EAGER: Development of a Prototype 2D Acoustic Tomography System for Rapid Temperature Measurements in Diffuse Hydrothermal Effluent

EAGER：开发用于快速测量扩散热液流出物温度的原型 2D 声学层析成像系统

• 批准号: 1744255
• 负责人: Eric Mittelstaedt
• 金额: $ 5.18万
• 依托单位: Regents of the University of Idaho
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-15 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1744255&HistoricalAwards=false
• 关键词: EAGER; Development; Prototype; 2D; Acoustic

Mid-ocean ridges, the boundaries between separating tectonic plates, are some of the most volcanically active features on Earth. Along many ridges, storage of molten magma in shallow chambers results in heating of seawater stored in the porous crust. Thermally buoyant, this seawater rises through the crust, is chemically altered by water-rock interactions, and finally exits the seafloor at hydrothermal vents. It is now recognized that this type of deep-sea hydrothermal circulation plays a key role in controlling long-term ocean chemistry, the thermal and chemical structure of the oceanic crust, and the evolution of unique and diverse chemosynthetic ecosystems found nowhere else on the planet. However, quantifying the biological and chemical impact of hydrothermal circulation requires knowledge of the volume, heat, and chemical fluxes exiting the seafloor, which are notoriously difficult to quantify. One large source of uncertainty in flux estimates lies in the widespread distribution of lower-temperature (/=100°C) diffuse hydrothermal fluids, which are commonly transparent and escape through fractures, porous rock, and sediment. It is estimated that diffuse fluids contribute more to the heat and volume fluxes of hydrothermal systems than higher temperature (300°C), "black smoker" style vents. Existing methods to quantify the flux of diffuse hydrothermal venting are limited by small measurement areas, measurements of a single quantity (e.g., only temperature or only velocity), or are invasive and alter the flow as they measure it. This project will develop new technologies for improved measurement of diffuse venting. Researchers will collaborate with University of Idaho, School of Engineering undergraduate students as part of their senior year Capstone Design Course to develop a prototype two-dimensional (2D) acoustic tomography system that can rapidly measure the temperature of anomalously warm, upwelling diffuse fluids across a ~1 m2 area. Development of these technologies will lead to future construction of a deep-sea measurement system targeting diffuse hydrothermal venting.The primary goal of this project is to develop a prototype two-dimensional (2D) acoustic tomography system capable of a time series of rapid (1 Hz) measurements of the temperature of anomalously warm, upwelling fluids at a spatial resolution of centimeters across a ~1 square meter area. The proposed system will consist of ~20-25 acoustic immersion transducers fixed at known spacing on a square, rigid frame. A subset of the transducers (~4-6) will emit staggered acoustic chirp pulses and the system will measure the pulse travel time between the emitting and receiving transducers. Using travel-time acoustic tomography, these travel times will be converted to sound speed and finally temperature throughout the 2D domain. Development will begin with a two-transducer, 1D system to test appropriate signal frequencies (e.g., kHz or MHz), chirp type (increasing or decreasing frequency), transducer shapes (e.g., spherical, planar, cylindrical), and effective measurement rates. Building upon these results, the final prototype will utilize the optimum transducer shape and pulse frequencies. After construction, the system will undergo submergence tests at the University of Idaho with controlled sources of warm, buoyantly rising fluids. To calculate fluid temperatures, we will evaluate several travel-time inversion methods including the algebraic reconstruction technique (ART), multiplicative algebraic reconstruction technique (MART), and simultaneous iterative reconstruction technique (SIRT). Final results will be compared to thermocouple measurements distributed throughout the 2D measurement domain.

大洋中脊，分离的构造板块之间的边界，是地球上火山活动最活跃的特征之一。沿着许多山脊，熔岩在浅层岩浆室的储存导致了多孔地壳中储存的海水的加热。由于热浮力，这些海水穿过地壳上升，在水-岩石的相互作用下发生化学变化，最后通过热液喷口排出海底。现在人们认识到，这种深海热液环流在控制长期的海洋化学、海洋地壳的热化学结构以及地球上其他地方所没有的独特而多样的化学合成生态系统的进化方面起着关键作用。然而，量化热液循环的生物和化学影响需要了解流出海底的体积、热量和化学通量，众所周知，这些都很难量化。通量估算的一个主要不确定性来源在于温度较低（/=100°C）的弥漫性热液流体的广泛分布，这些流体通常是透明的，并通过裂缝、多孔岩石和沉积物逸出。据估计，漫射流体对热液系统的热量和体积通量的贡献大于温度更高（300°C）的“黑烟”式通风口。现有的量化漫漫性热液喷口通量的方法受到测量面积小、测量单一量（例如，仅测量温度或仅测量速度）的限制，或者是侵入性的，并且在测量时改变了流量。该项目将开发新技术，以改进漫射通风的测量。研究人员将与爱达荷大学工程学院的本科生合作，作为他们高年级顶点设计课程的一部分，开发一个原型二维（2D）声学断层扫描系统，该系统可以快速测量1平方米区域内异常温暖的上升流扩散流体的温度。这些技术的发展将为未来深海漫漫性热液喷口测量系统的建设奠定基础。该项目的主要目标是开发一个二维（2D）声层析成像系统原型，该系统能够在1平方米面积内以厘米的空间分辨率对异常温暖的上升流流体的温度进行快速（1hz）的时间序列测量。所提出的系统将由约20-25个声浸泡换能器组成，固定在一个正方形的刚性框架上，以已知的间隔。换能器的一个子集（~4-6）将发出交错的声啁啾脉冲，系统将测量发射和接收换能器之间的脉冲旅行时间。利用声波传播时间层析成像技术，这些传播时间将被转换为声速，并最终转换为整个二维区域的温度。开发将从一个双换能器1D系统开始，以测试适当的信号频率（例如，kHz或MHz），啁啾类型（增加或减少频率），换能器形状（例如，球形，平面，圆柱形）和有效测量率。在这些结果的基础上，最终的原型将利用最佳的换能器形状和脉冲频率。建造完成后，该系统将在爱达荷大学进行水下测试，测试将使用受控的温暖、浮力上升的流体。为了计算流体温度，我们将评估几种旅行时间反演方法，包括代数重建技术（ART）、乘法代数重建技术（MART）和同步迭代重建技术（SIRT）。最终结果将与分布在整个二维测量域的热电偶测量结果进行比较。