Abstract
Abstract. Sediments containing gas hydrate dispersed in the pore space are known to show a characteristic seismic anomaly which is a high attenuation along with increasing seismic velocities. Currently, this observation cannot be fully explained albeit squirt-flow type mechanisms on the microscale have been speculated to be the cause. Recent major findings from in situ experiments, using the gas in excess and water in excess formation method, and coupled with high-resolution synchrotron-based X-ray micro-tomography, have revealed the systematic presence of thin water films between the quartz grains and the encrusting hydrate. The data obtained from these experiments underwent an image processing procedure to quantify the thicknesses and geometries of the aforementioned interfacial water films. Overall, the water films vary from sub-micrometer to a few micrometers in thickness. In addition, some of the water films interconnect through water bridges. This geometrical analysis is used to propose a new conceptual squirt flow model for hydrate bearing sediments. A series of numerical simulations is performed considering variations of the proposed model to study seismic attenuation caused by such thin water films. Our results support previous speculation that squirt flow can explain high attenuation at seismic frequencies in hydrate bearing sediments, but based on a conceptual squirt flow model which is geometrically different than those previously considered.
Highlights
Important mechanisms of wave attenuation in fluid-saturated porous media from seismic to ultrasonic frequencies, include friction between grain boundaries (Winkler and Nur, 1982), global flow or Biot’s mechanism (Biot, 1962), and waveinduced fluid flow at mesoscopic and microscopic scales (e.g., Müller et al, 2010)
The attenuation caused by global flow as well as that caused by wave-induced fluid flow at microscopic or mesoscopic scales are frequency dependent
The results demonstrate the high levels of seismic attenuation/dispersion that a range of variations of our conceptual model can cause
Summary
Important mechanisms of wave attenuation in fluid-saturated porous media from seismic to ultrasonic frequencies, include friction between grain boundaries (Winkler and Nur, 1982), global flow or Biot’s mechanism (Biot, 1962), and waveinduced fluid flow at mesoscopic and microscopic scales (e.g., Müller et al, 2010). Various researchers have attempted to mimic the natural environment of GH-bearing sedimentary matrices in laboratory experiments (Berge et al, 1999; Ecker et al, 2000; Dvorkin et al, 2003; Yun et al, 2005; Spangenberg and Kulenkampff, 2006; Priest et al, 2006, 2009; Best et al, 2010, 2013; Hu et al, 2010; Li et al, 2011; Zhang et al, 2011; Dai et al, 2012; Schicks et al, 2013) The results of this collective effort established a number of conceptual models for the role of GH embedded in its sedimentary matrix (Fig. 1). Our results support the suggestions that the estimation of GH saturation for GH occurring in a rather dispersed manner could be accomplished by using seismic wave attenuation as a tool for indirect geophysical quantification (Guerin and Goldberg, 2002; Priest et al, 2006; Best et al, 2013; Marin-Moreno et al, 2017)
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