Geophysical Properties of Oceanic Crust at Sites 768 and 770

Abstract

Velocity, density, resistivity, and neutron-porosity logs were recorded in virtually the entire 222-m-thick section of basaltic back-arc crust drilled at Site 768 and through the 106-m section of MORB crust at Site 770. Our analysis of these logs and comparison with measurements on discrete cores permit determination of interlog relationships and evaluation of the reliability of each log.Crustal porosity (Φ) at the two sites is most accurately determinable from transit time (At) and the inverse of velocity: Φ = 0.0054 At -0.259. Porosity is also closely related to resistivity (R o ), according to the Archie equation: R o -Rwd<f" 1 where /? w is resistivity of the formation fluid, and a and m are empirically determined "constants." Both logs and cores indicate that a and m range from 2 to 6.5 and 1.2 to 1.6, respectively. Density values estimated from sonic porosity are broadly similar to, but more reliable than, density logs. Neutron-porosity logs yield values 10%-20% higher than actual porosities; this error is caused primarily by lack of proper tool eccentralization and secondarily by the presence of hydrous alteration minerals in the rocks.These in-situ geophysical properties of oceanic crust 18 Ma and 42 Ma help to bridge the in-situ measurement gap between 6 Ma and 110 Ma at other sites. Observed velocities, densities, and porosities are generally similar to predictions from crustal aging models. We infer that this correspondence is largely coincidental; observed properties here are related more to style of volcanism than to crustal aging, and crustal heterogeneity is so high that a 100-200-m-interval is not representative of larger scale geophysical properties.

Physical properties of upper oceanic crust: Ocean Drilling Program Hole 801C and the waning of hydrothermal circulation

“…For upper crustal basalts, however, matrix density often decreases with increasing porosity, because of greater alteration in the more porous and therefore more permeable rocks. This pattern has been observed in several other studies [ Hamano , 1979; Christensen et al , 1980; Carlson and Herrick , 1990; Jarrard and Broglia , 1991], including prior analyses of the upper basalts from Hole 801C [ Busch et al , 1992; Jarrard et al , 1995]. Jarrard et al [1995] modified the equation above to include this effect, based on the upper 801C basalts, and we used their equation to convert the new Hole 801C density log into a porosity log.…”

“…Thirteen holes met both criteria: 395A, 396B, 417D, 418A, 504B, 556, 558, 564, 765D, 770C, 801C, 843B, and 896A (Table 2 and Figure 1). Basement logs for these sites [ Kirkpatrick , 1978; Salisbury et al , 1979; Cann and Von Herzen , 1983; Hill and Cande , 1985; Broglia and Moos , 1988; Moos , 1990; Jarrard and Broglia , 1991; Goldberg and Moos , 1992; Jarrard et al , 1995; Shipboard Scientific Party , 1978a, 1978b, 1979a, 1979b, 1983, 1985a, 1985b, 1985d, 1990a, 1990b, 1990c, 1993, 1998, 2000a] vary in length, from a minimum of 41 m at Site 843 to 1827 m at Hole 504B. Most sites have 90–200 m of velocity log (Figure 3), and only three (395A, 418A, and 504B) have more than 300 m. We confined our analyses to the top 300 m of basement, thereby preventing our intersite comparisons from being biased by intrasite velocity gradients.…”

“…Several other studies have examined the relationship between basalt porosity and velocity, based on core or log data. Those that have examined variations in matrix velocity have noted reduced matrix velocities at high porosity, attributed to alteration [ Carlson and Herrick , 1990; Jarrard and Broglia , 1991]. A velocity/porosity cross plot for Hole 801C (not shown) indicates that low‐alteration samples (based on LAS) are only subtly higher in velocity, by ∼200 m/s, than more altered samples.…”

“…Velocity/formation factor relationships for oceanic basalts have not been analyzed previously, but some porosity/formation factor log analyses have used sonic‐based porosities because density‐based porosities were poor or unavailable [ Jarrard and Broglia , 1991; de Larouzière et al , 1996]. At log scale, the velocity/formation factor relation may depend on alteration intensity [ de Larouzière et al , 1996].…”

Influence of sedimentation, local and regional hydrothermal circulation, and thermal rebound on measurements of seafloor heat flux

“…[19] Basement porosity and thermal properties assigned to various layers in the models that follow are based on observations from drilling and associated experiments [e.g., Bartetzko et al, 2001;Becker et al, 1983;Busch et al, 1992;Jarrard and Broglia, 1991;Pezard and Anderson, 1989;Shipboard Scientific Party, 2005]. The upper 100-1000 m of basement is thought to comprise the hydrothermal aquifer most responsible for advective heat loss on ridge flanks, and is assigned the highest porosity and lowest thermal conductivity (Table 1).…”