DOI: 10.5285/99f0d269-d8fa-48e0-89e6-358486d82330
title: Downhole distributed acoustic seismic profiles at SkyTrain Ice Rise, West Antarctica, January 2020

description: A distributed acoustic sensing (DAS) experiment was undertaken at SkyTrain Ice Rise in the Weddell Sea Sector of West Antarctica. The aim was to evaluate the use of DAS technology using existing infrastructure and for delineating the englacial fabric to improve our understanding of ice sheet history in the region. Three walkaway profiles were acquired at 45 degree intervals using a hammer and plate source. Both direct and reflected P- and S-wave energy, as well as surface wave energy, are observed using a range of source offsets recorded using fibre optic cable. Significant noise results from the cable hanging untethered in the borehole. At greater depth, where drilling fluid is present, signal strength is sufficient to measure seismic interval velocities and attenuation. Fieldwork was part of the BEAMISH Project (NERC AFI award numbers NE/G014159/1 and NE/G013187/1). John Michael Kendall was supported by additional funding from NERC award No. CASS-166. The Skytrain borehole and fibre optic cable are part of the University of Cambridge WACSWAIN Project (EU Horizon 2020 agreement No. 742224).
description: In the 2018/19 Antarctic field season, a 651 m ice core was collected at Skytrain Ice Rise, West Antarctica. A multi-mode fibre optic cable was deployed to allow DTS temperature measurements. In January 2020 the site was revisited to conduct a VSP experiment using the existing fibre optic cable and a Silixa iDAS interrogator unit with GPS timing. A 1.0 kVA petrol generator was used to power the interrogator unit. The interrogator was housed in a tent to protect it from the wind and maintain a working temperature. Data were recorded at 8000 Hz sampling frequency. Strain rate measurements were made at 1 m intervals along the fibre but with an effective resolution of 10 m due to the "gauge length" or strain rate averaging process. The fibre optic cable is 600 m long, with 595 m deployed down the borehole, and includes a minibend where an identical length of fibre within the cable is spliced and returned to the surface, enabling two measurements to be made at each depth and a raw data length of 1200 m. The DAS method measures longitudinal strain. Therefore, with the fibre optic cable vertical, only the vertical strain component of seismic waves propagating past the fibre is measured. Three walkaway profiles with seismic sources on the snow surface were acquired at 45 degrees to one another. Line names reference the orientation with respect to magnetic north. A 4.5 kg sledgehammer with a rigid polyethylene impact plate stamped into the snow surface acted as the energy source. Profiles were acquired out to 600 m offset from the borehole with hammer blows at 50 m intervals. Two hammer blows were made at each shot point out to 350 m and four hammer blows from 400 m and beyond. The interrogator unit recorded in continuous mode with shot times derived from impulsive arrivals on a continuous 1000 Hz sampled geophone recording made adjacent to the hammer plate. The fibre optic cable is hanging freely in the borehole, not frozen to the walls. Drilling fluid remains in the borehole from the previous season, filling approximately the lower half of the hole. The upper half of the fibre therefore hangs freely in air and is poorly coupled to the walls of the borehole. The lower half of the fibre is supported in the drilling fluid.
description: Seismic source: 4.5 kg hammer with rigid polyethylene impact plate. Seismic source timing: Reftek RT130 (serial no. bcbf) with 3-component geophone located adjacent to the hammer plate recording at 1000 sps. DAS: Silixa iDAS interrogator (serial number - 15040) with GPS timing recording at 8000 sps with 10 m gauge length. Fibre optic cable: 2x50 μm multimode fibre 600 m long with minibend. All shots within a 600 m radius of the borehole. DAS data were recorded at 8000 Hz sampling frequency. Strain rate measurements were made at 1 m intervals along the fibre but with an effective resolution of 10 m due to the "gauge length" or strain rate averaging process. Shot times are derived from a geophone located adjacent to the plate, recorded at 1 ms sample interval. Raw data from the geophone located at the shot point are provided for verification. Shot locations were surveyed in from the borehole location using compass and tape measure (Line ST130) and handheld GPS (Lines ST040 & ST085) giving a location uncertainty of +/-5 m.
description: The fibre optic cable is deployed in an open borehole with drilling fluid in the lower half (approximately). The cable is not frozen-in to the hole or to the wall of the hole. The signal in the upper half, which hangs in air, is therefore dominated by harmonic noise and weak coupling. Signal from the lower half, which sits in the drilling fluid, is stronger and noise levels are lower.