Electrical Resistivity, Sound Velocity, Thermal Conductivity, Density-Porosity, and Temperature, Obtained by Laboratory Techniques and Well Logs: Site 462 in the Nauru Basin of the Pacific Ocean

Abstract

At Deep Sea Drilling Project (DSDP) Site 462, from mudline to 447 meters below the sea floor, Cenozoic nannofossil oozes and chalks have acoustic anisotropies such that horizontal sonic velocities are 0 to 2.5% faster than those in the vertical direction. In laminated chalk, anisotropy of 5% is typical, and in limestones, radiolarian oozes, porcellanites, and cherts, the anisotropies range from 4 to 13%. Middle Maestrichtian volcaniclastics from 447 meters to 560 meters below the sea floor have an acoustic anisotropy of 2 to 32%; 4 to 13% is typical. Basalt flows and sills occur between 560 meters and 1068 meters, and have no apparent anisotropy, but minor interbedded volcaniclastics have anisotropies from 0 to 20% (5% is typical). These volcaniclastics frequently have very small anisotropies, however, compared with the volcaniclastic sequence above the basalt section.Data in cross-plots of the laboratory-measured compressional sound velocity versus wet-bulk density, wet-water content, and porosity of sediments and sedimentary rock typically lie between the equations derived by Wyllie et al. (1956) and Wood (1941); the Wyllie et al. (1956) equation has a fair fit with similar basalt velocity cross-plots. Crossplots of thermal conductivity and sound velocity indicate only a fair correlation. Cross-plots of thermal conductivity versus porosity, wet-water content, and wet-bulk density correlate well with equations derived by Maxwell (1904), Ratcliff (1960), Parasnis (1960), and Bullard and Day (1961). Electrical formation factor versus porosity for sediments and sedimentary rock agrees with the Archie (1942) equation, with m values of 2.6; for basalt, an m of about 2.1 is typical. Basalt pore-water resistivities do not appear to be greatly different from sea water. Formation factors are greater than those derived from equations in Maxwell (1904), Winsauer et al. (1952), Boyce (1968), and Kermabon et al. (1969). An "apparent interstitial water resistivity" (i?J curve was derived from the density and induction logs. This R wa curve indicated an anomaly, at 393.5 to 396.5 meters, which could be interpreted as (1) 76% hydrocarbons, (2) relatively fresh pore water (1.8‰ salinity), or (3) low-grain-density (2.2 g/cm 3 ) semi-lithified porcellanite-chert. Porcellanite-chert is the most plausible interpretation.In situ temperatures measured by the Uyeda temperature probe were about 2 to 5°C (50%) higher than the equilibrium temperature (Lachenbruch and Brewer, 1959) extrapolated from two Gearhart-Owen continuous temperature logs; this discrepancy probably arises because the hole was washed out in this depth interval, so these extrapolated temperatures are probably not reliable. If one ignores all precautions as to temperature artifacts, then the equilibrium temperatures of the Gearhart-Owen temperature logs suggest that hydrothermal circulation is occurring in at least the upper 40 meters of the basalt section and heat is transferred by convection and not conduction. Hydrothermal circulation is probably not indica...

Basement Logs from the Mouth of the Gulf of California, Deep Sea Drilling Project Leg 65

“…At other sites in old crust, however, such as Site 462 in the Nauru Basin, the basement formation properties determined by logging were almost indistinguishable from those determined in the laboratory, suggesting that the basement consists of massive, relatively unfractured flows or sills (Boyce, 1981). Finally, on Leg 64, a series of logs run at Site 474, where the recovery was low, were of considerable importance in distinguishing the relative abundance and position of pillow basalts, massive basalts, and sediments in the section (Curray, Moore et al, in press).…”

Leg 78B: Background, Objectives, and Geological Setting

“…The tools have included those listed above, plus sonic velocity, temperature, and caliper logs. Although the quality of the data has occasionally been excellent (e.g., Legs 48, 61, 69, and 70;Montadert, Roberts, et al, 1979;Boyce, 1981;Cann and Von Herzen, 1983), many of the logs have suffered from poor hole conditions, operational difficulties, and tool problems. In general, good logs were obtained only by persistence.…”

Massive Early Cretaceous volcanic activity in the Nauru Basin related to emplacement of the Ontong Java Plateau

“…Furthermore, observed thicknesses of basalt sills and flows drilled on the OJP commonly exceed 20 m, reaching a maximum thickness of 65 m . Additionally, sonic logs from DSDP Site 462 showed velocities of 5.5 $ 6.0 km/s for basaltic rock samples [Boyce, 1981;Larson and Schlanger, 1981]. Free-air gravity data aid structural interpretations; a negative gravity anomaly at the transition of the NB and OJP suggests flexure that likely formed by the emplacement of the OJP onto preexisting NB crust (Figure 9), which in simplest terms would require a deeper interface between the base of the OJP and preexisting Late JurassicEarly Cretaceous oceanic crust.…”