HR: 1340h
AN: OS33B-1472 [Abstracts]
TI: Seismic Quantification of Marine Methane Hydrates and Free Methane at Northern Hydrate Ridge -- Final
Results of the HYDGAS Project
AU: * Petersen, J
EM: jpetersen@ifm-geomar.de
AF: IFM-GEOMAR Leibniz Institute of Marine Sciences, Wischhofstr. 1-3, Kiel, 24148
Germany
AU: Papenberg, C
EM: cpapenberg@ifm-geomar.de
AF: IFM-GEOMAR Leibniz Institute of Marine Sciences, Wischhofstr. 1-3, Kiel, 24148
Germany
AU: Klaeschen, D
EM: dklaeschen@ifm-geomar.de
AF: IFM-GEOMAR Leibniz Institute of Marine Sciences, Wischhofstr. 1-3, Kiel, 24148
Germany
AB:
In September 2000, high-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge
in the accretionary complex of the Cascadia subduction zone within the HYDGAS project. The experiment was specifically
designed for hydrate investigations.
Here we present a local quantification of methane hydrate and free methane in the sediment based on the analysis of four
component (4C) ocean bottom seismometer (OBS) data. A regional estimate is obtained by extrapolating the local result to a
dense net of single channel streamer (SCS) profiles.
In the local study, a detailed model of the elastic parameters at the bottom simulating reflector (BSR) interface beneath an
OBS is developed by using synthetic seismogram modeling. The main focus in this study is an amplitude-versus-offset (AVO)
analysis of PS-converted shear waves, which originate by conversion at the BSR. The AVO analysis enables the determination of
the shear wave velocity above the BSR. A velocity of 400 m/s above the BSR indicates that already small hydrate saturations
are sufficient to increase the shear modulus of the sediment. Information about the attenuation and the shape of the BSR
transition zone is obtained from the frequency-dependent amplitude behaviour of the BSR. 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 BSR transition
is shown to be of a gradual type and can be described by a 1.5--2.5 m wide zone. From rock physics modeling, the hydrate
saturation is locally estimated to be 4--10 %. Below the BSR, free gas saturations of 0.5 % and 8 % are determined. These
values depend on the model chosen for the free gas distribution.
The regional estimate combines the local quantification result with the BSR amplitude distribution observed in the regional
SCS data and takes also the occurrence of active vent locations into account. It is calculated for an area of 20 km2
around the northern summit. In this region, the total mass of methane stored in hydrate or being existent as free gas is
roughly assessed to be 4.8--10.3 Mt or 1.3--3.9 Mt/km3.
DE: 0935 Seismic methods (3025, 7294)
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005