The validity of the estimated hydrate amounts depends on the accuracy of the acoustic logs and reliability of in situ porosities. The log quality was degraded because of poor borehole conditions from washouts and borehole rugosity. Therefore, the data sets at ODP Leg 164 did not provide optimum data sets to test acoustic theories or to estimate accurate in situ hydrate amounts. However, the acoustic data with core porosity provided useful information in characterizing in situ hydrated sediments.
The average concentration of hydrate in the pore space at Sites 994, 995, and 997 using core porosities and acoustic logs are 3.9%, 5.7%, and 3.8% (Table 1), and these are about 2.2%, 3.3%, and 2.2% of the total sediment volume (vol%). These values are higher than those estimated from chloride anomalies, which are 1.3, 1.8, and 2.4 vol% for Sites 994, 995, and 997, respectively, and are similar to those estimates from resistivity logs, which are 1.9, 3.0, and 3.4 vol% (Collett and Ladd, Chap. 19, this volume). On the other hand, the hydrate concentrations based on porosities calculated from the density logs are much higher, which implies that the log porosity measurements are degraded greatly from the poor borehole conditions.
A weighted equation proposed by Lee et al. (1996) was never rigorously tested against hydrated sediments. To test this theory, a more accurate data set (acoustic and porosity) as well as shear-wave data are required as demonstrated for permafrost samples (Lee et al., 1996). However, similar estimations of hydrates in pore spaces compared to other methods strongly suggest that the weighted equation with W = 1.1 and an exponent n = 1 is adequate in describing the acoustic properties of hydrated sediments in the Blake Ridge area.
In conclusion, the estimated gas hydrate amount in all three sites using acoustic logs and core porosities averages about 2.6 vol% of the sediments, and this value is comparable to those estimates from chloride anomalies and resistivity log data.