Potentials of Distributed Acoustic Sensing in Seismic Imaging

Abstract

This thesis investigates the potentials of Distributed Acoustic Sensing in seismology. Distributed Acoustic Sensing (DAS) is an emerging method to measure strain along optical fibers. DAS systems are capable of measuring acoustic and elastic waves propagating along the fiber, and hence can be used as a seismological receiver, where its measurements are related to the symmetric part of the displacement gradient tensor. To investigate the potentials of DAS in seismic imaging, this thesis first establishes the properties of DAS measurements in an extensive instrument response study utilizing data from a wide range of experiments conducted as part of this thesis. Confirming the suitability of DAS measurements for seismological applications from the instrument response study, this thesis then connects strain to rotational measurements, related to the anti-symmetric part of the displacement gradient tensor, and introduces the concept of obtaining rotational observations from DAS recordings. This thesis then extends the existing theory of generalized ambient noise interferometry from displacements to gradient measurements. We show the potential to combine different seismic observables within the framework of interferometry, accounting for the observational effect on the interferometric wavefield due to spatial gradients. With the extended formulation of interferometry, we use adjoint-based methods to incorporate spatial gradient observations into full waveform ambient noise inversion. Based on theoretical investigations, numerical simulations and real-world examples, this thesis ultimately aims to incorporate gradient observations into existing geophysical workflows and to develop new methods utilizing such gradient observations, expanding the fields of theoretical, numerical and observational seismology.