Seismic boundary 2A/2B: 1) lithologic boundary between lavas and dykes or 2) alteration boundary

我们对洋壳的了解来自于陆地上的暴露(被称为蛇绿岩)、对其表面的疏浚、绘制水深和物性随深度变化的地球物理调查，以及通过少量的深钻孔。洋壳通常厚约6公里，最初分为沉积物(第一层)、挤压层(第二层)和侵入层(第三层)。在地震速度数据中，我们观察到大洋上部地壳的速度从大约2公里/S迅速增加到5公里/S。人们普遍认为，这种速度的变化与裂缝孔隙度的减少有关，这要么与熔岩向岩墙的地质转变(假设1：熔岩/岩墙)有关，要么与蚀变强度随深度的变化有关，在这种变化中，热液矿化与裂缝闭合有关(假设2：蚀变)。尽管约定各不相同，这里我们将指的是：A层是速度为3公里/S的层，高速度梯度带是2A/2B层的边界或过渡层，2B层是速度为4-5公里/S的低速度梯度层。在年轻洋壳上采集的地震反射数据上，我们观察到在2A/2B过渡范围内的广角到达波。这些到达波可以在常规反射叠加上进行处理以形成明亮的反射面，并使用速度数据转换为深度。这里的关键是，由折射和反射旅行时和/或叠加速度获得的速度既不准确，也不能很好地分辨，这导致了对2A/2B转变的速度和深度控制不佳。项目伙伴在赤道太平洋的洋壳中采集了两个地震数据集。一个位于扩展速率较快的Hess深裂谷(HDR)，另一个位于邻近的Blanco变换断层(BTF)，具有中等的扩展速率。这些剖面是在紧邻HDR和BTF断层陡坎围岩的高原上获得的，实验的目的是直接将地震层2A/2B边界与绘制的地质单元进行对比。初步结果表明，从2A到2B的地震转换接近于HDR绘制的岩墙的顶部，但在沿BTF的岩墙上方的熔岩流动中，这些联合观测表明假设2(蚀变)更有可能。然而，由于目前的速度模型缺乏良好的空间分辨率，对于2A/2B地震过渡的精确深度仍然存在不确定性。我们提出的研究的目的是使用一个模拟整个地震波场的反演程序，以提供一个更准确和更好分辨的速度模型。这将使我们能够更好地将深度约束到2A/2B边界，从而更自信地在两个假设之间做出决定。我们还将能够观察到2A/2B转变的精细结构及其横向变异性，并使用这些数据来推断跨越过渡区和沿过渡区的裂纹孔隙率的变化。该项目有更广泛的应用，包括提供数据，使我们能够利用全球范围内对第2A层厚度的观测，更好地解析大洋地壳结构，并将帮助我们限制洋壳的热收支。我们的结果将被用来规划未来更广泛的洋壳项目，以及确定波形反演码的未来应用。

Our understanding of oceanic crust is derived from exposures on land (termed 'ophiolites'), dredging of its surface, geophysical surveys that map bathymetry and changes in physical properties with depth, and through a small number of deep drill holes. Oceanic crust is typically around 6 km thick and was initially divided into sediments (layer 1), an extrusive layer (layer 2) and an intrusive layer (layer 3). In seismic-velocity data we observe a rapid increase in velocity from around 2 to 5 km/s in the upper oceanic crust. It is widely believed that this change in velocity is related to a decrease in crack porosity, and that this is either related to a geological transition from lavas to dykes (hypothesis 1: lave/dyke), or to a change in the intensity of alteration with depth, in which hydrothermal mineralisation is associated with crack closure (hypothesis 2: alteration). Although conventions vary, here we will refer to: layer 2A as the layer with a velocity of < 3 km/s, the high velocity-gradient zone as the layer 2A/2B boundary or transition, and layer 2B as the layer with a low velocity-gradient in which velocities are > 4-5 km/s. On seismic reflection data acquired across young oceanic crust, we observe wide-angle arrivals that turn within the 2A/2B transition. These arrivals can be processed to form a bright reflector on a conventional reflection stack, and converted to depth using velocity data. The crux here is that velocities obtained from refraction and reflection travel-times and/or stacking velocities are neither accurate nor well resolved, and this leads on to poor velocity and depth control for the 2A/2B transition. Two seismic datasets were acquired by the project partner across oceanic crust in the equatorial Pacific ocean. One is located at the Hess Deep Rift (HDR) which has a fast spreading rate, and the other is located at the adjacent Blanco Transform Fault (BTF) which has an intermediate spreading rate. The profiles were acquired on plateaus immediately adjacent to the walls of HDR and BTF fault scarps, and the purpose of the experiment was to compare directly the seismic layer 2A/2B boundary with mapped geological units. Initial results show that the seismic transition from 2A to 2B is close to the top of the mapped dykes at the HDR, but within the lava flows above the dykes along the BTF, and these combined observations suggest that hypothesis 2 (alteration) is more likely. However, there remains uncertainty in the precise depth of the 2A/2B seismic transition, as the current velocity model lacks good spatial resolution. The purpose of our proposed study is to use an inversion code that models the full seismic wavefield, to provide a more accurate and well-resolved velocity model. This will allow us to constrain better the depth to the 2A/2B boundary, and hence decide more confidently between the two hypotheses. We will also be able to observe the fine-scale structure of the 2A/2B transition as well as its lateral variability, and use these data to infer changes in crack porosity across and along the transition zone. This project has wider applications, including providing data that will allow us to use world wide observations of layer 2A thickness to better resolve oceanic crustal architecture, and will help us constrain the thermal budget of oceanic crust. Our results will be used to plan future broader projects on oceanic crust, as well as identify future applications of the waveform inversion codes.