Sediment porosities can be determined from analyses of recovered cores and from numerous borehole log measurements (reviewed by Serra, 1984). At Sites 994, 995, and 997 we have attempted to use data from the lithodensity (HLDT), neutron porosity (CNT-G), and electrical resistivity (DITE) logs to calculate sediment porosities. Core-derived physical property data, including porosities (Shipboard Scientific Party, 1996a, 1996b, 1996c), have been used to both calibrate and evaluate the log-derived sediment porosities.
On Leg 164, water content, wet bulk density, dry bulk density, and grain density were routinely determined from recovered sediment cores. Other related physical property data, including sediment porosities, were calculated from these "index properties" (Paull, Matsumoto, Wallace, et al., 1996). The core-derived porosities actually represent the measured total water content of the sediments, which include interlayer, bound, and free water. Most downhole logs also measure the total water content of the sediments; thus the core- and log-derived sediment porosities should be the same. Sediment core porosities determined from Sites 994, 995, and 997 are shown in Figure 6. In general, the core-derived sediment porosities decrease from about 80% near the top of each hole to about 50% at the bottom.
The HLDT log measurements of bulk density in Holes 994D, 995B, and 997B (Fig. 4A-C) are highly variable and range from a maximum of about 1.9 g/cm3 to a minimum value of about 1.2 g/cm3. Other physical property data from the Blake Ridge, including core-derived sediment wet bulk densities and porosities (Fig. 6) (Shipboard Scientific Party, 1996a, 1996b, 1996c), are not consistent with the density log measurements. The core-derived bulk densities are relatively constant with depth and are characterized by a relatively limited range of values. It is likely that the density logs from all three sites have been severely degraded by both the rugosity and the enlarged size of the boreholes. Before using the log-derived bulk-density data to calculate sediment porosities, we have attempted to systematically remove the erroneous data from the recorded density logs at Sites 994, 995, and 997. The detailed analysis of the recorded logs indicates that the density log yields erroneous low values when the borehole exceeded a diameter of about 36 cm. In addition, it appears that when the borehole size is reduced, below a diameter of about 28 cm, the density log yields erroneous high values. Therefore, we have systematically deleted all of the density log data from the portion of Holes 994D, 995B, and 997B where the caliper log from the density tool indicates that the hole diameter is more than 36 cm or less than 28 cm. The edited density log curve still contained numerous unreasonably low density "spikes" that usually consist of only one or two data points. Therefore, any log measured bulk density values of less than 1.6 g/cm3 were also deleted from the recorded log traces. The edited bulk-density (b) log measurements were then used to calculate sediment porosities () in Holes 994D, 995B, and 997B using the standard density-porosity relation: = (m-b)/(m-f) (Serra, 1984). Water densities (f) were assumed to be constant and equal to 1.05 g/cm3 for each hole; however, variable core-derived grain/matrix densities (m) were assumed for each calculation. The core-derived grain densities (m) in Holes 994C, 995A, and 997A range from an average value at the seafloor of about 2.72 g/cm3 to about 2.69 g/cm3 at the bottom of each hole (Shipboard Scientific Party, 1996a, 1996b, 1996c). The density log porosity calculations from all three sites yielded values ranging from about 50% to near 70% (Fig. 6). The density log-derived porosities are more variable and generally higher than the core-derived porosities also shown in Figure 6. It appears that the density log porosities overestimate both the range and absolute porosities for the sediments on the Blake Ridge. It is likely the high clay content and unlithified nature of the sediments on the Blake Ridge have contributed to the inability of the density tool to make good contact with the borehole wall, which leads to erroneous density log measurements that cannot be further corrected. Data from the density logs in Holes 994D, 995B, and 997B can be used to assess general porosity trends but not for quantitative calculations.
Because of poor hole conditions, the CNT-G was run in only two holes (Holes 994C and 995B) on the Blake Ridge. The CNT-G measures the amount of hydrogen within the pore space of a sedimentary sequence, which is mostly controlled by the amount of water that is present. The CNT-G has two pairs of detectors that indirectly measure both epithermal (intermediate energy level) and thermal (low energy) neutrons, which provide two porosity measurements. The recorded neutron porosity log data from Holes 994C and 995B reveal an average thermal porosity of about 50%, and the epithermal porosity averages about 100%. The thermal and epithermal porosity logs are calibrated to read 50% and 100%, respectively, in water (no sediment). Therefore, the neutron porosity log in Holes 994C and 995B only detected the hydrogen in the borehole waters and the porosity data from the neutron log is of no value. In "standard" industry applications the CNT-G is run with a bowspring that keeps the tool near the wall of the hole, thus reducing the effects of an enlarged borehole. Because of the size limitation of running the logs through the drillpipe, it is impossible to use a bowspring on the CNT-G in ODP holes.
One approach to obtaining sediment porosities from downhole logs is to use the electrical resistivity logs (Fig. 4A-C) and Archie's relationship between the resistivity of the formation (Rt) and porosity (): Rt/Rw = a -m, where a and m are constants to be determined and Rw is the resistivity of the pore-waters (Archie, 1942).
The resistivity of pore-waters (Rw) is mainly a function of the temperature and the dissolved salt content (salinity) of the pore waters. Pore-water salinity data from Sites 994, 995, and 997 are available from the analyses of interstitial water samples collected from recovered cores at each site (Shipboard Scientific Party, 1996a, 1996b, 1996c). The interstitial water salinity trends at all three core sites mimic the interstitial water chloride trends discussed earlier in this report (Fig. 5). In general, the core-derived interstitial water salinities decrease with depth from a maximum value of about 35 ppt near the sediment-water interface to about 31 ppt within the upper part of logging Unit 2. Similar to the chloride profiles in logging Unit 2, the interstitial water salinities are also more variable within this inferred gas hydrate-bearing sedimentary section. Formation and seabed temperatures have been directly measured at all three core sites on the Blake Ridge, as described in Shipboard Scientific Party (1996a, 1996b, 1996c). Listed in Table 3 are the measured seabed temperatures and geothermal gradients for Sites 994, 995, and 997 (modified from Shipboard Scientific Party, 1996a, 1996b, 1996c). Arps' formula (Serra, 1984) was used to calculate the pore-water resistivity (Rw) at each site from the available core-derived interstitial water salinities and measured formation temperatures (Table 3; Fig. 7). In general, the calculated pore-water resistivities (Rw) reach a maximum of about 0.34 m within 100 m of the seafloor and decrease with depth to a value below 0.20 m at the bottom of each borehole. The small increase in water resistivities in logging Unit 2, depicted in Figure 7, are a result of the presence of freshwater in the analyzed cores, which was expelled from gas hydrate that had disassociated in the cores. To avoid introducing errors into the subsequent resistivity porosity calculations, the gas hydrate-affected pore-water resistivities from logging Unit 2 have been excluded and the pore-water resistivities from logging Units 1 and 3 have been used to statistically project undisturbed pore-water resistivities (Rw) for logging Unit 2 (Fig. 7).
To determine the Archie constants a and m, we used the method described by Serra (1984) and the log-measured resistivities and core-derived porosities (Paull, Matsumoto, Wallace, et al., 1996) from each site drilled on the Blake Ridge. The log-measured resistivity data from logging Unit 2 in each hole have been omitted from this calculation of the Archie constants to avoid introducing an error caused by using resistivity log-measurements that have been affected by the occurrence of in situ gas hydrate. In addition, log-measured resistivities from expected free-gas zones in Unit 3 of each hole have also been omitted from the determination of the Archie constants. Linear trends in resistivity log and core porosity data from logging Units 1 and 3 (exclusive of logging Unit 2) in each hole have been used to calculate representative (100% water saturated) formation resistivities (Ro) and porosities (). From these representative values the slope, m, and the intercept, ln a, of the function ln (Ro/Rw) = -m ln + ln a were calculated for each of the logged boreholes. The calculated a and m Archie constants for Holes 994D, 995B, and 997B have been listed in Table 3. The Archie constants (a and m) calculated for Holes 995B and 997B are similar and fall within the "normal" range of expected values (Serra, 1984). However, the values of the a and m constants for Hole 994D fall outside of the "normal" range of values. The cause of these anomalous Archie constants in Hole 994D is likely because of poor hole conditions in logging Unit 1, which has contributed to degraded resistivity log measurements. Because all three boreholes penetrated similar lithologic sections and because they are located in relatively close proximity to each other, we decided to use an average value (calculated from Holes 995B and 997B) for the a and m Archie constants throughout this study of the Blake Ridge (Table 3: a = 1.05, m = 2.56).
Given the Archie constants (a and m) and pore-water resistivities (Rw), we can now calculate sediment porosities () from the resistivity log using Archie's relation. The results of these calculations are the porosity logs shown in Figure 8. The calculated resistivity porosities should be considered "apparent" porosity values because the Archie relation assumes that all of the void space within the sediments are filled with water (no gas hydrate), which is not true. In all three holes, the resistivity-derived porosities decrease with depth (Fig. 8) though this is not the normal exponential consolidation trend that would be expected if the pore space were decreasing with depth primarily from the weight of increasing overburden (Lee and others, 1993); instead we see an almost linear porosity decrease. Relative to Units 1 and 3, Unit 2 exhibits a baseline shift to higher resistivities and lower calculated resistivity porosities. The assumption that all of the pore space within the sediments of Unit 2 is filled with only water is not valid. Some of the pore space in Unit 2 is occupied by gas hydrate that exhibits higher electrical resistivities and would contribute to an "apparent" reduction in resistivity-derived porosities.
The comparison of core- and log-derived porosities in Figure 6 and Figure 8, reveals that the resistivity log-derived porosities are generally similar to the core porosities. The density log-derived porosities, however, are generally higher than the core porosities. It is likely that the density log measurements have been degraded by poor borehole conditions. The resistivity log-derived porosities in Figure 8 are the best downhole-derived porosity logs for all three holes on the Blake Ridge. However, because of gas hydrate-induced resistivity effects in logging Unit 2, the resistivity-derived porosity data from Leg 164 should be used with caution and the core-derived sediment porosities are the best available porosity data from the Blake Ridge.