Local seismic quantification of gas hydrates and BSR characterization from multi-frequency OBS data at northern Hydrate Ridge.

Petersen, Carl Jörg, Papenberg, Cord and Klaeschen, Dirk (2007) Local seismic quantification of gas hydrates and BSR characterization from multi-frequency OBS data at northern Hydrate Ridge. Earth and Planetary Science Letters, 255 (3/4). pp. 414-431. DOI 10.1016/j.epsl.2007.01.002.

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Abstract

High-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge in the accretionary complex of the Cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment. We present a detailed local analysis of four component (4C) ocean bottom seismometer (OBS) data and show the importance of multi-frequency and shear wave data for determining hydrate reservoir properties. A detailed model of the elastic parameters at the bottom simulating reflector (BSR) is developed by using synthetic seismogram modelling. The main focus in this study is an amplitude-versus-offset (AVO) analysis of shear waves, which originate by PS-conversion at the BSR in 73 m below the seafloor (bsf). The AVO analysis enables the determination of the shear wave velocity above the BSR. A velocity of 400 m/s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the BSR. Information about the attenuation and the shape of the BSR transition zone is obtained from the frequency-dependent reflection amplitudes of the BSR. The BSR is shown to be a gradual type transition zone of 1.5–2.5 m thickness. Average Q factors of Qp = 150 for P-waves and of Qs = 35 for S-waves are determined within the gas hydrate stability zone (GHSZ). The low Qs factor points to a pronounced attenuation of S-waves in the uppermost sediments. From rock physics modelling, the hydrate concentration is estimated to vary locally between 3–12% of the pore space. Below the BSR, free gas concentrations of 0.5% and 8% are determined for homogeneous and patchy distributed gas, respectively.

Document Type: Article
Keywords: Seismic modelling, Gas hydratebottom simulating, reflector, PS-converted wave, AVO, Multi-frequency data
Research affiliation: OceanRep > SFB 574 > A1
OceanRep > SFB 574
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
Open Access Journal?: No
DOI etc.: 10.1016/j.epsl.2007.01.002
ISSN: 0012-821X
Projects: SFB574, Future Ocean
Expeditions/Models/Experiments:
Date Deposited: 09 Dec 2009 11:02
Last Modified: 30 Jan 2017 12:28
URI: http://oceanrep.geomar.de/id/eprint/5792

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