A better understanding of the effect of methane hydrate morphology and saturation on elastic wave velocity of hydrate‐bearing sediments is needed for improved seafloor hydrate resource and geohazard assessment. We conducted X‐ray synchrotron time‐lapse 4‐D imaging of methane hydrate evolution in Leighton Buzzard sand and compared the results to analogous hydrate formation and dissociation experiments in Berea sandstone, on which we measured ultrasonic P and S wave velocities and electrical resistivity. The imaging experiment showed that initially hydrate envelops gas bubbles and methane escapes from these bubbles via rupture of hydrate shells, leading to smaller bubbles. This process leads to a transition from pore‐floating to pore‐bridging hydrate morphology. Finally, pore‐bridging hydrate coalesces with that from adjacent pores creating an interpore hydrate framework that interlocks the sand grains. We also observed isolated pockets of gas within hydrate. We observed distinct changes in gradient of P and S wave velocities increase with hydrate saturation. Informed by a theoretical model of idealized hydrate morphology and its influence on elastic wave velocity, we were able to link velocity changes to hydrate morphology progression from initial pore‐floating, then pore‐bridging, to an interpore hydrate framework. The latter observation is the first evidence of this type of hydrate morphology and its measurable effect on velocity. We found anomalously low S wave velocity compared to the effective medium model, probably caused by the presence of a water film between hydrate and mineral grains.

KEY POINTS 14We present a method to calculate continuously saturations of pore phases during hydrate 15 formation/dissociation from pressure and temperature. 16In our experiment up to 26% hydrate co-existed with about 12% gas in three hydrate formation 17 cycles with 10 and 55 MPa differential pressure 18We suggest the dominant mechanism for gas and hydrate co-existence in our experiment is 19 formation of hydrate-enveloped gas bubbles. 20 21 ABSTRACT 22Methane hydrate saturation estimates from remote geophysical data and borehole logs are needed 23 to assess the role of hydrates in climate change, continental slope stability, and energy resource 24 potential. Here, we present laboratory hydrate formation/dissociation experiments in which we 25 determined the methane hydrate content independently from pore pressure and temperature, and 26 from electrical resistivity. Using these laboratory experiments, we demonstrate that hydrate 27 formation does not take up all the methane gas or water even if the system is under two phase 28 water-hydrate stability conditions and gas is well distributed in the sample. The experiment 29 started with methane gas and water saturations of 16.5% and 83.5% respectively; during the 30 experiment, hydrate saturation proceeded up to 26% along with 12% gas and 62% water 31 remaining in the system. The co-existence of hydrate and gas is one possible explanation for 32 discrepancies between estimates of hydrate saturation from electrical and acoustic methods. We 33 suggest that an important mechanism for this co-existence is the formation of a hydrate film 34 enveloping methane gas bubbles, trapping the remaining gas inside. 35 36 3

The majority of presently exploitable marine methane hydrate reservoirs are likely to host hydrate in disseminated form in coarse grain sediments. For hydrate concentrations below 25–40%, disseminated or pore‐filling hydrate does not increase elastic frame moduli, thus making impotent traditional seismic velocity‐based methods. Here, we present a theoretical model to calculate frequency‐dependent P and S wave velocity and attenuation of an effective porous medium composed of solid mineral grains, methane hydrate, methane gas, and water. The model considers elastic wave energy losses caused by local viscous flow both (i) between fluid inclusions in hydrate and pores and (ii) between different aspect ratio pores (created when hydrate grows); the inertial motion of the frame with respect to the pore fluid (Biot's type fluid flow); and gas bubble damping. The sole presence of pore‐filling hydrate in the sediment reduces the available porosity and intrinsic permeability of the sediment affecting Biot's type attenuation at high frequencies. Our model shows that attenuation maxima due to fluid inclusions in hydrate are possible over the entire frequency range of interest to exploration seismology (1–106 Hz), depending on the aspect ratio of the inclusions, whereas maxima due to different aspect ratio pores occur only at sonic to ultrasound frequencies (104–106 Hz). This frequency response imposes further constraints on possible hydrate saturations able to reproduce broadband elastic measurements of velocity and attenuation. Our results provide a physical basis for detecting the presence and amount of pore‐filling hydrate in seafloor sediments using conventional seismic surveys.