Potential of PSsurveys

The study of shear-wave propagation through gas-hydrate-bearing sediments is still in its infancy, so this topic remains speculative. Much depends on how strongly gas hydrates affect sediment Vs. Assuming that Vs is more sensitive to gas hydrates in the pore space, lateral variations of gas hydrate concentration should have a more pronounced effect on P-S traveltimes than on P-wave traveltimes. This will be tested during a planned PS-survey with OBSs across the Blake Ridge crest, which will allow us to compare PS-traveltimes to P-reflections.

P-to-S conversion at BSR

distance (km)

Fig. 11: PS-waves recorded with a three-component OBS on the Blake Ridge (white arrows in horizontal component). Plotted with reduction velocity of 2500 m/s, i.e., source-receiver-offset/(2500 m/s) is subtracted from arrival time. First arrival is the direct wave from the airgun to the OBS. The key in identifying arrivals as PS-waves is that they do not appear in the pressure component recorded by hydrophone a short distance above the OBS, because water does not transmit S-waves. Lower plots: Modeling of traveltimes using a Vp model from the VSP and an empirical relationship between Vp and Vs (Castagna, 1985) to estimate Vs assuming P-to-S conversion at the BSR. Calculated and observed traveltimes match extremely well considering that the Castagna-relation is only empirical. Although this does not prove conversion takes place at the BSR, it demonstrates that this is a realistic suggestion. Considering the uniform sediments beneath most of the Blake Ridge, most likely only parameters that may lead to P-to-S conversion are variations in gas hydrate concentration and free gas occurrence. After Pecher et al. (1997).

+ traveltime picks — computed traveltimes

P-to-S conversion at BSR

distance (km)

Fig. 11: PS-waves recorded with a three-component OBS on the Blake Ridge (white arrows in horizontal component). Plotted with reduction velocity of 2500 m/s, i.e., source-receiver-offset/(2500 m/s) is subtracted from arrival time. First arrival is the direct wave from the airgun to the OBS. The key in identifying arrivals as PS-waves is that they do not appear in the pressure component recorded by hydrophone a short distance above the OBS, because water does not transmit S-waves. Lower plots: Modeling of traveltimes using a Vp model from the VSP and an empirical relationship between Vp and Vs (Castagna, 1985) to estimate Vs assuming P-to-S conversion at the BSR. Calculated and observed traveltimes match extremely well considering that the Castagna-relation is only empirical. Although this does not prove conversion takes place at the BSR, it demonstrates that this is a realistic suggestion. Considering the uniform sediments beneath most of the Blake Ridge, most likely only parameters that may lead to P-to-S conversion are variations in gas hydrate concentration and free gas occurrence. After Pecher et al. (1997).

The greater potential, however, may be in the detection of layers of high gas hydrate concentration, which should cause P-to-S conversion. Furthermore, because Vs is lower than Vp, shear-waves have shorter wavelengths than P-wave (at a given frequency), so P-to-S surveys often have better resolution than P-wave surveys. PS-arrivals may be particularly well suited to study the BGHS. There, the waveform of the PS-arrival may enable us to determine the fine Vs-structure across the BGHS, just as waveform inversion of P-waves has elucidated the fine Vp-structure (Fig. 4). Unlike Vp, Vs will be almost unaffected by the underlying free gas (Fig. 6). Vs may therefore be directly related to gas hydrate. Vs below the BGHS may thus be useful as a reference value of hydrate-free sediment, since the composition of sediments that host gas hydrates may be identical to that immediately below the BSR. We caution that quantitative PS-analysis has many pitfalls, in particular the high attenuation of S-waves in shallow marine sediments. We nevertheless think that PS-analysis has the potential of leading to a breakthrough in gas hydrate quantification.

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