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

这个项目将开发一种测量地球应变的新方法。地球应变是一种测量由各种原因引起的地面变形的方法，包括地震过程、火山爆发、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情况的模型。测量这些微小的变化需要一个非常特殊的传感器，并且在将其连接到地球上时非常小心。在这个项目中，将测试一种相当新型的应变仪；保护电缆中的光纤将在数百米长的沟槽中拉伸并埋下。精密激光光学将感知埋地光纤长度的微小变化，从而探测到地球应变。如果成功，这种类型的应变计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。除了使许多新的地球物理研究成为可能外，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找到应用。几年前，我们组设计并安装了一个250米长的钻孔光纤垂直应变仪。该仪器通过干涉法记录固定在长钻孔的顶部和底部两端的拉伸光纤的长度变化来检测地球应变。随后，我们在一个180米长1米深的沟槽中开发了一个水平光纤应变仪的原型。当在远震和潮汐带进行分析时，发现光纤应变记录与位于南加州野外站点Piñon Flat Observatory （PFO）附近的730米长的参考激光真空应变仪的记录非常吻合。注意到部署在沟槽而不是井眼中可以节省成本，我们已经确定了水平沟槽传感器的几个重要改进，包括双光纤热补偿方案，简单的支撑-纪念碑耦合系统，以及紧凑，低成本的光电系统，我们有信心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，并开发一种强大，广泛部署，廉价和敏感的光纤长基线应变计，我们将在PFO进行评估。我们的一个长期计划是申请额外的资金来部署这种类型的仪器，一旦证实，研究哥斯达黎加的慢滑事件（ses）， GPS测量表明，在俯冲带的深层和浅层都有频繁的慢滑。在沿海部署仪器将非常接近浅层sse区域，使它们能够以前所未有的细节记录下来。

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.