Development of a Temperature Compensated Optical Fiber Strainmeter For Detecting Slow Slip Events

该项目将开发一种测量地球应变的新方法。地球应变是对各种原因引起的地面变形的测量，包括地震过程、火山、潮汐和地下流体运动(例如，石油和天然气生产中的水力压裂)。探测地球应变并绘制其在时间和位置上的变化有助于建立地下深处正在发生的事情的模型。测量这些微小的变化需要一个非常特殊的传感器，并在将其连接到地球上时极其小心。在这个项目中，将测试一种相当新型的应变仪，将保护电缆中的一根光纤拉伸到数百米长的沟槽中并埋入地下。精密激光光学系统将探测到埋在地下的光纤长度的微小变化，从而探测到地球应变。这种类型的应变仪如果成功，将比其他应变测量方法便宜得多，并允许更广泛地使用应变测量来研究地震和人为应变源。除了可能进行一系列新的地球物理调查外，这项工作还将推动光纤传感器技术的发展，并可能在其他学科中得到应用。几年前，我们团队设计并安装了一台250米长的钻孔光纤垂直应变仪。该仪器通过干涉记录固定在长钻孔顶端和底端的张力光纤的长度变化来检测地球应变。随后，我们研制了一台长180米、深1米的水平光纤应变仪样机。当对远震和潮汐带进行分析时，我们发现光纤应变记录与我们南加州皮尼翁平坦天文台(PFO)的730米长的参考激光真空应变仪的记录非常吻合。注意到部署在沟渠中而不是井眼中可以节省成本，我们确定了水平沟槽传感器的几项重要改进，包括双光纤热补偿方案、简单的支撑-纪念碑耦合系统和紧凑、低成本的光电系统，我们相信这些系统将使水平传感器的性能与井眼传感器一样好。我们建议整合这些改进，开发一种坚固耐用、可广泛部署、价格低廉且灵敏的光纤长基线应变仪，我们将在PFO上对其进行评估。我们的长期计划之一是申请额外的资金来部署这种类型的仪器，一旦得到证实，就可以在哥斯达黎加研究慢滑事件(SSE)，那里的GPS测量表明，在俯冲带的深部和浅层，慢滑都很频繁。该仪器的沿海部署将非常接近浅层SSE的区域，使它们能够以前所未有的细节进行记录。

This project will develop a new method for measuring strain in the Earth. Earth strain is a measure of deformation in the ground from a variety of causes including earthquake processes, volcanoes, tides, and underground fluid movement (for example, from hydraulic fracturing in oil and gas production). Detecting Earth strain and mapping its variation in time and location can contribute to models of what is happening deep underground. Measuring these tiny changes requires a very special sensor and extreme care in attaching it to the Earth. In this project, a fairly new type of strainmeter will be tested; an optical fiber in a protective cable will be stretched in a trench several hundred meters in length and buried. Precision laser optics will sense minute changes in the length of the buried optical fiber and hence detect Earth strain. This type of strainmeter, if successful, will be significantly less expensive than alternative methods of strain measurement and allow wider use of strain measurements to study earthquakes and man-made sources of strain. As well as making possible a number of new geophysical investigations, this work will advance the technology of optical fiber sensors and likely find applications in other disciplines.Several years ago, our group designed and installed a 250-m-long borehole optical fiber vertical strainmeter. The instrument detects Earth strain by interferometically recording length changes in a tensioned optical fiber held fixed at the top and bottom ends of a long borehole. Subsequently we developed a prototype horizontal optical fiber strainmeter in a 180-m-long 1-m-deep trench. When analyzed in the teleseismic and tidal bands, excellent agreement is found between both optical fiber strain records and those from the 730-m long reference laser-vacuum strainmeters sited adjacent to them at our southern California field site, Piñon Flat Observatory (PFO). Noting the cost savings from deploying in a trench rather than a borehole, we have identified several important improvements for the horizontal trench sensor, including a dual-fiber thermal compensation scheme, a simple braced-monument coupling system, and a compact, low-cost electro-optical system, that we are confident will make the horizontal sensor perform as well as the borehole sensor. We propose to integrate these improvements and develop a robust, widely deployable, inexpensive, and sensitive optical fiber long-baseline strainmeter that we will evaluate at PFO. One of our long term plans is to request additional funding to deploy this type of instrument, once proven, to study Slow Slip Events (SSEs) in Costa Rica where GPS measurements have indicated frequent slow slip, at both deep and shallow levels of the subduction zone. A coastal deployment of the instrument would be quite close to the zone of shallow SSEs, allowing their recording with unprecedented detail.