The discrepancies between experimental and theoretical velocity profiles can be related to concentration of gas hydrates and free gas in the pore space. Where we observe measured velocities greater than theoretical velocities we infer the presence of gas hydrate with variable concentration in the pore spaces. To obtain the amount of clathrates in the sediments, we progressively increase, in the theoretical formula, the parameter related to the concentration of gas hydrates (ch), until we fit the trend of the experimental downhole logging velocity curve. An analogous procedure is followed when the measured velocities are lower than the theoretical velocities. In this case, the theoretical velocity is obtained assuming a mixture of water and free gas in the pore spaces, with variable saturation (sw and sg, respectively). At the end of this calculation, we built a discrete model of layers where only water- and gas hydrates- and/or free gas-bearing sediments, with different concentration, are present. Table 2 summarizes the parameters we used for the computation. The results obtained for the three holes are shown in Figure 6, displaying gas hydrate concentrations vs. depth. At Site 994, no free gas has been revealed, as confirmed by other direct and indirect measurements, and the presence of methane hydrate is mainly confined between 280 and 450 mbsf. Small oscillations in gas hydrate concentrations below 450 mbsf are mostly because of velocity variations. At Site 995 we interpret a progressive increase of clathrate concentration from 260 to 440 mbsf (up to 12%), whereas the free gas amount is no more than 1%. At Site 997 we observed a trend similar to the previous site in the methane hydrate and free gas quantities, but the percentages are greater for both components (20% and 4%, respectively).
Indirect techniques for estimating gas hydrate concentrations (results of Cl- content of pore water; Paull, Matsumoto, Wallace, et al., 1996) revealed that some samples contained as much as 8% gas hydrate, and average concentration was 1% in a 200- to 250-m-thick zone above the BSR. Beneath 450 mbsf, chloride values are nearly constant. Gas-volume determinations (from the Pressure Core Sampler) at 17 depths at Sites 995 and 997, revealed that the pore space contains more than 12% gas bubbles in the free gas zone beneath the BSR, assuming that gas exists in oversaturated pore water (Dickens et al., 1997).
Quantitative differences between the theoretical and experimental concentration curves for gas hydrates depend mainly on two factors. The first is related to some parameters (the rigidity and the compressibility) in the calculation of theoretical velocity, which are difficult to measure and which significantly influence the results. The second is related to some experimental errors in the downhole logging velocity, as discussed, which are fundamental for the estimation of the concentrations. The comparison between gas hydrate determined from theoretical and chloride concentrations (see Fig. 6) shows that the trends are quite similar. For free gas concentrations in the sediment pores, the velocity trend is very sensitive to small variations of free gas concentration. Moreover, the downhole logging velocities are not very accurate in the free gas zone because of poor borehole conditions, as previously reported. That explains the significant discrepancies between the quantities of free gas in the experimental and theoretical calculations, and the nonuniform gas distribution in the sedimentary section (Domenico, 1977).