Distribution of subglacial sediment layers around a DAS-instrumented borehole, Store Glacier, Greenland
Authors: Booth Adam , Christoffersen Poul , Chapman Joseph , Schoonman Charlotte , Hubbard Bryn , Chudley Thomas , Doyle Samuel , Law Robert , Clarke Andy , Chalari Athena 
Published: March 2021

&lt;p&gt;Distributed acoustic sensing (DAS) involves detecting seismic energy from the deformation of a length of optical fibre cable, offers considerable potential in the high-resolution monitoring of glacier systems. Subglacial conditions and sediment properties exert a strong control on the basal sliding rate of glaciers, but identifying the connectivity of drainage pathways and their hydraulic conductivity remains poorly understood. This is due in part to the limitations of instrumental methods to monitor these processes accurately, whether by locating cryoseismic emissions in passive seismic records or actively imaging the subglacial environment in seismic reflection surveys.&amp;#160; Here, we explore the application of a borehole survey geometry for constraining the thickness and distribution of subglacial sediment deposits around a DAS installation on Greenland&amp;#8217;s Store Glacier.&lt;/p&gt;&lt;p&gt;Store Glacier is a fast-moving outlet of the Greenland Ice Sheet. The instrumented borehole is drilled near the centre of a drained supraglacial meltwater lake, 28 km upstream of the Store Glacier terminus, and within 100 m of an active moulin, representing a continuous supply of water to the glacier bed. The borehole, which&amp;#160;terminates at the glacier bed at a depth of 1043 m depth, is instrumented throughout its length with Solifos BruSENS fibre-optic cable, and monitored with a Silixa iDAS&lt;sup&gt;TM&lt;/sup&gt; interrogator. A suite of ~30 vertical seismic profiles (VSPs) was recorded at various azimuths and offsets (up to 500 m) from the borehole, using a 7 kg sledgehammer source.&amp;#160;&lt;/p&gt;&lt;p&gt;Initial analyses of VSP data implied a 20 [+17, -2] m thickness of sediment immediately beneath the borehole. These analyses are refined by considering the full suite of VSP data, to map spatial variations in the thickness of subglacial sediment layers.&amp;#160; This is undertaken using an iterative ray-tracing scheme, which seeks to minimise the differences in the arrival-time of direct seismic energy and subglacial reflections received at various depths in the borehole. Englacial compressional (P-) wave velocities are measured from cross-correlating direct arrivals (= 3700 &amp;#177; 75 m/s in the upper 800 m of the glacier, 4000 &amp;#177; 75 m/s between 880-950 m, 3730 &amp;#177; 75 m/s through basal ice). For the subglacial sediment, we use a P-wave velocity of 1839 m/s, consistent with a value constrained in nearby surface seismic reflection data. To improve the definition of subglacial reflections and the constraint of their arrival times, data are first enhanced using frequency-wavenumber filtering.&lt;/p&gt;&lt;p&gt;Our approach suggests that sediment thickness is ~30 m directly beneath the borehole, potentially thinning by 10 m approximately 75 m further south. In reality, the seismic velocity through the sediment layer is unconstrained, but travel-time variations are themselves indicative of changes in either P-wave velocity and/or sediment thickness. Our work further highlights the interpretative potential of borehole DAS approaches, in support of conventional surface-based seismic analysis.&lt;/p&gt;