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陆地上的暴露（称为“蛇绿岩”），其表面的疏浚，绘制水深和物理性质随深度变化的地球物理调查，以及通过少量的深钻孔。洋壳一般厚约6公里，最初分为沉积层（第1层）、挤压层（第2层）和侵入层（第3层）。在地震速度数据中，我们观察到上洋壳的速度从2公里/秒左右迅速增加到5公里/秒。人们普遍认为，这种速度的变化与裂缝孔隙度的降低有关，这与从熔岩到岩墙的地质过渡有关（假设1：熔岩/岩墙），或者与蚀变强度随深度的变化有关，其中热液矿化与裂缝闭合有关（假设2：蚀变）。尽管惯例各不相同，但在这里我们将把2A层称为速度< 3 km/s的层，高速度梯度区称为2A/2B层边界或过渡区，2B层称为速度> 4-5 km/s的低速度梯度层。在年轻的洋壳上获得的地震反射数据，我们观察到在2A/2B过渡内转向的广角到达。这些波至可以被处理以在常规反射叠加上形成亮反射体，并且使用速度数据转换成深度。这里的关键是，从折射和反射走时和/或叠加速度获得的速度既不准确也不好分辨，这导致2A/2B过渡的速度和深度控制不良。项目伙伴在赤道太平洋的洋壳上获得了两个地震数据集。一个位于扩展速率较快的赫斯裂谷（HDR），另一个位于扩展速率中等的相邻布兰科转换断层（BTF）。剖面是在紧邻HDR和BTF断层崖壁的高原上采集的，实验的目的是将地震层2A/2B边界与绘制的地质单元直接进行比较。初步结果表明，从2A到2B的地震过渡接近HDR映射的堤坝顶部，但在沿着BTF的堤坝上方沿着的熔岩流内，这些综合观测结果表明假设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，2A/2B地震转换的精确深度仍存在不确定性。我们提出的研究的目的是使用一个反演代码，模拟完整的地震波场，提供一个更准确和良好的分辨率的速度模型。这将使我们能够更好地将深度限制在2A/2B边界，从而更有信心地在两个假设之间做出决定。我们还将能够观察到2A/2B过渡区的精细结构及其横向变化，并使用这些数据来推断过渡区上和沿着过渡区的裂纹孔隙度变化。该项目具有更广泛的应用，包括提供数据，使我们能够利用全球范围内对2A层厚度的观测来更好地解决海洋地壳结构，并将帮助我们限制海洋地壳的热收支。我们的研究结果将被用来规划未来更广泛的项目上的海洋地壳，以及确定未来的应用程序的波形反演代码。

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.