Formation Vp Derived From LWD Sonic Data at IODP Sites U1325-U1329, Cascadia Margin

LWD sonic data were acquired in very low-velocity, hydrate-bearing formations at five sites drilled during IODP Exp 311 on the Cascadia Margin. Sonic velocity logs provide one of the best means to investigate the physical properties and porosity of gas hydrates, although caution must be exercised when interpreting LWD sonic data in shallow sediments. Current LWD sonic technology is challenged to recover accurate P-wave velocity where values are extremely low, close to the fluid velocity, as in shallow marine sediments where gas hydrates typically occur. Low formation Vp makes the analysis of LWD sonic data difficult because of the strong effects of wave modes linked to the presence of a tool in the borehole, such as dipole and leaky-P modes, which typically have high amplitudes and are dispersive. Slowness vs. frequency analysis using Prony's method is used to examine the frequency dispersion of borehole leaky-P modes and establish the minimum depth below seafloor for accurate Vp estimates. A fast downhole computation method is also compared as a useful approximation for future real-time applications. During Exp 311, real time data processing was configured to identify the wave propagating through the borehole fluid for monitoring of possible gas occurrences by a loss of waveform coherence. Post-processing of the dispersive leaky-P modes allows for accurate estimation of formation Vp profiles deeper than 57-100 m below the seafloor, at the different Exp 311 LWD sites. Vp increases from ~1.6 to ~2.0 km/s at total depth (350 mbsf for the deepest hole). Gas hydrate-bearing intervals show up as high-Vp anomalies over this steady increase with depth. Vp estimates from leaky-P modes below the minimum depth compare in general with wireline sonic logs and VSP interval velocities in nearby Exp 311 holes but differ over short intervals due to local formation heterogeneity. A low-Vp transition observed at the bottom of hydrate-bearing intervals, associated with the presence of free gas, was measured using LWD but not by wireline sonic tools deployed at some sites.