International Conference on Gas Hydrates (ICGH) (6th : 2008)


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  NEPTUNE-CANADA BASED GEOPHYSICAL IMAGING OF GAS HYDRATE IN THE BULLSEYE VENT   E.C. Willoughby * , R. Mir, C. Scholl, R.N. Edwards Department of Physics, University of Toronto 60 St George St., Toronto, ON, M5S 1A7 CANADA   ABSTRACT Using the NEPTUNE-Canada cable-linked ocean-floor observatory we plan continuous, real-time monitoring of the gas hydrate-associated, “Bullseye” cold vent offshore Vancouver Island. Our group inferred the presence of a massive gas hydrate deposit there, based on the significant resistivity anomaly in our controlled-source electromagnetic (CSEM) dataset, as well as anomalously heightened shear moduli, from seafloor compliance data. This interpretation was confirmed by drilling by IODP expedition 311 (site U1328), which indicated a 40 m thick gas hydrate layer near the surface. Sporadic venting and variations in blanking in yearly single- channel seismic surveys suggest the system is evolving in time. We are preparing two stationary semi-permanent imaging experiments: a CSEM and a seafloor compliance installation. These are designed not only to assess the extent of the gas hydrate deposit, but also for long-term monitoring of the gas hydrate/free gas system.  The principle of the CSEM experiment is to input a particular electromagnetic signal at a transmitter (TX) dipole on the seafloor, and to record the phase and amplitude of the response at several seafloor receiver (RX) dipoles, at various TX-RX separations. The data are sensitive to the underlying resistivity, which is increased when conductive pore water is displaced by electrically-insulating gas hydrate. The experiment is controlled onshore, and can be expanded to include a downhole TX. Repeated soundings at this site, over several years, will allow measurement of minute changes in resistivity as a function of depth, and by inference, changes in gas hydrate or underlying free gas distribution. Similarly, the displacement of pore fluids by solid gas hydrate will affect elastic parameters. Thus, seafloor compliance data, the transfer function between pressure and seafloor displacement time series, most sensitive to shear modulus as a function of depth, will be gathered continuously to monitor the evolution of the gas hydrate distribution.  Keywords: marine gas hydrates, NEPTUNE, CSEM, seafloor compliance, ocean observatory, Bullseye Vent, cold vent, methane seep, electrical resistivity, shear modulus, remote sensing    *  Corresponding author: E-mail NOMENCLATURE A  tortuosity constant (0.5<A<2.5) [unitless] fc cut-off frequency [Hz] g acceleration due to gravity [m/s 2 ] H height of the water column [m] k() wavenumber [m-1] m cementation factor (1.5<m<3) [unitless] N a constant  (½ < N < 2) [unitless] n is the saturation exponent [unitless] S fraction of water-filled pores [unitless] uz vertical displacement [m]  Lamé parameter [Pa] Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  shear modulus (Lamé parameter) [Pa] () normalized compliance [Pa-1] σ conductivity [S/m] σw is the conductivity of pore water [S/m] zz vertical stress [Pa]  porosity [unitless]  angular frequency [rad/s]  INTRODUCTION Geological setting Naturally-occurring marine gas hydrates are known to be abundant on continental slopes and margins, where there is both a source of primarily biogenic methane and the correct thermo-baric conditions for stability. Their presence offshore Western Canada was initially inferred from observations of Bottom Simulating Reflectors (BSRs), associated with the acoustic impedance contrast at the base of the gas hydrate stability zone, in reflection seismic data. However, seismic reflection data alone cannot define the total mass of gas hydrate contained in sediments, so other geophysical imaging methods must be employed to assess and monitor gas hydrate deposits.    Figure 1 Map showing geophysical surveys for gas hydrates in the cold vent zone with regional tectonics map inset.  For the last two decades, regular field campaigns, employing a full gamut of geophysical and geochemical methods, have delineated widespread gas hydrate distribution offshore Vancouver Island (see summaries in [1] and [2]).  In this region there is a thick sedimentary wedge called an accretionary prism which is created by sediments which  are scrapped off as the Juan de Fuca plate subducts under the North American plate. The inferred high abundance of hydrates in the accretionary sedimentary prism is explained by methane-rich pore fluids in the sedimentary section on the subducting Juan de Fuca plate tectonically expelled upward into the gas hydrate stability zone [3] (see Figure 1).  The Cold Vent Field and the Bullseye Vent In addition to widespread BSR reflectivity, likely associated with low gas hydrate concentrations, seismic reflection studies revealed regions without coherent reflections (known as „wipeouts‟ or seismic blank zones) which were associated with a cold vent field, near the Ocean Drilling Program (ODP) Site 889/890 ([4, 5, 6]). The most prominent of these four blank zones called Blank Zone 1 (BZ1) is known as the Bullseye Vent. Extensive evidence for gas hydrate deposits at the Bullseye vent (BZ1) includes 2D and 3D single and multi-channel seismic surveys, heat flow, exploration by a remotely-operated vehicle and samples recovered from piston coring [7]. During the 2004 field season, a joint geophysical research program over these vent structures included single-channel seismic reflection, deep-towed controlled-source electromagnetic (CSEM) [8] and compliance surveys [9] (see Figure 1).  CSEM data reveal the electrical resistivity of underlying sediments which are increased when conductive saltwater is displaced by insulating gas hydrate. The compliance data are most sensitive to the shear modulus of the sediments, which is highly sensitive to the inclusion of ice-like gas hydrate which displaces fluid in pore space. Both the CSEM and the compliance studies showed anomalies associated with the Bullseye vent, which were interpreted as evidence of massive gas hydrate at this site.  The methods are complimentary. The blanking in reflection seismic data, which can be caused by free gas or gas hydrate, was the first indication that of the presence of the cold vents. Unlike seismic reflection data, neither CSEM nor compliance is hampered by the presence of free gas within sediments. The CSEM data indicate the presence of resistors in the subsection and has finer spatial resolution than seafloor compliance data. Though it would be difficult to explain such massive resistivity anomalies with free gas alone, the CSEM data cannot rule out a combination of free gas and gas hydrate in the cold vent system [8]. The compliance data mainly are sensitive to the shear modulus as a function of depth over a wide area laterally, but are not significantly affected by variations in pore fluids, as neither pore water nor free gas contribute to the shear modulus. Thus the significant compliance anomaly at the Bullseye Vent corroborates the interpretation of the resistivity anomaly as due to massive gas hydrate near the surface [9, 10]. The extensive geophysical, geological and geochemical dataset on this marine gas hydrate region has encouraged scientific drilling operations including the recent Integrated Ocean Drilling Program (IODP) Expedition 311 [11]. A transect of holes was drilled across the margin. The site U1328 at the Bullseye Vent (shown on Figure 1), revealed a 40 m thick gas hydrate layer immediately below the seafloor. Now, two permanent seafloor observatories are planned at gas hydrate sites offshore Vancouver Island as part of the North-East Pacific Time-Series Undersea Networked Experiments (NEPTUNE Canada) cabled ocean floor observatory, including a node “ODP 889” servicing ultra-long time series experiments at the Bullseye Vent (see Figure 2).   Figure 2 Map of NEPTUNE Canada's cable and nodes offshore Vancouver Island [12].  NEPTUNE-based Long Term Monitoring NEPTUNE Canada will be the world‟s largest cable-linked seafloor observatory. It has an 800 km ring of power and fibre optic cable laid on the northern part of the Juan de Fuca plate, offshore Vancouver Island. There are six nodes planned at sites of scientific interest, which will support a wide variety of long-term instrument deployments, with real time data and control from shore [12]. The node called “ODP889” will be connected to a junction box near the Bullseye Vent. This in turn will be connected to our two long-term monitoring experiments: a CSEM array and a compliance installation. It is the purpose of this paper to explain the underlying physics of these remote- sensing monitoring experiments and the plans for their installation.  Figure 3 Example of gas plume observer near the Bullseye Vent in 12 kHz sounder data (image courtesy of S. Taylor, GSC).  The nearest gas plume to the Bullseye Vent will be monitored by the sector-scanning sonar experiment indicated on Figure 5. This will be the first time, such a gas-hydrate rich cold vent site will be subject to long term, continuous investigation, and data will be available on-line. Repeated single-channel seismic surveys at 25 m line-spacing suggest the volume subject to seismic blanking, interpreted as indicating the presence of scatterers of acoustic energy like free gas or, more likely, gas hydrate, varies from year to year [13]. Further, during a joint GSC-U of T-UVic research cruise in 2006 we observed active venting of gas plumes in 12 kHz sounder data, as shown in Figure 3; the plumes rose to a depth of ~500 m, suggesting that, as has been observed elsewhere (e.g. [14]), these plumes are likely methane bubbles with a thin methane hydrate film which looses stability above ~500 m and hence is no longer visible.  Though this region is one of the most actively surveyed marine gas hydrate sites, such gas plumes have not previously been observed, suggesting they are sporadic in nature. Both variations in seismic reflections seen in repeated surveys and the suggestion of sporadic methane venting suggest that there are small, but observable variations in this system on the time scale of years. Detecting such minute variations with campaign geophysics is extremely difficult at sea due to variations in positioning, particularly with towed seismic and CSEM arrays. For the first time, we will gain direct observational evidence of how such gas hydrate systems behave over time with these stationary, long-term time series experiments. The nature of the large-scale NEPTUNE observatory, with inter-connected experiments over a variety of sites, will help enable us to investigate hypotheses about connections between different types of datasets. We can monitor the time evolution of the cold vent system, distribution of free gas and gas hydrate. These may be related to other physical phenomena, including seismicity. THEORY OF CSEM CSEM imaging methods map the electrical resistivity of the subsurface. A time-varying electromagnetic field is generated near the seafloor and induces what are known as „eddy‟ currents in the seawater and the seafloor sediments. Below the seafloor, the currents are transmitted by ions through the conductive salt water which fills sedimentary pore space. The progress of these currents with time is a measure of the electrical conductivity (or conversely, resistivity) of the subsection; the more resistive the material the shorter the travel time. Measurements of electric fields associated with these eddy currents are made at a remote location. The sedimentary resistivity structure can be deduced from these data.  The electrical conductivity of marine sediments is caused by the movement of ions in fluid filled pores. The empirical relation between formation conductivity σ and porosity  is Archie's law σf =  m S n  σw/A  (1) where m is the sediment cementation factor in the range 1.5<m<3, A is a constant in the range 0.5<A<2.5, S is the fraction of the pores filled with water, n is the saturation exponent, and σw is the conductivity of the pore water.  In any given area, suitable values for the various parameters may be obtained through laboratory based experiments, including experiments in which some fraction of the pore volume is filled with hydrate. Archie's law is often reduced to σf =  S 2  σw    (2) where σw takes an average value for sea water of 3 S/m, with the caution that subsequent systematic laboratory measurements of the electrical conductivity of sediments containing hydrate could modify the form of the reduced law presented but not the principal conclusions.  Since gas hydrates are electrically insulating and replace conductive pore water they can significantly increase the electrical resistivity. Gas hydrate concentration can be inferred from resistivity data, particularly if other geophysical information is available (such as typical porosity, depth of the base of gas hydrate stability, etc.). CSEM experiments provide data which are completely independent from seismic data and sensitive to the gas hydrate layer itself (where the BSR is mainly an indication of the base of gas hydrate stability). Further, unlike seismic reflection studies, CSEM data are not hampered by the presence of free gas, which can wipe-out coherent seismic reflections. CSEM data alone cannot distinguish between different causes of increased resistivity (including free gas, freshened pore water or reduced porosity).  Edwards employed this relation to show that resistivity data should show a measurable increase in gas hydrated sediments [15]. The method was employed offshore Vancouver Island by Yuan and Edwards and more recently Schwalenberg et al. [16, 8] and it showed that typically resistivities are increased from non-hydrated sediments over a wide area with large anomalies over the cold vents. These studies employed two receiver dipoles towed behind the transmitter dipole. For the NEPTUNE experiment, we plan a 5 receiver array which will allow more depth resolution of the subsection‟s resistivity as greater Tx-Rx offsets are in general more sensitive to deeper structure.  A more thorough description of the theoretical background and a full 3D model of the expected resolution of the NEPTUNE CSEM experiment are provided by Mir et al. in this volume [17]     THEORY OF SEAFLOOR COMPLIANCE  The theory of the compliance method has been presented by Crawford et al. [18], among others, and we follow their treatment. Ocean-surface, long-period gravity and infragravity waves obey a well-established dispersion relation 2 = gk() tanh( k() H),          (3)  where  is angular frequency, k is the wavenumber, g is acceleration due to gravity and H is the water depth. A given frequency is equivalent to specifying the wavenumber in water of known depth. These evanescent wind and coastline-excited waves, particularly common in the bowl-shaped Pacific basin, create a pressure field which propagates downward to palpitate the seafloor. Seafloor compliance is the frequency-, or equivalently wavenumber-, dependent transfer function between pressure and deformation at the seafloor.  Compliance data can be normalized by multiplying by the wavenumber of the source so the normalized compliance of an infinite half- space is constant.  In general, normalized compliance is given by: ()  = k() uz/zz = k() uz/(( x ux + z uz) + 2  z uz)                            (4) where uz is the vertical displacement, ux is the horizontal displacement zz is the vertical stress, jui  =  ui / xj denotes the partial derivative in the direction of the subscript,  and  (shear modulus) are the Lamé parameters. Longer wavelength gravity waves effectively penetrate deeper. Thus lower frequency compliance data relate to deeper structure and higher frequency data relate to shallower structure beneath the seafloor. The highest frequency waves observed to exert pressure on the seafloor have wavelengths between one half to twice the water depth and wavenumber k = 2/NH, where ½ < N < 2. The constant N is determined by the wave amplitude. There is thus a high-frequency cut-off frequency fc = (g/(2NH)) 1/2 . In water depths of 1.3 km, typical of the region studied, fc is between 0.024 and 0.049 Hz. The low frequency limit is determined by the length of the time series. For existing data sets in this region, compliance is measured at frequencies between 0.003 and 0.049 Hz. The NEPTUNE experiment will allow longer time series with increased signal to noise ratios. This in turn will allow us access to lower frequency data, which will sense deeper shear structure.  It should be noted that like most geophysical imaging techniques, interpretations of compliance data are non-unique. These data are most sensitive to the product of layer thickness and shear modulus, rather than the shear modulus itself [19]. This does not hinder the estimation of the total mass of a hydrate deposit, as total mass is a function of the thickness-shear modulus product. The University of Toronto group developed fully 3D finite element code to model the compliance response of sediments as a function of densities and elastic moduli (or equivalently, seismic velocities) [20].  EXPERIMENTAL METHODS CSEM arrays   Figure 4 Upper panel shows deployment of towed CSEM array [16, 8]. Lower panel shows ship-based deployment of compliance system [9].  The University of Toronto developed a deep- towed inline electric dipole-dipole array optimized for sensing resistivity anomalies due to gas hydrate in the uppermost sedimentary layers [16, 8]. A transmitter-receiver array is dragged along the seafloor. A shipboard transmitter sets up a square wave current signal which is transmitter via coaxial cable to the transmitter dipole (Tx) on the seafloor. The signal travels through the seafloor sediments to receiver dipoles towed at various offsets beyond the Tx tuned to target depth. The receiver dipoles (Rx1 and Rx2) consist of a pair of silver-silver chloride electrodes, each with its own amplifier, clock, data processor and storage. A typical deployment is illustrated in the top panel of Figure 4. Yuan and Edwards employed this device to survey a large area near ODP 889 and showed that resistivities were typically elevated, suggesting the presence of gas hydrate which was not correlated to BSR reflectivity maps [16]. Schwalenberg and others employed the same array to image the resistivity structure of the cold vent field and found background resistivities in the range of 1.1 to 1.5 Ωm were elevated to more than 5 Ωm over the seismic blank zones (including Bullseye Vent) suggesting massive gas hydrate in the upper tens of meters below the seafloor. These results were confirmed by downhole resistivity and coring results of IODP Expedition 311, which indicated a 40 m gas hydrate deposit at the top of the Bullseye Vent [11].  For the NEPTUNE we are adapting this methodology to a cabled seafloor observatory. We have built a seafloor transmitter, which will fit in a pressure cylinder 36 cm in diameter and 92 cm in length. This transmitter will connected to 400 V power supply from the NEPTUNE array at the Bullseye Vent Junction Box, and draw 5 kW during repeated several-hour-long monthly experiments. The transmitter will be completely controlled from shore, over the internet. The operator running any given experiment will be able to select a waveform to be set up on the transmitter dipole (or, in the future, the dipole of choice), such as a square wave, sine wave or pseudo-random binary sequence.  Like the towed array, the NEPTUNE array will be in-line with a transmitter dipole and multiple receivers, at least in its initial deployment. The system can be expanded to include a second transmitter dipole in another configuration. This was intended to allow for a downhole transmitter dipole, if a borehole is drilled at this site at a later date. The addition of a downhole transmitter dipole can improve the depth resolution of CSEM data [21, 22].  Unlike the towed array previously used, the system will be fully fibre optically linked. All five receivers will send the measured voltages to a receiver controller. From there the data will go to an Ethernet switch in the transmitter box and be uploaded onto the NEPTUNE network and be available in (near) real-time.  We have elected to align the CSEM array with an existing deep-towed reflection seismic array known as DTAG09 [4]. This line goes from the south-southeast to the north-northwest across the seismic blank zone at the Bullseye Vent and the interpreted gas hydrate cap [23] as shown in Figure 5. The Tx dipole tends to the north- northeast from the Tx box and the 5-receiver dipole array tends to the south-southeast, extending beyond the gas hydrate cap, as delineated by seismic data. This way, signals will travel through the cold vent hydrated sediments.   Figure 5 Basemap of planned instrument layout at the Bullseye Vent Junction Box. The Junction Box is marked JB. The connection to the CSEM Tx box is 70 m from the JB. The CSEM array is aligned with the DTAG09 seismic line. The Tx dipole is to the north- northeast of the Tx box and the 5 receiver array is to the south-southwest. The compliance apparatus is situated over the hydrate cap, but off to the northwest, where the cap dips lower below the seafloor. There will also be a broadband seismometer installation and a sector-scanning sonar device to monitor methane venting into the water column.   Compliance installation The measurement of compliance time series requires the measurement of very small variations in pressure at the seafloor, induced by surface gravity and infragravity waves, and measurement of the associate deformation of the seafloor. A typical gravity wave at the surface of the ocean might have an amplitude of the order of 10 cm and induce a pressure field at the seafloor under 1.2 km of water of the order of 1 Pa. This can be recorded by employing a differential pressure gauge (DPG) [24]. The associated displacement of the seafloor would be 0.1 microns and extremely difficult to measure directly. However in the frequency band of 10 -3  to 10 -1  Hz, displacements of this order correspond with accelerations of the order 10 -11  to 10 -7  m/s 2  (or .001 to 100 microgal), which are well within the range of high-precision gravimeter measurements.  A typical deployment scheme of compliance field studies is shown in the lower panel of Figure 4 [9, 19, 25, 26]. A high-precision self-leveling Scintrex CG-3 gravimeter was adapted for seafloor use and interfaced with a DPG to an in situ data logger. Time series of 8 to 12 hours in length were gathered at many sites in the vicinity of ODP Site 889 and the Bullseye Vent. Data showed that the shear moduli at and near the Bullseye Vent were greatly elevated with respect to values typical of the area. Willoughby et al. employed full 3D finite element modeling to show that the data were most consistent with a cylindrical intrusion with high concentrations of gas hydrate [9], rather than a competing gas chimney model.  Displacements or accelerations of the seafloor can also be measured with a broadband seismometer and hence these have been employed to gather seafloor compliance data (e.g. [18]). We selected the Scintrex CG-3 gravimeter because it is superior for these measurements in terms of both bandwidth (which extends lower than broadband seismometers) and resolution. For the NEPTUNE installation, we are adapting a migro g-Lacoste gPhone meter. Like the Scintrex CG-3 its sensor is based on a zero-length spring. It has even higher resolution (0.1 microgal or 10 -9  m/s 2 ) and as it was designed to record earth tides, it automatically outputs at an appropriate 1 Hz sampling rate. The gPhone will be interfaced with a DPG to a data logger and continuous data will be uploaded to the NEPTUNE network. The high precision monitoring of the Bullseye vent over very long time periods should allow us to infer variations in concentration and distribution of gas hydrate by observing the shear modulus as a function of depth over time.  At every NEPTUNE node there will be an earthquake seismology monitoring experiment, which will contain both a broadband seismometer and a DPG. The seismic installation at the Bullseye Vent Junction Box is shown in Figure 5. We will be able to „mine‟ the seismic and pressure time series for compliance data, allowing us a built-in reference site within a few hundred meters, adjacent to the Bullseye Vent.  CONCLUSIONS The NEPTUNE-Canada seafloor cabled observatory will allow a unique opportunity for monitoring a complex gas hydrate-related cold vent structure, the Bullseye Vent. Both observations of variations in the position and topology of the volume subject to blanking in seismic reflection data and the observation of sporadic gas vent suggests there are measurable changes in the physical properties of the Bullseye vent. Our group has developed two very promising geophysical imaging techniques for surveying the sub-seafloor for physical property anomalies related to concentrations of gas hydrate.  The CSEM method has proven apt for detecting resistivities in the uppermost few hundred meters below the seafloor, which are increased when resistive gas hydrate displaces conductive sea water in available pore space [16]. Spectacular anomalies are associated with the cold vent and the Bullseye Vent in particular. [8]. By adapting this technology to a multi-receiver stationary array to monitor the Bullseye Vent over a period of several years, we will be able to detect small variations in resistivity as a function of time at five Tx-Rx separations, each most sensitive to a different depth. These data will be sensitive to variations in total mass and distribution of resistors in the system, including free gas and gas hydrate.  The compliance data will depend on the elastic parameters: shear modulus, bulk modulus and density (in order of importance) as a function of depth, over time. The presence of gas hydrates in sedimentary pore space has the largest impact on the shear modulus, hence the use of the compliance method to infer gas hydrate content is apt. There is a significant anomaly in the shear modulus depth profile of the Bullseye vent with respect to nearby sediments [9]. The opportunity for long-term monitoring of this active system using seafloor compliance data should allow us to see any variations in shear modulus within the gas hydrate stability zone due to the time evolution of gas hydrate content and distribution. Further, the method will be sensitive to deeper structure and may help address outstanding questions about the underlying „plumbing‟ of the system. These data will be augmented by and contrasted with compliance data derived from broadband seismic and pressure data at the nearby earthquake seismology monitoring experiment.  ACKNOWLEDGEMENTS We thank NEPTUNE Canada for access to maps and for funding these experiments. We thank the Captains and crews of C.C.G.S. John P. Tully and science teams from the University of Toronto, the University of Victoria and the Pacific Geoscience Centre for invaluable help in the field. We thank our collaborators at node ODP 889 and in particular Micheal Riedel, George Spence and Roy Hyndman. We thank Scintrex Ltd./migro g- Lacoste for their on-going participation in collaborative R&D and their enthusiasm for making the NEPTUNE compliance experiment a reality.  REFERENCES [1] Hyndman, R.D., Spence, G.D., Chapman, N.R., , M., Edwards, R.N., Geophysical Studies of Marine Gas Hydrate in Northern Cascadia, In: Paull, C.K & Dillon, W.P., editors. Natural gas Hydrates, Occurrence, Distribution and Detection, Geophysical Monograph Series No. 124. Washington, D.C.: American Geophysical Union, 2001. p.273 -295. [2] Spence, G.D. Hyndman, R.D., Chapman, N.R., Riedel, M., Edwards, R.N., Yuan, J., Cascadia Margin, northeast PacificOcean: hydrate distribution from geophysical investigations. In: Max, M.D., editor. Natural Gas Hydrate in Ocean and Permafrost Environments.  Dordrect: Kluwer Academic Publisher, 2000. p. 183 – 198. [3] Hyndman, R.D., Davis, E.E. A mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion. J. Geophys. Res. 1992;97, 7025-7041. [4] Wood, W.T., Gettrust, J.F., Chapman, N.R., Spence, G.D., Hyndman, R.D. Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness. Nature 2002;420: 656- 660. [5] Riedel, M., Spence, G.D., Chapman, N.R., Hyndman, R.D., Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island. J. Geophys. Res. 2002; 107 (B9),2200, doi: 10.1029/2001JB000269. [6] Zühlsdorff, L., & V. Spiess. Three-dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing. Geology 2004;32 (2): 101-104. [7] Riedel, M., Novosel, I., Spence, G.D., Hyndman, R.D., Chapman, R.N, Solem, R.C., Lewis, T. Geophysical and geochemical signatures associated with gas hydrate related venting at the north Cascadia Margin. GSA Bulletin 2006; 128(1/ 2): 23-38, doi: 10.1130/B25720.1  [8] Schwalenberg, K., Willoughby, E.C., Mir, R., Edwards, R.N. Marine gas hydrate electromagnetic signatures in Cascadia and their correlation with seismic blank zones. First Break 2005; 23: 57-63. [9] Willoughby, E.C., Latychev, K., Edwards, R.N., Schwalenberg, K., Hyndman, R.D. Seafloor Compliance Imaging of Marine Gas Hydrate Deposits and Cold Vent Structures J. Geophys. Res. 2008. (in press) [10] Willoughby, E.C., Schwalenberg, K., Edwards, R.N., Spence, G.D., Hyndman, R.D. In: Assessment of Marine Gas Hydrate Deposits: A Comparative Study of Seismic, Electromagnetic and Seafloor Compliance Methods. In: Proceedings of the Fifth International Conference on Gas Hydrates, Trondheim, 2005. [11] Expedition 311 Scientists. Cascadia margin gas hydrates. IODP Preliminary Report. 2005; 311 :doi:10:2204/ [12] The North-East Pacific Time-Series Undersea Networked Experiments. April 1, 2008. [13] Riedel, M. 4D seismic time-lapse monitoring of an active cold vent, northern Cascadia margin. Mar. Geophys. Res. 2007: doi 10.1007/s11001- 007-9037-2 [14] Heeschen K. U, A. M. Tréhu, R. W. Collier, E. Suess, G. Redher. Distribution and height of methane bubble plumes on the Cascadia Margin characterized by acoustic imaging. Geophys. Res. Lett. 2003.; 30 (12): 1643.  [15] Edwards, R.N., 1988, Two-dimensional modeling of a towed in-line electric dipole-dipole seafloor electromagnetic system: The optimum time delay or frequency for target resolution. Geophysics 1988;53: 846-853. [16] Yuan, J., Edwards, R.N. The assessment of marine gas hydrate through electrical remote sounding: Hydrate with a BSR? Geophys. Res. Lett., 2000; 27(16): 2397-2400. [17] Mir, R., Scholl, C., Willoughby, E.C., Edwards, R.N. 3D Resistivity Modeling of a Randomly Distributed Gas Hydrate Deposit In: Proceedings of the 6 th  International Conference on Gas Hydrates, Vancouver, 2008. [18] Crawford, W.C., Webb, S.C., Hildebrand, J.A., Seafloor Compliance Observed by Long- Period Pressure and Displacement Measurements J. Geophys. Res. 1991; 96: 16151-16160. [19]Willoughby, E.C. 2003. Resource Evaluation of Marine Gas Hydrate Deposits Using the Seafloor Compliance Method: Experimental Methods and Results. Ph.D. Thesis, University of Toronto, 2003. [20] Latychev, K.,Edwards, R.N. On the compliance method and the assessment of three- dimensional gas hydrate deposits. Geophys. J. Int. 2003;155, (3): 923-952, doi: 10.1111/j.1365- 246X.2003.02090.x [21]Scholl, C., Edwards, R.N. Marine downhole to seafloor dipole-dipole electromagnetic methods and the resolution of resistive targets. Geophysics 2007; WA39 [22]Scholl, C., Edwards, R.N., Mir, R., Willoughby, E.C. Resolving Resistive Anomalies Due to Gas Hydrate Using Electromagnetic Imaging Methods. In: Proceedings of the 6 th  International Conference on Gas Hydrates, Vancouver, 2008.  [23] He, T. Mound and vent structures associated with gas hydrates offshore Vancouver Island: analysis of single-channel and deep-towed multichannel seismic data. Ph.D. Thesis, University of Victoria, 2007. [24] Cox, C., Deaton, T., Webb, S., A Deep-Sea Pressure Gauge. J. of Atmospheric and Oceanic Tech 1984; 1: 237-246. [25] Willoughby, E.C., Latychev, K., Edwards, R.N., Mihajlovic, G. Resource evaluation of marine gas hydrate deposits using seafloor compliance methods. In: Gas Hydrate: Challenges for the Future, G.D. Holder and P.R. Bishnoi, editors. Annals of the New York Academy of Sciences 912, 2000. p. 146-158. [26] Willoughby, E.C., Edwards, R.N. Shear Velocities in Cascadia From Seafloor Compliance Measurements. Geophys. Res. Lett. 2000; 27: 1021-1024.  


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