As expected, the seismic Q measurements, like the Vp measurements, are shown to be more sensitive to small quantities of gas than to small quantities of hydrate. The gas-charged sediments of the Blake Ridge exhibit very low values of Q, between 6 ± 1 and 90 ± 10, significantly lower than the 200-700 range expected for fine-grained marine sediments with no gas or hydrate. Conversely, the hydrate-bearing sediments containing 3%-4% by volume hydrate exhibited values of Q between 90 ± 10 and 500 ± 100. This is slightly lower than, but generally consistent with the expected value for nonhydrate-bearing sediment.
Despite these findings we note that several authors (Brienzo, 1992; Hamilton, 1976; Jacobson et al., 1981; and Bowles, 1997) have attempted to establish a typical attenuation profile with depth, employing mechanisms such as porosity decrease with depth. The possible presence of hydrate in these areas should not yet be discounted as an agent that affects attenuation. However, this may be more pertinent to smaller spatial scale studies, where higher concentrations of hydrates may be associated with localized fluid flow.
Regarding the inversion technique, it is clear that a simple spectrogram (as shown in Fig. 3B) is adequate to detect the top of the free-gas zone. However, a full inversion for Q may be useful in remotely quantifying free-gas concentration, but only when equipment such as the pressure core sampler (Paull, Matsumoto, Wallace, et al., 1996) can be used for reliable corroboration.