First field experiments

P-to-S converted waves were first detected in conjunction with gas hydrates during ODP Leg 164 (Pecher et al., 1997a). They were recorded during walkaway-VSPs, for which a three-component geophone was clamped in a borehole. A 13-km long seismic profile was shot over the borehole. At larger offsets, the angles of incidence were large enough to generate P-to-S conversion at a number of horizons. Because of the uniformity of Blake Ridge sediments, P-to-S conversion is likely associated with variations of gas hydrate concentration rather than lithologic boundaries (Pecher et al., 1997b). A second profile was acquired over a three-component OBS at the seafloor. In these data, a clear PS-arrival, apparently from the base of the hydrate stability zone, was recorded (Fig. 11). Gas may cause P-to-S conversion because of the associated density contrast. However, PS-energy would be predicted to be weak, especially given the low gas concentrations encountered on the Blake Ridge. Therefore, it is more likely that PS-conversion is caused by a decrease of sediment Vs at the base of the hydrate-bearing sediments.

Fig. 10: Predicted relative change of Vs and Vp as a function of gas hydrate saturation of pore space, assuming that gas hydrate replaces part of the sediment frame. Sediment composition and conditions roughly as expected at the BGHS on Hydrate Ridge, offshore Oregon. Vs is more sensitive than Vp mainly because of the low baseline Vs of unconsolidated hydrate-free marine sediments (typically, Vs: 100-500 m/s; Vp: 1500-2000 m/s).

0.0 0.25 0.5 0.75 gas hydr. saturation

Fig. 10: Predicted relative change of Vs and Vp as a function of gas hydrate saturation of pore space, assuming that gas hydrate replaces part of the sediment frame. Sediment composition and conditions roughly as expected at the BGHS on Hydrate Ridge, offshore Oregon. Vs is more sensitive than Vp mainly because of the low baseline Vs of unconsolidated hydrate-free marine sediments (typically, Vs: 100-500 m/s; Vp: 1500-2000 m/s).

After these encouraging observations, a recent OBC survey off Norway did not show any PS-conversion associated with the BGHS [Andreassen et al., 2000]. It is not yet clear why P-to-S conversion at the BGHS appears to take place on the Blake Ridge but not off Norway. Possible explanations include a possibly low hydrate concentration in the OBC area off Norway, or that strong PS-conversions from densely-spaced ping strata may hide conversion at the base of the hydrate stability zone. This discrepancy is still under investigation, and a new PS-study is planned on the Blake Ridge.

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