Future-proof seismic: high-density full-azimuth

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1 first break volume 28, June 2010 special topic Future-proof seismic: high-density full-azimuth Salva R. Seeni, 1* Scott Robinson, 1 Michel Denis, 2 Patrick Sauzedde 2 and Roger Taylor 2 detail the extensive preparation and operational challenges involved in the carrying out of an intensive seismic programme in the Dukhan field, Qatar requiring high-density, full-azimuth 3D data coverage plus the simultaneous acquisition of a number of 3D VSPs. T he Dukhan field is a major oil field in Qatar discovered in It contains more than 750 wells producing from four reservoirs. It has a complex history of production and development strategies, starting with natural pressure depletion for more than 20 years, followed by initial water flooding, power water injection since 1989 and gas cap cycling since As with other large mature Middle East oil fields, Dukhan has witnessed significant changes in technology over the last 70 years. In order to maximize the long-term economic recovery from the field, Qatar Petroleum (QP) is committed to applying the appropriate leading-edge technologies. As part of this, the company identified a state-of-the-art 3D seismic programme as essential for the updating of the reservoir models, enabling QP to continue the development of the Dukhan field for many years to come. Specific aims of the seismic programme were to provide: Enhanced fault imaging to locate bypassed oil associated with faulting Quantitative reservoir characterization to control the distribution of reservoir properties away from wells by means of seismic attributes Fracture identification to delineate areas of enhanced productivity, especially in tighter intervals Improved structural definition of deeper reservoirs As development of the field and surrounding industries continues, further obstacles to seismic acquisition over the field will only increase with time. While seismic data can always be reprocessed to take advantage of new technology in the future, it is possible that future reacquisition of the entire Dukhan field area may be difficult or impossible to justify. Therefore one of the key driving principles behind the seismic programme was that Qatar Petroleum future-proofed the survey by ensuring that it did not under-specify the source and receiver effort and the gathering of complementary geophysical data. This article is a case study of the seismic programme so far, including the acquisition of a large, ultra high-density and full wide-azimuth 3D seismic survey across the Dukhan field and the 3D vertical seismic profiles (VSPs) acquired simultaneously with it. Need for next-generation seismic Although existing seismic data over the Dukhan field was able to provide gross structural information for reservoir mapping and well planning, it was failing to provide the vertical and spatial resolution and quantitative attributes needed for enhanced reservoir characterization. This was due to reduced frequency content, noise contamination, low fold, and limited spatial coverage. Advancements in the seismic industry led QP to the belief that a new ultra high-density, full wide-azimuth 3D seismic dataset would fulfill the quantitative interpretation needs for the continued development of the field (Seeni et al., 2009). This resulted in a series of tests to prove the case for the new acquisition. QP acquired highresolution VSP data in two wells in 2003 which proved that seismic data with frequencies greater than 100 Hz could be obtained from borehole seismic in the Dukhan field. In 2006, QP conducted a pilot 3D land survey to evaluate whether the same high-fidelity signal could be achieved with surface seismic. The results showed improvements in bandwidth (up to 100 Hz) and image quality compared to the legacy 3D surveys, clearly demonstrating the benefits of a next-generation 3D seismic programme for Dukhan. The pilot 3D survey also provided valuable information on logistics and operational issues in managing the considerable acquisition effort and working around extensive surface infrastructure. The scope of the new seismic programme for a full-field Dukhan 3D seismic survey was an integrated project. Firstly it expanded on previous work by extending the acquisition boundaries so that the entire field area would be properly imaged with full migration aperture, comprising approximately 860 km 2 of land, including sabkha (salt flats) and transition zone (Figure 1). Secondly, complementary data would be acquired to enhance the processing and allow for detailed and robust reservoir calibration. This would include uphole velocity surveys and 3D VSPs recorded simultaneously during the acquisition of the surface seismic (Figure 2). Some of the key design features were: 1 Qatar Petroleum. 2 CGGVeritas. *Corresponding author, seeni@qp.com.qa 2010 EAGE 79

2 special topic first break volume 28, June 2010 Point-source and single geophone point-receiver acquisition Ultra high-density spatial sampling for de-aliased recording of signal and noise Uniform, full wide-azimuth coverage using 36 x 5 km receiver lines and 1480 source points per km 2 25,000 live channels with 40,000 channels deployed on the ground High productivity, high-fidelity HFVS vibroseis sources sweeping up to 120 Hz Simultaneous recording of eight 3D VSPs during the surface seismic programme Comprehensive geophysical wireline logging programme in all new wells drilled Full areal coverage of uphole surveys with 130 planned across the field The step-change in the acquisition parameters and the resulting trace density compared to a conventional land 3D survey are summarized in Table 1. Figure 1 Location map for the Dukhan full-field 3D seismic project. The source type is shown for the three different environments encountered: Vibroseis, airgun for shallow water and explosives for the sabkha areas. Table 1 The high-density full-azimuth geometry used for Dukhan generates nearly 100 times the amount of traces per km 2 as a conventional 3D land survey. HSE and quality For a survey of this size, project management and strict adherence to quality and HSE procedures were the key to achieving quality, safety and high productivity. During the planning stages prior to mobilization, a full HSE hazard identification and risk assessment was carried out by CGGVeritas and QP considering the various operational modes. A crew HSE plan was prepared, procedures were established, implemented and audited by QP together with a bridging document in accordance with international guidelines. The main hazards included the ever-present risks relating to road transportation and those specific to the survey area: toxic gas, cement factories, quarries, and ordnance. HSE was a prime consideration for the survey and initiatives were taken to minimize exposure, such as moving the base camp location as the survey progressed to reduce road traffic exposure. To allow safe access around the various industrial sites, coordination with QP downstream activities in Dukhan and external parties (e.g. police, military, quarry operators, cement company, etc.) was planned well in advance. This allowed close access around infrastructure (Figure 3) for both sources and receivers and contributed significantly to providing nearly uninterrupted full-fold, full-azimuth coverage. In addition, this close liaison succeeded in minimizing industrial noise by shutting down these operations where possible during the survey. The outcome of this preparation has been more than 3.8 million man-hours of operations without a Lost Time Incident being accrued to date. In addition to hazards, the scope of the survey also includes environmentally-sensitive areas such as pristine desert, reefs, and sea-grass plains. The same rigorous approach to safety was also applied to environmental concerns. This EAGE

3 first break volume 28, June 2010 special topic Figure 2 Uphole coverage maps on the left with existing data sites in blue and new acquisition sites in red (123 locations). Planned VSP locations displayed on the right, with 2 km radius superimposed for reference. included restoration of uphole sites after surveying, strict waste management on the crew, and a marine mammal watch for the shallow water and transition zone phase of the survey. Due to the complex nature of this survey and the enormous volumes of data generated, QP chose to pursue an aggressive QC policy with a team of specialists from Jaguar Exploration to help oversee and manage the data acquisition and processing. The contracted QC specialists included geodetics, acquisition, in-field processing, and data processing as well as an overall field supervisor. Field operations overview Field operations span an extensive area of some 860 km 2 onshore (including sabkha) and a transition zone area. During the survey design the survey was split up into eight sub-areas (or zippers) related to the source type (Figure 4): Zipper 1, 2, 3, 4: Land Vibroseis source Zipper 5, 6, 7: Combined land and shallow water Vibroseis and airgun source Zipper 8: Sabkha explosive source In addition to source logistics, the choice of the zippers was intended to facilitate the most efficient deployment of the recording equipment, as it was not possible to span the full survey width (up to 14 km) with one set of equipment. To achieve full-fold coverage, a significant overlap of receiver stations or source points was required. In most locations, for operational efficiency, it was elected to re-occupy source locations in preference to re-laying receiver lines. For the complete project it was estimated that an additional 40% VPs and 315% marine source POPs would be taken to achieve the required overlap coverage. As a result, some source locations were occupied only once, others a maximum of four times. GPS stakeless surveying was used for Vibroseis source point positioning. Real-time GPS surveying has several advantages over and above the improved positioning accuracy that it achieves. It improves the efficiency of the front crew and allows real-time vibrator navigation along source lines which are scouted to pre-plan offsets and detours due to obstructions. It also eliminates surveying debris, reduces the HSE exposure of surveying teams, and allows real-time QC 2010 EAGE 81

4 special topic first break volume 28, June 2010 Figure 3 Close access to infrastructure by source and receiver crews was achieved by careful coordination with facility owners and ensured the maximum possible coverage. Top: front crew lay out in cement factory premises. Bottom: Vibrators operating inside the Jaleha main degassing station. Figure 4 Source occupancy and acquisition zipper map. Zipper 8 lies within the sabkha area of zipper 4 and will be acquired using explosives sources. To achieve full fold coverage it is necessary to reoccupy source points up to four times in some of the zippers. of positioning data. In this case variances from pre-planned locations were immediately identified and any VP which was outside positioning specification could be flagged for re-acquisition. Previous 3D surveys of the Dukhan field had been characterized by significant data gaps due to insufficient source coverage for oilfield and industry infrastructure as well as avoidance of sabkha and marine areas. In this survey, the crew was mandated to acquire every possible source point within defined safety limits, including those within hydrocarbon plants and industrial premises. Of the 660 km 2 of vibrator area, more than 99% of the pre-plot VP locations have been occupied at least once. Careful planning achieved 90% nominal fold coverage with good azimuthal distribution even through obstructed areas. The 160 km 2 of shallow water and transition zone acquisition began in early January this year. A shallow water crew equipped with custom-built shallow draught hydrojet vessels works concurrently with the vibroseis crew to provide the continuous coverage required across the land, transition zone, and shallow water zones in zippers 5-7. The final stage of the project will be the acquisition of data across the sabkha area in zipper 8. As the sabkha will not support the weight of vibrators it is planned to utilize explosives as a source. The field parameter testing for hole depth and charge size will be conducted soon to provide the necessary lead time for the order, approval, and delivery of the materials. Holes will be pre-drilled and maintained with plastic casing allowing for this phase to be completed ahead of blasting. Charges will only be loaded on the day they will be detonated (i.e. no pre-loading) to minimize risk. During the acquisition in zipper 8 up to five shooting crews will operate at the same time, but will operate in daylight hours only. High-density full-azimuth seismic Onshore 3D surveys have generally been limited to sparse geometries for both technical and economic reasons. The EAGE

5 first break volume 28, June 2010 special topic result is that typical land seismic data have been poorly sampled and noisy, despite the use of vibrator and geophone arrays to improve the signal-to-noise ratio. With the advent of high channel-count recording systems and high productivity Vibroseis techniques, onshore seismic is undergoing a high-density revolution. By using single geophone receivers and single vibrator sources it is possible to go beyond the limitations of working with arrays and provide isotropic spatial sampling of the seismic wavefield at a much higher density. This provides the following key benefits: Significantly reduced acquisition footprint noise Un-aliased sampling of signal and coherent noise Data free from array effects such as azimuthal sensitivity and intra-array statics The Dukhan survey utilizes precisely this kind of ultra high-density source and receiver geometry (Figure 5) to achieve these aims. The high-specification super-crew utilizes 40,000 single geophone channels with 25,000 live for recording. High-fidelity vibratory seismic (HFVS) (Allen et al. 1998) was chosen as the point-source high productivity Vibroseis method and the crew operates three fleets of four Sercel Nomad 65 (62,000 lb force) with a Hz sweep (boosted by 6 db from 90 Hz). The vibrators operate on a 24 hr basis with an average productivity of 4000 VPs per day. HFVS is a simultaneous source technique which enables a fleet of four vibrators to acquire four independent VPs. Four sweeps of eight seconds, each with a listening period, are recorded from each vibrator in a phase-encoded sequence. The phase encoding allows the separation of the individual VPs from the mother record by an inversion process after convolution. The recorded ground force is utilized to remove distortion and harmonic noise from the record during this process. The acquisition sequence is to deploy each fleet of four vibrators on separate source lines. The vibrators are spaced two source points apart and move up alternately one point and then leapfrog eight points to acquire the 7.5 m spaced VPs (Figure 5). With three groups of vibrators available it is possible to deploy one group in challenging areas, where progress is slow, and the remaining two groups in open terrain to ensure consistently high daily production. Furthermore, the flexibility of the HFVS system allows distance separation of the vibrators within a fleet and does not require the full complement of four units. The individual vibrators can therefore manoeuvre separately and continue to acquire VPs within congested residential and industrial areas, maximizing the coverage. To keep pace with the source effort, six front crew teams, each of five men, lay out the cables and geophones. The single geophone channels are planted and covered Figure 5 Acquisition geometry. A nominal receiver patch for a VP contains 36 lines of 672 channels and covers 4.2 x 5km. The staggered move-up for the four vibrator HFVS fleets is also illustrated. Three fleets of vibrators operate at the same time on separate source lines EAGE 83

6 special topic first break volume 28, June 2010 shallow water (up to a depth of 15 m) for the simultaneous vibroseis and airgun operations in the shallow water and transition zone zippers. Figure 6 The recording truck has the capacity to record 30,000 channels and is equipped to QC around 97 million traces (3.5 TB of data) a day. with sand to reduce the impact of wind-generated noise. Where required, holes are drilled in rocky ground to provide good coupling for the geophone spikes. In hazardous areas, such as quarry cliffs, specially trained teams lay out the cables using approved techniques so that the maximum number of receiver points can be deployed while ensuring safe operating conditions. The use of field digitizing units (FDUs) with waterproof links allows a seamless deployment of receivers from land to The technology challenge While high-density acquisition in this diverse and obstructed survey area created a wide range of logistical and operational challenges, the sheer volume of data generated by the survey created another set of technological challenges. For comparison, the numbers of traces recorded in one day on this project are more than the channels recorded in some complete contemporary 3D surveys. With the emphasis on quality and strict QC, a priority for the crew was to have access to all the geophone records, vibrator recorded force, and GPS stakeless survey coordinates in real time. The crew has been equipped with an acquisition system configured to record up to 30,000 channels at 2 milliseconds sample rate. To handle the throughput of data from the 25,000 live channels a fibre-optic backbone is used to support the receiver spread. The recording truck and vibrators were equipped with a powerful wireless (wi-fi) communications system to allow real-time transmission of the required vibroseis QC attributes, GPS coordinates, and recorded ground force so that the performance of the sources could be verified in real time. To accommodate the recording, pre-processing, QC and analysis of the large volumes of data, the recording truck Figure 7 QC application for the HFVS inversion. This location is flagged as having a bad conditioning vector EAGE

7 first break volume 28, June 2010 special topic Figure 8 QC of raw data. Gathers (left), crossline section from the brute stack (centre) and timeslice through brute stack (right) show good reflection strength from the primary and secondary targets, bandwidth up to 105 Hz and spatial resolution of faults. and in-field processing centre received a new suite of IT equipment. This included a NAS disk storage system for the recording truck (Figure 6), a 16-node PC cluster, a further 104TB of NAS disk storage, and high-specification QC workstations for the in-field center. One of the key in-field processing steps is the implementation of the separation of the HFVS data with the recorded ground force for each vibrator and each sweep. This is a computation-intensive process to which 10 of the 16 PC cluster nodes were dedicated. Once the recorded ground forces have been checked, the inversion of the mother record is performed and the quality of the inversion is based on a QC attribute called the conditioning vector (Figure 7). Should this vector be out of specification, the source points are flagged to be re-acquired. The distortion effect is analyzed at that stage and is quantified with the results of the HFVS inversion as part of the QC. Within the in-field processing system the data is progressively processed and stacked in order to build a field brute stack for data QC purposes. Source and signal attribute maps are generated on a daily basis, and become a useful visual measure of data quality and acquisition progress, though there is nothing like a brute stack to allow a 3D investigation of the data (Figure 8). Surface and borehole seismic programme To complement the surface seismic acquisition a suite of additional data was acquired which included upholes, new wireline log and core data, and 3D VSPs. This wider geophysi EAGE cal programme is designed to support the processing and interpretation of the surface seismic. Uphole data are crucial to defining the velocity profile of the highly variable weathered zone which is required for the statics sequence. Previous uphole velocity data surveys were sparse and in some cases too shallow to identify the base of the weathering layer. A total of 130 uphole velocity surveys are planned to provide full coverage of the onshore Dukhan 3D project area (including the sabkha) for this project. The locations and depth (nominally 100 m) of these surveys were based on modelling of the near surface velocity layers from previous work. A combination of air (shallow) and water-based mud (deep) drilling was used to complete the uphole wells. Borehole seismic data and in particular 3D VSPs provide a wealth of information about the reservoir interval and over-burden through in-situ downhole three-component seismic data. This can be used to calibrate and guide the surface seismic processing as well as to deliver a more detailed picture of the reservoir through high-resolution imaging and reservoir characterization. During the design of the Dukhan campaign QP specified the acquisition of eight 3D VSPs covering the crest and the flanks of the structure for the length of the field with the following objectives: N Optimized vertical seismic calibration to refine the processing parameters of the surface seismic N Identification of inter-bed multiple reflections in the surface seismic 85

8 special topic first break volume 28, June 2010 Figure 9 Baker Atlas VSP well site operations (top) using a crane for deployment of the VSP tool in pre-prepared wells. The Sercel MaxiWave 100-level VSP tool is shown during a system test with the retaining arm deployed. High-resolution wavelet calibration and identification of thin reservoir units Combine with core, wireline log and image log data to extract high-resolution petrophysical and fracture data Guide quantitative rock property and fracture estimation from the surface seismic Derivation of vertical-transverse-isotropy anisotropy parameters, Epsilon and Delta, and their variation with azimuth Processing seismic data acquired over carbonate strata has unique problems associated with the relatively high rock velocities and the nature of carbonate and evaporate strata to generate inter-bed multiples or reflections which contaminate the seismic record. Much of the processing sequence is based on algorithms that reduce or eliminate inter-bed multiples. With the simultaneous acquisition of 3D VSPs we are able to clearly identify where multiples contaminate the surface seismic data allowing parameters in the de-multiple algorithms to be tuned to the most aggressive level without compromising the true signal. Including eight simultaneously recorded 3D VSPs in the programme would appear to be prohibitively expensive in terms of direct cost and have the potential of adding weeks to the surface seismic acquisition. However, with planning, closely coordinated operations, and the use of the latest acquisition technology, this extensive programme was cost-effective by design and the schedule of the surface seismic was not affected. On the planning side, QP adjusted the drilling programme so that a series of vertical injection and observation wells were drilled prior to seismic acquisition which would be made available for the VSPs. These wells were completed with a short kill string to enable safe operations without the need for a drilling rig. This allowed the VSPs to be acquired from a crane rather than a rig, reducing cost. The VSPs were simultaneously recorded with the surface seismic by Baker Atlas using its SeisXplorer system (Figure 9) which includes the 100-level Sercel MaxiWave wireline VSP tool. With 100 three-component geophone stations spaced at 50 ft levels the system deploys an array which spans 1485 m in the borehole. An array this size can easily satisfy imaging and reservoir characterization requirements in a single pass, without the need to redeploy at several different depths and repeat the acquisition of surface seismic source positions. This is crucial for simultaneous operations as repetition of source locations would significantly increase the cost and duration of the whole programme. Geophysically, this one-pass approach also ensures consistent coupling and response of both the sources and receivers which will benefit both imaging and analysis of the data for reservoir characterization. In this way the 3D VSPs can easily be acquired as the surface seismic acquisition rolls past the well locations and records a full-azimuth circle of high-density shot points. Operations typically lasted 10 days per well and the final 2 km radius datasets contained around 20,000 shots each. Like the surface seismic, the VSPs were recorded using HFVS which, after correlation and inversion in the field, provided the same high-frequency and high-fidelity point source data. Data from the field is of high quality with excellent vector fidelity and the full range of P- and S-wave modes visible (Figure 10). For each 3D VSP, a high-frequency zero-offset VSP was also recorded using a 200 Hz sweep, in order to have the best tie with the well logs and the most accurate velocity information for calibration of the surface seismic data. After initial technical and mechanical challenges on the first survey, six of the eight 3D VSPs have now completed with excellent data quality. Processing for high-density WAZ High-density wide-azimuth datasets, such as the one being acquired over the Dukhan Field, are an opportunity to provide a far more meticulous seismic processing solution than is normally possible with the typical sparsely sampled land datasets. For example, higher-dimensional algorithms designed specifically for wide-azimuth surveys can be employed. These recognize and preserve the azimuthal variations present in the EAGE

9 first break volume 28, June 2010 special topic Figure 10 VSP data quality from the field is excellent, with a high bandwidth and the full range of P- and S-wave modes visible. A shot record (offset 1505 m) from the first survey is shown in order from the left: Raw Vz, Hx and Hy components; radially rotated components H1, H2; vertically rotated components E1, E2. data and are more effective at discriminating between signal, noise, and multiples. When we combine this technology with dense spatial sampling, which allows the recording of unaliased surface waves, we are in a position to provide the most effective removal of ground roll and guided waves. For Dukhan, a rigorous statics and noise attenuation sequence, including proprietary techniques, has been implemented prior to digital array forming (DAF) to take full advantage of the excellent spatial, azimuthal, and offset sampling. The pre-daf sequence was performed in the crossspread domain, which provides the finest spatial sampling of the data where source and receiver increments along lines are much smaller than the line intervals. A 3D adaptive groundroll attenuation algorithm (AGORA) (Le Meur et al., 2008) formed the main part of the noise attenuation sequence. It adapts to the velocity and dispersion characteristics of the groundroll from shot to shot and removes it (and guided wave energy if applicable) while preserving the amplitudes of the primary events. As well as naturally handling wide-azimuth data, AGORA can accommodate irregular spatial sampling which means that it can be used without the need for data regularization. Intra-array statics were based on a cross-correlation technique targeting refracted arrivals (Gulunay et al., 2009). Radial mixing (Gulunay and Benjamin, 2008) was used to generate pilot traces for the cross-correlation to derive the relative time-shifts for the individual traces in the cross-spread. The key step in this approach is the surface-consistent decomposition (Gulunay, 2005) of the individual trace statics into source line and receiver line statics on each cross spread. After performing this digital array forming sequence in the field, the data is shipped to Doha for full processing, which is ongoing. A final note on the processing of high-density pointreceiver data relates to the concept of digital array forming. This can be considered just as a convenient way of compressing the data for later stages of the signal processing and imaging workflows where compute power and data management is currently a bottle-neck. In the near future IT infrastructure will enable us to retain all the data at its original density all the way through to imaging. Achievements to date The Dukhan full-field seismic programme has achieved exceptional data density and quality. By taking a long-term perspective on data requirements, this survey has been designed not only to give a significant immediate product, but also to stand the test of time and to be able to be effectively reprocessed and reanalyzed as new processing and 2010 EAGE 87

10 special topic first break volume 28, June 2010 reservoir characterization technologies evolve. Some of the key achievements of the Dukhan campaign so far include: Over 3.8 million man hours without LTI during 24-hour operations in a range of environments Achieving 90% nominal fold coverage and azimuthal distribution within a diverse and obstructed survey area Acquiring, QC-ing and pre-processing through to brute stack around 97 million traces of HFVS data (3.5 TB of data) per day Integrating eight 100-level 3D VSPs into the surface seismic campaign in a cost-effective manner Seismic processing to take full advantage of the high quality, high-density data Delivering an integrated future-proof seismic programme Acknowledgements The authors would like to thank the management of Qatar Petroleum for granting permission to publish this work. We would like to acknowledge the efforts of Ardiseis Crew QAT3373 in making the survey a success along with our colleagues at Qatar Petroleum, CGGVeritas, VSFusion, Baker Atlas, and Jaguar Exploration. References Allen, K.P., Johnson, M.L. and May, J.S. [1998] High Fidelity Vibratory Seismic (HFVS) Method for Acquiring Seismic Data. 68 th SEG Annual Meeting, Expanded Abstracts 17, 140. Gulunay, N. [1985] A new method for the surface-consistent decomposition of statics using diminishing residual matrices (DRM). 55 th SEG Annual Meeting, Expanded Abstracts, 4, Gulunay, N. and Benjamin, N. [2008], Poststack driven prestack deconvolution (PPDEC) for noisy land data and radial trace mixing for signal enhancement. 78 th SEG Annual Meeting, Expanded Abstracts, 27, Gulunay, N., Khalil, A., Leveque, A., Seeni, S.R. and Robinson, S.W. [2009] Intra Array Statics Derived in the Cross-Spread Domain for a High Density, High Resolution, Wide Azimuth 3D Land Data Currently Being Acquired in Qatar. 79 th SEG Annual Meeting. Presentation in the International Showcase. Le Meur, D., Benjamin, N., Cole, R. and Al Marthy, M., [2008], Adaptive Groundroll Filtering. 70 th EAGE Conference & Exhibition, Expanded Abstracts G036. Seeni, S., Robinson, S., Denis, M. and Sauzedde, P. [2009] Dukhan 3D: An Ultra High Density, Full Wide Azimuth Seismic Survey for the Future. IPTC, Doha, Expanded Abstracts, IPTC EAGE