Fiber Optic Distributed Strain Sensing for Seismic Applications
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Fiber Optic Distributed Strain Sensing for Seismic Applications Thomas Reinsch1, Philippe Jousset1 and Charlotte M. Krawczyk2,1 1 GFZ German Research Centre for Geosciences, Potsdam, Germany 2 Institute for Applied Geosciences, Technical University Berlin, Berlin, Germany
In recent years, fiber optic sensor technologies have been increasingly used for seismic applications. Different acquisition strategies have been applied to measure the subsurface deformation: measurements at a single point using specially engineered fiber optic point sensors, quasi-distributed measurements along an optical fiber with an array of point sensors, or fully distributed, where deformation is detected using interaction of light with the molecular structure of the fiber itself. We will consider fully distributed technologies in the following, only.
Definition Optical Fibers Fiber optic distributed strain sensing relies on the interrogation of an optical fiber using an appropriate light source, typically a laser. When light travels through an optical fiber, it interacts with the fiber glass structure. The interaction can be elastic (Rayleigh) or inelastic (Raman or Brillouin) scattering phenomena. Among other, the scattered light can carry information about the current elongation of the optical fiber. Comparing the elongation from successive measurements, strain changes can be detected with a high temporal and spatial resolution. If a fiber optic cable is coupled to the ground, these strain changes are a measure of the ground motion and can hence be used for seismic applications.
Introduction To determine crustal properties distribution, seismic source processes, and wave propagation mechanisms, the acquisition of dense seismic and ground motion datasets is required (e.g., Jousset et al. 2018). Seismic and ground-motion datasets are typically acquired measuring acceleration, velocity (e.g., geophones and broadband sensors), or position (e.g., GNSS sensors) with individual sensors located in a favorable geometry. A high-resolution image of the subsurface, thereby, requires a dense spatial coverage of sensors.
When light is coupled into an optical fiber, it is typically guided within the core of the fiber. Optical fibers are mainly made of doped silica glass to engineer a desired refractive index. The core of a fiber is surrounded by a clad, also made of silica but with a lower refractive index. For a given angle of incidence, light propagating within the core towards this interface fulfils the condition for total reflection, is reflected back towards the core and hence confined within the fiber. If only a single mode of propagation is possible for a given wavelength, these types of fibers are called “single-mode fibers,” in contrast to “multi-mode fibers” where multiple propagation modes are possible. Single-mode fibers that are mostly used for distributed strain sensing have typical diameters for the core/clad of 8–10/125 mm, multi-mode fibers often 50 or 62.5/125 mm, respectively (Refi 1998). In order to protect the optical fiber from external forces and to
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