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Towards airborne measurements of ground displacement
Rapstine, Thomas D.
Rapstine, Thomas D.
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2020
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Abstract
Valuable resources and risk assessments are realized by mapping variations of the physical composition and structure at the surface and in the subsurface of the Earth. A key measurement for realizing these benefits is measuring ground displacement as a function of time, which enable geophysicists to conduct seismic studies for inferring subsurface structure and composition, and enable geomorphologists to study and monitor landslides. It is difficult to measure ground displacement due to seismic waves or landslide failure since such phenomena evolve rapidly and exhibit large dynamic ranges. In addition, logistic, environmental, and safety concerns limit when and where seismic or landslide measurements can occur. These issues motivate the need for rapid, accurate, remote, and possibly airborne measurements of displacement time series. Simultaneously addressing all of these issues is challenging since conventional sensors capable of accurately measuring ground displacement, such as a geophones, require physical coupling to the ground and only measure displacement at one location. However, advancements in the field of computer vision, together with rapid development of drone and camera hardware, provide a foundation for rapid and accurate airborne displacement measurements, thus aiding geophysicists and geomorphologists in their respective studies. In this thesis, I describe multiple theoretical advances that enable remote video-based measurements of subtle rapidly varying ground displacement for applications in geophysics and geomorphology. Based on the methods and applications discovered in this thesis, I draw four main conclusions. (1) Earthquake signals can theoretically be measured using drone-borne stereo cameras, provided that the camera position is known with sufficient accuracy. (2) Video cameras can be used to remotely and accurately measure ground surface displacement and velocity during a rapidly evolving landslide. (3) When static markers are present, the position and velocity of a debris-flow front can be measured with accuracy comparable to that of lidar, but with much denser spatial distribution. (4) Using measurements acquired from an airborne platform, ground displacement can be recovered accurately under reasonable assumptions about the platform motion. These conclusions demonstrate that airborne video measurements are on the brink of becoming a viable solution for obtaining ground displacement. Airborne measurements are faster and safer than the state of art, cover a wider area, and dramatically reduce environmental impact of seismic acquisition surveys while maintaining the accuracy needed for a multitude of geophysical applications.
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