Downhole acoustic data were acquired in very low-velocity, hydrate-bearing formations at five
sites drilled on the Cascadia Margin during the Integrated Ocean Drilling Program (IODP)
Expedition 311. P-wave velocity in marine sediments typically increases with depth as porosity
decreases because of compaction. In general, Vp increases from ~1.6 at the seafloor to ~2.0 km/s
~300 m below seafloor at these sites. Gas hydrate-bearing intervals appear as high-velocity
anomalies over this trend because solid hydrates stiffen the sediment. Logging-while-drilling
(LWD) sonic technology, however, is challenged to recover accurate P-wave velocity in shallow
sediments where velocities are low and approach the fluid velocity. Low formation Vp make the
analysis of LWD sonic data difficult because of the strong effects of leaky-P wave modes, which
typically have high amplitudes and are dispersive. We examine the frequency dispersion of
borehole leaky-P modes and establish a minimum depth (approx 50-100 m) below the seafloor at
each site where Vp can be accurately estimated using LWD data. Below this depth, Vp estimates
from LWD sonic data compare well with wireline sonic logs and VSP interval velocities in
nearby holes, but differ in detail due to local heterogeneity. We derive hydrate saturation using
published models and the best estimate of Vp at these sites and compare results with independent
resistivity-derived saturations.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/1619 |
| Date | 07 1900 |
| Creators | Goldberg, David, Guerin, Gilles, Malinverno, Alberto, Cook, Ann |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | text |
| Rights | Goldberg, David |
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