An Acoustic Array At Axial Seamount for Geodesy and Autonomous Vehicle Support

对各种火山的研究对于了解火山的工作原理和提高预测火山喷发的能力非常重要。轴向海山是一座海底火山，位于俄勒冈州海岸300英里外，人们对它的研究已经有近40年的历史了。这座火山高出周围的海底约3000英尺，其顶部有一个5英里乘2英里的火山口，深300英尺。这座火山分别在1998年、2011年和2015年喷发，最后一次喷发是由海洋观测站倡议（Ocean Observatories Initiative）的一个有线观测站捕捉到的。该观测站包括一个地震探测网络和海底压力传感器，这些传感器显示，岩浆进入火山时，火山口底部在喷发期间缓慢上升，岩浆释放时，火山口底部迅速下沉。海底的上下运动有些是由于火山口下方断层的滑动造成的，有些是由于火山像气球一样膨胀和收缩造成的。哪种机制是最重要的，以及当火山接近喷发时，这种机制是否会随着时间的推移而变化，目前还不得而知。该项目将安装一个为期10年的4站声波转发器网络，监测声波在站之间传播的时间，以测量站之间水平距离的变化。一个观测站将连接到电缆观测站，以便实时获得测量结果。这些测量将限制断层运动和岩浆房膨胀和收缩的相对作用。声学网络还将支持实验，学习如何用声学命令控制潜艇机器人，并将一家商业公司制造的校准压力传感器与天文台上已经安装的两个学术校准压力仪器进行比较。该项目将从代表性不足的群体中培养一名研究生和至少一名本科生实习生。在未来，该项目开发的技术可以应用于其他环境，如俯冲带和不稳定的海底斜坡。在各种各样的环境中研究火山活动，是提高我们对火山和火山爆发预测的认识的最好方法。事实上，洋中脊火山可能代表了一些最容易理解的系统，因为它们有浅岩浆系统，相对统一的岩石学，以及已知的地壳厚度。轴向海山是全球洋中脊网络中研究最广泛的地点之一。在1998年、2011年和2015年观测到三次喷发，最近的一次是由地震台网和海洋观测倡议（OOI）区域电缆阵列（RCA）上的底部压力和倾斜仪器记录的。校准过的海底压力传感器记录了近20年的连续记录，记录了膨胀和快速同步喷发的收缩周期，这与火山喷发是可预测的假设是一致的。然而，垂直测地观测不能完全区分火山口的广泛分布的膨胀和隆起，这是由埋在轴向火山口东、西壁下的外倾断层的运动所调节的。该项目将在轴向海山安装一个为期10年的4站声学网络，该网络将覆盖每对应答器，并包括每个应答器上的压力表、温度传感器和测速仪。一个站将连接到OOI-RCA，实现实时数据采集和应答器阵列的控制。了解火山口的动力学和环形断层在调节应变和调节喷发中的作用是火山研究的一个重要课题。声波网络实现的水平应变测量，将与正在进行的海底压力观测的垂直测量和重复的AUV测绘相结合，确定岩浆房膨胀和收缩以及位于东西火山口壁下的向外倾斜断层的运动的相对作用。这些观测结果将用于确定向外倾斜的断层是否在地震率较低的再膨胀周期早期被锁定，或者它们是否发生了地震滑动。然后，地震目录可以用来推断耦合水平在喷发周期中是否变化。水平范围也将提供数据，以支持比迄今为止获得的更详细的岩浆室膨胀模型，并将限制沿声学基线传播的岩脉的宽度。第二个目标是提供一个声学网络，以支持演示和优化auv的远程通信和导航，这是部署远程操作的auv的重要一步，可以捕捉未来的火山喷发。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Studies of all kinds of volcanoes are important to understand how volcanoes work and improve the ability to forecast eruptions. Axial Seamount is a submarine volcano that lies 300 miles off the coast of Oregon that has been studied for nearly four decades. The volcano rises about 3000 ft above the surrounding seafloor and its summit is characterized by a 5-mile by 2-mile caldera that is 300 ft deep. The volcano erupted in 1998, 2011 and 2015, with the last eruption captured by a cabled observatory that is part of the Ocean Observatories Initiative. The observatory includes a seismic network that detects earthquakes and seafloor pressure sensors that show that the caldera floor rises slowly between eruptions as magma moves into the volcano and sinks rapidly during eruptions when magma is released. Some of the up and down movement of the seafloor occurs by slip on faults that underlie the caldera and some occurs because the volcano inflates and deflates like a balloon. Which mechanism is most important and whether this changes with time as the volcano nears an eruption is unknown. This project will install a 10-year 4-station network of acoustic transponders that will monitor the time for sound waves to travel between stations to measure changes in the horizontal distance between stations. One station will be connected to the cabled observatory so that measurements will be obtained in real time. These measurements will constrain the relative roles of fault motions and magma chamber inflation and deflation. The acoustic network will also support experiments to learn how to control submarine robots with acoustic commands and compare a calibrated pressure sensor built by a commercial company with two academic calibrated pressure instruments that are already on the observatory. The project will train a graduate student and at least one undergraduate intern from an underrepresented group. In the future, the techniques developed for this project can be applied in other settings such as subduction zones and unstable submarine slopes. Efforts to advance our understanding of volcanoes and eruption forecasting are best served by studying volcanism in a wide variety of settings. Indeed, mid-ocean ridge volcanoes may represent some of the most tractable systems to understand because they have shallow magmatic systems, relatively uniform petrology, and known crustal thickness. Axial Seamount is one of the most extensively studied sites on the global network of mid-ocean ridges. Three eruptions have been observed in 1998, 2011 and 2015, with the most recent recorded by the seismic network and bottom pressure and tilt instruments on the Ocean Observatories Initiative (OOI) Regional Cabled Array (RCA). There is a near continuous 20-year record from calibrated seafloor pressure sensors recording cycles of inflation and rapid syn-eruptive deflation that are consistent with the hypothesis that the eruptions are inflation predictable. However, the vertical geodetic observations cannot fully discriminate between broadly distributed inflation and uplift of the caldera that is accommodated by movement on the buried outward-dipping faults that underlie the east and west walls of the Axial caldera. This project will install a 10-year 4-station acoustic network at Axial Seamount that will range between each pair of transponders and include a pressure gauge, temperature sensor and velocimeter on each transponder. One station will be connected to the OOI-RCA enabling real time data acquisition and control of the transponder array. Understanding the dynamics of calderas and the role of ring faults in accommodating strain and modulating eruptions is an important topic in volcano research. The horizontal strain measurements enabled by the acoustic network will, in conjunction with ongoing vertical geodesy from seafloor pressure observations and repeat AUV mapping, determine the relative roles of magma chamber inflation and deflation and motion on the buried outward dipping faults that lie beneath the east and west caldera walls. These observations will be used to determine whether the outward dipping faults are locked early in the reinflation cycle when earthquake rates are low or whether they slip aseismically. The seismic catalog can then be used to infer whether the level of coupling varies during the eruptive cycle. The horizontal ranging will also contribute data to support more detailed models of magma chamber inflation than have been obtained to date and will constrain the width of dikes that propagate across the acoustic baselines. A secondary objective is to provide an acoustic network that can support efforts to demonstrate and optimize remote communication and navigation of AUVs, an essential step toward the deployment of remotely operated AUVs that can capture future eruptions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.