New observations of the shallow seismic structure of young oceanic crust
New observations of the shallow seismic structure of young oceanic crust
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年轻洋壳浅层地震结构的新观测
DOI：
10.1029/jb092ib09p09351
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发表时间：
1987
影响因子：
--
通讯作者：
G. Purdy
中科院分区：
文献类型：
--
作者：
G. Purdy
本文介绍了在北纬23°附近的大西洋中脊进行的三项新实验的结果，这些实验采用了一种独特的方法来研究大洋地壳最上层几百米的地震结构。这些数据是使用固定的海底水听器接收器和可控的爆炸源收集的，爆炸源被拖到崎岖的海底地形几十米以内。这些1至2公里长的折射线首次直接观测到年轻火成岩地壳最上部200-300米的纵波速度结构。在海洋钻探计划的648 B孔的位置上，在MAR中谷内的一座小火山上进行了一次实验，观测到海底速度为2.1 km s-1，其下有一个近似线性的4 s-1速度梯度。鉴于我们知道这个位置的地壳由新鲜的玄武岩熔岩组成，实验室测量的速度超过5.8 km s−1，推断海底的孔隙率高达30-50%。另外两个实验位于7百万年以上在MAR以西395号深海钻探项目现场附近的一个古老地壳，横向仅相隔14公里，但水深超过1 400米。该实验位于100米厚的沉积物池中，并位于南部14公里处的一个突出的地形高点上，产生的结果在数据分辨率范围内是无法区分的。最上层玄武岩基底的速度为4.1 km s−1，速度梯度小于0.5 s−1。这两个地点之间的速度差为0.2km s-1，被认为是地壳物理性质与年龄有关的变化的结果。如果主要变化仅是总孔隙度，则需要减少15-20%来解释这些观察结果。这似乎不太可能，因为任何这种普遍的过程都将在地球物理和采样数据中观察到（例如，离轴火山作用、次生矿物沉积、构造挤压）。要使孔隙率降低，必须将许多过程结合起来。如果裂缝和空隙的几何形状以正确的方式进行修改，那么不需要总孔隙率的变化来解释速度差异。这些海底折射剖面图被证明是实用的实验，可以提供适合海底许多地质特征的精确信息。它们产生的结果可以以有意义的方式与海洋钻探和井下测量的结果相关联。
The results are presented of three new experiments carried out on the Mid-Atlantic Ridge (MAR) near latitude 23°N using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust. The data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography. These 1- to 2-km-long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200–300 m of the young igneous crust. One experiment was carried out over the site of hole 648B of the Ocean Drilling Program on a small volcano within the median valley of the MAR. The seafloor velocity was observed to be 2.1 km s−1 underlain by an approximately linear velocity gradient of 4 s−1. Given that we know the crust at this location consists of fresh basalt lavas with laboratory-measured velocities in excess of 5.8 km s−1, porosities at the seafloor of as high as 30–50% are inferred. The two other experiments were located over 7-m.y.-old crust near Deep Sea Drilling Project site 395 west of the MAR, separated by only 14 km laterally but by over 1400 m in water depth. The experiment positioned in the ∼100-m-thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable. The velocity of the uppermost basaltic basement was 4.1 km s−1, and the velocity gradient was less than 0.5 s−1. This ∼2 km s−1 difference in velocity between the two sites is assumed to be a consequence of age-related modifications to the physical properties of the crust. If the primary change is in only the total porosity, then a 15–20% reduction is required to explain these observations. This seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data (e.g., off-axis volcanism, secondary mineral deposition, tectonic compression). A combination of many processes acting in unison would be necessary to produce such a decrease in porosity. If the geometry of the cracks and voids were modified in just the right way, then no change in total porosity is required to explain the velocity difference. These on-bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor's geologic features. They produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements.