6th International Conference on Gas Hydrates


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3-D TRAVEL TIME TOMOGRAPHY INVERSION FOR GAS HYDRATEDISTRIBUTION FROM OCEAN BOTTOM SEISMOMETER DATAMykhail M. Zykov, N. Ross Chapman  and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. Spence and G. D. SpenceSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesSchool of Earth and Ocean SciencesUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of VictoriaUniversity of Victoria3800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P63800 Finnerty Rd, Victoria, BC, V8W 3P6CANADACANADACANADACANADACANADACANADAABSTRACTABSTRACTABSTRACTABSTRACTABSTRACTABSTRACTABSTRACTABSTRACTThis paper presents results of a seismic tomography experiment carried out at the Bullseye coldvent site offshore Vancouver Island. In the experiment, a seismic air gun survey was recorded onan array of five ocean bottom seismometers (OBS) deployed around the vent. The locations of theshots and the OBSs were determined to high accuracy by an inversion based on the shot traveltimes. A three-dimensional tomographic inversion was then carried out to determine the velocitystructure around the vent, using the localized source and receiver positions. The inversionindicates a relatively uniform velocity field around and inside the vent. The velocities are close tothe values expected for sediments containing no hydrate, which supports previous claims that thebulk concentrations of gas hydrates are low at the site. However, the largest resolved velocityanomalies of + 25 m/s are spatially within the limits of the acoustic blank zone seen inmultichannel seismic data near the Bullseye vent. The velocity inversion is consistent with zonesof high concentration (15-20 % of the pore space) in the top 50-100 m of sediment.Keywords: gas hydrates, ocean bottom seismometers, cold vent, seismic tomography                                                         Corresponding author: Phone: +1 250 472 4340 Fax +1 250 472 4620 E-mail: chapman@uvic.caINTRODUCTIONThis paper presents results from a seismic studyusing Ocean Bottom Seismometers (OBS) carriedout at the Bullseye vent site located on theaccretionary margin of the northern Cascadiasubduction zone offshore Vancouver Island,Canada.The major objective was to develop a 3-D velocitymodel around the Bullseye vent zone by means oftravel time inversion tomography in order todetermine the distribution and quantity ofhydrates.  The area has received close attentionafter seismic data showed acoustic blankingfeatures [1,2], and the recovery of massive hydratefrom the vent site. The site was the target ofnumerous multi-  and single-channel [3-5] seismicsurveys and various other experiments includingpiston coring, electromagnetic studies, andscientific drilling (Ocean Drilling Program (ODP)Leg 146 in 1993 and Integrated Ocean DrillingProgram (IODP) Expedition 311) [6,7].  The blankzones in the vicinity of the Bullseye vent (blankzone 1) are shown in Figure 1. The OBS data set consisted of 22 parallel seismicsurvey lines at 200 m spacing with threeperpendicular crossing lines, recorded on five OBSstations. The OBS experimental geometry requiredcorrections for the coordinates of sources andreceivers initially obtained in the field. A newcomprehensive source and receiver localizationtechnique was developed for the case of stationaryhydrophones and multiple seismic lines [8].  Theinversion for the experimental geometry was donein a separate travel time inversion.A simulation study was carried out first todetermine the resolution of the OBS array, usingtwo possible models for the hydrate distribution inthe vent zone.   The inversion results for theexperimental data indicate a uniform velocity fieldaround and inside the vent zone. Velocities areProceedings of the 6th International Conference on Gas Hydrates (ICGH 2008),Vancouver, British Columbia, CANADA, July 6-10, 2008.approximately equal to values expected forsediments containing no hydrate, which supportsthe hypothesis that the bulk concentrations of gashydrates are low at the site. The largest velocityanomaly, with velocity increase of +25 m/s, isspatially associated with the limits of the blankzone. The anomaly suggests greater hydrateconcentrations inside the vent zone than outside.Low vertical resolution of the model limited theinformation on the detailed structure of the depthdistribution of the hydrate. However, the resultsare consistent with a zone (or perhaps multiplezones) of high hydrate concentration (15?20% ofthe pore space) associated with a hydrate lens,located at the top of the sediment section.Figure 1.  Acoustic blanking from high resolutionseismic survey with the high resolution DTAGSarray (Gettrust et al. [9]).The paper is organized as follows.   The OBSexperiment is described in the next section.  Thetravel time inversion method is then outlined, andthe inversion results are presented and discussed.OBS TOMOGRAPHY EXPERIMENTLocal lithological structureThe location of the experiment is shown in Figure2.  In the upper 300 m of the sediment section twodistinct lithological units can be identified at thesite using the 2-D and 3-D seismic data [3,4]. Thefirst unit is characterized by slope sediments whichdisplay well-defined parallel bedding. It can becorrelated with Unit I at the ODP 889 drill site [6].The second unit with almost complete loss ofcoherency of the seismic signal can be attributedto the Unit III at the drill site. The lower unitrepresents accreted sediments which are moreconsolidated, but highly fractured compared to theundisturbed sediments of the upper unit [6].The upper boundary of the accreted sedimentcomplex is highly variable in terms of its depthbelow the seafloor. It is possible to outline threesmall well expressed ridge structures within thelimits of the modelled area [3.4] (Figure 2). Theslope sediments were deposited on top of theaccreted sediments. The bedding of the unit isundisturbed and almost parallel at the top of thesection. The thickness of the unit mostly dependson the configuration of the surface of the accretedsediment complex, rather than bathymetry. Thethickness is greatest for the area associated withthe depression between the two accreted sedimentridges and may reach 300 m. The thickness of theslope sediments unit can be as small as 50 m in thenorthern corner of the modelled area.Figure 2.  Locations of the five OBS (open circles)relative to the bathymetry and the ODP 146 drillsites.  The box indicates the limits of the inversionarea.  The Bullseye vent is the light-shaded regionnear the centre of the OBS array, and the hatchedareas represent buried ridges of accreted sediment.Seismic surveyThe seismic data were obtained during theCOAMS-99 cruise that took place from July 26until August 11 of 1999 [3].   Five OBS weredeployed in the vicinity of the Bullseye vent site asindicated in Figure 2.  The OBS array formed asquare with sides 1.4 km and one at the centre.The seismic sound source was a single 40 in3(0.655 l) air gun that used a wave-shape kit.  Theair gun depth was ~2 m, and it was triggered to fireevery 19 m along the survey lines.The OBS array recorded 22 10-km lines spaced at100 m spacing from a pseudo 3-D survey at thesite, and three cross lines as shown in Figure 3.Single channel normal incidence seismic data werealso recorded along the same survey tracks, andthese data were used with the OBS data in theinversion.The OBSs were obtained from the DalhousieSeismic Group, Dalhousie University.   Theseismometer sensing system consisted of onepressure sensor (hydrophone) and a 3-componentgeophone package. The analog signals from thefour sensors were filtered using an anti-alias filterwith corner frequency of 200 Hz, and then weredigitized with 1.433 ms sampling interval (698.8Hz) and 16-bit dynamic range. The activeoperational period with the system parametersused was limited by the storage capacity toapproximately 2.5 days.   Only the hydrophonedata were used in the inversion.The recorded seismic data were tied to an absolutetimescale using a precise clock with an expectedtime drift rate less than 1 ms per day. However,the actual average drift for the internal timingsystems was up to 10 ms per day, and the clockdrift rates for stations A and F were not steady.Inversion of experimental geometryThe source-receiver geometry was first refinedfrom the nominal estimates from the survey usinga regularized linear inversion based on travel timesof the direct path arrivals at the OBS array [10,11].A special procedure that assumed linear driftsegments was developed so that information fromthe two sensors with faulty clocks could beincluded in the inversion [8].   The RMS (rootmean square) change of shot positions in theinversion was ~ 6.7 m, and the mismatch of OBSdepths was less than 0.1 %.   The invertedgeometry was used to define the shot and OBSpositions in the tomographic velocity inversion.3-D TOMOGRAPHIC INVERSIONInversion methodThe Jive3D tomographic inversion code wasapplied [12,13].  Jive3D uses a linearized iterativeapproach for the travel time inversion that allowscombined inversion of refractions and reflections.The inversion is implemented in a layer-interfaceformalism.  The cell size of a gridded model of thesub-bottom structure is allowed to vary from layerto layer, which provides flexibility in choosing thenumber of cells required for simulating differentlayers. The final model that provides a satisfactoryfit to the data is obtained through a series ofrefinements of a starting model.Figure 3.  Survey track lines that were recorded bythe OBS array.Jive3D produces a minimum-structure model thatfits the given travel time data to a requiredaccuracy using the simplest model possible forgiven input parameters. The desired simplicity ofthe model (amount of structure) is controlled by asmoothing factor, which is a trade-off parameterbetween the accuracy of fitting of the input dataand the smoothness of the generated model.The model that was constructed for this inversionconsisted of 5 layers and 5 interfaces. The toplayer (or, strictly speaking, half-space) representedthe water column, and the top interface simulatedthe sea bottom. The bottom of the model or thedeepest interface represented the BottomSimulating Reflector (BSR). There was nopossibility to model any deeper layers, as therewere no continuous deeper reflectors in the datathroughout the modelled area.Travel time dataThe travel times of rays were obtained using singlechannel seismic data (normal incidence) and datarecorded by OBSs.  The following events werechosen; the events are described in terms of twoway travel time relative to the seafloor reflectionfor the single channel data near OBS B):Event 1: 90 msEvent 2: 180 msEvent 3: 235 msEvent 4: 280 ms (BSR)Event #3 was not traceable in the vicinity of theOBS C, because the layered structure of the slopesediments is strongly disturbed there and theaccreted sediments package lies closer to the seasurface. All other events were observed (althoughsometimes weakly) in all seismic sections on bothsingle channel and OBS data.The chosen events were traced from one surveyline to another using the cross lines. The events forpicking on the seismic data recorded by OBSswere identified with the help of the single channelseismic data, as indicated in Figure 4. The seismicsections were aligned along the same shot trace(with the minimum offset for the OBS) such thatthe seafloor reflection for the normal incidencedata coincided with the direct water wave arrivalon the OBS section, and then compared.In general, the uncertainty in the arrival time pickswas different for each event, and for each type ofdata.  The uncertainties for each event (in ms)listed in Table 1.Event OBS SingleChannel1 0.75 3.02 1.5 3.03 2.5 3.04 3.0 4.5Table 1.  Uncertainties (in ms) for arrival timepicks of the OBS and single channel data.Figure 4.  Example of the seismic sections alignedfor picking.  The middle panel represents singlechannel normal incidence data, and the left andright panels are OBS B and F, respectively.  Timeis two-way travel time.Inversion modelThe modelled area is 3 km x 2.7 km with OBS Bat approximately the centre (Figure 5). Thevertical span of the model is from -1220 m (thehighest seafloor point) to -1620 m (the lowestpoint of the BSR). The thickness of the invertedportion varied according to the depth of the BSRbelow the seafloor and was about 220?240 m. Thecell size for the interface grid was 50 m x 50 mand for the inverted velocity grids 50 m x 50 m x20 m, so the horizontal dimensions of the modelwere 60 x 54 cells with about 20 cells in verticaldirection. With the chosen cell size the vent zone(600 m x 400 m in horizontal dimensions) isrepresented by 12 x 8 cells.The starting model for the velocities was based ona simple velocity gradient in the sediment,assumed to be ~ 1 s-1 with a velocity of 1500 m/sat the sea floor.  There were no specific anomaliesintroduced in the profile.Inversion of synthetic dataAn inversion was carried out using synthetic datato investigate the resolution performance of theOBS array in resolving the distribution of hydratein the vicinity of the cold vent.  Two probablemodels of hydrate distribution were selected,based on information of hydrate distribution fromprevious experiments.The first model, (A), assumed increased velocityuniformly within the whole volume of the ventzone. To model this situation, a positive velocityanomaly with a magnitude of 30 m/s wasintroduced to the background velocities within thevent zone over the whole vertical span of themodelled sediment section. Such a small value forthe increase does not necessarily represent theactual velocity increase due to anomalous gashydrate presence, but was chosen mainly to testthe method sensitivity.  The other hypothesis (B)assumed a localized volume of sediments (15?20m in thickness) with significantly higher P-wavevelocities. This body was located close to the seasurface and did not extend deeper; that is, thevelocity beneath it was the same as the velocitiesoutside of the vent zone. This model is consistentwith the hydrate cap model.  The two physicalmodels are shown in Figure 6.Figure 5.  Gridded model of the Bullseye ventenvironment for the Jive3D inversion.The results of the inversion of the synthetic traveltime data set are shown in the right panel of Figure7. The test showed that the inversion was not ableto localize a thin body velocity anomaly near theseafloor (model B). Instead, it spread the region ofthe increased velocity vertically. Most likely this isthe result of the sparse OBS geometry of theexperiment. The results for the two differentphysical models are almost undistinguishable fromeach other. However, the anomaly tends todecrease with depth for the thin body case, whilefor the uniform anomaly the increase of thevelocity reaches the same level for all depths in theresulting velocity field.From the model A results, the limits of theanomaly are not so well determined and the trueanomalous velocity (+30 m/s) was not fullyrecovered by the inversion (only +20 m/s). Thiscan be explained by the need to use a largesmoothness factor in the inversion. This parameteracts as a trade off between higher resolution andreliability in the inversion results. For the sparseOBS geometry in this experiment, a largesmoothness factor is required. Otherwise, falseanomalies due to noise in the data may appear inthe poorly constrained areas.Figure 6.  Physical models of hydrate distribution.Figure 7.  The velocity models (left panels) andthe recovered velocity profiles (right panels).The results of the inversion with syntheticexperimental data can be summarized as follows:? The final velocity models inverted for the twodifferent physical models of a thin anomalousbody and an anomaly with a uniform verticalextent are almost indistinguishable from eachother in the case of small anomalous velocities.? The horizontal limits of small amplitudeanomalies cannot be accurately determined but aresmoothed over distances of 1 to 2 cell sizes.? The maximum anomalous velocity increasetends to be underestimated.Inversion of experimental dataThe results for the inversion at the Bullseye ventare presented in vertical and horizontal slices fromthe 3-D velocity model.  Examination is focusedon the central regions which are better constrainedby the data.   The vertical slices, as shown inFigure 8 (N/S slices) and Figure 9 (E/W slices),reveal small positive anomalies that are in goodspatial agreement with the limits of the blankzones.  Although the increase does not exceed +25m/s, the results indicate that the sediments withinthe blank zones have higher velocities than thoseoutside.  The negative velocity gradients seen inthe peripheral regions of the slices are believed tobe artifacts due to insufficient ray coverage at theextremities of the grid.Figure 8.  Vertical slices in N/S direction (bottompanel shows the slice through OBS E, B and F).Figure 9. Vertical slices in W/E direction (middlepanel shows the slice through OBS A, B and C).The average vertical velocity profile for the centralpart of the model is shown in Figure 10. Sea floorvelocities are about 1500?1520 m/s, with auniform increase of 1 s-1 to the BSR, reachingvalues of 1700?1750 m/s immediately above.The spatial variations of the velocities are alsodisplayed in slices at constant depth below the seafloor.   Figure 11 shows the horizontal distributionof the upper portion of the sediment section at adepth of 50 m.   The small positive velocityanomalies correspond well with the bounds of theblank zones.Model uncertaintiesThe direct quality assessment of the inversionresults involves calculation of the uncertaintyvalue for each model parameter. The uncertaintyanalysis package for Jive3D estimatesuncertainties based on: (i) the estimated travel-time uncertainties of the input data, and (ii) thecorrelations introduced between neighbouringparameters as a result of the smoothingregularization [12]. The inclusion of the smoothingregularization into uncertainty analysis biases theuncertainty estimates, and makes them smaller byassuming that the true model actually has the sameamount of smoothness as was introduced by thesmoothness regularization term in the inversion.The values produced by uncertainty analysisrepresent one standard deviation in the Gaussianmodel.Figure 10.  Velocity versus depth profiles for thefinal model (thick line).  Results from the VSP(circles) and sonic log (thin line) from the ODP889 site are shown for comparison [6].Figure 11.  Final velocity distribution for the sliceat 50 m below sea floor.  The black dashed linesoutline the limits of blank zones.The spatial distribution of calculated uncertaintiesof the model are shown in Figure 12 for the depthof 50 m below the seafloor. As expected, thecentral part of the model is better resolved than itsflanks. From similar slices in the vertical, thesmallest values for the uncertainties are located inthe planes going through lines of three OBSs. TheN?S direction (along the depression) is betterresolved than W?E (across the depression). Theminimum uncertainty value for the velocityachieved for the model was as low as 20 m/s.Figure 12.  Uncertainty values for the final modelfor a horizontal depth slice of 50 m below the seafloor.DISCUSSION OF INVERSION RESULTSGas hydrate concentrationThe concentration of hydrate at different siteswithin the modeled area around the Bullseye coldvent was estimated by comparison of the invertedvelocity profiles with a background, no-hydratereference velocity profile [14]. Three sites wereselected within the modeled area, and the velocityprofiles extracted from the inversion are shown inFigure 13.  Profile A represents velocities at alocal minimum, where the base of the gas hydratestability zone (GHSZ) is above the surface of theaccreted sediment complex. The position isapproximately in the middle between OBS E andB. As can be seen, it follows the backgroundprofile for the slope sediments within a few m/s.This fact suggests virtually no presence of gashydrates at the point. The velocity profile at thispoint can be used as a new background.Velocity profile B is taken at a point associatedwith a local maximum. At this point, the sedimentswithin the modelled thickness are representedexclusively by slope sediments. The increase inthe velocity compared to profile A is minor: 20m/s at the seafloor increasing down the sedimentsection to 40 m/s at the BSR depth. If the changein the velocity is completely attributed to thechange in gas hydrate presence, these numberscorrespond to 5?6% of pore space occupied byhydrates (or 2?4% of the sediment volume).Figure 13.  Velocities from the tomographyinversion compared with the background velocity[14].  Also shown are the sonic log data from ODPsite 889B [6].The results of the travel time tomography studyargue in favour of no gas hydrates, or only a verysmall amount, present in the slope sediment unit.At the sampled points the section from the seafloorto the BSR is represented by slope sediments. Thegas hydrates may be distributed evenly in thevertical direction or they can be localized in thethin layers of more porous sediments [15]. Thelayers may be as thin as several tens ofcentimetres, well below the resolution of the traveltime tomography inversion.Profile C represents the velocity profile in part ofthe modeled area where accreted sediments enterthe GHSZ.   This profile matches the averagedsonic velocity data for depths below 120 m.  Thevelocity increases of greater than 40 m/s translateto hydrate concentrations of 10?20% of the porespace (or 5?7% of the total sediment volume).Physical model of the vent zoneA model that helps to explain most of theobservations at the vent site was suggested byRiedel et al. [16]. The model assumes the presenceof a hydrate lens at the top of the blank zone andthin layers of sediments containing gas hydrates insmall concentrations.  The methane is deliveredinto localized hydrate formation areas through anetwork of thin channels in either a dissolved formor as a free gas. In addition, Z?uhlsdorff and Spie?[17] suggested episodic focused fluid flow. Theepisodic nature of the phenomena is justified bythe necessity to build up the pore fluid pressurehigh enough to push the fluids through thesediments from the BSR depth to the sea bottom.The hydrate cap that consists of either layers withhigh hydrate concentration or dispersed hydratechunks and lenses is responsible for the pull up ofthe travel time for the seismic events inside theblank zone. The cap also acts as a diffractor.  For acap thickness of about 30 m, the observed pull upof 5 ms will be achieved if the velocity in theaffected sediments are 200 m/s more than thevelocity in the adjacent sediments. Such anincrease of the velocity translates into gas hydrateconcentrations of about 15?17% of total sedimentvolume.However, the results of the travel time tomographyinversion do not reveal any significant velocityanomaly associated with the blank zone.  Instead,the bulk velocity increase within the zone is low.If a high velocity zone is present at the top of theblanking feature, the rest of the vent zone isrepresented by sediments with a seismic velocitysimilar to that of the sediments outside.In addition to the diffractions associated with thehydrate cap near the sea floor, several otherdiffractions were observed in the OBS data [8].The presence of the strong diffractions in closeproximity to the BSR that are observed on theseismic sections recorded by OBS B suggests aphysical condition similar to the one at the top ofthe vent. A structure like the hydrate cap can be inplace at the BSR inside the limits of the blankzone. These hydrates were most likely formed byfree gas entering the GHSZ from below.Overall, the results of the inversion are consistentwith a physical model that consists of thin layersof hydrate located at specific depths within thestability zone.  It is worth noting also that thevertical axis of the blank zones passes through theintersection of the BSR and accreted sedimentboundary beneath the younger slope sediments.The coincidence of these two boundaries may bethe controlling factor for the blank zone location.The accreted sediments are more compacted thanthe slope sediments, and at the same time they aremore permeable due to the presence of cracks. Onthe other hand, filling the cracks with gas hydratescan greatly reduce their permeability. That is,below the bottom of the GHSZ, accretedsediments are more permeable than slopesediments, but above this boundary the situationmay reverse forcing the fluids to penetrate throughthe slope sediment section.Comparison with results from IODP 311In October 2005, IODP Expedition 311 drilled atseveral holes at the Bullseye vent site [7].  Thedrilling revealed the presence of massive hydratein thin sections localized to three depths: (i)between the sea floor and 46 m; (ii) at 160 m; and(iii) in an interval 10 m thick above the BSR.These findings support the structure of the blankzone and the distribution of gas hydrates withdepth proposed in this study based on the velocitymodel from the tomographic inversion andanalysis of the seismic data: a layer of several 10?sof metres thick at the top of the sediment section;no presence of hydrates in large concentrationsbelow the layer to the BSR; and occurrence of gashydrates just above the BSR.CONCLUSIONSThe seismic data set recorded by five OBS stationswas used in conjunction with conventional seismicdata for the investigation of the structure of theBullseye cold vent zone. A 3-D velocity modelwas obtained by means of travel time tomographyinversion for the region around the vent. Themodel does not support the presence of largevelocity anomalies. The anomaly associated withthe vent zone was found to be in the range of +25m/s, which suggests no anomalous high gashydrate concentration inside the blank zone.Unfortunately, higher resolution (vertically) ofmore detailed velocity structure was not possibledue to limitations of the experimental geometry.Nevertheless, a combination of the tomographyresults with available seismic and otherinformation about the site suggests that a layer ofhigh gas hydrate concentration (15% or more) islocated at the top of the vent zone. The rest of theblank zone may contain local concentrations ofhydrates in small amounts.The average velocity profile for the site starts withcompressional velocities close to those in the seawater, and increases virtually linearly reachingabout 1750 m/s at the BSR. The velocity profiledoes not reveal any significant variations from thebackground no-hydrate velocity profile for theslope sediments, which suggests that slopesediments outside the blank zone contain no orvery little gas hydrate down to the BSR.ACKNOWLEDGEMENTSThe authors thank Dr. Keith Louden, DalhousieUniversity, for his support with the ocean bottomseismometers.REFERENCES[1] Chapman, N.R., J.F. Gettrust, R. Walia, D.Hannay, G.D. Spence, W.T. Wood, R.D.Hyndman (2002). High-resolution, deep-towed,multichannel seismic survey of deep-sea gashydrates off western canada. Geophysics,67(4):1038?1047.[2] Wood, W.T., J.F. Gettrust, N.R. Chapman,G.D. Spence, and R.D. Hyndman (2002).Decreased stability of methane hydrates in marinesediments owing to phaseboundary roughness.Nature, 420(6916):656?660.[3] Hyndman, R.D., G.D. Spence, R. Chapman,M. Riedel, and R.N. Edwards (2001). Geophysicalstudies of marine gas hydrate in NorthernCascadia. 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