Abstract
A better understanding of seismic wave attenuation in hydrate-bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates, important for climate change, geohazard and economic resource assessment. Hence, we conducted a series of small strain (<10−6), seismic frequency (50–550Hz), laboratory resonant column experiments on synthetic methane hydrate-bearing sands under excess-water seafloor conditions. The results show a complex dependence of P- and S-wave attenuation on hydrate saturation and morphology. P- and S-wave attenuation in excess-water hydrate-bearing sand is much higher than in excess-gas hydrate-bearing sand and increases with hydrate saturation between 0 and 0.44 (the experimental range). Theoretical modelling suggests that load-bearing hydrate is an important cause of heightened attenuation for both P- and S-waves in gas and water saturated sands, while pore-filling hydrate also contributes significantly to P-wave attenuation in water saturated sands. A squirt flow attenuation mechanism, related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate, is thought to be responsible for the heightened levels of attenuation in hydrate-bearing sands at low hydrate saturations (<0.44).
Highlights
Detection and quantification of seabed methane is important for predicting greenhouse gas fluxes between the seabed, the water column and the atmosphere and their impact on future climate change
We present novel, laboratory resonant column results for seismic compressional and shear wave attenuation measured in water saturated, synthetic methane hydrate-bearing sand created under excess-water conditions at an effective pressure of 500 kPa and a temperature of 10 1C, representative of shallow sub-seabed hydrates
In an attempt to study the possible contribution of different hydrate morphologies to intrinsic attenuation, we introduce the Hydrate Effective Grain (HEG) model based on the notion of microporous hydrate grains
Summary
Detection and quantification of seabed methane is important for predicting greenhouse gas fluxes between the seabed, the water column and the atmosphere and their impact on future climate change. Methane gas and methane hydrate quantification techniques are needed for assessing seafloor geohazards (e.g., landslides associated with hydrate dissociation on continental slopes) and hydrate energy resources (hydrate reservoir characterisation) (Riedel et al, 2010). Given a uniform sediment sample, isolating intrinsic loss mechanisms and accurately predicting their dependence on, for example, measurement frequency, effective pressure, temperature, pore fluid type and saturation, hydrate saturation and morphology, are major challenges
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