In situ measurements of P-wave attenuation in the methane hydrate- and gas-bearing sediments of the Blake Ridge

Abstract

Recent drilling on the crest of the Blake Ridge during Ocean Drilling Program Leg 164 has provided an opportunity to compare estimates of attenuation from seismic data with direct samples of hydrate and gas in this region with the objective of using attenuation to remotely quantify hydrate and gas. Hydrate formation at the sediment grain contacts rather than in the pore spaces may significantly decrease the seismic attenuation. Because attenuation may be estimated from single-channel data, it would be more useful in hydrate detection than velocity, which requires more expensive multichannel data. In this analysis both single-channel seismic data and vertical seismic profile (VSP) data were inverted using a spectral modeling technique. For the single-channel data, this was performed using a simulated annealing algorithm, and for VSP data, the model updates were implemented manually. The results from both independent data sets are consistent with each other in each of the hydrate-and gas-stability zones. Values of the quality factor Q for hydrate-bearing sediments fall within the range expected of nonhydratebearing, fine-grained marine sediments, ranging from ~90 to 600, suggesting that small amounts of hydrate do not significantly affect Q. All values of Q less than ~90 were associated with gassy sediments; some were as low as Q = 6. Q within the hydratestability field changes systematically, reaching a minimum directly below the ridge crest. As expected, Q in the gassy sediments appears to correlate inversely with reflection strength.

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

“…The Q values estimated in deeper layers (L2, L3, and L4) using these two methods are found to be in concordance with each other, and Q values in layers just above the BSR (L2 and L3) are in good agreement with the Q values normally observed in gas-hydratebearing marine sediments (Wood et al, 2000). The Q estimates in the first layer (L1) do not correspond well.…”

“…At seismic frequencies, analysis on attenuation normally refers to intrinsic attenuation (Mavko et al, 1998), which can be studied through spectral analysis (Jacobson et al, 1981). Because gas hydrate increases the stiffness of the matrix (Jung et al, 2012) and P-wave velocity, it was normally assumed that the sediments saturated with gas hydrates will show lower attenuation (Wood et al, 2000). Unlike P-wave velocity, no unique trend of seismic attenuation in gas hydrates can be observed from the literature; thus, making attenuation characteristic of the gas hydrate bearing sediments a debatable topic (Guerin et al, 1999;Wood et al, 2000;Chand et al, 2004;Rossi et al, 2007;Sain et al, 2009;Sain and Singh, 2011;Jaiswal et al, 2012;Dewangan et al, 2014).…”

“…Because gas hydrate increases the stiffness of the matrix (Jung et al, 2012) and P-wave velocity, it was normally assumed that the sediments saturated with gas hydrates will show lower attenuation (Wood et al, 2000). Unlike P-wave velocity, no unique trend of seismic attenuation in gas hydrates can be observed from the literature; thus, making attenuation characteristic of the gas hydrate bearing sediments a debatable topic (Guerin et al, 1999;Wood et al, 2000;Chand et al, 2004;Rossi et al, 2007;Sain et al, 2009;Sain and Singh, 2011;Jaiswal et al, 2012;Dewangan et al, 2014). Laboratory experiments in hydrate bearing sediments indicated increase of attenuation with hydrate saturation (Priest et al, 2006;Best et al, 2013), whereas attenuation estimates from field experiments on gas hydrates indicated contradicting results.…”

Seismic characterization of a gas hydrate system in the Gulf of Mexico using wide-aperture data

“…We corrected the maximum amplitude values from the top of the HRZ for spherical divergence. Attenuation in the sediment column was estimated using an average interval velocity of 1.54 km s −1 and a seismic quality factor Qp = 300 as a rough average between those measured in vertical seismic profiles on the Blake Ridge (Wood et al 2000) and in line with other published values from unconsolidated fine-grained sediments (e.g. Sheriff & Geldart 1995;Trehu & Flueh 2001).…”

Study on p-Wave Attenuation in Hydrate-Bearing Sediments Based on BISQ Model

“…In Mackenzie Delta, Canada, Guerin and Goldberg [1], Guerin et al [2], and Pratt et al [3] obtained high value of wave attenuation in hydrate-bearing sediments by logging data and crosshole seismic tomography results, respectively. In Nankai Trough, central Japan, Matsushima [4] also proved that elastic wave showed strong attenuation in hydrate-bearing sediments by logging data, in Blake Ridge and in the Makran Accretionary Prism, Arabian Sea, Wood et al [5] and Sain and Singh [6] obtained week attenuation of seismic wave in hydrate-bearing sediments by using seismic data. Although the wave attenuation mechanism in hydratebearing sediments is not yet well explained.…”