Physical properties of ODP Holes 124-768C and 124-770C (Table 1)

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 (Phi) at the two sites is most accurately determinable from transit time (Delta t) and the inverse of velocity: Phi = 0.0054*Delta t - 0.259. Porosity is also closely related to resistivity (Ro), according to the Archie equation: Ro = (Rwas**-m where Rw 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.

研究对768站位钻取的222米厚玄武质弧后地壳几乎全段，以及770站位106米厚洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB）地壳段，均开展了速度、密度、电阻率与中子孔隙度测井记录。通过对上述测井曲线的分析，以及与离散岩芯测量结果的对比，本研究得以明确各测井曲线间的相关关系，并评估单条测井曲线的可靠性。

两处站位的地壳孔隙度（Φ）可通过声波时差（transit time，Δt）与速度倒数进行最精准的计算，其关系式为：Φ = 0.0054×Δt - 0.259。根据阿尔奇公式（Archie Equation），孔隙度同样与电阻率（R_o）密切相关：R_o = (R_w/a)^m，其中R_w为地层流体电阻率，a与m为经验确定的"常数"。测井与岩芯数据均显示，a与m的取值范围分别为2~6.5与1.2~1.6。通过声波孔隙度估算得到的密度值，与密度测井结果整体相近，但可靠性更优。中子孔隙度测井所得数值较实际孔隙度偏高10%~20%，该误差主要源于测井仪器未进行恰当的偏心校正，其次则受岩石中含水蚀变矿物的影响。

本次测得的18 Ma与42 Ma洋壳原位地球物理属性，填补了其他站位6 Ma至110 Ma区间内原位测量数据的空白。观测得到的速度、密度与孔隙度数据，整体与地壳老化模型的预测结果相符。本研究认为该一致性很大程度上属于偶然：研究区观测得到的地球物理属性更多与火山活动样式相关，而非地壳老化过程；且地壳非均质性极强，100~200米的岩芯段无法代表更大尺度的地球物理特征。