The Leading Edge 05_2013_arctic Seismic Acq

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524 The Leading Edge May 2013

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High Arctic marine geophysical data acquisition

Despite record-low sea ice extents over the past five years,the high Arctic Ocean remains one of the most difficultoperational environments on Earth for marine geophysicaldata acquisition. Until 2006, the extent of seismic reflectiondata in the western Arctic Ocean (western, from a NorthAmerican perspective) amounted to ~3000 line-km.In 2008,the United States and Canada teamed up to embark on fouryears of joint marine operations to acquire in excess of 15,000line-km of geophysical data reaching to the farthest pointsnorth. Each nation contributed an icebreaker to operatejointly to acquire seismic reflection, seismic refraction,shipborne gravity, single and multibeam bathymetry, and

subbottom reflection data. Tis article presents some of theoperational aspects of data acquisition in perennially ice-covered seas and demonstrates some of the outstanding datathat resulted, focusing on the seismic components of theprogram. Te multibeam-sonar component of the program ispublished by Armstrong et al. (2012).

Data acquisition

In 2007, Canadian scientists embarked to the Canada Basinof the western Arctic Ocean to acquire seismic reflection andrefraction data. Te Canadian Coast Guard ship (CCGS)Louis S. St-Laurent (Louis), Canadas premier ice-breakingvessel, was retrofitted with necessary equipment to conduct

these operations. Because of towed seismic gear behind Lou-is, the vessel was not able to use its center propeller, i.e., ithad both limited maneuverability and power for workingin ice. Without the ability to utilize full power, operationsin even moderate ice cover were deemed impossible or, atbest, cause data quality to suffer. Te solution was to utilizetwo icebreakers with a lead icebreaker to clear a path for the

D. C. MOSHER, C. B. CHAPMAN, J. SHIMELD, H. R. JACKSON, D. CHIAN, andJ. VERHOEF, Geological Survey of CanadaD. HUTCHINSON, United States Geological SurveyN. LEBEDEVA-IVANOVA, Woods Hole Oceanographic Institute

R. PEDERSON, Department of National Defence

seismic vessel (Figure 1). Te U.S. Coast Guard Cutter Healyjoined the program in 2008 and between 2008 and 2011,four joint expeditions were conducted acquiring 15,481 line-km of seismic-reflection data, 171 seismic-refraction profilesand about 38,000 line-km of multibeam sonar, gravity andsubbottom profiler data. During seismic operations, Healywould break ice ahead of the seismic acquisition vessel, Louis.

Te seismic reflection system consisted of a source arrayof 2 500 and 1 150 in3Sercel G-guns, totaling 1150 in3

(Figure 2a). Pressurized air was supplied to the array by anair-cooled Hurricane compressor, model 6-276-44SB/2500,powered by a C13 caterpillar engine capable of developing a

total air volume of 600 SCFM at 2500 PSI. Te hydrophonereceiver was a GeoMetrics GeoEEL with a 100 m-long activesection consisting of 16 digital channels with a hydrophonespacing of 6.25 m (Figure 3). With stretch sections anddead sections, the entire length was 250 m with a maximumsource-receiver offset of 236 m. Te reflection program was

Figure 1. wo-ship operations in ice. Te USCGC Healyis in thelead breaking ice as the CCG Louis S. St-Laurentfollows behindacquiring seismic data.

Figure 2. (a) G-gun sled being deployed. (b) Photo is an underwatershot. Te sled is 5000 lbs and is towed 11.5 m deep immediatelybehind the stern of the vessel.


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bundle wrap securely. In this way, when contacted by ice, thebundle deflected behind the 1-in pull cable leaving the 1-inpull cable to protect the bundle from ice impact damage.

o prevent freeze-up, all on-deck air piping was coveredin heat tape and the high-pressure air manifold was containedin a heated cabinet. At timed intervals small amounts of bio-degradable antifreeze was injected into the high-pressure airreservoirs. Te injection system used the ships control air of80120 PSI to operate the injector pump. Tis action pre-vented freeze-up in the guns and lines leading to the guns. In2007, our longest air-gun deployment was eight hours, but it

complemented with ultrasonic expendable sonobuoys to pro-vide wide-angle seismic reflection and refraction data (Figure4). A single hydrophone from each sonobuoy was deployedto 60-m depth and received acoustic responses were radio-telemetered to a stacked-Yagi array on the ship. Coherentseismic signals at ranges out to 35 km were received by thesonobuoys and were digitized and stored in SEG Y format.Te sonobuoys self-scuttled after eight hours of operation.

Arctic adaptations

Aside from the necessity of two ships and redundancy of allequipment, a number of modifications to the seismic equip-ment permitted continuous (or at least lengthy) operationsin ice and freezing conditions. Te gun array sled was out-fitted with a cylindrical 3500-pound hydrodynamic depres-sor weight (originally a 16-inch artillery shell) for the arrayto tow vertically behind the stern of the vessel to protect itfrom ice (Figure 2). Te array was towed at a depth of 11.5m below sea level to keep it beneath the thickness of mostsea ice. At this depth, sound output was measured at 234 dBre 1Pa at 1 m over a usable bandwidth of 565 Hz (Figure5). One-inch steel cable supported the sled and a cable bun-dle consisting of air hoses and trigger lines. Bundle clampsrotated freely around the pull cable but grabbed the plastic

Figure 3. (a) Geometrics GeoEels on the winch drums. (b) Deploymentinvolved weighting the tail section to sink the streamer beneath theice immediately behind the stern. Te drogue would pull the weightoff the tail section once underway and the streamer would rise to ahorizontal (acquisition) position.

Figure 4. Expendable sonobuoy being deployed off the stern.

Figure 5. (a) Measured shot signature in the time domain (red line),showing a zero to peak amplitude of 5.135 bar-m or 234 dB re 1

Pa at 1 m. Blue line is the modeled shot signature. (b) Te frequencyspectrum plot for this trace, showing prominent power between 2 and60 Hz with notching occurring at 65 Hz, caused by the bubble pulse

period.


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Te hydrophone streamer was towed from the aft posi-tion of the gun sled, to keep it beneath the sea ice as well.Te streamer was deployed first with a weighted end to sinkit vertically, followed by the gun sled deployment (Figure 3).A parachute drogue was fitted to the weight so that, whenthe ship came up to speed, the resistance of the droguepulled the weight from the streamer and the keel rose to tow-ing configuration. No birds were used to prevent snaggingin ice on deployment and recovery. Variable ship speed inthe ice often resulted in extreme changes in streamer depth.In-streamer depth sensors commonly failed, so beginning in2009, streamer dynamics were monitored with miniatureconductivity, temperature, and depth (CD) sensors at threepositions along the array (Figure 6). Pin connections betweenstreamer sections proved to be a weak link, perhaps because ofthe severe dynamics on the array. Careful attention to the con-nectors and O-ring seals at assembly time as well as taping thejoints to prevent loosening minimized problems of water egress.

Seismic shots were based on time, not distance, because

of the irregular forward motion of the vessels in ice, so shotspacing is highly variable. Although the short seismic arraydid not provide moveout for seismic processing, multichan-nel data permitted binning data geographically, similar to 3Dseismic gathers, rather than in shot gathers as might be donefor short offset data in a conventional program. Other pro-cessing steps are detailed below.

Hull-mounted multibeam and sub-bottom profiler sys-tems as well as a gravimeter were available on the USCGCHealy. Tese were conventional systems with respect to theirinstallation, but survey patterns were adapted for ice con-ditions. If multibeam bathymetry was the priority or whenseismic operations were impossible in heavy ice, then Healy

would follow behind Louis to acquire relatively noise-freemultibeam data (e.g., Figure 7).

For seismic refraction data acquisition, ocean bottom seis-mometers were not an option because instrument recoverywas impossible in the 90 and 100% ice cover of the Arcticpack ice. As a result, expendable sonobuoys were used (Figure4). In order to acquire reversed profiles, some sonobuoys weredeployed with a helicopter in advance of the ship (Figure 8).

During the 2011 field program, multibeam data were alsoacquired from an autonomous underwater vehicle (AUV)(Figure 9). wo units were mobilized for redundancy. TeseAUVs were developed by International Submarine Engineering(ISE) specifically for under-ice operations (Crees et al., 2010).

Tey were outfitted with Knudsen 118 kHz single-beamechosounders and Kongsberg-Simrad EM2000 (200 kHz)multibeam-sonar systems. Tere were several unique demandson these systems as opposed to conventional AUV:

1) Tey were constructed in modules for transportability toremote locations in small aircraft if necessary. Tey werereassembled on-site.

2) Surfacing and recovery in ice is not always an option, sothey were designed to be recharged while remaining in thewater through a hole in the ice.

3) Te recovery position cannot be programmed in advance

Figure 7. Examples of shipboard data from the CCGS Louis S.St-Laurent: (a) Free-air gravity. Te green line is the regional gravityanomaly data from the IPY circum-Arctic grid; the blue line is theraw acquisition data from the shipboard gravimeter. Te top profilewas acquired while following a lead icebreaker while the bottom wasacquired during ice breaking. (b) Chirp subbottom profiler data. Tetop profile was acquired while following a lead icebreaker and thebottom profile was acquired during ice breaking.

Figure 6. Depth sensors (inset) on the streamer recorded streamerdynamics and showed extreme changes in depth, largely because of shipspeed changes dictated by ice cover. Tese changes result in variablereceiver ghosting. Prestack adaptive signature deconvolution based onreceiver channel depth was implemented in the processing scheme asa result. Water temperature and salinity measurements were recordedsimultaneously with these devices.

was typically much less. With these modifications, air gunsoperated continuously without need for servicing except forpreventative measures.


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because of ice drift. Even from anice-breaker, it is not feasible to sta-tion keep. As a result, the vehicleswere designed with several hom-ing systems to locate the recoverysite within a 50 km radius of pro-grammed survey end positions.

Results

15,481 line-km of high-quality, short-offset multichannel seismic reflectiondata were acquired over the course ofthis program (Figure 10). Tese datawere acquired in latitudes from 70 to88 N 11 min. N, much of which iswithin perennial sea ice several metersthick. Concurrent multibeam bathy-metric sonar, sub-bottom profilerand gravity data were acquired. One-

hundred and seventy-one sonobuoyswere deployed for refraction analysis.Regions of the Arctic Ocean, particu-larly along the continental margin ofthe Canadian Arctic islands, wereaccessed that have never before beenvisited by a surface ship. wo-ship op-erations and diminishing multiyear iceprovided this access.

Seismic reflection results

As mentioned, the hydrophone array was towed from the aftposition of the gun sled, at 11.5 m depth. No birds were used

and the ships speed varied significantly, depending on iceconditions, so streamer motion was exceptionally dynamicwhich caused degradation in data quality if not compen-sated. Depths were monitored at three positions along thestreamer. An example of these data is shown in Figure 6.Tese data were used to empirically model a deconvolutionoperator for prestack trace data, an integral part of the pro-cessing flow.

Seismic processing

Multichannel seismic data were processed with the followingsequence:

1) Read raw SEG D records and static shift for recording delays2) Extract shot navigation and design CMP bins at 12.5 m

intervals3) Integrate streamer depth data and interpolate receiver

group depth at each shotpoint. Apply source/receiver stat-ic corrections

4) Apply bandpass filter (3/8/140/240 Hz),f-kfilter (> 4 msper trace), 2 amplitude scaling and balance, debias

5) Noise is a large factor in ice-breaking because of propwash and cable strumming. ight tolerance on noisy trac-es would result in elimination of too much data. Instead,noise levels were quantified using instantaneous frequency

and traces were treated independently. Common receiversort, trace edit, develop noise model and substract, despike,FX-deconvolution

6) Minimum phase conversion using measured wavelets,adaptive subtraction of multiple model

7) Surface consistent gapped deconvolution, CMP sort,NMO correction, and residual statics

8) Diversity stack with square root normalization9) Zero-phase conversion, despike, FX decon, running trace

mix, time-varying band-pass filter10)Finite-difference migration with sediment velocity function

Figure 8.An example of a reversed sonobuoy refraction profile. Te expendable sonobuoy isdeployed ahead of the ship via helicopter and data are acquired as the ship steams toward and thenaway from the sonobuoy. Note strong refractions out to 30 km offset.

Figure 9.Autonomous underwater vehicle for under-ice operations.Te vehicle consists of seven modular sections for portability. It is justover 7 m in length. It is capable of diving to 5000 m with duration ofabout 48 hours and is able to relocate its return position within 50 kmof its preprogrammed mission end position.


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Figure 10.Map of the Arctic Ocean showing seismic track lines andsonobuoy drop locations (red dots). Tese data were acquired between

2007 and 2011, mostly as two-ship operations. Te yellow dotrepresents the North Pole.

Figure 11.A comparison of single-channel seismic reflection data acquired in Canada Basin in1993 with processed multichannel data acquired in 2010. Te two lines are adjacent. Note bothclearer imaging of reflections but also a distinct basement event on the 2010 data.

from sonobuoys,f-kfilter to eliminate wrap-around mul-tiples

11)ime-varying band pass, balance, add CMP navigation,resample (4 ms), SEG Y output

Seismic reflection profiles

Te objective of the acquisition program was to image thesedimentary section to the basement. Figure 11 shows a com-parison of single-channel seismic reflection data acquiredin 1993 (Grantz et al. 2011) using an analog streamer withair guns versus data acquired and processed during this lat-est program. Figure 12 shows a new profile transecting thelength of Canada Basin from the Alaska margin to AlphaRidge. As shown by these figures, the sediment column is

well-imaged at fine resolution and basement is readily dis-tinguishable, compared with the 1993 single-channel data.Tis data quality is typical for the entire data set and yet wasacquired in mostly 90 and 100% ice cover. Tese data permita detailed assessment of the sedimentary geology of the basinfor the first time (e.g., Mosher et al. 2011, 2012).

Sonobuoy resultsRaw signals from the sonobuoys were digitized and storedin SEG Y format. Sonobuoy data were used to calculate ve-locities in the sediment column. Expendable sonobuoys lackinternal location systems; only the position of deployment isknown. In order to calculate source-to-receiver offsets, thedirect water wave arrival was used. o convert traveltime todistance, however, the sound velocity in the water columnis needed. A regression of first arrival seismic travel timeagainst seafloor depth at the shot position determined fromthe IBCAO grid (Jakobsson et al., 2012) shows an averagewater column velocity of 1475 m/s. Te water column in

Canada Basin is highly structured, however, and water ve-locities were derived from a number of CD casts. Te bestfit equation derived from the CD casts yielded an averageempirical function X(w) of:

X (w) = 1.441 w + 0.00075 w20.006,

where X is offset in km and w is direct-wave arrival time inseconds (Lebedeva-Ivanova and Lizarralde, 2011). Tis func-tion accounts for the receiver depth at 60 m and substantiallyimproves offset estimates for the Canada Basin in compari-son to a constant-velocity assumption.

Seismic refraction analysisOnce geometries were restored, littleadditional processing was requiredfor refraction analysis, save for band-pass filtering and static correctionsfor various shot delays. Various sedi-mentary refractors were obvious (e.g.,Figure 8). A 2D forward modelingmethod (Zelt and Ellis, 1988; Zeltand Smith, 1992) that incorporatesboth reflection and wide-angle datainto a unified velocity model in bothtime and depth domains was used

(Figure 13). During the modeling,the sonobuoy profile and its corre-sponding reflection profile were geo-metrically aligned in distance traceby trace. Major horizons on the re-flection profile were digitized to gen-erate a preliminary model in the timedomain. Each layers velocity (V)and gradient should in general matchthe corresponding refraction slope ofthe sonobuoy wide-angle data by V= dX/d. Te slope is easily matched


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Figure 12.A composite seismic reflection profile transecting Canada Basin from south to north.Data are displayed as envelop of the amplitude, allowing the image to be extremely compressed.Notice the strong basement reflection, the correlateable reflection horizons of the sedimentarycolumn and along-strike amplitude changes.

Figure 13.An example of sonobuoy refraction analysis. (a) Sonobuoydata with reflections and refractions labeled. (b) Reflection profileconcerted to time domain with the refraction ray-trace model overlain.

by perturbing the reduction velocityof the sonobuoy data in an interac-tive process. Tis time-domain mod-el was converted to depth for ray-tracing, and the resulting traveltimecurves are compared to the observedrefraction phases. A few iterativeperturbations of a layer velocity areusually sufficient to arrive at a satis-factory match of the correspondingrefraction phase. Tis same step goesfrom the first layer down in a layer-stripping method.

Semblance analysis

Sedimentary velocities were alsoderived using a semblance veloc-ity analysis technique using the sonobuoy data (Figure 14).Semblance velocity analysis determines the normal-moveo-

ut (NMO) velocity for individual seismic reflection events(Yilmaz, 1987). For flat-lying sediments, the NMO velocityis approximately equal to the root-mean-square (rms) averagevelocity from the sea surface to the reflection horizon. For ex-ample, a dip of 5 affects velocity for less than 0.5%. In mostof the Canada Basin, reflection sequences are exceptionallyflat lying. NMO velocities can be recalculated into intervalvelocity (and depth) using the Dix equation (Dix, 1955).Te Dix equation is designed for common-midpoint (CMP)gathers used in shallow water, rather than for receiver gath-ers in deep-water data. Because of the spatial consistency ofreflections in Canada Basin, however, it was felt that receivergathers were still functionally appropriate. Semblance analy-

sis was not conducted on sonobuoy data with dipping reflec-tions such as along the continental margins. After modelingsynthetic data at the frequencies acquired in the sonobuoys,optimum offsets for semblance analysis were determined tobe 7 km. Te processing flow is as follows:

noise spike removal shot-receiver geometries restored using the direct water

wave arrival times broad minimum phase band-pass filter (2/380/120 Hz) predictive deconvolution for the entire length of the trace time and spatially variant minimum phase band-pass fil-

ters in a range of 3.550 Hz

top muting above first reflections, and automatic gain control semblance-based wide angle reflection velocity analysis of

the sedimentary section

Semblance velocity analysis was able to provide velocityinformation in the sedimentary section, including the up-permost section where refraction information is sparse be-cause of short offsets. It was also a quick way to implementproduction-mode analysis in horizontal/subhorizontal sedi-ments where receiver gathers could be substituted by CMPgathers. Semblance velocity results were used in a case studywithin the central Canada Basin to compare with refraction

results (Figure 15). Strong agreement between the two dif-ferent techniques within the 07 km offset range providesconfidence in their results.

Semblance analysis also permitted testing for layer inducedvelocity (VI) anisotropy in the sedimentary section. Te bestpractical way of identifying VI medium based on seismic datais by comparing reflection (semblance) and refraction velocities,because reflection velocity is mainly dependent on the verti-cal component while refraction velocity is mainly dependenton the horizontal component. No significant VI anisotropywas observed, at least on the regional scale that these data


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provide, as there is strong agreement between the different

techniques and there is little deviation from the Dix assump-tion (i.e., by applying Dix NMO correction to the sonobuoydata and testing for differences in the solutions).

Autonomous underwater vehicle

In 2010, the first trial of an under-ice AUV was undertaken.Te system was launched near Borden Island of the Cana-dian Arctic archipelago 400 km under the ice to be recoveredat a drifting ice camp. It maintained a height of approxi-mately 100 m above the seafloor and acquired single-beambathymetric data during its voyage. It was recharged at a re-mote camp and sent back to its base camp acquiring dataon its return voyage. In 2011, the system was launched and

recovered from the Louis. It traveled 110 km under ice andacquired multibeam data along its track, traveling over dif-ficult terrain during its transect of a feature known as SeverSpur (Figure 16). Te Louishad drifted about 10 km from itsdeployment position during the mission, but the AUV wasable to return within meters of the vessel.

Summary

Perennially ice-covered seas present obvious obstacles toshipborne geophysical data acquisition. For the most part,modifications to conventional technologies overcame thesedifficulties and permitted successful acquisition of >15,000line-km of seismic reflection and refraction data in some of

the most difficult operating conditions on earth. Paramount

amongst these solutions was the cooperation and collabora-tion between two icebreakers; one from the United Statesand one from Canada. Te data acquired will open the doorto understanding the Arctic Oceans tectonic and sedimen-tologic history.

ReferencesArmstrong, A. A., L. A. Mayer, and D. C. Mosher, 2012, Gathering

multibeam bathymetry data aboard icebreakers: Sea echnology,53, http://www.sea-technology.com/features/2012/1012/icebreak-ers.php.

Crees, ., C. D. Kaminski, J. Ferguson. 2010, UNCLOS under-icesurvey: A historic AUV deployment in the high Arctic: Sea ech-nology, 51, no. 12, 3944.

Dix, C. H., 1955, Seismic velocities from surface measurements: Geo-physics, 20, 6886, http://dx.doi.org/10.1190/1.1438126.

Grantz, A., P. E. Hart, and V. A. Childers, 2011, Chapter 50Geol-ogy and tectonic development of the Amerasia and Canada Ba-sins in Arctic Ocean, in Geological Society of London Memoirs,771799, doi: http://www.dx.doi.org/10.1144/M35.50.

Jakobsson, M., 2012, Te internationa l bathymetric chart of the Arc-tic Ocean (IBCAO) version 3.0: Geophysical Research Letters, 39,L12609, doi:10.1029/2012GL052219.

Lebedeva-Ivanova, N. and D. Lizarralde, 2011, An empirical direct-wave travel time equation for Arctic sonobuoy data: Te Sixth In-ternational Conference on Arctic Margins.

Lebedeva-Ivanova, N., D. Chian, P. Hart, J. Shimeld, D. Lizarralde,

Figure 14. Example of semblance analysis. (a) Part of the MCS line at the sonobuoy deployment location, marked by the arrow. (b) Coherenceplot of the sonobuoy velocity analysis. (c) Te sonobuoy processed data, and (d) processed sonobuoy data with NMO correction.


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D. Mosher, and D. Hutchinson, 2012, Sonobuoy-based velocityfunctions for sediment thickness calculation in the deep CanadaBasin, the Arctic Ocean: 82nd Annual International Meeting,SEG, Expanded Abstracts.

Mosher, D. C., D. Hutchinson, J. Shimeld, R. Jackson, D. Chian, andN. Lebedova-Ivanova, 2012, Canada Basin revealed: AC, paper23797.

Mosher, D. C., J. W. Shimeld, D. Hutchinson, N. Lebedeva-Ivano-va, and C. B. Chapman, 2012, Submarine landslides in ArcticSedimentation: Canada Basin, in Y. Yamada, K. Kawamura, K.Ikehara, Y. Ogawa, R. Urgeles, D. Mosher, J. Chaytor, and M.Strasser, eds., Submarine mass movements and their consequencesV, Advances in Natural and echnological Hazards Research, 31,147158: Springer.

Yilmaz, O., 1987, Seismic data processing: Invest igations in Geophys-ics 2: SEG, http://dx.doi.org/10.1190/1.9781560801580.

Zelt, C. A. and R. M. Ellis, 1988, Practical and efficient ray tracingin two-dimensional media for rapid traveltime and amplitude for-

ward modeling: Journal of the Canadian Society of ExplorationGeophysicists, 24, 1631.

Zelt, C. A. and R. B. Smith, 1992, Seismic traveltime inversion for 2Dcrustal structure: Geophysical Journal International, 108, 1634.

Acknowledgments: Te authors express their appreciation to thechief scientists of the Healyduring these joint missions: L. A.

Figure 15. (a) Crossplot of time-to-depth pairs for the sedimentarysection derived from refraction analysis and from semblance analysis.(b) Ratio of velocities derived from the different techniques. Tis plot

shows their degree of similarity is within 10%.

Figure 16.Multibeam data acquired by the AUV under ice, flyingabout 100 m above the seafloor in 30003800 m water depth. otal

AUV track is 110 km, acquired during about 24 hours of operation.Inset is an example of the raw sounding data.

Mayer, J. Childs, and B. Edwards, as well as all of the scientificstaff on both the Healyand the Louis. We also thank the officers

and crews of the Healy and the Louisduring these missions. With-out the efforts of such a large team, these programs would not havebeen at all successful.

Corresponding author: [email protected]

The Leading Edge 05_2013_arctic Seismic Acq

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