The quality of the log measurements were described by shipboard scientists at Sites 994 and 997 as significantly degraded by the size and rugosity of the borehole, whereas the log measurements at Site 995 were described as moderately degraded. Borehole conditions were especially degraded at Site 997 below the BSR, resulting in large-scale shifts in the electrical resistivity log. Despite poor borehole conditions, downhole logging along the transect did define areas of gas hydrate accumulation and free gas below the BSR. Three logs are plotted on the seismic section; electrical resistivity, acoustic velocity, and sediment porosity.
Electrical resistivity at each site was logged by a phasor dual induction-spherically focused resistivity tool. Because gas hydrate is an electrical insulator, the resistivity of gas hydrate and gas hydrate-bearing sediment is higher than that of surrounding sediment. The contrast is sharpest in sediments that contain salty water, such as marine sediments. Acoustic velocity well logs can be used to measure the amount of gas hydrate within a drilled sedimentary unit. On Leg 164 this measurement was done with a long-spaced sonic tool. Because gas hydrate has a high compressional velocity, the measured acoustic velocity increases where gas hydrate occurs. Conversely, acoustic velocity decreases where free gas bubbles exist, clearly seen below the BSR at Site 995.
Electrical resistivity and acoustic velocity well logs, when used together (Paull, Matsumoto, Wallace, et al., 1996) clearly show occurrences of gas hydrate in this transect. For example, at Site 994 two spike-shaped large-scale increases in electrical resistivity and acoustic velocity occur between about 220 and 250 mbsf (use internal depth scale for reference). Using this relationship, clear stratigraphic horizons of gas hydrate-rich sediments can be traced from Site 994 to 995, and possibly to Site 997.
Sediment porosity curves were plotted from measurements made directly from the cores as described in the "Explanatory Notes" chapter of the Leg 164 Initial Reports (Paull, Matsumoto, Wallace, et al., 1996).
Leg 164 vertical seismic profiles (VSPs) were acquired at all three sites, and the details of the experiment with results are found in Holbrook et al. (1996). VSPs accurately measure the seismic velocity of sediments and sharply delineate sediment containing free gas relative to sediment containing nonfree gas. These relationships are clearly visible when plotted on the seismic section, because the occurrence of seismic velocity drops dramatically below the BSR at Sites 995 and 997. Interpretations of where zones of free gas-containing sediment occur are made based primarily on VSP data and secondarily on acoustic velocity.
In situ temperature measurements were made to depths as great as ~415 mbsf on the Blake Ridge in part to determine the gas hydrate stability field boundary. The measurements were made primarily by the Adara temperature tool and, in deeper sediment, by the water-sampling temperature and pressure tool (WSTP). Details of these experiments can be found in the Leg 164 Initial Reports (Paull, Matsumoto, Wallace, et al., 1996). One of the primary conclusions of these studies is that there is a discrepancy between the observed BSR and predicted base of gas hydrate stability (BGHS; Paull, Matsumoto, Wallace, et al., 1996). Our present understanding assumes that these horizons should be equivalent. The discrepancy between the BSR and the predicted BGHS is not only offset by at least 50 m; it is also quite variable between sites. Ruppel (1997) suggested the discrepancy may be due to capillary effects in pore spaces that inhibit gas hydrate formation.
Gas hydrate decomposition during core recovery releases water and methane into the interstitial pores, resulting in a freshening of the pore water. The general chloride profile may represent a combination of several processes; however, the negative spikes of chloride concentrations can best be explained as artifacts of gas hydrate decomposition during core recovery. The relative amplitudes of the chloride anomalies are proportional to the amount of gas hydrate that dissociated within the sample during core recovery. In some cases, chlorinity spikes can be correlated from site to site.
During Leg 164, a pressure coring system tool was deployed several times, in part to measure in situ concentrations of methane. The results, compiled by Dickens et al. (1997), are reproduced in between Sites 995 and 997. A line denoting methane concentration at saturation for the range of depths is given as a reference. The maximum concentration occurs at the level of the BSR where free gas is trapped beneath gas hydrate-bearing sediment.