HR: 10:20h
AN: OS42A-01    [Abstracts]
TI: Structure of the Bullseye Blank Zone in Northern Cascadia: Results of Velocity and Amplitude Studies Using OBS Data.
AU: * Zykov, M M
EM: zmm@uvic.ca
AF: University of Victoria, School of Earth and Ocean Sciences, E-hut, 3800 Finnerty Rd., Victoria, BC V8T 3S1 Canada
AU: Chapman, R N
EM: chapman@uvic.ca
AF: University of Victoria, School of Earth and Ocean Sciences, E-hut, 3800 Finnerty Rd., Victoria, BC V8T 3S1 Canada
AB: Vertical 'blanking' or 'wipe-out' zones are common for gas hydrate sites, being zones of decreased seismic signal strength. At the gas hydrate site in northern Cascadia region this phenomenon is expressed in a number of vertical or sub-vertical 'chimneys'. The biggest one, Bullseye vent, has horizontal dimensions of 400 by 600 meters. Vertically, it extends from the BSR to the sea floor and is proved to be associated with fluid flow. Several models for blanking exist for the site while no single one fully satisfies all available data. During COAMS 99 seismic cruise an extensive data set was collected, including Ocean Bottom Seismometers (OBS) and normal incidence single channel recordings. The collected data set allowed to conduct a 3-D travel time tomography study and amplitude analysis around the Bullseye vent. The newly obtained results put more constraint on an applicable model for blanking. Tomography results suggest a small (20 m/s) positive anomaly that spatially corresponds with blanking zone. While the anomalous value Is around the value of the resolution limit of the inversion it discounts the presence of a negative anomaly that could be caused by presence of free gas inside the blank zone. On the other hand, substantially higher gas hydrate concentrations inside the blank zone compared to ones outside of it are also not supported. The OBS seismic data were also processed to obtain a high-resolution map of sea floor reflection coefficients. The reflection coefficient inside the sea floor expression of the blank zone is up to two times lower than average. The negative anomaly is fringed by a positive anomaly about 100 m wide. Previously available data will be combined with these recently obtained results to generate the most probable model for blanking for the vent site.
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 3653 Fluid flow
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005