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.