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。 地球应变是一种测量地面变形的方法，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可以有助于建立地下深处发生的事情的模型。 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时非常小心。 在该项目中，将测试一种相当新型的应变仪;保护电缆中的光纤将被拉伸到数百米长的沟槽中并被掩埋。 精密的激光光学器件将感测埋在地下的光纤长度的微小变化，从而检测地球应变。 这种类型的应变仪，如果成功的话，将大大低于其他方法的应变测量，并允许更广泛地使用应变测量来研究地震和人为的应变源。 这项工作不仅使许多新的地球物理调查成为可能，而且将促进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，我们小组设计并安装了一台250米长的钻孔光纤垂直应变仪。 该仪器通过干涉法记录固定在长钻孔顶端和底端的张紧光纤的长度变化来检测地球应变。 随后，我们开发了一个原型的水平光纤应变计在180米长的1米深的沟槽。 当分析在潮汐带，光纤应变记录和730米长的参考激光真空应变仪位于邻近他们在我们的南加州现场，皮农平天文台（PFO）之间的协议被发现非常好。注意到部署在沟槽而不是钻孔中的成本节省，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，低成本的电光系统，我们相信这将使水平传感器的性能与钻孔传感器一样好。 我们建议整合这些改进，并开发一个强大的，广泛部署，廉价，灵敏的光纤长基线应变仪，我们将在PFO评估。 我们的长期计划之一是要求额外的资金来部署这种类型的仪器，一旦得到证实，研究哥斯达黎加的慢滑事件（SSE），GPS测量表明在俯冲带的深和浅层次经常发生慢滑。 该仪器的沿海部署将非常接近浅层SSE区域，从而能够以前所未有的细节记录它们。

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.