Collaborative Research: Quantitative Estimates of Oceanic Turbulence and Temperature Structure from Seismic Reflection Data

本课题组最近的工作表明，使用低频(10-150赫兹)地震反射剖面法可以非常详细地成像海洋精细结构，这是一种广泛用于固体地球成像的技术，但直到最近才被系统地应用于海洋内部结构的研究。这些发现开辟了物理海洋学的一个新领域，我们称之为“地震海洋学”(SO)。这项新技术发展成为一种工具，可以产生物理海洋学家感兴趣的关于动力学性质的有用(和可信的)信息，正在进入关键阶段。在头三年的努力中取得了很大进展。我们已经对海洋中低频声反射的起源有了基本的物理理解。我们小组和其他人在许多环境中制作了迷人的精细结构图像，包括锋面、徽章、温跃层内透镜、暖核环、水团边界和温盐阶梯，其中一些提出了关于控制海洋精细结构分布的过程的意想不到的问题。我们已经证明，关于内波频谱和温度对比的信息可以从地震数据中收集到。然而，尽管取得了这些初步的成功，但对于从地震数据中收集有用、可靠、定量的信息的程度(以及在什么情况下)，仍然存在相当大的不确定性。地震海洋学还没有找到自己的定位。在这项研究中，要求提供资金用于开发和应用新的二氧化硫分析技术，以帮助确定这一利基。我们将解决两个基本问题：(1)从精细结构的地震图像中可以收集到关于耗散、内波和温度结构的哪些定量信息？(2)该方法的局限性和不确定性是什么？初步结果令人振奋。一种新的等周期斜率水平波数(Kx)谱理论表明，地震反射图像可能特别适合于估计湍流耗散，因为kx谱的湍流子范围扩展到令人惊讶的水平尺度(100米)，这很容易在地震中成像。我们的计算表明，确实可以从地震图像中得到对湍流耗散的合理估计。我们对SO数据进行的全波形反演的初步试验表明，典型的温度精细结构可以用通常采集的频率上的地震数据来分辨，尽管目前不确定性的表征很差。我们建议继续进行方法开发，并将这些技术应用于来自各种海洋环境的大约12个公开可用的遗留地震数据集。通过将我们的结果与来自重合的XBT/CTD信息的地面真相控制进行比较，将产生在大约20个地震剖面上进行完全处理的图像、从kx谱估计耗散以及反演地震波形以估计温度剖面。智力优势：我们正处于开发可能成为海洋精细结构成像的非常有用的工具的形成阶段。初步工作表明，这个工具可以提供关于湍流消散的独特信息，这可能有助于提高我们测量海洋混合和绘制混合“热点”的能力。这里提出的工作将把地震海洋学推向更定量的状态，因此是确定低频声波成像在研究海洋结构和动力学方面的能力和局限性的合乎逻辑的下一步。广泛的影响：这项研究将开发一种新的方法来成像对地球气候起主要控制作用的海洋结构和动力学。我们的研究将对理解海洋混合以及海洋内部热和盐的分布等过程具有重要意义。我们的工作在许多方面服务于NSF更广泛的目标，超越了增加物理海洋学知识的特定领域。具体地说，我们将(1)推进一个新的跨学科领域，以改善海洋地质学、地球物理学和物理海洋学；(2)在这个迅速发展的领域帮助培养一批新的研究生和博士后；(3)让一名本科生参与尖端研究；(4)通过培训两名女学生，支持科学领域的性别多样性。

OCE-0647573Recent work by our group shows that oceanic finestructure can be imaged in great detail usinglow-frequency (10-150 Hz) seismic reflection profiling, a technique that is widely used to image the solid earth but until recently has not been systematically applied to studies of oceanic internal structure. These findings open up a new field of endeavor in physical oceanography, which we call "seismic oceanography" (SO). Development of this new technique into a tool that can produce useful (and trusted) information on dynamical properties of interest to physical oceanographers is entering a critical stage. Much progress has been made in the first three years of effort. We have achieved a basic physical understanding of the origin of low-frequency acoustic reflections in the ocean. Our group and others have produced fascinating images of finestructure in numerous settings, including fronts, Meddies, intrathermocline lenses, warm-core rings, water-mass boundaries, and thermohaline staircases, some of which raise unexpected questions about the processes controlling the distribution of oceanic finestructure. We have shown that information on internal-wave spectra and temperature contrasts can be gleaned from seismic data. Yet despite these early successes, there remains considerable uncertainty about the extent to which (and under what circumstances) useful, reliable, quantitative information can be gleaned from seismic data. Seismic oceanography has yet to find its niche. In this study funds are requested for development and application of new SO analysis techniques that will help determine that niche. We will address two fundamental questions: (1) What quantitative information about dissipation, internal waves, and temperature structure can be gleaned from seismic images of finestructure? (2) What are the limitations and uncertainties of the method? Preliminary results are promising. A new theory of horizontal wavenumber (kx) spectra of isopycnal slopes suggests that seismic reflection images may be especially well suited to estimating turbulence dissipation, as the turbulent subrange of kx spectra extends to surprisingly large horizontal scales (100 m), which are easily imaged seismically. Our calculations show that reasonable estimates of turbulence dissipation can indeed be derived from seismic images. Our preliminary tests of full-waveform inversion to SO data shows that typical temperature finestructure can be resolved by seismic data at commonly acquired frequencies, though uncertainties are currently poorly characterized. We propose to undertake continued method development and apply these techniques to about a dozen publicly available legacy seismic data sets, from a variety of oceanic environments. Fully processed images on ~20 seismic sections, estimate dissipation from kx spectra, and invert seismic waveforms to estimate temperature profiles will be produced by comparing our results to ground-truth control from coincident XBT/CTD information.Intellectual Merit: We are in the formative stages of developing what may become a very useful tool for imaging oceanic finestructure. Preliminary work shows that this tool can provide unique information on turbulence dissipation that may help improve our ability to measure oceanic mixing and map "hotspots" of mixing. The work proposed here will advance seismic oceanography to a more quantitative state and therefore is a logical next step in determining the capabilities and limitations of low-frequency acoustic imaging in study ocean structure and dynamics.Broader Impacts: This study will develop a new approach for imaging ocean structure anddynamics, which exert a major control on the Earth's climate. Our study will have implications for understanding such processes as ocean mixing and the distribution of heat and salt in the ocean's interior. Our work serves NSF's broader goals in numerous ways that go beyond the specific realm of increasing physical oceanographic knowledge. Specifically, we will (1) advance a new cross-disciplinary field to the improvement of both marine geology and geophysics and physical oceanography; (2) help train a new cadre of graduate students and postdocs in this rapidly developing field; (3) involve an undergraduate student in cutting-edge research; and (4) support gender diversity in science, by training two female students.