4 research outputs found
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
Application of virtual seismology to DAS data in Groningen
In this report we investigate whether and under what conditions virtual
seismology via the acoustic Marchenko method can be applied to DAS data from a
survey in the province of Groningen, The Netherlands. Virtual seismology allows
to retrieve the band-limited Green's function between a virtual source at an
arbitrary focal point in the subsurface, while accounting for all orders of
multiples. The method requires the reflection response at the surface and an
estimate of the traveltime between the surface and focal point. However, in
order to successfully apply the method the reflection response needs to be free
from surface waves and other direct waves, and properly scaled in order for the
Marchenko scheme to converge. These limitations severely complicate the
application of the Marchenko method to field data, especially seismic surveys
on land. This report considers a full 2D geophone survey as well as a 1.5D
approximation for a DAS survey, and compares the results of the virtual sources
with an actual dynamite source. The results show that virtual seismology can be
used to recreate the reflections recorded at the surface from the dynamite
source using either geophone or DAS data.Comment: 7 pages, 6 figure
SARS-CoV-2 vaccination modelling for safe surgery to save lives: data from an international prospective cohort study
Background: Preoperative SARS-CoV-2 vaccination could support safer elective surgery. Vaccine numbers are limited so this study aimed to inform their prioritization by modelling.
Methods: The primary outcome was the number needed to vaccinate (NNV) to prevent one COVID-19-related death in 1 year. NNVs were based on postoperative SARS-CoV-2 rates and mortality in an international cohort study (surgical patients), and community SARS-CoV-2 incidence and case fatality data (general population). NNV estimates were stratified by age (18-49, 50-69, 70 or more years) and type of surgery. Best- and worst-case scenarios were used to describe uncertainty.
Results: NNVs were more favourable in surgical patients than the general population. The most favourable NNVs were in patients aged 70 years or more needing cancer surgery (351; best case 196, worst case 816) or non-cancer surgery (733; best case 407, worst case 1664). Both exceeded the NNV in the general population (1840; best case 1196, worst case 3066). NNVs for surgical patients remained favourable at a range of SARS-CoV-2 incidence rates in sensitivity analysis modelling. Globally, prioritizing preoperative vaccination of patients needing elective surgery ahead of the general population could prevent an additional 58 687 (best case 115 007, worst case 20 177) COVID-19-related deaths in 1 year.
Conclusion: As global roll out of SARS-CoV-2 vaccination proceeds, patients needing elective surgery should be prioritized ahead of the general population