Experiences with Distributed Acoustic Sensing using both straight and
helically wound fibers in surface-deployed cables -- a case history in
Groningen, The Netherlands
Distributed Acoustic Sensing (DAS) has been limited in its use for
surface-seismic reflection measurements, due to the fiber's decreased
sensitivity when the fiber is deployed horizontally. Deploying the fiber in a
helically wound fashion has the promise of being more sensitive to broadside
waves (e.g. P-wave reflections) and less sensitive to surface waves than
straight fiber. We examine these claims by burying a set of straight fibers
(SF) and helically wound fibers (HWF) with different wrapping angles, using
standard and engineered fibers. These fibers were buried in a 2 m deep trench
in a farmland in the province of Groningen in the Netherlands. They are linked
up to two interrogating systems and an electrically driven vibrator was used as
a seismic source. We observe in our field data that using HWF has a destructive
effect on the surface-wave amplitudes. Our data confirmed the effect of the
wrapping angle on the polarity of the surface-wave arrival and the dampening
effect of the helical winding, both behaving in quite a predictable fashion.
Apart from the effect of the wrapping angle, the different design choices, e.g.
cable filling and material type, did not show a significant effect on the
amplitude of the signals. As for P-wave reflections, we observe that both
engineered SF and HWF provide reflection images comparable to those obtained
from the geophone data despite the straight fiber's decreased broadside
sensitivity. A polarity reversal and an amplitude difference between SF and HWF
fibers are observed. Finally, we show that the combined use of SF and HWF
proved to be useful since SF showed better sensitivity in the shallower part
and HWF in the deeper part.Comment: This manuscript has been submitted to GEOPHYSICS journa