MRI: Acquisition of a fiber optic distributed acoustic sensing instrument for hydrological and seismological research

地球在流体压力、构造板块运动以及月球和太阳引力的作用下不断变形。这种变形或应变的浅显表达具有重要的意义。例如，地下流体压力变化引起的地质材料应变必须考虑到地下水的可持续性、碳氢化合物的提取和地热井田的效率。构造力引起可测量的应变，最终导致地震。在建筑物和道路的设计中必须考虑地面变形。月球和太阳的自然引力引起的地球潮汐对人类几乎没有直接影响，但可以用来表征地球材料的力学性质。然而，测量地球变形的一个挑战是，尽管应变分布在很长的距离上，但典型的测量设备(如应变仪)只能测量较短的间隔。为了这个项目，开发了一种传感系统，可以在几公里的距离内以亚米的间隔测量分布应变。虽然该仪器在商业上是可用的，但这种应用是全新的，将揭示以前在浅层地下没有观察到的变形。它迅速被石油和安全行业采用，但由于与服务公司签订合同的成本很高，学术界对此知之甚少。这项收购MRI的目的是将这种下一代商业技术带到学术研究人员和学生手中，这样它就可以用来促进对自然和工程地球系统中地球运动的理解。由加州州立大学长滩分校、加州理工学院和南加州大学组成的财团将共享这台仪器。这个联盟将由加州州立大学长滩分校领导，这是一所非授予博士学位的拉美裔服务大学，大约一半的人口被认为是NSF代表不足的少数族裔(URM)学生。另外两个成员，加州理工学院和南加州大学是授予博士学位的顶尖机构。共享仪器和培训将使URM学生接触到他们通常无法接触到的研究文化和经验。为期两天的DAS培训讲习班将为联盟外的研究人员和学生提供了解DAS技术的机会。核磁共振项目将为URM学生提供机会，让他们成为全国首批亲身体验DAS成像的学生之一。这项由主要研究仪器(MRI)收购的光纤分布式声学传感(DAS)将支持与构造和水力强迫相关的地面运动测量。DAS是一种用于测量地球环境中的应变和振动的变革性技术。利用激光干涉术，动态应变沿光缆的整个长度被询问。DAS通常具有每25厘米1 kHz的采样分辨率，沿长度可达数十公里的光缆。这项技术允许测量通信网络和深井中的地震运动，通常是沿着已经存在的光纤电缆。该系统测量可以安装在深井或浅沟中的光纤电缆的应变。可以检测到每米小于1纳米(十亿分之一米)的菌株。它背后的技术被称为分布式声波传感(DAS)，是为了测量对声波或地震波的响应而开发的。这里的进步之处在于，应变是按接近一天的周期测量的，而振动通常是以不到一秒的周期测量的。一个特别的研究重点将是将频率分辨率扩展到微赫兹范围，在那里可以观察到潮汐强迫和超低频地面运动。这一奖项反映了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。

The earth deforms constantly in response to fluid pressure, the movement of tectonic plates, and the gravitational pull of the moon and sun. The shallow expression of this deformation, or strain, has important implications. For example, strain in geologic material due to changes in subsurface fluid pressure must be considered in the sustainability of groundwater, the extraction of hydrocarbons, and the efficiency of geothermal well fields. Tectonic forces cause measurable strain that ultimately result in earthquakes. Ground deformation must be considered in the design of buildings and roads. The natural pull of the moon and sun cause earth tides which have little direct effect on humans, but can be used to characterize the mechanical properties of earth materials. A challenge in measuring earth deformation, however, is that while strain is distributed over large distances, typical measurement devices such as strain meters measure only over short intervals. For this project, a sensing system is developed that can measure distributed strain at sub-meter intervals over the distance of kilometers. Although the instrument is available commercially, this application is entirely new and will reveal deformation not previously observed in the shallow subsurface. It has been adopted rapidly by the petroleum and security industries but is largely unknown in academia due to the high cost of contracting service companies. This MRI acquisition is intended to bring this next-generation commercial technology into the hands of academic researchers and students so it can be used to advance the understanding of earth movements in both natural and engineered earth systems. A consortium of California State University Long Beach, California Institute of Technology, and University of Southern California will share the instrument. This consortium will be led by a California State University Long Beach, which is a non-PhD granting and Hispanic Serving university with about half the population considered NSF Underrepresented minority (URM) students. The other two members, California Institute of Technology and University of Southern California are top PhD-granting institutions. Sharing of the instrument and training will expose URM students to research cultures and experiences to which they would not normally have access. A two-day DAS-training workshop will provide researchers and students outside of the consortium and opportunity to learn about DAS technology. The MRI program will provide URM students the opportunity to be among the first students in the country to have hands-on experience with DAS imaging.This Major Research Instrumentation (MRI) acquisition of a Fiber Optic Distributed Acoustic Sensing (DAS) will support measurement of ground motion related to tectonic and hydraulic forcing. DAS is a transformative technology for measuring strain and vibration in Earth environments. Using laser interferometry, dynamic strain is interrogated along the entire length of a fiber optic cable. DAS typically has sampling resolution of 1 kHz every 25 cm along a fiber optic cable that can be tens of kilometers in length. This technology allows the measurement of seismic movements along communication networks and in deep boreholes, often along fiber-optic cable that is already present. The system measures strain on a fiber-optic cable that can be installed, for instance, in deep boreholes or in shallow trenches. Strains of less than 1 nanometer (one billionth of a meter) per meter can be detected. The technology behind it is called distributed acoustic sensing (DAS) and was developed for measuring vibration in response to sound or seismic waves. The advancement here is that strain is measured over periods approaching a day, while vibrations are usually measured at periods of less than a second. A particular focus of research will be extension of frequency resolution into the microHertz range, where tidal forcing and ultra-low-frequency ground motions can be observed.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.