuation that can-not be resolved by other technologies. A fully braced acoustic network provides accurate posi-tioning of the in-sea equipment. Q-Fin steering devices accurately control the depth and lateral position of the streamers, facilitating constant streamer separation. DSC dynamic spread con-trol technology adds steerable sources and auto-matic vessel, source and streamer steering to achieve the best possible match with planned source and receiver positions. The Coil Shooting technique requires no special adaptation of the equipment used for conventional 3D surveys, and vessels can easily switch between linear and cir-cular acquisition programs.

Coil Shooting surveys have been acquired in several regions and, in areas of complex geology, the results from these circular geometry FAZ sur-veys have been superior to those from conven-tional 3D surveys and comparable to or better than WAZ data acquired using multiple vessels. This article describes recent imaging successes from offshore Indonesia, Brazil, Angola and the Gulf of Mexico.

Imaging in IndonesiaThe first full commercial survey using the Coil Shooting technique was acquired in 2008 for the E&P division of Eni SpA on the Tulip project in the Bukat production-sharing contract block east of Kalimantan, offshore Indonesia. Several unfavorable geologic conditions conspire to cause poor seismic response in the area.4 The target has low P-wave impedance contrast, thus has only weak seismic reflectivity. The seafloor is characterized by rough geomorphology, with can-yons and irregularities that cause uneven illumi-nation in the subsurface and complex 3D raypaths for surface and internal multiples.5 A bottom-simulating reflector (BSR) below the seabed gen-erates several orders of multiples that further degrade subsurface illumination.6 The presence of free gas below the BSR causes sudden fre-quency and amplitude decay of primary reflec-tions. Complex subsurface geology further complicates the scenario. Combined, these con-ditions lead to diffraction, absorption, scattering and weak transmission of seismic signal energy. Such effects have been observed in the results of conventional narrow azimuth towed streamer (NATS) 3D seismic surveys, which have delivered poor illumination of the target reservoir.

Eni sought to improve imaging through inno-vative acquisition. Engineers performed a feasi-bility study using ray tracing on an existing Tulip velocity-depth model to evaluate the potential of

various single-vessel towed streamer geometries. Multivessel WAZ and RAZ options were not con-sidered because it was important to record near offsets—data with short source-receiver separa-tion—to image the undulating seabed. The study concluded that a Coil Shooting survey would provide the best illumination of the targets. In addition, mobilizing several vessels to the survey area would have been logistically and financially challenging.

The selected survey design consisted of 145 circles of radius 6,500 m [21,300 ft] with circle centers spaced 1,000 m [3,280 ft] apart. The seis-mic vessel Geco Topaz, equipped with eight streamers each 6 km [3.7 mi] long and separated by 100 m [328 ft], acquired the 563-km2 [217-mi2] survey during August and September 2008. Approximately 260,000 shotpoints were acquired.

Acquisition of the data from the original pro-grammed circles was completed more quickly than expected, and some additional lines were acquired to fill in areas of low illumination. At the end of the survey, after infill shooting, the actual target illumination was slightly more uniform than the planned one. WesternGeco completed the Tulip field Coil Shooting survey in 49 days. By comparison, a three-azimuth MAZ survey was predicted to require 60 days, and a four-azimuth survey 75 days.

Seismic engineers began processing the Tulip Coil Shooting survey data onboard Geco Topaz in August 2008 and completed the process in

Jakarta in February 2010. The Coil Shooting method results in many acquisition and imaging benefits but introduces challenges in data pro-cessing because some standard processing work-flows were designed for data with linear geometry. Prior to acquisition of the full survey, WesternGeco geophysicists generated a subset volume of 3D synthetic data with coil geometry and processed it to verify the efficacy of the proposed algorithms and workflow.

An important step in preparing data for the processing workflow is the removal of multiples. WesternGeco has developed the 3D GSMP gen-eral surface multiple prediction process, which has proved highly effective for attenuating multi-ples while preserving the integrity of primary energy.7 Apart from stacking velocities, the algo-rithm requires no a priori knowledge of the sub-surface and can handle all orders of surface multiple in the presence of complex geology and irregular acquisition geometries.8 The 3D GSMP technique predicts the multiples at true azimuth, ensuring that the modeled multiples accurately match the multiples in the input data. The tech-nique is most effective when applied to data from a wide range of azimuths, so optimal performance is achieved when applied to the FAZ data pro-vided by the Coil Shooting approach. In the Tulip dataset, 3D multiples were predicted almost per-fectly in phase, and the algorithm reduced their amplitudes by approximately 25 dB [94%].

> Azimuthal tomography workflow. A tilted transverse isotropic (TTI) velocity model derived from time domain processing formed the starting model. The Tulip Coil Shooting dataset was divided into three azimuthal groups for prestack depth migration (PSDM). Traveltimes from the PSDM were compared with those predicted by the model using tomography inversion and ray tracing techniques, leading to an updated velocity model.

Tomography inversionRay tracing with actual azimuths

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High stacking fold, and therefore improved S/N, is one of the many benefits of the Coil Shooting method. However, fold can vary considerably from one bin to another, and this variation must be addressed to avoid introducing anomalous variations in the amplitude of traces after stacking them together. Using a weighting system that computed scale factors based on the spatial distribution of traces within a 3D bin, engineers applied processes to regularize fold and offset contributions within the full range of azimuths. Subsequent analysis of amplitudes at the target depth indicated that normalization was successful, rendering the dataset suitable for processes such as amplitude variation with offset analysis or inversion.

Azimuthal information also provides opportu-nities for building more-accurate models of sub-surface seismic velocity, which in turn enable more-accurate depth images of subsurface 3D structures. The models are built using tomogra-phy—an inversion process that attempts to con-struct an estimate of the 3D velocity structure of the Earth based on observed measurements of traveltimes associated with seismic reflections, often including some geologic constraints. The analysis is usually performed on 2D sections and is an iterative process that moves toward a best-fit solution between observed traveltimes and those predicted from the 3D velocity model.

As input for the azimuthal tomography work-flow, the Tulip dataset was split into three azimuth sectors, each representing a range of 60° (previous page). Prestack depth migration (PSDM) was applied to each sector and the prestack results were output on a 50-m × 50-m [164-ft × 164-ft] grid for analysis.9 The initial tilted transverse isotropic (TTI) velocity model was derived from time domain processing. Analysis of the PSDM data indicated where adjustments to the velocity model were required, and the process was iterated until the model matched the observed traveltimes. The resulting anisotropic velocity model showed a good match with interval velocities derived from a verti-cal seismic profile (VSP) that had been previously acquired in the survey area. The TTI model was also consistent with geologic boundaries and velocities observed in a well, and it identified areas of low velocity below the seabed that were probably caused by the presence of free gas.

The final Coil Shooting PSDM results show several improvements in imaging at the target level and at greater depths compared with data from a previous narrow azimuth survey over the same area (above right). In particular, the conti-nuity, visibility and sharpness of the dipping events are clearly evident.

Seeing Through Salt Offshore BrazilIn early 2010, an operator was looking for an opportunity to evaluate the coil acquisition tech-nique as a tool to improve imaging of presalt tar-gets offshore Brazil. The company invited WesternGeco to implement the technology in an oil field located in deep water in the Santos basin.10 The reservoirs in this field are up to 6,000 m [20,000 ft] below the ocean surface and

vary in thickness from tens to hundreds of meters. The overburden includes a complex dip-ping salt layer up to 2,000 m [6,600 ft] thick that consists of homogeneous halite bodies and lay-ered evaporites. The S/N in existing seismic data from the area is poor at the reservoir level. In addition, strong surface and interbed multiple energy interferes with primary reflections from the presalt target.

4. Buia M, Vercesi R and Tham M: “Coil Shooting on Tulip Discovery: Seismic Processing Challenges, Opportunities and Results,” paper SPE 134222, presented at the SPE Annual Technical Conference and Exhibition, Florence, Italy, September 19–22, 2010.

5. A multiple is a seismic arrival that has incurred more than one reflection in its travel path. Many multiples involve reflections from the seabed and the sea/air interface. Others involve reflections between subsurface reflectors. Multiples can interfere with or obscure primary reflections and usually need careful suppression during processing.

6. A bottom-simulating reflector (BSR) is a seismic reflection often seen in seismic sections from deepwater areas. Studies indicate that it is primarily caused by the acoustic impedance contrast where free gas is trapped at the base of a gas hydrate zone.

> Tulip Coil Shooting data. Comparison of an example line from a previous NATS 3D survey (left) with equivalent data from the new Coil Shooting prestack depth migrated dataset (right) demonstrates improvements in imaging, particularly in the continuity, visibility and sharpness of the dipping events in the deeper section.

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7. Moore I and Dragoset B: “General Surface Multiple Prediction: A Flexible 3D SRME Algorithm,” First Break 26, no. 9 (September 2008): 89–100.

8. Stacking is a key stage in seismic processing in which the traces in a bin are combined. Before stacking, traces require individual corrections based on their source-receiver offsets and an estimate of subsurface seismic velocities to bring them to a common time horizon before stacking, or summing.

9. Migration is a step in seismic processing in which reflections are moved from their recorded two-way traveltimes to an estimate of their true position in space based on a model of subsurface seismic velocities.

10. Cooke A, Le Diagon F, De Marco R, Amazonas D, Bunting T, Moldoveanu N, Klug S and Mattos E: “Full-Azimuth Towed-Streamer Seismic: An Exploration Tool for Pre-Salt Hydrocarbon Exploration Offshore Brazil,” paper SGS 1.6, presented at the 82nd SEG Annual International Meeting and Exposition, Las Vegas, Nevada, USA, November 4–9, 2012.

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The survey was centered on the planned loca-tion of a future well, with the objective of using the new dataset to help optimize well placement. The survey area also enclosed an active drilling rig. While this rig prevented acquisition of con-tiguous coil data over the entire survey area, it facilitated acquisition of a spiral 3D VSP dataset. Processing specialists used these measurements to validate the earth model used for depth imag-ing of the surface seismic dataset. The operator plans to merge the 3D VSP data with the surface seismic data volume, which will improve cover-age in the area obscured by the presence of the drilling rig.

Modelers conducted a survey design study to compare the expected results of several potential acquisition geometries. The expected processing sequence was applied to synthetic datasets gen-erated using 3D ray tracing through an existing earth model. The study confirmed that, compared with linear geometries, Coil Shooting acquisition would deliver improved S/N, better attenuation of multiple energy and better continuity of reflec-tions at the target level. After considering several potential coil geometries, the companies agreed on a plan to acquire 78 circles of 6.25-km [3.88-mi] radius over an area of 600 km2 [230 mi2] (below). In an effort to make the acquisition geometry slightly less regular and reduce offset clustering within 3D bins, the coil centers were randomly distributed within a pre-defined tolerance.11

The survey was acquired from November 2010 to January 2011 in water depths ranging from 2,000 to 2,300 m [6,600 to 7,500 ft]. The seismic vessel was equipped with 12 streamers, each 8,000 m [26,250 ft] long, towed 120 m [394 ft] apart. Two source arrays 60 m [197 ft] apart were fired alternately every 37.5 m [123 ft].

The seismic crew generated displays onboard to confirm the quality of the seismic data. Source and receiver position data were transferred to offices in Rio de Janeiro, where analysts pro-duced near real-time fold and illumination maps for comparison with the original plan (above). The crew made some changes to the arrangement of swaths during the survey, particularly in the area around the rig. These changes were neces-sitated by the magnitude and direction of currents

as well as an extension of the exclusion zone around the rig. The circular geometry allowed for easy operational adjustments. The vessel acquired more than 92,000 shots and conducted one less circle than originally planned.

Before acquisition began, analysts tested a workflow that included initial noise attenuation and some wavelet processing, which was subse-quently applied in near real time onboard the seismic vessel. The workflow effectively removed high-amplitude noise with no apparent impact on signal amplitudes (next page, top).

Processing specialists applied corrections to account for variations in the acoustic velocity of the seawater during the survey because of changes in temperature and salinity. Such varia-tions cause anomalies in the traveltimes of

> Brazil survey source positions. The red coils show the actual source positions. The squares represent the survey area boundary, 180° and 360° azimuth areas. The drilling rig location is indicated by the black circle.

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Area of 360° coverage> Fold and illumination. Total fold of coverage (top) calculated for 25-m � 25-m bins. The planned fold (left) and actual fold (right) are in close agreement. Target illumination, or hit count (bottom), also shows a close match between planned (left) and actual illumination (right).

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reflection events that can impact imaging, atten-uation of multiples and some other processes applied to data sorted into 3D bins. The varia-tions are likely to be most significant in deep water and in areas of rapidly changing currents, which are both features of the waters offshore Brazil. Many different coils, each with its own water column characteristics, contribute data to each bin; thus when significant velocity varia-tions occur, they must be corrected.

Water velocity corrections were applied using a layer-replacement approach.12 Because a single coil sail line with a radius of 6.25 km [3.9 mi] may sample significantly varying current regimes in the survey area, geophysicists divided each coil into separate segments and estimated the water velocity for each segment. They then applied dynamic corrections to adjust the dataset to a single water velocity function.

In this area of the deepwater Santos basin, surface multiples from both the seafloor and the complex top-of-salt horizons coincide with the arrivals of the weaker base-of-salt and subsalt reflections. Therefore, it was critical to attenuate this energy without corrupting the primary ampli-tudes. One of the characteristics of coil acquisi-tion is that it has a higher shot density than conventional NATS surveys and delivers more near-offset information than multivessel WAZ sur-veys. These features provide data that more closely meet the requirements of techniques for

attenuating multiples such as the 3D GSMP method that was applied.

Data coverage in terms of offset, azimuth and midpoint (the surface position equidistant between source and receiver) is inherently irregular over the area of a coil survey. For some data processing algorithms—such as tomo-graphic velocity model building—3D bins need to have regularly spaced midpoints, azimuths and offsets. Several methods are available for

11. Moldoveanu N: “Random Sampling: A New Strategy for Marine Acquisition,” Expanded Abstracts, 80th SEG Annual International Meeting and Exposition, Denver (October 17–22, 2010): 51–55.

12. Carvill CV: “A New Approach to Water Velocity Estimation and Correction,” paper U027, presented at the 71st European Association of Geoscientists and Engineers Conference and Exhibition, Amsterdam, June 8–11, 2009.

13. Schonewille M, Klaedtke A and Vigner A: “Anti-Alias Anti-Leakage Fourier Transform,” Expanded Abstracts, 79th SEG Annual International Meeting and Exposition, Houston (October 25–30, 2009): 3249–3253.

regularization, and various methods were used as appropriate for different parts of the data pro-cessing sequence for the Santos basin survey.

Producing regularly sampled data from irregu-larly sampled data requires interpolation. For the offshore Brazil dataset, matching pursuit Fourier interpolation produced a fully regularized dataset in offset, midpoint and azimuth for building the velocity model in the sediments (below).13 This method can efficiently interpolate in multiple

>Noise attenuation. The raw shot record (left) contains high-amplitude noise, which is effectively attenuated after onboard processing (right).

Shot Gathers: Raw Shot Gathers: Filtered

> Data regularization. This section from a common offset, common azimuth volume (top) exhibits gaps and “jitter” between traces caused by azimuthal variation. After regularization, the gaps are filled and jitter is reduced (bottom). The blue plots (inset) show the fold coverage for the offset-azimuth volume before and after regularization. The yellow lines indicate the location of the seismic sections.

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dimensions to improve the regularization of sparsely sampled data. Data are transformed from the space-time domain to the space-frequency domain by a fast Fourier transform. For each fre-quency slice, data are transformed from the spa-tial domain to the spatial Fourier, or wavenumber, domain. Once the Fourier domain has been com-puted, data can be transformed back to any loca-tion in the spatial domain—in this case onto a dense and regular grid—using an inverse discrete Fourier transform.

For depth imaging in complex geologic areas such as this Santos basin field, an accurate velocity model is essential to correctly place reflections in their true subsurface positions. With full azimuth acquisition, multiazimuth tomographic methods may be used for velocity model updating. The introduction of additional information from multiple azimuths reduces uncertainty and increases confidence in veloc-ity model updates.14

Geophysicists updated the Santos basin field velocity model in several steps: They first split the coil dataset into three azimuth volumes for tomography: 0° to 60°, 60° to 120° and 120° to 180° and their opposite azimuths. They next updated the anisotropic velocity model sequen-tially in three zones: sediment, intrasalt and pre-salt. Validation of the resulting earth model included 3D VSP traveltime analysis, in which measured and modeled arrival times were com-pared to produce an indication of confidence in the model (left).

Results of depth migration of the Coil Shooting data using an intermediate velocity model pro-vided significant imaging improvement compared with those from a previous 2D depth migrated dataset in the area (below left). The new FAZ data have delivered high-quality images of the base of the salt and are allowing confident inter-pretation of presalt structures. Further improve-ments are expected when the data are migrated using the full anisotropic earth model.

Coil Acquisition Offshore AngolaThe first Coil Shooting survey acquired in the West African subsalt province was acquired in Block 33 Angola over the Calulu Predevelopment Area (PDA) for Total E&P Angola (TEPA) and its partners (next page, top right). Water depth in this block ranges from 1,500 to 2,500 m [4,900 to 8,200 ft]. The area is characterized by complex geology related to extended salt canopies that cover most of the block. The main reservoir levels are turbidite sands located in highly structured near-salt and subsalt areas.

A NATS 3D seismic survey was acquired over the area in 1999; however, the quality of the resulting data was insufficient to correctly image the complex salt tectonic structures and the steeply dipping anticlinal flanks. In addition, the presalt targets were characterized by poor S/N and poor illumination. Limited 3.5-km [2.2-mi] offsets and a restricted range of azimuths con-tributed to these unsatisfactory results.

To address these concerns, TEPA and part-ners decided to acquire two new 3D datasets: a 1,284-km2 [496-mi2] NATS survey with long off-

> Brazil survey traveltime residuals. Geophysicists validated the velocity model for seismic imaging by comparing traveltimes through the model with actual traveltimes from a 3D VSP acquired at the same time as the Coil Shooting survey in the Santos basin. The VSP was acquired with spirally distributed source positions around the rig and receivers in the borehole at the center of the spiral. Colors indicate the difference between the modeled and actual traveltimes, with aqua indicating the closest match. The magenta surface beneath and to the right of the spiral is the top of salt.

–32–40 –24 –16 –8 8 16 24 32 400Traveltime residuals, ms

> Seismic lines and a time slice from the Brazil Coil Shooting 3D volume. The new Coil Shooting data have delivered high-quality images of the base of salt and are enabling confident interpretation of presalt structures.

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14. Dazley M, Whitfield PJ, Santos-Luis B, Sellars A, Szabo P, Nieuwland F and Lemaistre L: “Solving Short-Wavelength Velocity Variations with High-Resolution Hybrid Grid Tomography,” paper C001, presented at the 69th European Association of Geoscientists and Engineers Conference and Exhibition, London, June 11–14, 2007.

15. Khaled N, Capelle P, Bovet L, Tchikanha S and Hill D: “A Coil Shooting-Acquisition Case Study in the Angolan Deep Offshore,” paper X027, presented at the 74th European Association of Geoscientists and Engineers Conference and Exhibition, Copenhagen, Denmark, June 4–7, 2012.

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sets—7.5 km [4.7 mi]—over the full area, and an 860-km2 [332-mi2] Coil Shooting survey over a subset of the area. The company’s objective in conducting the NATS survey was to improve the imaging of the deep Oligocene, Cretaceous and presalt sections through better signal penetra-tion, longer offsets, higher fold and a longer recording time than the 1999 survey. The new data would enable reevaluation of the deep post-salt series and a first 3D interpretation of the presalt series. The objective of the Coil Shooting survey was to improve the imaging of the deep Oligocene and Cretaceous subsalt structures to provide a better understanding of trap geometry and reduce uncertainty about the presence or absence of a reservoir. Comparison of the results of the two types of survey provided an opportunity to assess the added value of the Coil Shooting technique.

Before data acquisition, a feasibility study was performed to select the optimal Coil Shooting parameters to adequately image a 38-km2 [15-mi2] target area at the center of the survey.15 The study resulted in a survey design comprising 72 circles with centers in a rhombic pattern spaced 2,500 m [8,200 ft] apart in both inline and crossline directions (right). For the selected design, the fold of the 12.5-m × 12.5-m [41-ft × 41-ft] bins was up to 567, and the azimuthal coverage was up to 360° within the target area.

In comparison with a NATS survey of the same size acquired with exactly the same in-sea equip-ment configuration, in the coil design 95% of bins have a higher fold, and 70% have more than twice the fold. A single vessel Coil Shooting survey records more than double the data volume of a NATS survey, although the Coil Shooting survey records slightly shorter far offsets because of the curvature of the streamers (below).

> Location of the Calulu Predevelopment Area (PDA), Block 33, offshore Angola.

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> Calulu Coil Shooting survey design. The survey design comprised 72 coils (left) with their centers (red dots) in a rhombic pattern (right). The survey delivered high-fold, full azimuth coverage over the target area (red box).

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> Comparison of NATS and single vessel Coil Shooting survey fold and offsets. The histogram of fold (left) shows the constant fold of a NATS survey (red) and the variable but large fold of a coil survey (blue). In offset (right), a NATS survey offers slightly longer offsets, but a coil survey of the same size records more than double the number of traces.

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The seismic vessel WesternGeco Amundsen acquired the two surveys between February and April 2011. The Calulu PDA coil survey acquisition was completed in 21 days. On several occasions, the vessel acquired data continuously for periods longer than 24 hours with no NPT. Despite the much smaller area of the Coil Shooting survey, the size of its prestack dataset was equivalent to that of the NATS survey; each survey comprised approx-imately 120,000 shots.

Quality control of coverage in a NATS survey is usually based on the fold and range of offsets represented in each bin of the 3D grid. Seismic traces whose midpoints lie within a bin are assumed to belong to that bin. This assumption is correct for flat reflectors and isotropic velocities, which are often appropriate for the seabed and shallow geology but becomes less accurate with increasing geologic complexity. Ocean crosscur-rents cause streamers to feather, or deviate from planned positions, which may lead to gaps, or areas of bins with low fold. If the gaps are consid-ered detrimental to the imaging objective, addi-tional infill lines are acquired over these areas. Because the Calulu survey was designed to over-come illumination challenges in a structurally

complex subsalt target, QC using 3D ray trace modeling, in addition to the conventional acqui-sition QC, was performed in near real time onboard the vessel to compare the expected and actual illumination at the target reservoir level. At the end of the programmed survey, the achieved illumination proved to be essentially equivalent to the expected illumination, indicat-ing that streamer feather, vessel deviation and other factors that can lead to a requirement for infill acquisition did not impact illumination of the target (above). Based on this analysis, the team determined that no infill was necessary for the Coil Shooting survey, although the NATS survey required 6.4% more time for infill lines compared with that used for the programmed acquisition (next page, top).

Towing the streamers in a curve in the pres-ence of strong crosscurrents created high levels of acoustic interference known as crossflow noise that required special processing. Because geo-physicists needed to design a workflow for effec-tive crossflow noise attenuation during the main Coil Shooting survey—scheduled to be performed after the NATS survey—several preliminary cir-cles were acquired before the NATS survey to

assess the magnitude of the crossflow noise. The crossflow noise for this Coil Shooting survey reached levels more than 10 times those of average straight line surveys.

Q-Marine technology facilitates effective removal of crossflow noise by leveraging advances in electronics and fiber-optic networks to provide high channel count recording systems. Its single-sensor field data are sampled at 3.125-m [10.25-ft] intervals along each streamer, provid-ing adequate sampling of the signal and most noise. The first stage of the onboard processing sequence is digital group forming (DGF). In DGF, engineers apply data-adaptive algorithms to the shot records from each streamer to recognize and suppress the crossflow noise while preserving the integrity of the seismic signal. Further noise attenuation is achieved during subsequent pro-cessing stages.

Seismic engineers tested parameters for 3D GSMP processing early during acquisition to determine the optimal workflow and facilitate rapid processing turnaround. They selected parameters based on ray tracing through a TTI velocity model provided by TEPA. Various types of multiples were modeled, including some related to

> Target illumination. Expected illumination based on 3D ray trace modeling (left) closely matches actual illumination (right) at the target level. Illumination values, the number of raypaths passing through a bin, are color-coded: Low values are blue, and high values are red.

1,050525

Illumination

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the sea/air surface and others occurring between subsurface reflectors. The water bottom and top-of-salt horizons were defined as the most signifi-cant generators of multiples. The Coil Shooting and NATS data were processed with similar 3D GSMP parameters.

TEPA’s field evaluation schedule required rapid turnaround for the processing and prelimi-nary imaging. A raw TTI reverse time migration (RTM) prestack depth migration 3D volume was available four and five months after the last shotpoint was acquired for the NATS and Coil Shooting data, respectively. RTM is a prestack two-way wave equation migration algo-rithm suited to accurate imaging in and below areas with structural and velocity complexities. Until recently, companies considered RTM impractical because of its high computational requirements and sensitivity to velocity and reflectivity parameters. Now, large parallel com-puting clusters, coupled with new workflows able to build increasingly accurate velocity models, make RTM a more viable option in the imaging portfolio. Nevertheless, to complete the fast track processing on schedule, compromises were made. To produce the fast track images, geophys-icists selected 50% of the shots and migrated fre-quencies up to 25 Hz and 20 Hz for the NATS and Coil Shooting data, respectively. The Coil Shooting data were divided into four azimuth sectors before RTM, then the four partial-stack azimuth data sets were stacked with equal weighting. The Coil Shooting dataset provided an overall improvement in imaging, particularly in the areas of complex structures and steep dips, com-pared with the NATS dataset.

In some areas, steeply dipping reflectors appeared better imaged in the fast track NATS data than in the unweighted stack of the Coil Shooting data. In the presence of complex geology, illumination from different azimuths is likely to lead to images of varying quality. Combining datasets of different azimuths with-out consideration of image quality may lead to destructive stacking. Analysis of predicted sub-surface illumination using ray tracing showed large variations in the offsets and azimuths that would be expected to illuminate different areas and also indicated that destructive stacking was likely to occur in specific offset-azimuth ranges along specified target horizons (right).

To further investigate the variation in azi-muths required to illuminate reflectors around the salt structures, geophysicists applied PSDM

> Survey times. While prime acquisition took about the same time for each of the Calulu PDA surveys, line change and other nonproductive time are much greater for the NATS acquisition.

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> Offset-azimuth plots. Offset-azimuth plots are shown for four locations in a target horizon (center) around a salt diapir. For each circular plot, distance from the center indicates offset, clockwise deviation from vertical represents azimuth and color represents the illumination, or the number of raypaths passing through that location, with low values in blue and high values in red. The plots indicate large variations in the offsets and azimuths required to illuminate each area. Optimal stacking results are achieved by giving additional weight to data from azimuths that provide the best image.

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to part of the dataset and split the results into 18 azimuth ranges—each of 10°—for stacking. Analysis of the 18 datasets confirmed the corre-lation observed between the seismic imaging and the illumination study (above). The images produced using data from different azimuths exhibited significant differences, indicating that a more intelligent data-adaptive stacking scheme should create a better image than an unweighted method.

Processing experts developed an iterative intelligent stacking scheme to solve the problem of destructive stacking and create an optimal image throughout the 3D volume. Localized weights for each azimuthally migrated image were derived from comparison with a reference image. If the azimuthal image was locally similar to the reference image, its weight was increased, and if it was dissimilar, its weight was decreased. Preliminary results of this iterative stacking scheme suggest that it shows promise for produc-ing optimized results from multiazimuth data (middle left).16

Initial results indicate that the azimuthal richness of the Coil Shooting technique may be beneficial for enhancing imaging in the Calulu PDA. A comparison between the Coil Shooting fast track results and the full processing results of the NATS data acquired at the time demon-strates the effectiveness of the weighting approach (left). Areas of imaging improvements include targets below salt canopies that have

> The impact of azimuth on imaging. PSDM results on a subset of the Calulu PDA Coil Shooting survey were split into 18 azimuth ranges. Each panel illuminates different features. Because ordinary stacking averages these features and enhances only those signals that are common to all panels, it could degrade signal amplitude in the most difficult-to-image areas. This observation led geophysicists to devise a data dependent weighted stacking method that would give preference to azimuths that provided the best illumination.

0° to 10° 10° to 20° 20° to 30° 30° to 40° 40° to 50° 50° to 60° 60° to 70° 70° to 80° 80° to 90°

90° to 100° 100° to 110° 110° to 120° 120° to 130° 130° to 140° 140° to 150° 150° to 160° 160° to 170° 170° to 180°

> Intelligent stacking. Variations in azimuth and offset coverage necessitate a weighted stacking scheme to deliver optimal results. In some areas, data from the NATS survey (left) were similar in quality to those from the Coil Shooting fast track processing (middle). Applying data dependent weighting to azimuth ranges prior to stacking the Coil Shooting data (right) provided an optimized image throughout the dataset.

NATS Data Unweighted Coil Shooting Data Weighted Coil Shooting Data

> Comparing NATS final processing with preliminary Coil Shooting results. The fully processed narrow azimuth, towed streamer data (left) reveal a great deal of structural complexity, but the partially processed Coil Shooting data (right) show areas even more clearly imaged (green boxes). These include features that are poorly imaged in the NATS data such as layers below salt overhangs and deep reflectors that show better lateral continuity.

NATS Final Processing, 20 Hz Coil Shooting, Fast Track Processing, 20 Hz

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benefited from FAZ illumination, and deep targets that have benefited from improved S/N resulting from the higher fold. The ongoing final process-ing is focused on fully exploiting the azimuthal richness of the Coil Shooting technique.

Full Azimuth and Long Offsets in the Gulf of MexicoSome geologic environments lead to highly com-plex raypaths. In such environments, adequate illumination of the subsurface often requires not only a full range of azimuths but also very long offsets between seismic sources and receivers. This is the case in some subsalt hydrocarbon plays in deepwater areas of the Gulf of Mexico, which often present severe imaging challenges because of thick salt bodies with complex mor-phology. Multivessel WAZ methods have improved imaging in these areas, but many datasets have exhibited areas of low S/N and poor reflector con-tinuity, particularly beneath salt overhangs and where dip is steep. These areas of poor illumina-tion are often where imaging is most crucial for identifying drilling targets and performing field appraisal. Modeling studies of these areas have indicated that adequate subsalt imaging requires full azimuth coverage and source-receiver offsets of up to 14 km [8.7 mi] (above right).17

Modern 3D surveys in the Gulf of Mexico typi-cally use streamers 8 km [5 mi] long. Because deploying much longer streamers in a circular geometry would be logistically challenging, a single-vessel solution cannot meet the require-ments for long offsets. To deliver the required azimuth and offset ranges in this area, WesternGeco geophysicists designed a four-vessel coil system. It involves two recording vessels with their own sources and two separate source ves-sels sailing in 12.5-km [7.8-mi] diameter inter-linked circles (right). Each streamer vessel has 10 streamers 8 km in length and with 120-m streamer separation. This dual coil design deliv-ers a trace density approximately 2.5 times that of current WAZ survey designs, which improves S/N, further enhancing the imaging of weak sub-salt reflections. The first multivessel coil survey in the Gulf of Mexico was acquired in 2010, and today an area of more than 25,600 km2 [9,880 mi2]—equivalent to about 1,100 Outer Continental Shelf blocks—has been surveyed using the multivessel method. Survey locations have included heavily obstructed areas with cur-rents exceeding 2.5 knots [4.6 km/h, 2.9 mi/h].

The surveys are designed to have a random distribution of sources and receivers. The motiva-tion behind this is twofold: The random shot and

> Improving subsalt illumination with long offsets. Finite-difference acoustic modeling shows the effect of offset length on imaging a reflection package truncating against a salt keel (dashed yellow circles). In a commonly used WAZ recording configuration with a maximum inline offset of 8 km and maximum crossline offset of just over 4,200 m [13,800 ft], reflection truncation against the salt keel is indistinct (left). With a long-offset FAZ Coil Shooting design (right), the reflection truncation is more coherent.

Salt keel

Wide Azimuth Survey, Standard Offsets Coil Shooting Survey, Long Offsets

Reflectionpackage

> Dual coil survey design. The long-offset FAZ Coil Shooting surveys in the Gulf of Mexico have been acquired using two recording vessels with their own sources (S1 and S3) and two separate source vessels (S2 and S4) sailing in 12.5-km diameter interlinked circles.

S1

S3S2

S4

16. Zamboni E, Tchikanha S, Lemaistre L, Bovet L, Webb B and Hill D: “A Coil (Full Azimuth) and Narrow Azimuth Processing Case Study in Angola Deep Offshore,” paper X025, presented at the 74th European Association of Geoscientists and Engineers Conference and Exhibition, Copenhagen, Denmark, June 4–7, 2012.

17. Moldoveanu N and Kapoor J: “What Is the Next Step After WAZ for Exploration in the Gulf of Mexico?,” Expanded Abstracts, 79th SEG Annual International Meeting and Exposition, Houston (October 25–30, 2009): 41–45.

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receiver distribution removes any clustering or repeating pattern that may appear in the cover-age, and compressive sampling theory states that if data are undersampled, the seismic wavefield is better reconstructed if measurements are ran-domly distributed.18 Seismic data acquired in conventional NATS marine surveys are typically

undersampled for both sources and receivers and regularly distributed along parallel lines. Therefore, in any part of the processing sequence that requires interpolation or regularization, the Coil Shooting randomly sampled data will per-form better than conventional data.

One Gulf of Mexico survey was located in an area with several production and drilling installa-tions, representing exclusion zones that required consideration when planning coil locations. To acquire surface seismic data under such obstruc-tions involves deploying sources on one side and receivers on the opposite side of the restricted area—a method called undershooting. The dual coil configuration lends itself to undershooting because a four-boat coil unit can enclose an area with a diameter of approximately 9 km [5.6 mi] without modification. Careful planning of the coil locations enabled the survey crews to undershoot most of the production and drilling facilities without the need to reconfigure. For the three largest obstructions in the survey area, the coil diameter was enlarged to accommodate the exclusion zones. An automated steering and posi-tioning system accurately controlled the loca-tions of the vessels, sources and streamers, which is particularly critical when making passes close to obstructions.

Flexibility in survey design in terms of the shape of the area to be covered is another impor-tant feature of the Coil Shooting technology. While NATS surveys are generally rectangular or have other regular geometric shapes, Coil Shooting survey designs can accommodate any shape so can be optimized to address the res-ervoir or exploration target area. Also, survey areas can be easily extended in any direction after acquisition of an initial program, for exam-ple if interesting new features are identified or the initial survey is completed faster than expected (above left).

The dual coil Gulf of Mexico FAZ datasets have been processed using vertical transverse isotropy or TTI RTM schemes appropriate for the complex geology and steep dips around the sub-salt targets. Processing included 3D prestack acoustic full waveform inversion (FWI), which uses a two-way wave equation method, to build high-resolution velocity models. Full waveform inversion performs forward modeling to compute the differences between the acquired seismic data and the current model and carries out a pro-cess similar to RTM on the residual dataset to compute a gradient volume and to update the velocity model. When combined with imaging using RTM, model building with FWI improved the final product because consistent wavefield solutions were applied throughout the depth imaging workflow. Initial results from the Gulf of Mexico Dual Coil Shooting surveys show signifi-cant improvements over linear WAZ surveys in the same areas (left).

> Coil Shooting design flexibility. The source positions (red coils) of a dual coil survey in an obstructed area of the Gulf of Mexico demonstrate the ability to extend the survey area in any direction. Obstructions are denoted by yellow circles.

> Dual Coil Shooting results. Two datasets were fast track processed using the same preliminary velocity model. The linear WAZ dataset (left) and Dual Coil Shooting dataset (right) both show a strong reflection at the top of salt. The Dual Coil Shooting dataset exhibits improved imaging of the base of salt and better continuity of reflections (dashed yellow circles) beneath the salt body.

Linear WAZ Data

Top of Salt

Dual Coil Shooting Data

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> Slanting the streamer. In a Gulf of Mexico test, Coil Shooting data from a horizontal streamer (top) and a slant streamer (bottom) demonstrate the broader bandwidth achievable with a slant streamer. Deep reflectors in complex structures, such as those in the yellow boxes, are more clearly imaged with the slant streamer.

Horizontal Streamer

Slant Streamer

To date, the multivessel Coil Shooting tech-nique has focused on imaging challenges in the western Gulf of Mexico; however, it is applicable in other difficult-to-image geologic environments such as where thick layers of basalt are present or where carbonates distort seismic raypaths.

Investigations are ongoing into the use of simultaneous sources to improve source sampling and productivity in multivessel seismic acquisi-tion. The four sources in the current Gulf of Mexico projects are fired sequentially at intervals 17 s apart. By firing four sources at the same time, data density is quadrupled for no extra acquisi-tion cost as long as the resulting wavefields can be recorded separately. With sources spaced at least 12.5 km apart on opposite sides of a coil, there is no overlap of the wavefields for large por-tions of a simultaneous shot record. Modeling studies and a field feasibility test have indicated

that multivessel coil data acquired with simulta-neous sources can be processed effectively.19 Further investigations are underway to confirm that simultaneous shooting can be applied on future multivessel Coil Shooting projects.

Extending the BandwidthCoil acquisition geometries deliver FAZ data and may be configured to deliver long offsets, both of which contribute to improved illumination. Improving resolution, another key objective in the quest to enhance seismic imaging, requires extending the range of usable signal frequencies at both high and low ends.

One of the limiting factors in seismic resolu-tion for marine towed streamer acquisition is the so called “ghost” effect, which results from the receivers being deployed several meters below the sea surface. The ghost effect causes attenua-tion of certain frequencies depending on the receiver depth. The attenuation is caused by interference between the upgoing seismic wave-field and its ghost—the reflection of the wave-field bouncing back from the sea surface above the streamer. Conventional towed streamer marine seismic acquisition systems typically deploy streamers at depths between 6 and 12 m [20 and 39 ft]. Shallower towing would preserve medium and high frequencies but attenuate low

frequencies. Shallower towing would also make the data more susceptible to environmental noise such as waves, swell and wind. Towing streamers at greater depths would reduce environmental noise and preserve low frequencies but would attenuate higher frequencies.

Recent developments in seismic acquisition and processing technologies have enabled the successful implementation of several solutions to address the receiver ghost problem. One solution is to tow the streamers at a slant, resulting in variable receiver depths—and thus attenuating a variable range of frequencies—from one end of the streamers to the other.20 Stacking and migra-tion combine data from different parts of the streamers, attenuating the receiver ghost. However, before these processes, attenuation of multiples and velocity model building must be performed, for which a uniform wavelet is required. To facilitate this, a new algorithm was developed that performs prestack receiver deghosting, and this is applied at an early stage in processing. The source ghost is addressed by a newly developed calibrated marine broadband family of seismic sources.

WesternGeco conducted a feasibility field test with a slant streamer configuration during the multivessel coil acquisition program in the Gulf of Mexico, in which streamers are usually towed at a depth of 12 m. Acquisition of one coil was repeated with the streamers deployed in a slant mode such that receiver depths ranged from 12 m to 32 m [39 to 105 ft]. Comparison of prestack depth migration results for horizontal streamer and slant streamer data indicated that the ObliQ slant streamer acquisition and processing technique enhanced the low frequencies while preserving medium and high frequencies (above left). The preservation of lower frequencies is important not only for imaging deep or steep targets but also for building high-resolution velocity models using FWI. To date, two single-vessel Coil Shooting surveys have been acquired using slant stream-ers, one in Europe and one in Asia. A multivessel Coil Shooting survey using slant streamers has been acquired in the Gulf of Mexico.

Since the first Coil Shooting feasibility tests in 2007, the technique has proved to be a cost-effective and efficient solution for better illumi-nation and improved seismic imaging in complex geologic environments around the world. Further improvements are expected from the implemen-tation of innovative acquisition configurations, advanced processing technologies and new work-flows that will extract more information from seismic measurements to enhance our under-standing of the subsurface. —JK

18. Moldoveanu, reference 11. 19. Moldoveanu N, Ji Y and Beasley C: “Multivessel

Coil Shooting Acquisition with Simultaneous Sources,” paper ACQ 1.6, presented at the 82nd SEG Annual International Meeting and Exposition, Las Vegas, Nevada, November 4–9, 2012.

20. Moldoveanu N, Seymour N, Manen DJ and Caprioli P: “Broadband Seismic Methods for Towed-Streamer Acquisition,” paper Z009, presented at the 74th European Association of Geoscientists and Engineers Conference and Exhibition, Copenhagen, Denmark, June 4–7, 2012.