6th International Conference on Gas Hydrates


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3-D TRAVEL TIME TOMOGRAPHY INVERSION FOR GAS HYDRATE DISTRIBUTION FROM OCEAN BOTTOM SEISMOMETER DATA Mykhail M. Zykov, N. Ross Chapman and G. D. Spence School of Earth and Ocean Sciences University of Victoria 3800 Finnerty Rd, Victoria, BC, V8W 3P6 CANADA ABSTRACT This paper presents results of a seismic tomography experiment carried out at the Bullseye cold vent site offshore Vancouver Island. In the experiment, a seismic air gun survey was recorded on an array of five ocean bottom seismometers (OBS) deployed around the vent. The locations of the shots and the OBSs were determined to high accuracy by an inversion based on the shot travel times. A three-dimensional tomographic inversion was then carried out to determine the velocity structure around the vent, using the localized source and receiver positions. The inversion indicates a relatively uniform velocity field around and inside the vent. The velocities are close to the values expected for sediments containing no hydrate, which supports previous claims that the bulk concentrations of gas hydrates are low at the site. However, the largest resolved velocity anomalies of + 25 m/s are spatially within the limits of the acoustic blank zone seen in multichannel seismic data near the Bullseye vent. The velocity inversion is consistent with zones of 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.ca INTRODUCTION This paper presents results from a seismic study using Ocean Bottom Seismometers (OBS) carried out at the Bullseye vent site located on the accretionary margin of the northern Cascadia subduction zone offshore Vancouver Island, Canada. The major objective was to develop a 3-D velocity model around the Bullseye vent zone by means of travel time inversion tomography in order to determine the distribution and quantity of hydrates.  The area has received close attention after seismic data showed acoustic blanking features [1,2], and the recovery of massive hydrate from the vent site. The site was the target of numerous multi-  and single-channel [3-5] seismic surveys and various other experiments including piston coring, electromagnetic studies, and scientific drilling (Ocean Drilling Program (ODP) Leg 146 in 1993 and Integrated Ocean Drilling Program (IODP) Expedition 311) [6,7].  The blank zones in the vicinity of the Bullseye vent (blank zone 1) are shown in Figure 1.  The OBS data set consisted of 22 parallel seismic survey lines at 200 m spacing with three perpendicular crossing lines, recorded on five OBS stations. The OBS experimental geometry required corrections for the coordinates of sources and receivers initially obtained in the field. A new comprehensive source and receiver localization technique was developed for the case of stationary hydrophones and multiple seismic lines [8].  The inversion for the experimental geometry was done in a separate travel time inversion. A simulation study was carried out first to determine the resolution of the OBS array, using two possible models for the hydrate distribution in the vent zone.  The inversion results for the experimental data indicate a uniform velocity field around and inside the vent zone. Velocities are Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. approximately equal to values expected for sediments containing no hydrate, which supports the hypothesis that the bulk concentrations of gas hydrates are low at the site. The largest velocity anomaly, with velocity increase of +25 m/s, is spatially associated with the limits of the blank zone. The anomaly suggests greater hydrate concentrations inside the vent zone than outside. Low vertical resolution of the model limited the information on the detailed structure of the depth distribution of the hydrate. However, the results are consistent with a zone (or perhaps multiple zones) of high hydrate concentration (15–20% of the pore space) associated with a hydrate lens, located at the top of the sediment section. Figure 1.  Acoustic blanking from high resolution seismic survey with the high resolution DTAGS array (Gettrust et al. [9]). The paper is organized as follows.  The OBS experiment is described in the next section.  The travel time inversion method is then outlined, and the inversion results are presented and discussed. OBS TOMOGRAPHY EXPERIMENT Local lithological structure The location of the experiment is shown in Figure 2.  In the upper 300 m of the sediment section two distinct lithological units can be identified at the site using the 2-D and 3-D seismic data [3,4]. The first unit is characterized by slope sediments which display well-defined parallel bedding. It can be correlated with Unit I at the ODP 889 drill site [6]. The second unit with almost complete loss of coherency of the seismic signal can be attributed to the Unit III at the drill site. The lower unit represents accreted sediments which are more consolidated, but highly fractured compared to the undisturbed sediments of the upper unit [6]. The upper boundary of the accreted sediment complex is highly variable in terms of its depth below the seafloor. It is possible to outline three small well expressed ridge structures within the limits of the modelled area [3.4] (Figure 2). The slope sediments were deposited on top of the accreted sediments. The bedding of the unit is undisturbed and almost parallel at the top of the section. The thickness of the unit mostly depends on the configuration of the surface of the accreted sediment complex, rather than bathymetry. The thickness is greatest for the area associated with the depression between the two accreted sediment ridges and may reach 300 m. The thickness of the slope sediments unit can be as small as 50 m in the northern corner of the modelled area. Figure 2.  Locations of the five OBS (open circles) relative to the bathymetry and the ODP 146 drill sites.  The box indicates the limits of the inversion area.  The Bullseye vent is the light-shaded region near the centre of the OBS array, and the hatched areas represent buried ridges of accreted sediment. Seismic survey The seismic data were obtained during the COAMS-99 cruise that took place from July 26 until August 11 of 1999 [3].  Five OBS were deployed in the vicinity of the Bullseye vent site as indicated in Figure 2.  The OBS array formed a square with sides 1.4 km and one at the centre. The seismic sound source was a single 40 in 3 (0.655 l) air gun that used a wave-shape kit.  The air gun depth was ~2 m, and it was triggered to fire every 19 m along the survey lines. The OBS array recorded 22 10-km lines spaced at 100 m spacing from a pseudo 3-D survey at the site, and three cross lines as shown in Figure 3. Single channel normal incidence seismic data were also recorded along the same survey tracks, and these data were used with the OBS data in the inversion. The OBSs were obtained from the Dalhousie Seismic Group, Dalhousie University.  The seismometer sensing system consisted of one pressure sensor (hydrophone) and a 3-component geophone package. The analog signals from the four sensors were filtered using an anti-alias filter with corner frequency of 200 Hz, and then were digitized with 1.433 ms sampling interval (698.8 Hz) and 16-bit dynamic range. The active operational period with the system parameters used was limited by the storage capacity to approximately 2.5 days.  Only the hydrophone data were used in the inversion. The recorded seismic data were tied to an absolute timescale using a precise clock with an expected time drift rate less than 1 ms per day. However, the actual average drift for the internal timing systems was up to 10 ms per day, and the clock drift rates for stations A and F were not steady. Inversion of experimental geometry The source-receiver geometry was first refined from the nominal estimates from the survey using a regularized linear inversion based on travel times of the direct path arrivals at the OBS array [10,11]. A special procedure that assumed linear drift segments was developed so that information from the two sensors with faulty clocks could be included in the inversion [8].  The RMS (root mean square) change of shot positions in the inversion was ~ 6.7 m, and the mismatch of OBS depths was less than 0.1 %.  The inverted geometry was used to define the shot and OBS positions in the tomographic velocity inversion. 3-D TOMOGRAPHIC INVERSION Inversion method The Jive3D tomographic inversion code was applied [12,13].  Jive3D uses a linearized iterative approach for the travel time inversion that allows combined inversion of refractions and reflections. The inversion is implemented in a layer-interface formalism.  The cell size of a gridded model of the sub-bottom structure is allowed to vary from layer to layer, which provides flexibility in choosing the number of cells required for simulating different layers. The final model that provides a satisfactory fit to the data is obtained through a series of refinements of a starting model. Figure 3.  Survey track lines that were recorded by the OBS array. Jive3D produces a minimum-structure model that fits the given travel time data to a required accuracy using the simplest model possible for given input parameters. The desired simplicity of the model (amount of structure) is controlled by a smoothing factor, which is a trade-off parameter between the accuracy of fitting of the input data and the smoothness of the generated model. The model that was constructed for this inversion consisted of 5 layers and 5 interfaces. The top layer (or, strictly speaking, half-space) represented the water column, and the top interface simulated the sea bottom. The bottom of the model or the deepest interface represented the Bottom Simulating Reflector (BSR). There was no possibility to model any deeper layers, as there were no continuous deeper reflectors in the data throughout the modelled area. Travel time data The travel times of rays were obtained using single channel seismic data (normal incidence) and data recorded by OBSs.  The following events were chosen; the events are described in terms of two way travel time relative to the seafloor reflection for the single channel data near OBS B): Event 1: 90 ms Event 2: 180 ms Event 3: 235 ms Event 4: 280 ms (BSR) Event #3 was not traceable in the vicinity of the OBS C, because the layered structure of the slope sediments is strongly disturbed there and the accreted sediments package lies closer to the sea surface. All other events were observed (although sometimes weakly) in all seismic sections on both single channel and OBS data. The chosen events were traced from one survey line to another using the cross lines. The events for picking on the seismic data recorded by OBSs were identified with the help of the single channel seismic data, as indicated in Figure 4. The seismic sections were aligned along the same shot trace (with the minimum offset for the OBS) such that the seafloor reflection for the normal incidence data coincided with the direct water wave arrival on the OBS section, and then compared. In general, the uncertainty in the arrival time picks was different for each event, and for each type of data.  The uncertainties for each event (in ms) listed in Table 1. Event OBS Single Channel 1 0.75 3.0 2 1.5 3.0 3 2.5 3.0 4 3.0 4.5 Table 1.  Uncertainties (in ms) for arrival time picks of the OBS and single channel data. Figure 4.  Example of the seismic sections aligned for picking.  The middle panel represents single channel normal incidence data, and the left and right panels are OBS B and F, respectively.  Time is two-way travel time. Inversion model The modelled area is 3 km x 2.7 km with OBS B at approximately the centre (Figure 5). The vertical span of the model is from -1220 m (the highest seafloor point) to -1620 m (the lowest point of the BSR). The thickness of the inverted portion varied according to the depth of the BSR below the seafloor and was about 220–240 m. The cell size for the interface grid was 50 m x 50 m and for the inverted velocity grids 50 m x 50 m x 20 m, so the horizontal dimensions of the model were 60 x 54 cells with about 20 cells in vertical direction. With the chosen cell size the vent zone (600 m x 400 m in horizontal dimensions) is represented by 12 x 8 cells. The starting model for the velocities was based on a simple velocity gradient in the sediment, assumed to be ~ 1 s -1  with a velocity of 1500 m/s at the sea floor.  There were no specific anomalies introduced in the profile. Inversion of synthetic data An inversion was carried out using synthetic data to investigate the resolution performance of the OBS array in resolving the distribution of hydrate in the vicinity of the cold vent.  Two probable models of hydrate distribution were selected, based on information of hydrate distribution from previous experiments. The first model, (A), assumed increased velocity uniformly within the whole volume of the vent zone. To model this situation, a positive velocity anomaly with a magnitude of 30 m/s was introduced to the background velocities within the vent zone over the whole vertical span of the modelled sediment section. Such a small value for the increase does not necessarily represent the actual velocity increase due to anomalous gas hydrate presence, but was chosen mainly to test the method sensitivity.  The other hypothesis (B) assumed a localized volume of sediments (15–20 m in thickness) with significantly higher P-wave velocities. This body was located close to the sea surface and did not extend deeper; that is, the velocity beneath it was the same as the velocities outside of the vent zone. This model is consistent with the hydrate cap model.  The two physical models are shown in Figure 6. Figure 5.  Gridded model of the Bullseye vent environment for the Jive3D inversion. The results of the inversion of the synthetic travel time data set are shown in the right panel of Figure 7. The test showed that the inversion was not able to localize a thin body velocity anomaly near the seafloor (model B). Instead, it spread the region of the increased velocity vertically. Most likely this is the result of the sparse OBS geometry of the experiment. The results for the two different physical models are almost undistinguishable from each other. However, the anomaly tends to decrease with depth for the thin body case, while for the uniform anomaly the increase of the velocity reaches the same level for all depths in the resulting velocity field. From the model A results, the limits of the anomaly are not so well determined and the true anomalous velocity (+30 m/s) was not fully recovered by the inversion (only +20 m/s). This can be explained by the need to use a large smoothness factor in the inversion. This parameter acts as a trade off between higher resolution and reliability in the inversion results. For the sparse OBS geometry in this experiment, a large smoothness factor is required. Otherwise, false anomalies due to noise in the data may appear in the poorly constrained areas. Figure 6.  Physical models of hydrate distribution. Figure 7.  The velocity models (left panels) and the recovered velocity profiles (right panels). The results of the inversion with synthetic experimental data can be summarized as follows: – The final velocity models inverted for the two different physical models of a thin anomalous body and an anomaly with a uniform vertical extent are almost indistinguishable from each other in the case of small anomalous velocities. – The horizontal limits of small amplitude anomalies cannot be accurately determined but are smoothed over distances of 1 to 2 cell sizes. – The maximum anomalous velocity increase tends to be underestimated. Inversion of experimental data The results for the inversion at the Bullseye vent are presented in vertical and horizontal slices from the 3-D velocity model.  Examination is focused on the central regions which are better constrained by the data.  The vertical slices, as shown in Figure 8 (N/S slices) and Figure 9 (E/W slices), reveal small positive anomalies that are in good spatial agreement with the limits of the blank zones.  Although the increase does not exceed +25 m/s, the results indicate that the sediments within the blank zones have higher velocities than those outside.  The negative velocity gradients seen in the peripheral regions of the slices are believed to be artifacts due to insufficient ray coverage at the extremities of the grid. Figure 8.  Vertical slices in N/S direction (bottom panel shows the slice through OBS E, B and F). Figure 9. Vertical slices in W/E direction (middle panel shows the slice through OBS A, B and C). The average vertical velocity profile for the central part of the model is shown in Figure 10. Sea floor velocities are about 1500–1520 m/s, with a uniform increase of 1 s -1  to the BSR, reaching values of 1700–1750 m/s immediately above. The spatial variations of the velocities are also displayed in slices at constant depth below the sea floor.  Figure 11 shows the horizontal distribution of the upper portion of the sediment section at a depth of 50 m.  The small positive velocity anomalies correspond well with the bounds of the blank zones. Model uncertainties The direct quality assessment of the inversion results involves calculation of the uncertainty value for each model parameter. The uncertainty analysis package for Jive3D estimates uncertainties based on: (i) the estimated travel- time uncertainties of the input data, and (ii) the correlations introduced between neighbouring parameters as a result of the smoothing regularization [12]. The inclusion of the smoothing regularization into uncertainty analysis biases the uncertainty estimates, and makes them smaller by assuming that the true model actually has the same amount of smoothness as was introduced by the smoothness regularization term in the inversion. The values produced by uncertainty analysis represent one standard deviation in the Gaussian model. Figure 10.  Velocity versus depth profiles for the final model (thick line).  Results from the VSP (circles) and sonic log (thin line) from the ODP 889 site are shown for comparison [6]. Figure 11.  Final velocity distribution for the slice at 50 m below sea floor.  The black dashed lines outline the limits of blank zones. The spatial distribution of calculated uncertainties of the model are shown in Figure 12 for the depth of 50 m below the seafloor. As expected, the central part of the model is better resolved than its flanks. From similar slices in the vertical, the smallest values for the uncertainties are located in the planes going through lines of three OBSs. The N–S direction (along the depression) is better resolved than W–E (across the depression). The minimum uncertainty value for the velocity achieved for the model was as low as 20 m/s. Figure 12.  Uncertainty values for the final model for a horizontal depth slice of 50 m below the sea floor. DISCUSSION OF INVERSION RESULTS Gas hydrate concentration The concentration of hydrate at different sites within the modeled area around the Bullseye cold vent was estimated by comparison of the inverted velocity profiles with a background, no-hydrate reference velocity profile [14]. Three sites were selected within the modeled area, and the velocity profiles extracted from the inversion are shown in Figure 13.  Profile A represents velocities at a local minimum, where the base of the gas hydrate stability zone (GHSZ) is above the surface of the accreted sediment complex. The position is approximately in the middle between OBS E and B. As can be seen, it follows the background profile for the slope sediments within a few m/s. This fact suggests virtually no presence of gas hydrates at the point. The velocity profile at this point can be used as a new background. Velocity profile B is taken at a point associated with a local maximum. At this point, the sediments within the modelled thickness are represented exclusively by slope sediments. The increase in the velocity compared to profile A is minor: 20 m/s at the seafloor increasing down the sediment section to 40 m/s at the BSR depth. If the change in the velocity is completely attributed to the change in gas hydrate presence, these numbers correspond to 5–6% of pore space occupied by hydrates (or 2–4% of the sediment volume). Figure 13.  Velocities from the tomography inversion compared with the background velocity [14].  Also shown are the sonic log data from ODP site 889B [6]. The results of the travel time tomography study argue in favour of no gas hydrates, or only a very small amount, present in the slope sediment unit. At the sampled points the section from the seafloor to the BSR is represented by slope sediments. The gas hydrates may be distributed evenly in the vertical direction or they can be localized in the thin layers of more porous sediments [15]. The layers may be as thin as several tens of centimetres, well below the resolution of the travel time tomography inversion. Profile C represents the velocity profile in part of the modeled area where accreted sediments enter the GHSZ.  This profile matches the averaged sonic velocity data for depths below 120 m.  The velocity increases of greater than 40 m/s translate to hydrate concentrations of 10–20% of the pore space (or 5–7% of the total sediment volume). Physical model of the vent zone A model that helps to explain most of the observations at the vent site was suggested by Riedel et al. [16]. The model assumes the presence of a hydrate lens at the top of the blank zone and thin layers of sediments containing gas hydrates in small concentrations.  The methane is delivered into localized hydrate formation areas through a network of thin channels in either a dissolved form or as a free gas. In addition, Z¨uhlsdorff and Spieß [17] suggested episodic focused fluid flow. The episodic nature of the phenomena is justified by the necessity to build up the pore fluid pressure high enough to push the fluids through the sediments from the BSR depth to the sea bottom. The hydrate cap that consists of either layers with high hydrate concentration or dispersed hydrate chunks and lenses is responsible for the pull up of the travel time for the seismic events inside the blank zone. The cap also acts as a diffractor.  For a cap thickness of about 30 m, the observed pull up of 5 ms will be achieved if the velocity in the affected sediments are 200 m/s more than the velocity in the adjacent sediments. Such an increase of the velocity translates into gas hydrate concentrations of about 15–17% of total sediment volume. However, the results of the travel time tomography inversion do not reveal any significant velocity anomaly 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 the blanking feature, the rest of the vent zone is represented by sediments with a seismic velocity similar to that of the sediments outside. In addition to the diffractions associated with the hydrate cap near the sea floor, several other diffractions were observed in the OBS data [8]. The presence of the strong diffractions in close proximity to the BSR that are observed on the seismic sections recorded by OBS B suggests a physical condition similar to the one at the top of the vent. A structure like the hydrate cap can be in place at the BSR inside the limits of the blank zone. These hydrates were most likely formed by free gas entering the GHSZ from below. Overall, the results of the inversion are consistent with a physical model that consists of thin layers of hydrate located at specific depths within the stability zone.  It is worth noting also that the vertical axis of the blank zones passes through the intersection of the BSR and accreted sediment boundary beneath the younger slope sediments. The coincidence of these two boundaries may be the controlling factor for the blank zone location. The accreted sediments are more compacted than the slope sediments, and at the same time they are more permeable due to the presence of cracks. On the other hand, filling the cracks with gas hydrates can greatly reduce their permeability. That is, below the bottom of the GHSZ, accreted sediments are more permeable than slope sediments, but above this boundary the situation may reverse forcing the fluids to penetrate through the slope sediment section. Comparison with results from IODP 311 In October 2005, IODP Expedition 311 drilled at several holes at the Bullseye vent site [7].  The drilling revealed the presence of massive hydrate in 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 blank zone and the distribution of gas hydrates with depth proposed in this study based on the velocity model from the tomographic inversion and analysis of the seismic data: a layer of several 10’s of metres thick at the top of the sediment section; no presence of hydrates in large concentrations below the layer to the BSR; and occurrence of gas hydrates just above the BSR. CONCLUSIONS The seismic data set recorded by five OBS stations was used in conjunction with conventional seismic data for the investigation of the structure of the Bullseye cold vent zone. A 3-D velocity model was obtained by means of travel time tomography inversion for the region around the vent. The model does not support the presence of large velocity anomalies. The anomaly associated with the vent zone was found to be in the range of +25 m/s, which suggests no anomalous high gas hydrate concentration inside the blank zone. Unfortunately, higher resolution (vertically) of more detailed velocity structure was not possible due to limitations of the experimental geometry. Nevertheless, a combination of the tomography results with available seismic and other information about the site suggests that a layer of high gas hydrate concentration (15% or more) is located at the top of the vent zone. The rest of the blank zone may contain local concentrations of hydrates in small amounts. The average velocity profile for the site starts with compressional velocities close to those in the sea water, and increases virtually linearly reaching about 1750 m/s at the BSR. The velocity profile does not reveal any significant variations from the background no-hydrate velocity profile for the slope sediments, which suggests that slope sediments outside the blank zone contain no or very little gas hydrate down to the BSR. ACKNOWLEDGEMENTS The authors thank Dr. Keith Louden, Dalhousie University, for his support with the ocean bottom seismometers. 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 gas hydrates 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 marine sediments 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). Geophysical studies of marine gas hydrate in Northern Cascadia. In: Natural Gas Hydrates: Occurrence, Distribution, and Detection. G e o p h y s i c a l Monograph 124. AGU, 2001. [4] Riedel, M., G.D. Spence, N.R. Chapman, and R.D. Hyndman (2002). Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island. Journal of Geophysical Research, 107(B9), EPM5, 1-16. [5] Spence, G.D., T.A. Minshull, and C.R. Fink (1995). Seismic studies of methane hydrate offshore Vancouver Island. In: B. Carson, G.M.Westbrook, R.J. Musgrave, and E. Suess (eds.), Proceedings of the Ocean Drilling Program, Scientific Results. College Station, TX (Ocean Drilling Program), 1995. [6] Westbrook, G.K., B. Carson, and Shipboard Scientific Party (1994). Summary of Cascadia drilling results. In: G.M. Westbrook, B. Carson, and R.J. Musgrave (eds.), Proceedings of the Ocean Drilling Program, Initial Reports, volume 146, pages 389–396. College Station, TX (Ocean Drilling Program), 1994. [7] Expedition 311 Scientists, 2006. Expedition 311 summary. In Riedel, M., Collett, T.S., Malone, M.J., and the Expedition 311 Scientists. Proc. IODP, 311: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.311.101.2006 [8]  Zykov, M.M., 3-D travel time tomography of the gas hydrate area off shore Vancouver Island based on OBS data, PhD thesis, University of Victoria, Victoria, BC, 2005. [9] Gettrust, J.F., W.T. Wood, and S.E. Spychalski (2004). High-resolution MCS in deep water. The Leading Edge, 23(4):374–377. [10] Dosso, S.E. and B.J. Sotirin (1999). Optimal array element localization. J. Acoust. Soc. Am., 106(6):3445–3459. [11] Dosso, S.E. and N.E. Collison (2001). Regularized inversion for towed-array shape estimation. In: M. Taroudakis and G. Makrakis (eds.), Inverse Problems in Underwater Acoustics, chapter 6, pages 77–103. Springer-Verlag, 2001. [12] Hobro, J.W.D. (1999). Three-dimensional tomographic inversion of combined reflection and refraction seismic travel-times data. PhD thesis, University of Cambridge,1999. [13 ]  Hobro, J.W.D., S.C. Singh, and T. A. Minshull (2003). Three-dimensional tomographic inversion of combined reflection and refraction seismic traveltime data. Geophysical J. International, 152(1):79–93. [14] Yuan, T., R.D. Hyndman, G.D. Spence, and B. Desmos (1996). Seismic velocity increase and deep-sea gas hydrate concentration above a bottom-simulating reflector on the northern cascadia continental slope. Journal of Geophysical Research, 101 (B6):13,665–13,671. [15] Tr´ehu, A.M., P.E. Long, M.E. Torres, G. Bohrmann, F.R. Rack, T.S. Collett, D.S. Goldberg, A.V. Milkov, M. Riedel, P. Schultheiss, N.L. Bangs, S.R. Barr, W.S. Borowski, G.E. Claypool, M.E. Delwiche, G.R. Dickens, E. Gracia, G. Guerin, M. Holland, J.E. Johnson, Y.J. Lee, C.S. Liu, X. Su, B. Teichert, H. Tomaru, M. Vanneste, M. Watanabe, and J.L. Weinberger (2004). Three- dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204. Earth and Planetary Science Letters, 222(3-4):845–862. [16] Riedel, M., I. Novosel, G.D. Spence, R.D. Hyndman, R.N. Chapman, R.C. Solem, and T. Lewis (2006). Geophysical and geochemical signatures associted with gas hydrate related venting at the North Cascadia Margin. Geological Society of America Bulletin, 118(1-2):23–38. doi:10.1130/B25720.1. [17] Z¨uhlsdorff, L. and V. Spieß (2004). Three- dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing. Geology, 32(2):101–104.


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