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On the potential for fiber-optic distributed magnetic sensing in near-surface geophysics
Snyder, Tomas H.
Snyder, Tomas H.
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2024
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Abstract
Breakthroughs in distributed fiber optic sensing have enabled continuous recording of seismic and temperature data, resulting in unparalleled spatial resolution and coverage at a more affordable cost in remote areas. Electromagnetic data has proven to be useful in providing additional insight into near-surface applications, however, distributed electromagnetic sensing systems are still in the prototype stage. This thesis explores the attributes of a multi-physics optical fiber that records seismic waves and magnetic fields simultaneously and its potential for application in selected near-surface problems.
Current applications of magnetic geophysical methods are discussed and used to inform potential uses of the distributed magnetic sensing fiber, particularly for improved monitoring of seawater intrusion, mine drainage, and lithium brines. Simulations of these potential groundwater application areas are explored with a hypothetical survey design and computational simulation. Preliminary magnetic field sensitivity requirements of the fiber are established based on simulation results.
Laboratory experiments to determine the fiber sensitivity to magnetic fields are performed in addition to field tests that are used to discuss practical application of the fiber in geophysical surveys. Additional testing was performed to provide insight into the variation of the fiber signal over time. Studies are conducted using a fiber with Bragg gratings as well as a fiber without Bragg gratings to guide future fiber design and selection.
The magnetostrictive effect underlies the basic measurement principle of the proposed distributed magnetic sensing. Two-dimensional and three-dimensional modeling of the magnetic fiber based on micromagnetic dynamics and magnetostriction are explored in this thesis to improve the understanding of the mechanisms causing a response in the fiber and ensure data can be reliably modeled, even in the face of nonlinearity. Model sensitivity to source magnetic field amplitude, source frequency, environmental temperature, initial conditions, and the Gilbert damping parameter is explored. Comparisons of model amplitude spectra to laboratory-measured amplitude spectra along with model prediction of fiber sensitivity offer insight into the reliability of the models.
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