Publication Date:
2023-01-24
Description:
One of the key questions related to the prediction of the hydrate distribution and quantity is the nature of the interface at the base of the gas hydrate stability zone (GHSZ), where it is generally considered that free gas is present below. The presence of gas below the gas hydrate stability zone is a major contributor to the formation of a bottom simulating reflector (BSR) present in seismic profile records. Improved geophysical methods were used in September 2000 during cruise Sonne SO-150 to seismically investigate the nature of the BSR across Hydrate Ridge, which is part of the Cascadia accretionary complex, offshore the Oregon continental margin. The experiments were carried out by using a variety of source and receiver acquisition systems. Within the scope of Ocean Drilling Program (ODP) Leg 204, Southern Hydrate Ridge was drilled in the year 2002 providing additional information to verify the results derived from the seismic data and supporting ongoing analyses with relevant acoustic log data. The analyses, presented in this study, focus on deriving P-wave velocities from the seismic data to quantify the gas hydrate content within the GHSZ. By combining ocean bottom hydrophone (OBH) data with surface (SCS) and deep tow streamer data, an accurate velocity-depth distribution is determined, which is constrained by the acoustic logs of Ocean Drilling Program (ODP) Leg 204 and the depth of the BSR is predicted within a 5 m uncertainty. In general, the P-wave velocities are below 1600 m/s, averaged over the entire GHSZ, which leads to an estimate of about 1% of hydrate of pore space. This is in general below the average value of 2% resulting from ODP analyses, but is within an uncertainty of 1%, when considering thin hydrated layers affecting the mean velocity distribution. The analyses of the frequency dependent amplitude variations of the BSR signal, result in the presence of a thin gas bearing layer below the BSR with an average thickness of 8 ± 4 m. The existence of a thick gas zone of several 100 m beneath the GHSZ, as observed in other hydrate provinces, is ruled out by P-wave velocity considerations. The presence of a thin gas layer leads to the assumption that amplitude variation along the BSR, in addition to a change in the gas concentration, is caused by tuning effects. Amplitude and velocity anomalies are not necessarily the result of a change in gas hydrate saturation. Further, it is assumed that this thin gas layer is an indication for gas hydrate in the GHSZ being recyled due to tectonic uplift and a major contributor for the free gas below.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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