The local acoustic velocity field obtained with the tomography method, and the velocity trends derived from in situ (borehole logging, VSPs) and laboratory-related measurements, can be compared to the theoretical velocity function in water-filled porous media. Significant positive deviations between theoretical and experimental curves can be related to presence of clathrates, and negative deviations can be associated to free gas-bearing sediments.
To derive the theoretical velocity trends, we used Gaussman's equations with an explicit dependence on differential pressure and depth. The compressional (Vp) and shear (Vs) velocities expressed for water-filled porous media, in which there are two solid components (matrix and gas hydrates), and for fluid-filled porous media, where the fluid is a mixture of free gas and water, are those given by Domenico (1977). The Vp and Vs velocities are given by the following formulas:
and
Parameters are defined in Table 1.
To compute the theoretical velocity, we used the density and porosity trends available from laboratory measurements at the three sites (index properties of recovered cores), instead of downhole logging measurements. Core-measured densities are generally greater than the downhole logging values at the three Blake Ridge sites (Paull, Matsumoto, Wallace, et al., 1996), contrary to typically observed results, which yield lower laboratory values, because of the "elastic rebound" affecting the sediment specimens after recovery. This discrepancy has been attributed to the degraded borehole conditions at the three sites. The curves of porosity vs. depth obtained from core measurements and logging runs (Paull, Matsumoto, Wallace, et al., 1996) are generally more comparable than the density curves, because the resistivity-derived porosities are less affected by poor borehole conditions. The progressively increasing discrepancies between the index properties and borehole logging curves found from Site 994 to Site 997 raises the possibility that gas hydrate dissociation and core disturbance during drilling degraded the boreholes (variations in size and rugosity), with significant loss of quality of the log measurements. The experimental error that is associated with the elastic parameters we used propagates in the velocity determination, and therefore is difficult to predict.
Figure 5 relates theoretical velocities derived from index properties data (for water-bearing sediments) with logging velocity vs. depth at the three Blake Ridge sites. The tomographic velocity and the VSP-derived velocity trends have also been included in the plot for comparison. The logging velocity curve at Site 994, where no BSR is observed, shows a decrease starting from below 400 mbsf, with values ranging from 1700 to 1800 m/s. No significant increases in velocity have been found in the interval 200-400 mbsf, where direct and indirect techniques revealed small amounts of hydrates present in the sediments (Paull, Matsumoto, Wallace, et al., 1996). This confirms that irregularly disseminated, small-crystal gas hydrates cannot produce a recognizable increase in the acoustic velocity in the sedimentary column. At Site 995, two main minima in the logging velocity curve testify to the presence of free gas in the sediment pores at levels below the BSR, with the lower velocities occurring at 600 mbsf (1600 m/s). In this hole, logging velocity values diverge from the theoretical curve in the interval 200-440 mbsf, where direct and indirect methods revealed gas hydrates present in the sediment section. At Site 997, major fluctuations in the downhole logging velocity curve occur (see Fig. 5). In particular, two spikes in the function are found at the BSR depth and at 600 mbsf, indicating free gas accumulations in the sediment pores. In this hole, the logging velocity curve gradually diverges from the theoretical curve in the depth interval between 260 and 450 mbsf, indicating an increasing abundance of gas hydrates vs. depth.
