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

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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.