To assess the geometry of cracks in highly altered peridotites, we analyzed the ultrasonic velocity of serpentinized dunites and harzburgites collected by the Oman Drilling Project (Holes BA1B, 3A, and 4A). First, we estimated the hydrated fraction from grain density to obtain the porosity-free matrix velocity, which indicated complete serpentinization at shallow depths and decreasing hydration at greater depths. We assume that the difference between the solid matrix and measured onboard ultrasonic velocity is attributed to cracks with a spheroidal shape in the samples. Application of the effective medium theory to onboard data, such as P-wave velocity and porosity, indicates that the average pore aspect ratio is mostly between 0.1 and 0.01, and crack density varies from 0.58 to 0.02. We found a positive relationship between aspect ratio and hydrated fraction, suggesting a change in crack shape related to dissolution–precipitation processes during hydration. The relatively high aspect ratio and hence high fluid flux at shallow depths are also consistent with the onboard resistivity data and present-day hydration processes inferred from the borehole fluid chemistry. The inversion of ultrasonic data provides a series of elastic moduli that can be used to make a rough approximation of Poisson's ratio from the onboard data, which is a key physical property for interpreting geophysical observations in the oceanic lithosphere.

为了评估高度蚀变橄榄岩中裂缝的几何形状，我们分析了阿曼钻井项目（BA1B、3A 和 4A 孔）收集的蛇纹石化纯英岩和菱镁矿的超声波速度。首先，我们从颗粒密度估计水化部分以获得无孔隙度的基质速度，这表明在浅层完全蛇纹石化，在更深的深度减少水化。我们假设固体基质和测量的机载超声波速度之间的差异归因于样品中的球状裂纹。将有效介质理论应用于纵波速度和孔隙度等船上数据表明，平均孔隙纵横比大多在0.1~0.01之间，裂缝密度在0.58~0.02之间变化。我们发现纵横比和水合分数之间存在正相关关系，这表明与水合过程中的溶解-沉淀过程相关的裂纹形状的变化。相对高的纵横比和因此在浅层的高流体通量也与船上电阻率数据和从钻孔流体化学推断的当今水化过程一致。超声波数据的反演提供了一系列弹性模量，可用于从船上数据粗略近似泊松比，这是解释海洋岩石圈地球物理观测的关键物理属性。相对高的纵横比和因此在浅层的高流体通量也与船上电阻率数据和从钻孔流体化学推断的当今水化过程一致。超声波数据的反演提供了一系列弹性模量，可用于从船上数据粗略近似泊松比，这是解释海洋岩石圈地球物理观测的关键物理属性。相对高的纵横比和因此在浅层的高流体通量也与船上电阻率数据和从钻孔流体化学推断的当今水化过程一致。超声波数据的反演提供了一系列弹性模量，可用于从机载数据粗略近似泊松比，这是解释海洋岩石圈地球物理观测的关键物理属性。