Evidence From Three‐Dimensional Seismic Tomography for a Substantial Accumulation of Gas Hydrate...

8/12/2019 Evidence From ThreeDimensional Seismic Tomography for a Substantial Accumulation of Gas Hydrate in a Fluid E

1/24

Evidence from threedimensional seismic tomography

for a substantial accumulation of gas hydrate in a fluidescape

chimney in the Nyegga pockmark field, offshore Norway

Andreia PlazaFaverola,1,2 Graham K. Westbrook,3 Stephan Ker,1 Russell J. K. Exley,3

Audrey Gailler,3,4

Tim A. Minshull,5

and Karine Broto6

Received 27 October 2009; revised 24 March 2010; accepted 14 April 2010; published 20 August 2010.

[1] In recent years, it has become evident that features commonly called gas chimneysprovide major routes for methane to pass through the methanehydrate stability zonein continental margins and escape to the ocean. One of many such chimneys lying beneath

pockmarks in the southeastern Vring Plateau off Norway was investigated with ahighresolution seismic experiment employing a 2D array of sixteen 4component ocean

bottom seismic recorders at approximately 100 m separation and a dense network of shotsto define the 3D variation of the chimneys structure and seismic properties. Thetomographic model derived from Pwave travel times shows thatPwave velocity inside

the chimney is up to 300 m/s higher than in the surrounding strata within the methanehydrate stability zone. The zone of anomalously high velocity, about 500 m wide near its

base, narrowing to about 200 m near the seabed, extends to a depth of 250 m belowthe seafloor. The depth extent of this zone and absence of high velocity beneath the base ofthe methanehydrate stability field make it more likely that it contains hydrate rather thancarbonate. If a predominantly fracturefilling model is appropriate for the formation ofhydrate in lowpermeability sediment, the maximum hydrate concentration in the chimney isestimated to be 14%27% by total volume, depending on how hostsediment propertiesare affected by hydrate formation. Doming of the strata penetrated by the chimney appears to

be associated with the emplacement of hydrate, accompanying the invasion of the gashydrate stability zone by free gas.

Citation: PlazaFaverola, A., G. K. Westbrook, S. Ker, R. J. K. Exley, A. Gailler, T. A. Minshull, and K. Broto (2010),

Evidence from threedimensional seismic tomography for a substantial accumulation of gas hydrate in a fluidescape chimney in

the Nyegga pockmark field, offshore Norway, J. Geophys. Res., 115, B08104, doi:10.1029/2009JB007078.

1. Introduction

[2] The escape of pore water and gas from continentalshelves, through seafloor features known as pockmarks hasbeen investigated for many years [King and MacLean, 1970].The discovery of pockmarks in deeper water, within thegas hydrate stability field [e.g., Vogt et al., 1994], led to agrowing appreciation that the chimneylike features in thesedimentary strata beneath the pockmarks provide a meansfor methane beneath the hydrate stability zone to escape to theocean, accompanied by the formation of gas hydrate [e.g.,

Riedel et al., 2006;Chand et al., 2009;Hustoft et al., 2009a].The methane release through these chimneys in response toclimate changes may be more significant than the methanereleased by submarine slides, which are commonly invokedas the mechanism for releasing methane from submarinehydrate [McIver, 1982; Kvenvolden, 2002].

[3] A pockmark field of about 2000 km2 in the Nyeggaregion, north of the Storegga slide in the midNorwegiancontinental margin (Figure 1), where hydraterelated bottomsimulating reflectors commonly occur [Mienert et al., 1998;Bnz et al., 2003], contains hundreds of pockmarks underlain

by chimneylike features, usually referred to as gas chimneys.In seismic reflection sections, chimneys are represented byzones of low coherence, scattering, and low amplitude thatis, at least in part, a consequence of the seismic scattering inthe shallowest parts of the chimneys. The surrounding strataappear truncated at the margins of the zone of incoherenceand may also be flexed upward in the flanks of the chimney.Some of the truncation may only be apparent because ofseismic visibility loss in the zone of incoherence, but in othercases, diffractions from points where strata meet the zoneof incoherence show that truncation is real. In the uppermost

1Dpartement Gosciences Marines, Ifremer, Plouzan, France.2Department for Geology, University of Troms, Troms, Norway.3School of Geography, Earth & Environmental Sciences, University of

Birmingham, United Kingdom.4Universit Europenne de Bretagne, Brest, CNRS UMR 6538, Institut

Universitaire Europen de la Mer, Plouzan, France.5National Oceanography Centre, Southampton, United Kingdom.6Institut Franais du Ptrole, RueilMalmaison, France.

Copyright 2010 by the American Geophysical Union.01480227/10/2009JB007078

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B08104,doi:10.1029/2009JB007078, 2010

B08104 1 of24
http://dx.doi.org/10.1029/2009JB007078http://dx.doi.org/10.1029/2009JB007078


2/24

200 m or so beneath the seabed, the chimneys (lowcoherencezones) become wider with depth, and their width is typicallygreater than their depth. Deeper than the base of the gashydrate stability zone (GHSZ), the chimneys may be under-lain by strata with very little disturbance or be shown to

continue downward by the disturbance of the strata throughwhich they pass. In many cases, however, the deeper part ofthe chimney is represented by a zone of very low amplitudeincoherent reflections, which may be caused by amplitudeloss in the upper part of the chimney producing a seismicshadow in the zone beneath. In some cases, the illusorynature of the apparent deeper continuation of chimneys hasbeen demonstrated with seismic data with large shotreceiveroffsets that can undershoot the scattering zone. The flux ofmethane through these features in the past is indicated by theoccurrence in the pockmarks of methanederived authigeniccarbonate [Hovland et al., 2005; Mazzini et al., 2006] andshallow gas hydrate [Ivanov et al., 2007], while variations inthe depth beneath the seabed of the sulfatemethane transi-

tion indicate different methane flux rates inside and outsidepockmarks [Paull et al., 2008].[4] Flares of bubbles of methane in the water column

have not been observed in the Nyegga area during thecruises, submarine dives, and remotely operated vehicle(ROV) operations carried out over the last 10 years. Hence,the chimneys at Nyegga are believed to be currently of verylow activity or inactive in terms of the amount of free gasbeing released to the water column [Hovland et al., 2005;Hustoft et al., 2007;Ivanov et al., 2007;Paull et al., 2008].However, seepage of dissolved methane has been observedat the G11 [Hovland et al., 2005] and CNE03 pockmarks

[Nouz and Fabri, 2007]. Also, the CNE03 pockmark wasknown from previous seismic imaging to exhibit significantlocal upwarping of reflectors that might be caused either bydeformation related to fluid escape or by a seismic velocityanomaly caused by the presence of hydrate. As part of the

HERMES (Hotspot Ecosystem Research on the Margins ofEuropean Seas) integrated project to study gas seeps sys-tems, a highresolution seismic experiment was carried out inJune 2006 to investigate the chimneylike features beneaththe G11 [Jose et al., 2008] and the CNE03 [Westbrook et al.,2008b] pockmarks.

[5] In this paper we present the results of a detailed 3DPwave reflection tomography study of the chimney beneaththe CNE03 pockmark, using ocean bottom seismometer(OBS) data, which provides evidence for the occurrence ofhighvelocity material inside the chimney. The extent towhich the interiors of the chimneys of the NyeggaStoreggaregion are occupied by hydrate, carbonate or gas, as well aswhether the internal strata suffered upward doming, had not

been determined prior to the seismic investigation reportedhere. The results of the tomographic experiment constitute,therefore, a valuable contribution to the knowledge of theinternal structure of chimneys in the Norwegian continentalmargin and to further understanding of their formation.

2. Geological Setting

[6] The chimney studied is one of the fluidoutflow fea-tures associated with pockmarks in the Nyegga pockmarkfield. The Nyegga region is located at around 64N, 5E. Itlies above the Helland Hansen arch, which separates two

Figure 1. (a) Location and (b) bathymetry of the CNE03 pockmark at the midNorwegian continentalmargin. The CNE03 pockmark lies a few kilometers north from the Storegga slide (outlined in red inFigure 1a) and close to the northern limit of the prevalent BSR area (outlined in black in Figure 1a) [afterBnz et al., 2003]. OBS sites are displayed around the pockmark. Only a limited amount of data fromOBS 9 (at the center of the pockmark) was recovered.

PLAZAFAVEROLA ET AL.: GAS HYDRATE IN A FLUIDESCAPE CHIMNEY B08104B08104

2 of 24


3/24

NESW trending Cretaceous basins: the Vring to the north-west and the Mre basin to the southeast [Brekke, 2000]. Itseastern and western limits are the Trndelag platform andthe NWSE trending Jan Mayen fracture zone respectively.The latest Cenozoic deposition was controlled by glacial andinterglacial periods. The PlioPleistocene sedimentary wedgecan beup to1.75 kmin thickness [Hjelstuen et al., 1999]. Thewestward progression of the wedge and its fast depositiongenerated differential compaction in the underlying sedi-ments, causing lateral fluid flow and fracturing of regions,depending on their position with respect to the front of thewedge [Reemst et al., 1996; Kjeldstad et al., 2003; Gmezand Vergs, 2005]. The sedimentation rate decreased in theQuaternary [Hjelstuen et al., 1999].

[7] During Pleistocene glacial stages, thick sequencesof glacigenic debris flows (GDFs) were deposited on theNorwegian continental shelf and slope. GDFs are composedof glacigenic material interfingered with very fine grainedsediments [Hjelstuen et al., 2005]. Along the Vring margin,these glacigenic sequences are restricted to the uppermostcontinental slope [Hjelstuen et al., 2005]. The CNE03chimneylike feature, investigated here, is located far from

the thick glacigenic sequences, which are mainly character-istic of the Naust units S and T [Berg et al., 2005]. Thelack of thick sequences of glacigenic debris flow is importantbecause glacigenic debris flows are characterized by anom-alous high seismic velocities.

[8] The sedimentary sequence containing the structuresinvestigated with the seismic experiment lies within the Naustformation, for which we use the nomenclature and ages fromRise et al. [2006] (Table 1). Bottom to top, the units arenamed N, A, U, S, and T (02.8 Ma). Naust unit N (1.52.8 Ma) represents dominantly glaciofluvial and marineprocesses [Rise et al. , 2006]. Unit A (0.61.5 ma) representsa period where the ice sheets reached the paleoshelf edge. Itconsists of hemipelagic sediments and remnants from land

based glaciers [Berg et al., 2005].[9] Sampled sediments from NaustU (0.40.6 Ma) arepredominantly hard clays with variable sand and gravelcontent [Berg et al., 2005]. The shallower part of NaustU isdescribed as distal glacial marine together with hemipelagicdeposition [Berg et al., 2005]. According to borehole data,this upper sequence (NaustU) has relatively high organicdebris content and water content compared to the overlyingstrata [Hustoft et al., 2007].

[10] NaustS (0.20.4 Ma) represents predominantlyglacial marine to normal marine conditions with glacial debrisdeposits on the slope [Berg et al., 2005]. The transition from

U to S is characterized by a decrease of water content andplasticity. NaustS lower sequences show more coarsegrained and unsorted sediments. However, the clay contentincreases again in the upper S sequence [Berg et al., 2005].

[11] Finally, the top of Naust unit T (00.2 Ma) in the areaof CNE03 is mainly glacial marine, with tills on the shelf anddebris flow on the slope [Berg et al., 2005]. Water contentand clay content in the marine clay sediments within Naust

are reported to be 30%60% [Bnz and Mienert, 2004] and50%60%, respectively [Berg et al., 2005].

[12] A gas hydraterelated bottom simulating reflector(BSR) has been mapped over an area of about 4000 km2 ofthe gas hydrate province [Bnz et al., 2003]. The BSR marksthe transition between gas hydratebearing sediment aboveand sediments containing free gas below [Bouriak et al. ,2003;Bnz et al., 2005;Westbrook et al., 2008a]. The BSRis easier to see where the slope of the seabed causes the BSRto cut across the stratigraphy.

[13] Two major subbottom layers are inferred from P wavevelocity (Vp) and seismic amplitude anomalies in Nyeggaand its adjacent regions to be undercompacted and containoverpressured fluid [Bnz et al. , 2005; Westbrook et al.,

2008a; PlazaFaverola et al., 2010]. Hydraulic fracturing

has been inferred to play a major role in the upward trans-portation of fluids in this region [Berndt et al., 2003]. TheNyegga pockmark field in the eastern part of the mappedregion of the BSR shows the highest density of seabed fluidventing to the north of the Storegga Slide [Bouriak et al.,2000;Bnz et al., 2003; Hovland et al., 2005; Hovland andSvensen, 2006]. Some of the pockmarks have beendescribed as complex structures with faunal communities andcarbonate edifices associated with them [Hovland et al. ,2005; Hovland and Svensen, 2006; Mazzini et al., 2006;Paull et al., 2008].

[14] From the reported geothermal gradient in the region[Sundvor et al., 2000; Mienert et al., 2005], the variation in

depth of the BSR with seabed depth 15 km to the southwest[Bnz et al., 2003; Westbrook et al., 2008a] and the mea-sured seabed temperature at the location of CNE03 [Nouzand Fabri, 2007], the depth of the presentday base of theGHSZ at CNE03 is predicted to be at about 230 m below theseafloor (mbsf) (Figure 9).

3. Experiment and Data

[15] The aim of the highresolution seismic reflectiontomography was to resolve the 3D structure and variationof Vp in the chimney beneath the CNE03 pockmark. Thetomographic experiment was part of an investigation thatincluded data from a deeptowed 100 kHz side scan sonar

and 5 kHz subbottom profiler. Both, single

channel seismic(SCS) and ocean bottom seismic (OBS) data were used inthe tomographic inversion.

[16] An array of 16 OBSs was deployed around thepockmark (Figure 1) by lowering each OBS by cable, underguidance from acoustic navigation, to a height of 50 m abovethe seabed before releasing the OBS to fall to the seabed.This approach provided relatively precise positioning of theinstruments in relation to the 300 m wide pockmark in awater depth of around 725 m [Westbrook et al., 2008b]. Eight4component OBSs were recorded at a sampling interval of0.4 ms. Eight 2component OBSs were recorded at a 2 ms

Table 1. Naust Formations Stratigraphical Terminology, Unit

Ages, and Lithology

NaustUnits

Rise et al. [2006]Age (Ma) Berg et al.[2005] Lithology

T 00.02 Glacialmarine with tilland glacigenic debris flow

S 0.20.4 Glacialmarine to normal marine.Glacigenic debris.

U 0.40.6 Hard clays with variable sand and gravelHemipelagic and glacialmarine at the base

A 0.61.5 Hemipelagic and remants from glacialsN 1.52.8 Glaciofluvial and marine processes


3 of 24


4/24

sampling interval. The spacing of the OBSs was about 100 m.Only a small part of the data from one OBS located at thecenter of the pockmark could be recovered because of a faultwith the recorder, and the data from this OBS were not used inthe modeling. The seismic sources were minigeneratorinjector (GI) guns deployed as a single gun in true GI mode(configured as 13 cubic inch generator and 35 cubic inchinjector) for recording part of the seismic lines set with a

maximum resolution (shot spacing 8 m; line spacing 50 m)and as two guns in harmonic mode to record lines with a betterpenetration (shot spacing 12 m; line spacing 100 m). Theseismic signal had a dominant frequency of 120 Hz.

[17] Singlechannel seismic (SCS) reflection data were alsorecorded along the shot lines, including circles (Figure 2),which were designed to provide a good coverage of azimuthsat farther offset ranges (Figures 2 and 3). Processing ofthe OBS data included a bandpass filtering (2040280300 Hz.) to improve the data quality for picking. This filterremoved the low frequency noise from the ship. Shots andOBSs were acoustically relocated using the direct wave traveltimes. The number of median residuals (between measuredand predicted travel times) for each shot to all OBS out of

the range 0.5 to +0.5 ms was negligible. At a shotOBSoffset of 500 m, a change in the expected direct wave traveltime of 0.5 ms is produced by a change in range of 1.35 m[Westbrook et al., 2008b].

Figure 2. Geometry of the center of the seismic experi-ment, showing the seismic lines and OBS sites (stars), con-centrated around the chimney.

Figure 3. Selected seismic reflection sections from the OBS seismic experiment at the CNE03 pockmarkshowing the diffractions interfering with primary reflections. Arrivals from NS, NWSE, NESW, andcircular seismic lines were recorded by an array of 15 OBSs (represented by stars). The seven seismicreflection events used for tomographic modeling are labeled.


4 of 24


5/24

[18] Detailed bathymetry of the CNE03 pockmark with avertical resolution of 0.5 m, acquired by the ROV Victor[Nouz and Fabri, 2007], provided visualization of theOBSs location with respect to the CNE03 pockmark mor-phology (Figure 1).

4. Modeling Methodology

4.1. P

Wave Travel Time Picking[19] Pwave reflections were identified in 45 seismic lines

recorded by 15 OBSs and a singlechannel streamer. Toensure the same reflector was picked in all data sets, the SCSand OBS profiles were correlated (Figure 4). To facilitate thepicking of travel times, the reflectors in the record sectionswere flattened or had their curvature reduced by applying ahyperbolicmoveout correction. The maximum offset rangefor picked arrivals was 2 km. Automatic and semiautomaticpicking could be implemented, but spurious irregular timepicks needed to be corrected manually. By visual inspection,1 and 2 ms were set as errors in picking the data sampled at0.4 and 2 ms, respectively. We have therefore considered2 ms as a maximum data uncertainty (1/5 of the dominant

period of the signal).[20] Seven seismic reflectors were interpreted and corre-

lated with the published stratigraphy of the region [Rise et al.,2006; Hustoft et al., 2007]: reflector H30 corresponds tointra Naust T (Figure 4). Horizons H60 and H70 are withinNaust S. H100 is at the transition of Naust S and Naust U.This reflector is characterized by strong amplitudes andreverse polarity in most of the seismic sections. It is com-plicated by triplications and diffractions from the flanks of thechimney (Figures 3 and 4). H120 is the base of a lowvelocitylayer within Naust U [Bnz et al., 2005; Westbrook et al.,2008a]. H150 and H160 are the top and base, respectively,of a layer correlated with a layer within Naust unit A that isinterpreted to be overpressured [Reemst et al., 1996; Bnz

et al., 2005].[21] To enhance deeper reflectors (below the base of GHSZ),

a technique consisting of the summation of the hydrophoneand vertical component of the geophone (PZ summation) wasimplemented. This technique enhances the amplitude of theupgoing waves containing the reflected arrivals and sup-presses the amplitudes of the downgoing waves containingnoise primarily. Events H120, H150, and H160 were pickedfrom the PZsummed profiles.

[22] Picking travel times ofPwaves reflections toward theNE of the chimney above the base of the GHSZ was com-plicated by the presence of diffracted events and by seismicattenuation. Picking reflector H120 (right below the base ofGHSZ) was mainly affected by blanking inside the chimney.

Picking the deepest two reflectors was only limited by theblanking inside the chimney. Travel times of rays crossing thechimney were included in the tomography. These travel timeshelped constrain velocities in zones where the density ofseismicray impact points was poor (e.g., at the flanks andchimney interior).

4.2. Inversion and Parameterization

[23] To build the velocity model, we used TomoInv, pre-stack travel time tomography software developed at InstitutFranais du Ptrole (IFP) and industrialized in a partnershipbetween IFP and Parallel Geosciences Corporation (PGC).

Travel time tomography aims to determine the subsurfacevelocity model that best satisfies the travel times of seismicwaves that propagate through the subsurface (Appendix A).4.2.1. Velocity Distribution Representation

[24] The tomographic model uses a blocky representationfor the velocity distribution. The model is divided intoblocks with smoothly varying interface depths and velocitycontrolled by Bspline functions (Figure B3) (Appendix B)

to ensure the continuity of derivatives with respect to themodel up to second order [Clarke, 1996]. The tops andbases of the blocks correspond to seismic reflectors (sevenin our study, Figure 4) that were chosen as the boundaries ofthe layers in the model (six in our study). Since eachvelocity block is characterized by its own smooth velocitydistribution, the blocky representation provides the possi-bility of properly modeling discontinuous velocity variationsand hence discontinuous travel times after ray tracing [Laillyand Sinoquet, 1996]. For the tomographic model of theCNE03 chimney, the velocity within a single layer re-mained vertically invariant.4.2.2. Ray Tracing

[25] Ray tracing is performed by the bending method

[e.g., Jurado et al., 1996]. This method has advantages interm of its speed and offers a sufficient accuracy comparedwith other ray tracing methods [Jurado et al., 1996]. Aninitial raypath linking source and receiver and obeying theSnellDescartes law at each intersection between the trajec-tory interfaces and the reflector (impact points) is estimated.A raypath is retrieved by moving the impact point along thereflector until the initial trajectory satisfies the Fermat prin-ciple, i.e., the time function is stationary (Figure 5). The totaltime from source and receiver is then the ray travel time[Jurado et al., 1996].4.2.3. Regularization

[26] The tomographic inversion is an iterative process.The current velocity model is updated in order to minimize

the misfits between observed and calculated travel times(equation A3, Appendix A). One major difficulty encoun-tered when trying to solve the tomographic problem is thatthe solution, although providing the best match betweenobserved and computed travel times, does not necessarilyyield a model that is probable on the basis of geological andother geophysical information, often because errors in thedata generate spurious smallscale details in the model. Theprogress of the inversion towards the optimum global solu-tion (minimizing residual times, Tcalculated Tobserved) can behalted by becoming trapped in local minima, especially whenthe starting model is far from the real geology (expected finalmodel).

[27] To reduce this underdetermination, a dedicated

approach based on the introduction, through regularization,of a priori information about the model (more precisely onits roughness), as well as a quasiautomatic management ofthe resulting regularization weights was employed. Hence,the tomographic algorithm provides control of the roughnessand variability of expected surfaces and velocities by meansof regularization weights. In particular, with this approach,one can find progressively less and less smooth models asthe calculated travel times get closer to the observed traveltimes (Appendix A). In addition to the regularization, con-straints can be placed on the model, such as an a priori rangeof velocity values or interface depths obtained from well


5 of 24


6/24

Figure 4. (top) Correlation between the OBS and streamer data. The interpreted reflectors have beencorrelated with the published seismic stratigraphy of the area [Rise et al., 2006]. (bottom) OBSs 7 and10 profiles recording from the western and eastern flanks of the chimney, respectively, show the seismicattenuation and diffractions impeding picking of the main reflectors at both sides of the chimney. Theinterpreted reflectors are indicated (dashed lines).


6 of 24


7/24

data, forward modeling, or any other kind of velocity dataavailable in the area [Delbos et al., 2006]. Information onthe resolution of the tomographic model of the CNE03chimney can be found in Appendix B.

[28] The effectiveness of the tomography software and themethodology presented here have been demonstrated on

several real data sets and in different geological contextssuch as subsalt imaging, subchalk imaging with P wavereflections and PtoS converted waves and vertical trans-verse isotropic symmetry anisotropy estimation, and foot-hills imaging [e.g., Ehinger et al., 2001, Broto et al., 2003,Jardin et al., 2006].4.2.4. Parameterization

[29] The 3D tomographic model for CNE03 has dimen-sions 4 4 1.4 km in x, y, and z, respectively. The cellsize is 40 40 m in x and y (100 100 per layer). The cellsize was chosen taking into account the shot spacing (12 m).During the course of the inversions, the sensitivity of thecalculated velocities to cell size was investigated with check-erboard tests (Appendix B). Seven major reflectors within

the upper 500 mbsf were included in the model. The firstlayer in our model is the water column. The seafloor inter-face and water velocity were not derived by tomographicinversion. The seafloor interface was derived by the depthconversion of the seafloor reflector map in the SCS, usingthe average velocity of the water column, 1.475 km/s, whichwas derived from inversion of the direct wave travel timesduring acoustic relocation of the OBSs [Westbrook et al.,

2008b]. Flat interfaces were used to initialize the inversionof all the layers. Initial velocities were taken from 1D and2D models from previous studies a few tens of kilometerssouth [PlazaFaverola et al., 2010] and southwest [Bnzet al., 2005; Westbrook et al., 2008a] from CNE03. Detailsof the analysis of the residuals and model uncertainty arepresented in Appendix B.

5. Results

5.1. Vp Model at CNE03

[30] The subseafloor layers in the model will be referredto, from shallow to deep, as L30, L60, L70, L100, L120,L150, and L160.

[31] Considerable differences exist between the lateralvariation of Vp in layers above the base of the GHSZ(230 mbsf) and in layers below it (Figure 6). The upper230 m of sediments exhibit lateral velocity changes, with Vpincreasing toward the chimney center. In some cases the Vpincreases coincide with doming of the upper interfaces ofthe layers (Figures 6 and 7). In contrast, the layers underlyingthe GHSZ do not show large lateral changes in velocity.

[32] Although the travel times of rays with offsets(sourcereceiver horizontal distance) of up to 2 km wereincluded in the inversion, the zone in which crossing raysoccur is controlled by the positions of the OBS and by thedepths of the reflecting interfaces. The large thickness ofthe water layer in comparison with the subseabed depths of

Figure 5. Schematic representation of the ray tracing,modified from Jurado et al. [1996].

Figure 6. Velocity distribution in the EW and NS directions. See location in Figure 2. Each section islabeled with the identifiers of the interpreted layers. Vertical and horizontal scales are in kilometers. Vp isin kilometers per second. A cluster of raypaths is displayed to show the extent of the zones with wellconstrained velocity in the model. For display purposes, the number of rays is decimated by a factor of100. Velocities in the pale colored zones are undetermined by the inversion. The dashed black linecorresponds to the calculated base of the GHSZ.


7 of 24


8/24

Figure 7. (left) Top view of the modeled interfaces depths and (right) velocities for layers above eachinterface focusing on their relief and anomalous lateral Vp changes within the GHSZ (H30, H60, H70,and H100). The color scales representing depth and velocity are different for each layer (the color scaleis normalized to the maximum and minimum values). Contours are at 1 m intervals for depth and 10 m/sintervals for velocity. The dashed lines encircle the wellconstrained regions for each modeled layer. Thethree upper layers are characterized by doming of their basal surfaces.


8 of 24


9/24

the reflectors place the impact points much closer to the OBSthan to the shots. The radius of the zone in which crossingrays occur increases with depth from about 350 m for layerL30 to about 800 m for layer L160. Outside this radius, thelow density or absence of rays toward the borders of themodel leads to poor constraints of interface and velocities(border effect), resulting in an inaccurate Vp estimate. Theborder effect is minimal inside this radius, and a high con-

centration of observed travel times provides robust resultsfor this region.

[33] Layer thicknesses in the model vary between 30 and100 m (Figure 6). The shallowest reflector used in the model(base of L30, intra Naust T) is located at 80 mbsf (Figure 6).The interface is characterized by a gentle relief. Velocities inL30 need care in interpretation, because, although thevelocity beneath the pockmark in the model is about 40 m/sgreater than background, the impact points on the reflectorsat its base (H30) cluster closely around each OBS, and sovelocity is patchily defined in the layer.

[34] Positive relief of the second and third subseabedinterfaces follows a NESW trend. Here, the magnitude ofthe relief of the model increases with depth (Figure 7). The

maximum relief (measured respect to the flat part of thereflectors) is approximately 14 m at H30 and 22 m at H70.At H60 the relief coincides with a velocity increase toward thecenter of the chimney from 1580 up to 1880 m/s (Figure 7).The velocity increase is larger at the depth of H70. Vpoutside the chimney is 1650 m/s on average, and it is up to2000 m/s at the interior. For L70, the pattern of anomalouslyhigh velocity does not correlate so closely with the mor-phology of the base of the layer (e.g., the location of maxi-mum Vp does not coincide with the location of maximuminterface relief) as it does for the layers above (Figure 7).

[35] Layer L100 has a similar velocity distribution to L70,with an increased Vp of up to 2000 m/s inside the chimney.The basal reflector of L100 is 20 m deeper than the theo-

retical base of the GHSZ (see section 6). The appearance ofhigh Vp beneath the base of the GHSZ (Figure 6) is,therefore, related to the choice of the L100 basal interfacefor inversion. Excluding this possibly hydratefree zone(beneath the calculated base of the GHSZ) from the L100layer inversion would probably lead to higher predictedanomalous velocities in the overlying layer (most of L100).Modeling was not attempted, however, for lack of a welldefined reflector above H100 close to the top of the poten-tially hydratefree zone and because of the thinness of thatzone.

[36] The basalinterface of L100 has a gentle concave shapethat reaches a maximum depression of10 m with respect tothe flat sediments towards the southeast of the chimney

interior (Figure 7). Modeling of this interface is not optimalbecause impact points cover only half the area of the centraldepressed part of the base (see Figure B1, Appendix B).Consequently, the concave shape of the interface may, inpart, result from a velocitydepth tradeoff. The thickness ofthis layer (80100 m) and wide range of incidence anglesmakes it less prone to this tradeoff. If, however, the tradeoff were enough to depress a truly flat base, the true maxi-mum velocity of L100 would be 1800 m/s.

[37] Below the GHSZ, a 50 m thick layer (L120) showsvelocities of less than 1550 m/s (Figure 6). A 100 m thicklayer (L150) with a nearly flat base, and homogenous lateral

distribution of Vp separates L120 from L160, a second lowvelocity zone (LVZ) (Figure 6). The velocity of this secondLVZ is not well determined, as it is a thin layer of about30 m thickness at a depth of 470 mbsf. Both LVZs, however,can be correlated with highamplitude negativepolarityreflections at their tops and with the two LVZs found withinadjacent parts of the Nyegga area during previous studies[Bnz et al., 2005;Westbrook et al., 2008a; PlazaFaverola

et al., 2010].

5.2. Correlation Between Seismic Anomalies and theLateral Extent of the L70 HighVelocity Zone

[38] To qualitatively evaluate the nature of the chimneyboundaries in the tomographic model, we correlated theextent of the anomalously highvelocity zone (HVZ) in layerL70 in the model with the seismic character of rays reflectedfrom the base of L70 that crossed the area of the chimneyfrom different azimuths. For this approach, we implementeda simple methodology that consisted of tracing rays in a 2Dplane linking the OBS sites to shots along the circular shotlines from which the seismic records showed evidence oflateral discontinuity, such as the origins of scattered waves,

truncated reflectors, and the onset of distinct velocity pullup,to form a polygon circumscribed by the rays that had grazedthe margins of the chimney. The polygon was projected onthe Vp map for L70 layer (Figure 8).

[39] The area enclosed by crossing polygons correspondsto the anomalous HVZ (Figure 8e). Scattering of the wavescrossing the chimney makes it difficult to recognize thevelocity pullups at some locations, e.g., at the western flankof the chimney (Figures 8a and 8d). However, at otherlocations and for some azimuths (e.g., at the eastern flankwhen the waves cross the structure in a NESW direction),the velocity pullup can be recognized in spite of the seismicattenuation (Figure 8d, left of D3). The diffractions recordedat the flanks and front of the chimney (e.g., the phase reversal

diffraction SD in Figure 8) also show differences related tothe azimuth of the trajectory of the waves passing throughthe velocity anomaly inside the chimney (Figures 8a, 8b, 8c,and 8d). In the seismic profiles recorded by OBSs 1 and 17,recording from the NE and SW flanks, respectively, theshallowest diffraction at the top of the chimney (SD) dips tothe east (Figures 8b and 8d). This same shallow diffraction(SD) is symmetrical with respect to the center of the chimneyin OBSs 6 and 11 (recording from WNW and ESE, respec-tively) (Figures 8a and 8c). The observed azimuthrelateddifferences indicate that the material inside the chimney thatcauses seismic scattering is heterogeneously distributed.

6. Discussion

6.1. Internal Structure of the Chimney

[40] The tomographic models interfaces above H100dome upward beneath the pockmark (Figure 7). This is alsoshown by the seismic sections after migration and depthconversion using the velocity field of the model (Figure 9).The seabed is domed upward around the central depression ofthe pockmark. The geometry of the shallowest (


10/24

Figure 8. Seismic sections recorded by OBSs (a) 11, (b) 1, (c) 6, and (d) 17 from the outermost circularshot line (Figure 2). The sections show the width of the region of attenuation inside the chimney andtravel time thinning (pullup) of the reflectors. Lines linking the OBS sites with beginning and end of theregion of attenuation (thick black dots) are plotted inside the circular seismic line over the (e) top view ofH70 Vp. The length of the seismic record sections shown in Figures 8a, 8b, 8c, and 8d is indicated in thecircle in Figure 8e by the corresponding labels in the seismic sections. A major diffraction at the front ofthe chimney is indicated by SD, and it is compared in the four OBS seismic sections (Figures 8a, 8b, 8c,and 8d).


10 of 24


11/24

interfaces (Figures 6 and 7) H30, H60, and H70 in the modelare 14, 19, and 22 m, respectively. The amplitudes shownby the seismic reflectors H30, H60, and H70 on the depthsection (Figure 9) are 23, 23, and 22 m, respectively. Thecorrespondence between the model and the reflectors is good

for H60 and H70 but not for H30. The probable reason forthe difference is that the H30 interface in the model has noray impact points in the center of the dome (Figure B1), andbecause of the smoothness imposed by regularization in theinversion, the model underestimates the curvature of the H30surface. Consequently, it appears that the amplitude of thedome is about the same from H70 to H30 but is only half ashigh at the seabed. With the exception of L100, the layershardly change thickness across the dome or, in the case ofL30, thin slightly. This is additional verification that thevelocity changes within these layers are not a consequence ofa tradeoff between velocity and thickness in the modeling.

[41] Layer L100 is thickened in the chimney. The base ofthe modeled layer (H100) lies at the top of a lowpermeability

unit above what is inferred to be a highporosity unit con-taining free gas, because of its low velocity. In the regionaround the chimney, the H100 reflector is clearly strati-graphic in origin and displays no bottomsimulating char-acteristics. The predicted base of the GHSZ lies about 20 mabove the H100 reflector. With no appreciable gas content inthe lowpermeability unit, the base of the GHSZ does notexhibit a BSR. The probable absence of hydrate in thebottom 20 m of layer L100 will dilute the effect of hydratein the rest of L100 on the L100 velocity, which is verticallyinvariant for each cell of the model. The variation in velocitywithin L100 shows some correlation with the depth of its

base (Figure 7), and so one cannot exclude the possibilitythat for L100 there is some tradeoff between velocity andthickness in the inversion, but particularly when one takesinto account the probable absence of hydrate in the bottom20 m of L100, the magnitude of the lateral variation of

velocity within L100 is too large to be explained solely by atradeoff between velocity and thickness.

[42] Beneath the chimney, one would expect the basal partof L100, below the GHSZ, to be invaded by gas in fractures,locally reducing its velocity. The increase in travel timecaused by this region of gas invasion could be modeled as athickening of the layer and may be the cause of the slightdeepening of the lower boundary of L100 beneath thechimney. Also, if warm fluids continue to carry heat upwardthrough the base of the chimney, the increased temperaturewill move the base of the GHSZ locally higher, increasingthe low velocity volume that occupies the bottom part ofL100. It must be admitted that the detail of the chimneybase in L100 is poorly resolved by the seismic data and that

interpretation of this part of the tomographic model shouldbe approached with caution.

[43] Beneath H100, the boundaries exhibit no doming(Figures 6 and 9), an observation confirmed by an image ofthe chimney in an industry seismic section from data withsourcereceiver offsets of up to 2400 m. This implies thatwhatever the process responsible for the doming of thechimney, it occurs within the GHSZ or at its immediate base.

[44] It is an interesting question as to whether there is anyfree gas in the chimney at present. Seismic blanking andscattering, however, coincide with structural disturbance andanomalously high Vp (Figure 8). Differences in areas affected

Figure 9. Inferred CNE03 chimney structure presented on a NWSE oriented seismic reflection section(see Figure 2 for location) on which the Vp model has been overlain. The seismic attenuation is strongerat the western half of the chimney. The structural relief of reflectors increases only very slightly towardthe base gas hydrate stability zone (BGHSZ).


11 of 24


12/24

by blanking may indicate variation in the location and/oraccumulation of gas hydrate [Riedel et al., 2006]. Hence, we

conjecture that seismic attenuation may come from discretehydrate accumulations. If there is free gas in migrationpathways of locally very high salinity, the amount is likelyto be very small, and its effect on reducing Vp counteractedby the increased velocity produced by hydrate. The corollaryof this is that if free gas is present, the estimates of hydratecontent based on Vp will be too low.

6.2. Estimation of Gas Hydrate Concentration

[45] The increased seismic velocity at the chimney centrerelative to that of the surrounding strata can be used to esti-mate the concentration of hydrate present. There are severalpublished relationships between seismic properties andhydrate content in sediments, and most of them take into

account the habit of the hydrate within the pores of the hostsediment, such as porefilling, frameforming, cementing,and also in some cases the mineralogy of the host sediment[Helgerud et al., 1999; Lee et al., 1996; Jakobsen et al.,2000;Chand et al., 2004, 2006].

[46] All of them, however, assume that the hydrate occu-pies intergranular pore space, in one way or another. Hydratefound in cores at pockmarks 10 km from CNE03 [Ivanovet al., 2007; Akhmetzhanov et al., 2008] occupies beddingplanes and fractures. Also, it has been observed that hydrateoccupies networks with veins of a few centimeters separationin cores of finegrained clayrich mud sampled at in situpressure from offshore India and South Korea [Schultheisset al., 2009]. Given the finegrained, muddy character of

the NAUST S and T units (layers L30L100 in our model)[Berg et al., 2005; Rise et al., 2006], it is likely that hydratein the chimney beneath CNE03 is predominantly fracturefilling in its habit. Consequently, we have used a simple timeaverage approach to estimate the concentration of hydrate inthe chimney. For each of the three layers of the model withsignificant lateral variation in velocity, L60, L70, and L100,we took the maximum velocity and compared it with maxi-mum and minimum values of the background velocity ofthe layer in the wellconstrained part of the model to deriveestimates of the minimum and maximum proportion of sed-iment locally occupied by hydratefilling veins, assuming

the background seismic velocity represented the velocity forhydratefree sediment. This simple mixture model predicts

more hydrate for a given velocity anomaly than frameforming or cementing models for intergranular porosity but iscomparable to porefilling model predictions. We madehydratesaturation estimates based on two different endmember assumptions:

[47] 1. Hydrate is an addition to the host sediment, so themixture is between hydrate and unaltered host. This wouldbe the case if hydrate formed from methane in solution,where both the methane and the water that form the hydrateare introduced to the GHSZ, displacing the sediment with-out changing the sediments water content, porosity, ormechanical properties.

[48] 2. Only free gas is introduced into the veins, so thewater to create hydrate in the veins must come from the

pores of the host, reducing the water content and porosity ofthe host. In this case, one might assume that although theseismic velocity of the host is increased by the reduction inporosity, its matrix velocity is unaltered, but it appearsunlikely that the host could compact beyond its elastic limitwithout an increase in its matrix velocity. Hence, we prefer amodel in which the velocity of the host is increased by boththe reduction in its porosity and the increase of its matrixvelocity.

[49] Details of the derivations are given in Appendix C,and the results are shown in Table 2. Assumption (1) yields ahigher hydrate saturation estimate than assumption (2) becausethe increase in the velocity of the host, as well as the presenceof hydrate, contributes to the overall velocity increase, and so

less hydrate is needed under assumption (2) to produce thevelocity anomaly.

[50] In layer L100, the maximum velocity within thechimney is 1980 m/s, and the background velocity in thewellconstrained part of the model varies between 1680 and1700 m/s. This yields an estimate of maximum concentra-tion of hydrate of 26%27% of sediment volume with the(a) additionalwater model and 13%14% with the (b) waterfromhost model. Layer L70 yields similar values, and layerL60 gives a range from 9% minimum with the (b) waterfromhost model to 21% maximum with the (a) additionalwater model. For comparison, if only the porosity changes

Table 2. Results of the Estimation of Hydrate Concentration for Two Casesa

LayerMaximum Velocity

(km/s)Hydrate Velocity

(km/s)Minimum. Background

Velocity (km/s)Maximum Background

Velocity (km/s)

L60 1.800 3.8 1.585 1.595(1) Fraction of hydrate

(additionalwater model)0.21 0.20

(2) Fraction of hydrate(waterfromhost model)

0.10 0.09

L70 1.960 3.8 1.675 1.690(1) Fraction of hydrate(additionalwater model)

0.26 0.25


0.14 0.12

L100 1.980 3.8 1.680 1.700(1) Fraction of hydrate

(additionalwater model)0.27 0.26


0.14 0.13

aCase 1, additional water model; case 2, waterfromhost model. The estimations are done for the three layers presenting the highest Vp in thetomography model (L60, L70, L100). Hydrate concentration is given as a fraction of total volume, not as a fraction of pore space.


12 of 24


13/24

in the host for the waterfromhostmodel, the estimatesof hydrate concentration (from equations C3 to C5 ofAppendix C) are between 78% and 96% of those obtainedwith the (a) additionalwater model. The increase in thicknessof L100 where it achieves a maximum increase in velocityis only 20% greater than its normal thickness, whereas the(a) additionalwater model predicts that it should be about35% thicker if the hydrate occupies 26%27% of the vol-

ume. For the waterfromhost model, however, the maxi-mum increase in volume in L100 is 2.7% (4.9% if thepossible increase in host matrix velocity is ignored). Fur-thermore, layers L70 and L60 show no obvious thickeningaccompanying the increase in velocity in the core of thechimney. Consequently, we consider that the waterfromhost model, with its lower predictions of hydrate concentra-tion, is likely to represent the predominant mode of hydrateformation, although it is possible that, during the history offormation of the chimney, hydrate was also formed frommethane in solution, giving the hydrate mixed modes offormation.

6.3. Processes and Time Scale for Formation

of the Chimney[51] To evaluate the possible occurrence of carbonate and

gas hydrate in relation to the period of formation of theCNE03 chimney, we consider two scenarios: (1) the chim-ney grew over a long period of time, possibly with morethan one period of activity (gas venting) and inactivity, and(2) the chimney is relatively young and developed rapidlyduring a single gas venting episode, probably at the endof the last glaciation (1916 ka), during a period of fluidexpulsion in this region suggested by Hustoft et al. [2009b].6.3.1. LongPeriod Formation Scenario

[52] In this scenario, the chimney has been active forhundreds of thousands of years and paleoseepage featureshave been preserved through time within the structure of

the chimney. As the base of the zone of high Vp lies nearthe base of the Naust S unit (Figure 9), the minimum periodfor the chimneys formation would be about 350,000 years[Rise et al., 2006]. If the chimney has been active for about350 ka without ceasing its activity completely, or if it hadseveral long periods of activity during this time, we wouldexpect to see some stratigraphic evidence such as onlap ofreflectors against paleoseabed mounds created during theactive periods. However, there is no clear evidence for onlapof reflectors against the flanks of the CNE03 chimney(Figure 9) [Westbrook et al., 2008b].

[53] In a long period of activity scenario, carbonateformed at shallow depth would extend through the strati-graphic range of sediment deposited during the period of the

chimneys activity, and it is likely that carbonate wouldextend beneath the present GHSZ if the chimney had grownover a long period. No anomalous highvelocity materialoccurs beneath the chimney deeper than the presentdayGHSZ (Figure 6).

[54] The lack of evidence for carbonate preservationbetween 80 and 250 mbsf, where the highest Vp anomalyoccurs, and the lack of anomalously high Vp beneath thepresent GHSZ, make it unlikely that the chimney is 350 ka orolder and has sustained activity over that period (Figure 10).On the basis of the data and other evidence available, wecannot, however, exclude the possibility of very intermittent,

short periods of activity, creating a few thin deposits of car-bonate since 350 ka, the age of strata at the presentday baseof the GHSZ.6.3.2. ShortPeriod Formation Scenario

[55] In this scenario, formation of the CNE03 chimneybegan as a vigorous system with free gas entering the GHSZ.The beginning of the period over which chimney formationoccurred and was most active is likely to be governed by

external factors, such as rapid sediment loading in the latterstages of the last glacial leading to overpressured formationsaround 18 ka [Hustoft et al., 2009b].

[56] From the general downward increase of anomalouslyhigh seismic velocity at CNE03 (Figures 6, 7, and 9), higherhydrate concentrations are predicted near the base of theGHSZ. This is consistent with formation of hydrate from theinvasion of the GHSZ by free gas, comparable in its struc-ture to the highflux methane vent feature described byHaacke et al. [2009] offshore Korea. Gas invasion of thelower part of the GHSZ, probably in a series of periodicpulses to allow time for sufficient gas pressure to build upfor each pulse, is the starting point for the model of chimneyformation proposed by Liu and Flemings [2007], in which

the upward migration of gas and the formation of hydrate ispromoted by the increased salinity of pore water caused bythe exclusion of ions from water when it forms hydrate. Themodel byLiu and Flemings[2007] provides a good basis forunderstanding how we believe the CNE03 chimney mayhave formed, with the qualification that the formation offractures and fracture flow [Jain and Juanes, 2009] arelikely to have played a more important role in the formationof CNE03 because the low permeability of the muddyhemipelagic sediment of the Naust units [Rise et al., 2006]is lower than those used in Liu and Flemings models,which were for sand and silt. Consequently, a predominantlyfracturefilling mode of hydrate emplacement appears most

probable, although formation of hydrate locally in the inter-

granular pore space of permeable lithologies will also beexpected.

[57] Later in the development of the chimney, such as atthe present day, when there is very little or no flux of gas-eous methane from the CNE03 pockmark, but evidence ofmethane seepage [Nouz and Fabri, 2007] and shallow gashydrate [Akhmetzhanov et al., 2008], there can be continuedformation of hydrate in the shallow part of the chimney frommethane in solution carried upward by aqueous advection.This is supported by biochemical sampling at differentlocalities within Nyegga. Carbonate at the G11 pockmark(southwest of CNE03) is associated with microseepage oflight hydrocarbon [Hovland et al., 2005]. Sampled pock-marks to the southeast of Nyegga [Mazzini et al., 2005]

and pockmarks located about 12 km west of CNE03 [Paullet al., 2008], appeared to have 13C depleted (decrease in 13Cmolecules) and 18O enriched (increase in 18O molecules)authigenic carbonate. Positive values of 18O indicate con-tribution of 18O enriched water that may be related to gashydrate dissociation [Naehr et al., 2007].

[58] Advecting pore water saturated in methane can formhydrate throughout the depth range of the chimney becausethe solubility of methane in aqueous solution decreases withdecreasing pressure [Xu and Ruppel, 1999; Zatsepina andBuffet, 1997] in the GHSZ. Undersaturated water enteringthe base of the chimney will become saturated by dissolving


13 of 24


14/24

hydrate in the lowest part of the chimney and then formhydrate at shallower depths, effectively transporting hydratefrom the deeper part of the chimney to the shallower part.

[59] The four stages we believe are important to the for-mation of the CNE03 chimney (Figure 10) can be summa-rized as follows:

[60] 1. Vertical migration of gas through polygonal faultsand fractures from deep reservoirs [Berndt et al., 2003;Bnzet al., 2003; Hustoft et al., 2007].

[61] 2. At a shallower level, gas migrates through themore permeableporous units, accumulating in conventionalstratigraphic and structural traps and at the base of the

Figure 10. Schematic representation of the inferred internal structure and formation mechanism of the

CNE03 chimney. In a shortperiod formation scenario, the chimney contains a fracture network wheremainly gas hydrate is currently emplaced. The formation is explained in four steps involving migration

of gas from deep sources and its passage through the GHSZ (see discussion). Two gas rich layers supplythe methane for hydrate formation inside the chimney. Doming of the strata seems to be related to hydrateaccumulation. Carbonates form above the chimney at the seafloor and immediate sediments.


14 of 24


15/24

GHSZ where it lies within permeable units, until it achievesa sufficient pressure to propagate fractures.

[62] 3. When the gas achieves a critical overpressure, itmigrates rapidly into the GHSZ by creating fractures andforming hydrate in veins [Hornbach et al., 2004; Jain andJuanes, 2009; Liu and Flemings, 2007] similar to thoseidentified in pressure cores offshore India and Korea[Schultheiss et al., 2009].

[63] 4. The input of gaseous methane decreases with timeas a consequence of diminution of the process driving gasmigration from deeper, but water with methane in solutioncontinues to migrate upward, forming small amounts ofhydrate throughout the GHSZ and authigenic carbonateclose to the seafloor and supporting chemosynthetic biota[Foucher et al., 2009]. This water is probably saturated inmethane because the layer beneath the chimney contains freegas, but even if it is undersaturated, it will become saturatedin methane by dissolving a proportion of the hydrate that itpasses through in the lower part of the chimney.

[64] The contribution to the chimney formation frommethane released by hydrate dissociation caused by postglacial climate change is probably negligible or very minor

because in the water depths greater than 700 m in this region,where the current seabed temperature is about 0.5C[Nouz and Fabri , 2007], the effect of sea level rise com-pensate d the effect of seabed temperature increase upon thethickness of the GHSZ [e.g., Mienert et al., 2005].

7. Conclusions

[65] A highresolution seismic tomographic experimenthas defined the main elements of the internal structure of afluidescape chimney and provided evidence for the pres-ence of gas hydrate within it:

[66] 1. The core of the CNE03 chimney has a zone ofanomalously high Pwave velocity 500 m in diameter at its

base and 200 m in diameter near the seabed that is restrictedto the GHSZ. Vp increases laterally toward the center of thechimney. Beneath the GHSZ, the velocity distribution ineach layer is homogenous with no major lateral changes invelocity.

[67] 2. The 230 m depth extent of the highvelocity zoneand the absence of a highvelocity zone beneath the GHSZ,extending downward from the chimney, make it likely thatthe presence of hydrate is the primary cause of the high velocity zone.

[68] 3. The seafloor and layers within the GHSZ aredomed over the center of the chimney, in contrast to theplanar strata surrounding and beneath the chimney. Strataldeformation beneath the GHSZ is minor. It appears that the

emplacement of hydrate into veins and fractures has, insome way, created the updoming, which is not inheritedfrom a deeper structure.

[69] 4. We suggest a predominantly fracturefilling modelis appropriate for the formation of hydrate in the finegrained sediments in which the chimney occurs. The hydrateconcentration calculated from anomalous Vp values is later-ally and vertically heterogeneous. The highest hydrate con-centration in the chimney is near the base of the GHSZ andmay locally be as high as 27% of the total volume. However,it is likely that the maximum hydrate concentration is muchless than 27% and may not exceed 14% of the total volume.

[70] We infer that the chimney is likely to have originatedas a vigorous gas venting system that generated a fracturenetwork in which hydrate formed, primarily in the lowerpart of the chimney. Two of the layers beneath the GHSZ,each of which have aPwave velocity that is less than that ofthe overlying unit and is laterally homogeneous, are inferredlevels for local gas migration and accumulation. At present,with low rates of methane seepage methane in solution from

the seabed, the chimney is likely to be dominated by theformation of hydrate from methane in aqueous solution.If the upwardly migrating pore water entering the base ofthe chimney is undersaturated in methane, dissolution ofhydrate within the chimney will bring the methane insolution to saturation, some of which will be released toform hydrate at shallower depths where methane solubilityis lower.

[71] The chimney has provided, through the formation ofhydrate within it, a reservoir of methane that is apparentlybeing released slowly at present, supporting chemosyntheticcommunities at the seabed and the formation of authigeniccarbonate in the shallow sediment beneath. The creation ofhydrate during the early stages of the chimneys formation,

which will have reduced the amount of gas reaching theseabed, and hydrate dissolution within the chimney, whichmaintains a supply of methane to the surface when the inputof methane from beneath the chimney has been reduced, actas a buffer on the supply of methane in solution to theseabed. If this is representative of this kind of chimney ingeneral, it appears that chimneys have an important role inbuffering, as well as enabling, the escape of methane fromsources beneath the GHSZ.

Appendix A: Principle of Tomography Software

[72] Travel time tomography aims at determining the

velocity model m that satisfies the travel times t

obs

pickedfrom the seismic data. This means that model m must satisfy

tobs t m ; A1

wheretis the physical law that links the data space and theparameter space.

[73] As mentioned in section 4.2, the tomography softwareis based on a blocky velocity representation of the subsurface.The modelm is thus made as follows:

mv

Z

0@

1A; A2

whereZ is the vector containing the mathematical represen-tation of each interface and vectorv contains the mathemat-ical representation of the velocity law of each velocity block.Since velocities are assumed vertically invariant within alayer for the model used for the CNE03 inversion, velocitylaws for each block are limited to lateral velocity variations.

[74] The travel times supplied to the tomographic inver-sion are generally corrupted by errors, such as those fromthe finite frequency spectrum of seismic data, errors intemporal sampling, and interference of primary reflectionswith other seismic events (multiples, converted modes,surface waves, diffractions, etc). It is assumed that the travel


15 of 24


16/24

time errors have a Gaussian distribution, and a leastsquaresformulation is used to solve equation A1. Also, the inverseproblem of travel time tomography is generally ill posed.Only a discrete number of traces are recorded, and the raysfrom which the travel times are taken may not have sampledreflectors regularly or continuously, and in any case, will havea limited incidenceangle range. To stabilize the inversion, aprioriinformation about the expectedmodel may be introduced

and a regularized leastsquares problem employed. As shownbyDelpratJannaud and Lailly [1993], a priori information onthe second derivatives of the model is necessary and sufficientto obtain a mathematically wellposed problem.

[75] Hence, the travel time tomography implementationthat we used aims at determining the velocity model m thatbest satisfies the interpreted travel times tobs and whosediscrepancy with a priori model mprior is characterized by asmall roughness. Equation A1 is, therefore, reformulated asthe minimization of the following cost function:

C m XNTi1

2i ti m tobsi

2

XNZi1

ZW

"2i;Z x;y D2 Zi x;y Zi x;y prior

2

dxdy

XNVi1

Z

W

"2i;v x;y D2 vi0 x;y vi0 x;y prior

2dxdy;

A3

where NT is the number of travel times, siis the uncertaintyof the observed travel times ti

obs (i = 1, , NT), NZ is thenumber of interfaces of the model, ei,z is the regularizationweight on the roughness of interface Zi with respect to apriori interface Zprior

i (i = 1, , NZ), NV is the number oflateral velocity laws, ei,vis the regularization weight on theroughness of the lateral velocity law v

0

i with respect to apriori lateral velocity law v0prior

i (i = 1, , NV), k k standsfor the L2 norm, D2 is the total differential with respect tospatial coordinates of second order, and W is the modeldefinition domain.

[76] Since the physical law tused to estimate calculateddata for a given model m is nonlinear, the cost functiondescribed in equation A3 is nonquadratic. This nonlinearproblem is solved iteratively with the GaussNewton method.For each iteration, the travel time function t is linearizedaround the current model. A model perturbation that mini-mizes the resulting quadratic cost function is computed (witha conjugate gradient algorithm) and used to update the currentmodel. However, in some cases, the line search method is

not able to ensure a satisfactory convergence. In order toimprove the stability of the inversion process and accuracyof the inversion results, atrust regionhas been introduced[Sebudandi and Toint, 1993;Delbos et al., 2001].

[77] Solving equation A3 requires defining regularizationweights for each inverted lateral velocity law and eachinverted interface. An inappropriate choice of those weightsmay have a strong impact on the solution of the inverseproblem. Indeed if strong regularization weights are used,the regularization term will be predominant, and the solutionmodel will not satisfy the travel time information sufficientlywell. On the other hand, in case regularization weights that

are too small, the inversion problem will become ill condi-tioned and the inversion process will become unstable. Toovercome those difficulties, tomography software imple-ments an approach proposed by Renard and Lailly [1999].This method consists of obtaining a first solution of theinverse problem by starting with strong regularization weights.Then, every p iteration of the GaussNewton process, theregularization weights are decreased, by dividing by a factor

t. This procedure is repeated until satisfactory travel timeresiduals are obtained that are not reduced further by lowerregularization weights.

[78] In addition to classical regularization, informationabout the expected model may also be introduced as linearequality or inequality constraints. These constraints maycorrespond to geological or geophysical knowledge includingwell data, for example, by preventing nonphysical solutionsand ensuring that interfaces do not cross each other. Theconstraints are generally used for managing expected velocityvalues or reflector depth, as well as their firstorder andsecondorder derivatives (reflector dips, velocity gradients,reflector roughness, and velocity variations). The constraintscan be applied globally or locally for a specific area. The

leastsquare problem under constraints is solved by theoptimization algorithm Sequential Quadratic ProgrammingAugmented Lagrangian developed at IFP [Del bos et al. ,2006].

Appendix B: Analysis of Residuals and ModelAssessment

B1. Residual Travel Times

[79] The distribution of travel time residuals and RMSvalues of residuals are used as a measure of the improve-ment in the travel times predicted by the models after eachiteration. The residuals are presented at the location of theimpact points (IPs) over each interface in the model(Figure B1). In general, the density of impact points is poor

at the west half of the chimney.[80] High positive travel time residuals (Tcalculated Tobserved)

indicate model velocities that are too low or thicknesses thatare too great. Most of the residuals have values between 2and 2 ms and are distributed around zero, indicating that theinversion has been optimized (Figure B2). The magnitudes ofresiduals are reduced to less than 2 ms at the flat parts of thelayers. Slightly higher residuals (e.g., 5 and 5 ms) remain atthe eastern flank within the upper 3 layers (Figure B1).

[81] The fact that high negative and high positiveresiduals are observed at the same location suggests thatfor OBSs recording at different sides of the chimney, thevelocity is overestimated or underestimated depending onthe direction of the raypaths. This may be related to the

parameterization and regularization of the model (resolutionof the experiment). Since the cell size is 40 m, velocitieswithin 40 m (between two parameters in a layer) will beinterpolated according to the regularization used for inver-sion. Impact points of rays with travel times recordingpassage through faster material might not be optimized ifthey are close to a parameter node where lower velocities(than the average velocity of their path) have been interpo-lated due to the dominancy of travel times of rays that do notpass through the faster material (Figure B3). On the otherhand, travel times from rays with impact points close toparameter nodes with higher velocities may lead to over-


16 of 24


17/24

Figure B1. Residual distribution displayed at the locations of the impact points (IPs) for reflected arri-vals recorded by OBS, shown in the top views of the velocity distribution for all the layers in the tomog-raphy model. A variable decimation factor (between 50 and 200, depending on the number of observedtravel times at each layer) for the number of displayed IP, has been used to allow the visualization of thevelocity model.Xand Yscales are in kilometers. The velocity scales are in kilometers per second, and theresiduals are in milliseconds.


17 of 24


18/24

estimation of the velocity. At the depth of reflector H70(900 m), a 20 m/s velocity decrease/increase representstravel time differences of over 4 ms. This is consistentwith the range of confidence that we find from syntheticmodeling of H70.

[82] Residuals of reflectors H100H160 appear lessoptimized (remaining residuals > 6 ms) because of theuncertainties introduced by an incomplete modeling ofinterface H100. The uncertainties are inherited by deeper

events. The RMS values increase from about 1 ms afterinversion for the shallower reflectors to about 3 ms for thedeeper ones (Table B1). The number of travel times fromthe seismic streamer data is between 9000 and 16,000 forall the layers. The number of OBS data travel times foreach reflector included in the inversion is more variable.H70 and H100 have the highest number of observed traveltimes: 166,099 and 219,000, respectively. H60 and H120have the lowest number of available observed data: 92,700and 95,657, respectively (Table B1).

B2. Resolution and Uncertainty Tests

[83] In order to avoid misinterpretation of the structuresappearing in the tomographic model, the lateral resolution ofthe velocity variations must be known [Zelt, 1998]. At thescale of our investigation, the velocity and interface depthdo not show significant heterogeneity in flat and homoge-neous parts of the model outside the chimney. Velocity valuesand interface depths are more heterogeneous inside thechimney and in the close vicinity of the chimney flanks. We

have adapted the checkerboard test [Leveque et al., 1993;Schmelzbach et al., 2008; Zelt, 1998] to investigate the pos-sible resolution that is allowed by the parameterization andregularization chosen for our model.

[84] Three checkerboards were created for layer L70 at880 m depth (Figure B4), with square sizes of one cell (40 40 m), four cells (80 80 m), and nine cells (120 120 m).The velocity functions in the model for each square werealternately modified by plus or minus 5%, and the modifiedmodel was used to calculate synthetic travel times. Thesesynthetic travel times were then tomographically inverted to

Figure B2. Histograms with the residual distribution (Tcalculated Tobserved) after tomographic inversionfor each reflector in the model.


18 of 24


19/24

test how well the checkerboard pattern in the modifiedmodel was retrieved.

[85] The checkerboard pattern at 120 m resolution isretrieved well, with differences between the retrieved andexpected velocity of 20 m/s (1% of the velocities between1500 and 2000 m/s) if a reliable number of travel times isavailable; areas with only a few impact points are still imagedbut with velocity differences up to 50 m/s (Figure B4). Thepattern with 80 m resolution (equivalent to four cells) is stillresolved where there are sufficient travel times, with differ-ences in velocity of 1%2%. Where the number of travel

times is low, the velocity differences can be up to 5% at thisresolution (Figure B4). The pattern with a resolution of 40 m(one cell size) is barely retrieved. The B spline function doesnot accommodate the spatial frequency of velocity variationat 40 m.

[86] The effect of adding random noise with a 1 msstandard deviation to the synthetic data was evaluated for 80and 120 m resolution at the depth of H70 (Figure B5). At120 m resolution, seismically wellilluminated areas are notsignificantly affected. The velocities are retrieved with1.5%differences with respect to the expected velocities. At 80 mresolution, the match between expected and retrieved pat-terns is affected slightly, with differences up to 3% in wellilluminated areas, but the differences can be higher than 5%

in areas with a poor impact point density (Figure B5). Testingwith a different pseudorandom noise series, still with a 1 msstandard deviation, shows very similar results (Figure B5c).The effect of adding random noise is also shown in themagnitude of the misfit. Residuals, being all around zero inthe synthetic inversion without noise, are up to 3 ms aftersynthetic inversion of travel times with noise. The histogramshowing the distribution of residuals (Figure B5d) is com-parable with the histogram after inversion of H70 (Figure B2,H70).

[87] A second test evaluated the differences between theresulting and expected velocity and interface depth func-

tions without the effect of missing travel time picks, whichreduce the illumination. A synthetic inversion was run forH70 with travel times for all the sourcereceiver pairs. Thesynthetic travel times were calculated from the resultingH70 model. The differences between the expected andresulting interface and velocity functions were less than 1 mand 10 m/s, respectively, in the flat wellconstrained areas(Figure B6). In a layer of 30 m thickness with seismicvelocities around 1800 m/s, these differences represent 3%of thickness and less than 1% of velocity. They increase toup to 1.4 m (5%) and 25 m/s (1.5%), respectively, at the center

of the chimney where the interface is slightly more complex(relatively steep flanks and a relief of 20 m) causingdeviation of the rays and therefore decreasing the resolutionin shadow areas. After the addition of random noise, theresiduals are up to 4 ms, which is comparable to the misfitfor our resulting H70 model (Figures B6c, B16f, B2, H70).

Appendix C: Calculation of Hydrate ConcentrationFrom Changes in PWave Velocity

[88] In both cases used to estimate hydrate concentration,the hydrate is assumed to occupy fractures or veins cuttingthrough the host sediment.

Figure B3. Diagram to show the relationship between the raypath and the location of its impact pointrespect to the minimum or maximum of the Vp Bspline function within cells.

Table B1. List of RMS Values, Number of Ocean Bottom

Seismometer Travel Times, and Number of Streamer Travel

Times Used for Inversion of Each Layer in the Resulting Model

Layer RMS (ms) No. of OBS Rays No. of Streamer Rays

L30 1.02 156,309 10,715L60 1.06 92,700 15,912L70 1.925 166,099 15,857L100 2.275 219,000 14,820L120 2.15 95,657 9709L150 3.022 164,957 14,619L160 2.947 123,909 12,576


19 of 24


20/24

[89] 1. Hydrate is an addition to the host sediment, so themixture is between hydrate and unaltered host. The velocityof the mixture is a time average between that of the host andthat of hydrate, dependent on the fraction of the volumeoccupied by each.

1=vmix hyd=vhyd 1hyd=vhost; C1

where vhyd is the acoustic velocity through hydrate, vhost isthe velocity through the host sediment where it is unaffected

by hydrate formation,vmix is the velocity through the mix-ture of host and hydrate in veins, andhydis the fraction ofthe mixture that is hydrate.

[90] Hence, the fraction occupied by hydrate filled veins,

hyd 1=vmix1=vhost = 1=vhyd1=vhost

: C2

[91] 2. If only free gas is introduced into the veins, thewater to form hydrate must come from the pores of the host,

Figure B4. Uncertainty test: checker board principle for reflector H70. Three Vp lateral perturbationswere tried: (a) 40 (one cell size), (b) 80 (four cell size), and (c) 120 m (nine cell size). (d, e, and f) Thecalculated models are display next to the expected models for comparison. In Figures B4aB4f, the darkand light color squares represent a Vp perturbation of +5% and 5%, respectively. (g, h, and i) The dif-ference Vpexpected Vpcalculated is illustrating the range of uncertainty (nonuniqueness) of the calcu-lated models.


20 of 24


21/24

reducing the water content and porosity of the host. Thevelocity through the host is changed by the reduction inporosity caused by the withdrawal of pore water to create

hydrate in the veins. If it is assumed that the velocity throughthe host matrix is unaltered, the timeaverage velocity of themixture of hydratefilled veins and the altered host sedimentis given by

1=vmix hyd=vhyd ifhyd

=vwater

1i=vmatrix=1 1fhyd; C3

wherei is the initial porosity of the host sediment, fis thefraction of a unit volume of water required to form a unitvolume of hydrate and has the value 0.80 (assuming thatwater forms 87% of the mass of hydrate with a density of

920 kg/m3), vwateris the velocity through pore water, andvmatrix is the velocity through the matrix of the host. Thedenominator in equation C3 normalizes the proportions in

the mixture to account forthe volume increase caused by thetransformation of water into hydrate. Here vmatrix is derived byrearranging the timeaverage equation for the host with nohydrate present, 1/vhost= i/vwater+ (1i)/vmatrix. Hence,

vmatrix 1 i=1=vhost i=vwater: C4

So, by substituting the identity forvmatrixfrom (C4) into (C3)and rearranging terms,

hyd 1=vmix1=vhost=1=vhyd f=vwater 1 f=vmix:

C5

Figure B5. Top view of the velocity differences after synthetic modeling after addition of random noisefor H70. Results for (a) four cell size and (b) nine cell size velocity perturbation are shown. The randomnoise has a Gaussian distribution of 1 ms standard deviation. (c) A second random distribution of thenoise was tested for 4 cell size velocity perturbation. (d) The histogram of the residuals after syntheticinversion of noisy travel times can be compared with the histogram of the residuals of the resulting model(Figure B2, L70).


21 of 24


22/24

It is not necessary to know the initial porosity of the host toestimate hydrate content. However, it is unlikely that theelastic moduli of the matrix are unaffected by the reduction inporosity and compaction resulting from water loss. To includethe effect on the matrix velocity of a change in porosity in ourestimate of the change in the velocity of the host, we use the

velocitydensity relationship for marine terrigenous sedimentofHamilton[1978], assuming a grain density of 2700 kg/m3.

[92] The initial porosity of the host is given by

i 28901:135vhost=1700; C6

with the velocity vhostgiven in meters per second.

Figure B6. Uncertainty test 2: synthetic modeling completing the missing travel times for H70. (a and b)

Top views show the velocity and interface depth differences after synthetic modeling. (d and e) The samerandom noise as for test 1 has been added to compare with the synthetics without noise. (c and f) Noticethe changes in magnitude and distribution of the residuals after adding noise to the synthetic travel timesby comparing histograms with and without noise.


22 of 24


23/24

[93] The altered velocity of the host is then

vmod 2890=1:1351700i fhyd=1:135; C7

with

hyd 1=vmix1=vmod=1=vhyd 1=vmod: C8

The fraction of volume filled by hydrate hyd was solvediteratively by changing the value ofhyd in equation (C7)until it was within 107 of the value ofhyd yielded byequation (C8).

[94] Acknowledgments. This work was supported by the EuropeanCommission FP6 project HERMES (GOCECT2005511234) throughcontracts with Birmingham University, the Institut Franais de Recherche

pour LExploitation de la Mer (IFREMER), and the National OceanographyCentre (NOC), Southampton, by the Norwegian Research Council and Sta-toilHydro Petromaks projects (169514/S30 and 175969/S30) contracts withTroms University, and by StatoilHydro through a contract with Birming-ham University. The research of Andreia PlazaFaverola at IFREMER wasmade possible under the Memorandum of Understanding (MOU Ref. 05/1215838) between IFREMER and the Department of Geology, Universityof Troms. The data were collected during Leg 3 of TrainingThroughResearch Cruise 16 of the Professor Logachev in June 2006, and our thanksgoto Michael Ivanov, as cochief scientistof thecruise,for hisassistanceand

encouragement. Herv Nouz, formerly of IFREMER, took a major part inthe planning of the project and the acquisition of the data. The OBSs were

provided by the UK Ocean Bottom Instrumentation Consortium and IFRE-MER. Wes Wilson of PGC is thanked for his assistance with the installationand testing of the seismic tomography software package, and Tesmi Jose of

NOC, Southampton, for her assistance with relocation of the OBSs. MartinHovland provided encouragement and support for the creation of the project.We also thank the reviewers Ingo Pecher and Henrik Svensen and associateeditor Bill Waite for their suggestions for improvement of the manuscript.

ReferencesAkhmetzhanov, A. M., N. H. Kenyon, M. K. Ivanov, G. Westbrook, and

A. Mazzini (2008), Deepwater depositional systems and cold seeps ofthe Western Mediterranean, Gulf of Cadiz and Norwegian continentalmargins, IOC Technical Series 76, UNESCO.

Berg, K., A. Solheim, and P. Bryn (2005), The Pleistocene to recent geo-logical development of the Ormen Lange area,Mar. Petrol. Geol., 22(12),4556.

Berndt, C., S. Bunz, and J. Mienert (2003), Polygonal fault systems on themidNorwegian margin: a longterm source for fluid flow, SubsurfaceSediment Mobilization, 216, 283290.

Bouriak, S., M. Vanneste, and A. Saoutkine (2000), Inferred gas hydratesand clay diapirs near the Storegga Slideon thesouthernedge of the VoringPlateau, offshore Norway,Mar. Geol.,163(14), 125148.

Bouriak, S., A. Volkonskaia, and V. Galaktionov (2003), Split stratabounded gas hydrate BSR below deposits of the Storegga Slide and atthe southern edge of the Voring Plateau,Mar. Geol.,195(14), 301318.

Brekke, H. (2000), The tectonic evolution of the Norwegian Sea Continen-tal Margin with emphasis on the Voring and More Basins, in Dynamicsof the Norwegian Margin, pp. 327378, Geol. Soc. Publ. House, Bath,UK.

Broto, K., A. Ehinger, J. Kommedal, and P. Folstad (2003), Anisotropictraveltime tomography for depth consistent imaging of PP and PS data,

Leading Edge, February.Bnz, S., and J. Mienert (2004), Acoustic imaging of gas hydrate and free

gas at the Storegga Slide, J. Geophys. Res., 109, B04102, doi:10.1029/2003JB002863.

Bnz, S., J. Mienert, and C. Berndt (2003), Geological controls on theStoregga gas hydrate system of the midNorwegian continental margin,

Earth Planet. Sci. Lett., 209(34), 291307.Bnz, S., J. Mienert, M. Vanneste, and K. Andreassen (2005), Gas hydrates

at the Storegga Slide: Constraints from an analysis of multicomponent,wideangle seismic data, Geophysics, 70(5), B19B34.

Chand, S., T. A Minshull, D. Gei, and J. M Carcione (2004), Elastic veloc-ity models for gas hydratebearing sediments: A comparison,Geophys.

J. Int., 159(2), 573590.Chand, S., T. A. Minshull, J. A. Priest, A. I. Best, C. R. I. Clayton, and

W. F. Waite (2006), An effective medium inversion algorithm for gashydrate quantification and its application to laboratory and borehole mea-

surements of gas hydratebearing sediments, Geophys. J. Int., 166(2),543552.

Chand, S., L. Rise, D. Ottesen, M. F. J. Dolan, V. Bellec, and R. Boe(2009), Pockmarklike depressions near the Goliat hydrocarbon field,Barents Sea: Morphology and genesis, Mar. Petrol. Geol., 26(7),10351042.

Clarke, R. A. (1996), Ray tracing and tomography friendly geologicalmodel, KIM 1996 Annual Report, Institut Franais du Ptrole, Pau,France. (http://consortium.ifp.fr/KIM/)

Delbos, F., D. Sinoquet, J. Ch. Gilbert, and R. Masson (2001), Trustregion

GaussNewton method for reflect ion tomography, KIM 2001 AnnualReport, Institut Franais du Ptrole, Rueil, France.

Delbos, F., J. C. Gilbert, R. Glowinski, and D. Sinoquet (2006), Constrainedoptimization in reflection tomography: an GaussNewton augment edLagrangian method,Geophys. J. Int.,164, 670684.

DelpratJannaud, F., and P. Lailly (1993), Illposed and wellposed formu-lations of the reflection travel time tomography problem, J. Geophys.

Res., 98, 65896605.Foucher,J. P.,G. K.Westbrook, A.Boetius,S. Ceramicola,S. Dupre, J. Mascle,

J. Mienert, O. Pfannkuche, C. Pierre, and D. Praeg (2009), Structure andDrivers of Cold Seep Ecosystems, Oceanography,22(1), 92109.

Ehinger, A., K. Broto, and A. Jardin (2001) 3D tomographic velocitymodel determination for two North Sea case studies, 63rd Ann. Internat.Mtg., EAGE, Expanded Abstract.

Gmez, M., and J. Vergs (2005), Quantifying the contribution of tectonicsvs. differential compaction in the development of domes along the mid

Norwegian Atlantic margin,Basin Res., 17(2), 289310.Hamilton, E. L. (1978), Sound velocitydensity relations in sea floor

sediments and rocks, J. Acoust. Soc. Am., 63, 366.Helgerud, M. B.,J. Dvorkin,A. Nur, A. Sakai,and T. Collett (1999), Elasticwave velocity in marine sediments with gas hydrates: Effective mediummodeling,Geophys. Res. Lett.,26(13), 20212024.

Hjelstuen, B. O., O. Eldholm, and J. Skogseid (1999), Cenozoic evolu-tion of the northern Voring margin, Geol. Soc. Am. Bull., 1 11(12),17921807.

Hjelstuen, B. O., H. P. Sejrup, H. Haflidason, A. Nygard, S. Ceramicola,and P. Bryn (2005), Late Cenozoic glacial history and evolution of theStoregga Slide area and adjacent slide flank regions, Norwegian conti-nental margin, Mar. Petrol. Geol., 22(12), 5769.

Hornbach, M. J., D. M. Saffer, and W. S. Holbrook (2004), Critically pres-sured freegas reservoirs below gas hydrate provinces,Nature,427(6970),142144.

Hovland, M., and H. Svensen (2006), Submarine pingoes: Indicators ofshallow gas hydrates in a pockmark at Nyegga, Norwegian Sea, Mar.Geol., 228(14), 1523.

Hovland, M., H. Sve

Date post:	03-Jun-2018
Category:	Documents
Upload:	sriram-gullapalli
View:	218 times
Download:	0 times

Evidence From Three‐Dimensional Seismic Tomography for a Substantial Accumulation of Gas Hydrate...

Documents