Distributed Acoustic Sensing: Exploring the potential of unconventional fiber configurations

Abstract

Over the last decade, Distributed Acoustic Sensing (DAS) has
gained increased attention as a cost-effective and practical ground motion
monitoring technique, due to the unprecedented temporal and spatial sampling
possibilities. Considering these inherent advantages, DAS-collected strain rate
measurements can be beneficial as addition to traditional particle motion
sensors, or potentially replace them under certain circumstances. A DAS system,
consisting of an interrogator and attached optical fiber, is conventionally
employed in a single (inline) direction providing 1D longitudinal strain rate
measurements along the installed fiber. In this thesis, we explore
unconventional fiber configurations to retrieve additional wavefield
components. To investigate the potential of these unconventional fiber
geometries, we perform numerical tests, discuss real data from a field campaign
in Yverdon (Switzerland) and report on our analysis of synthetic and field
data. Both synthetic and field results show that a loop- and cross fiber
lay-out provide thepseudo-divergence, defined as the sum of the local spatial
velocity gradients (∂xvx+∂yvy), which is proportional to the full wavefield
divergence at the free surface. We confirm that the fiber loop & cross
shape predominantly capture Rayleigh surface waves, but are insensitive to Love
wave energy. An important factor regarding the loop & cross behaviour is
the configuration size with respect to the wavelength (λ). Reduced amplitudes
are recognizable for loop & cross dimensions spanning more than 1/5λ,
followed by additional phase distortions from 1/3λ and completely altered
waveforms for even larger loop & cross sizes.We show that a spherical
configuration (composed of 3 independent orthogonal fiberloops) is
theoretically attractive by enabling the retrieval of both the full divergence
and the individual spatial gradients in the three orthogonal directions. The
free surface-relation between the pseudo- and full divergence can be exploited
to approximate the local ratio of the shear and pressure wave velocity (VS/VP
), which is demonstrated with synthetic data by means of spectral element
modeling. However, the field results are less convincing due to, supposedly,
poor cable-ground coupling as a result of required dense cable winding.Finally,
we introduce a local approach to estimate the effective shear velocity (VS)
without the need for larger arrays. We achieve this by combining local
gradients and particle acceleration data, obtained from the fiber loop and one
centred 3C-geophone respectively. Our novel approach is validated with
synthetic data and we present promising field data results. The proposed
4C-configuration mitigates traditional finite-difference errors related to
tilted or inaccurately spaced ground motion sensors. Furthermore, our novel
arrangement offers the advantage of azimuthal independence, which can be of
interest for VS-estimations from earthquake-, traffic- or potentially ambient
noise recordings. The local character of the effective VS-computation is
dependent on the incident P-wavelength and is therefore potentially exploitable
with regard to 1D shear velocity subsurface models.The presented synthetic and
field results in this thesis show the potential of employing unconventional
fiber configurations with regard to multi-component gradient sensing. This work
further contributes to the development of attractive DAS applications at
various scales, especially related to wavefield separation (ground-roll
suppression), shallow static corrections and near-surface characterization.