Double BSR on the Nankai slope (Japan): seismic imaging and physical property estimation
Abstract
Bottom Simulating Reflectors (BSR) commonly occur on the slope of the Eastern Nankaï margin, in water depths ranging from 0.6 km south of Kodaïba Knoll to 2.5 km landward of Yukie Ridge. We draw our attention to the "double BSR area" on the south-eastern flank of Kodaïba Knoll in water depths of 600-800 m. We present a high resolution seismic data set acquired over the double BSR area. The data was recorded with a 4.5 Km long digital streamer (SFJ French-Japanese Program, 2000). If previous campaigns did show the presence of a double BSR, this data set clearly enhances this feature and the images greatly benefit from large source-receivers offsets. The seismic sections clearly show two reflectors having the usual characteristics of BSR and running parallel to each other approximately 50 m apart. The upper BSR, at the current lower limit of the hydrate stability zone (HSZ) in the sediments, is viewed as an active BSR. The lower BSR is associated with high frequency seismic attenuation which suggests rather large amounts of gas in the sediments below it. The high frequency content of the SFJ data was a key factor in obtaining sharp and clear images of this area, but not the only one. The multiplicity of the data improves dramatically the signal to noise ratio, and the long offsets allowed the use of slope tomography followed by residual move-out analysis, leading to very high resolution velocity sections. These velocity profiles suggest the presence of free gas even in the HSZ. We suggest that the BSR on the survey area is unstable, and migrates upwards. These results confirm that the Nankaï margin is a key site for studies of gas hydrates occurrence and formation.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....6556L