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喷发的能力很重要。轴向海山是一座海底火山，位于俄勒冈州海岸300英里处，已经被研究了近40年。这座火山高出周围的海底约3000英尺，其顶端有一个5英里乘2英里的300英尺深的火山口。这座火山分别于1998年、2011年和2015年喷发，最后一次喷发是由一个有线天文台捕捉到的，该天文台是海洋天文台倡议的一部分。天文台包括一个探测地震的地震网络和海底压力传感器，这些传感器显示，随着岩浆进入火山，火山口底部在喷发之间缓慢上升，并在喷发期间迅速下沉，当岩浆释放时。一些海底的上下运动发生在火山口下方的断层上，另一些则是因为火山像气球一样膨胀和放气。哪种机制是最重要的，以及随着火山接近喷发，这种机制是否会随着时间的推移而变化，目前尚不清楚。该项目将安装一个为期10年的4个站的声波应答器网络，该网络将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。一个站将连接到有线天文台，以便实时获得测量结果。这些测量将限制断层运动和岩浆室膨胀和收缩的相对作用。声学网络还将支持学习如何用声学命令控制潜艇机器人的实验，并将一家商业公司制造的校准压力传感器与天文台上已有的两台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。未来，为该项目开发的技术可以应用于其他环境，如俯冲区和不稳定的海底斜坡。在各种各样的环境中研究火山作用是促进我们对火山和喷发预测的理解的最好方法。事实上，大洋中脊火山可能代表了一些最容易理解的系统，因为它们具有浅层岩浆系统，相对统一的岩石学，以及已知的地壳厚度。轴向海山是全球大洋中脊网络中研究最广泛的地点之一。1998年、2011年和2015年观察到了三次喷发，最近一次喷发是由海洋观测站倡议区域电缆阵列上的地震台网和海底压力和倾斜仪记录的。校准的海底压力传感器几乎连续20年记录了通货膨胀和快速同时爆发的通货紧缩的周期，这与喷发是可预测的通货膨胀的假设相一致。然而，垂直大地测量不能完全区分广泛分布的喷口膨胀和喷口抬升，这些喷口是通过埋藏在轴向喷口东西两面墙下的向外倾斜断层上的运动来调节的。该项目将在Axial Semount安装一个为期10年的4站声学网络，该网络将在每对应答器之间覆盖范围，并在每个应答器上安装压力计、温度传感器和速度计。一个站点将连接到OOI-RCA，以实现对应答器阵列的实时数据采集和控制。了解火山口的动力学和环形断层在调节应变和调节喷发方面的作用是火山研究中的一个重要课题。声波网络实现的水平应变测量将结合海底压力观测的持续垂直大地测量和重复的AUV测绘，确定岩浆室膨胀和收缩以及位于东西破火山口墙下的埋藏的向外倾斜断层上的运动的相对作用。这些观测将被用来确定当地震率较低时，向外倾斜的断层是否在再膨胀周期的早期被锁定，或者它们是否在地震中滑动。然后，地震目录可以用来推断耦合程度是否在喷发周期中发生变化。水平测距还将提供数据，以支持比迄今获得的更详细的岩浆室膨胀模型，并将限制跨声波基线传播的岩脉的宽度。第二个目标是提供一个声学网络，以支持展示和优化AUV远程通信和导航的努力，这是部署能够捕捉未来喷发的远程操作AUV的关键一步。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.