At Ocean Drilling Program Site 859 in the vicinity of the Chile triple junction, the source of the bottom simulating reflection (BSR) at the base of the gas hydrate layer has, for the first time, been logged to reveal the nature of the impedance contrasts producing the reflection. We estimate from the P-wave velocity (Vp) that hydrate occupies no more than 18% of the pore space just above the BSR and is not concentrated enough to cause the reflections. The BSR is caused by a sharp drop in Vp, and presumably density, from ∼1950 to 1600 m/s (on average) within an 8 m interval. Seismic modeling of wave form and amplitude vs. offset of the BSR at Site 860 indicates that the BSR is produced by a 12 m interval with low Vp and shear-wave velocities that are consistent with small quantities of free gas (∼1% of pore space) in the interval.

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