Jökull


Jökull - 01.01.2021, Page 53

Jökull - 01.01.2021, Page 53
Bedrock and tephra layer topography within the Katla caldera were reviewed. Most observed mismatch was due to the limitation of the 2D migrated RES-profiles. When profiles are not driven parallel to the maximum slope of a steep bed, the traced bed reflection may origi- nate from cross-track bed reflections up-slope from the measurement location, hence the obtained bed re- flections appear higher than the actual bed directly be- neath the profile (e.g. Lapazaran et al., 2016). This is clearly demonstrated by comparison between traced reflections from 2D and 3D migrated RES-profiles around K6 (Figure 5). At locations where this ex- plained the mismatch, the profile more closely match- ing the bed slope direction was kept unchanged while the data from the crossing profiles was either omitted or shifted between 5 m and 130 m cross track in up slope direction to fit the bed elevation of the cross- ing profile (Figure 3). In the few cases where neither profile followed the bed slope direction, both profiles where shifted (< 50 m) cross-track to obtain a match at the crossing point. At a few locations where the mismatch could not be related to the shortcoming of the 2D migrated RES-profiles, the bed-tracing was re- vised. This usually revealed discrepancy in the inter- preted bed reflections for the crossing profiles. In such cases the tracing that seemed more likely to be correct given the surrounding data was kept. The revision of the 2D migrated bedrock data reduced the maximum difference of crossing profiles from 108 m to 8 m. There are various ways to construct bedrock DEM from RES-profile data. In recent studies on Ice- landic glaciers (Magnússon et al., 2012; 2016) the fi- nal bedrock DEMs have been produced by manually modifying elevation contours of a preliminary DEM obtained with kriging interpolation of the RES-data and then interpolating the final product using the mod- ified contours as input for another kriging interpola- tion. This has been done to reduce artefacts of krig- ing interpolation from discrete RES-profiles as well as artefacts caused by the shortcoming of the 2D mi- grated RES-data described above. Applying this ap- proach for a large and dense data set requires time- consuming manual work. The artefacts of the kriging methods are also substantially less prominent than in the case of more discrete RES-profiles, particularly after taking care of mismatch at crossing profiles as explained above. Alternatively, more sophisticated in- terpolation schemes, constrained by physical models and other input data such as glacier surface topogra- phy and velocity observations (e.g. Morlighem et al., 2011; Fürst et al., 2017), could be adopted. However, for a large portion of the study area these schemes would fail without substantial improvements, due to strong basal melting beneath ice cauldrons, which strongly affects the surface topography and motion. Due to the complications of the above interpola- tion methods it was decided follow a relatively sim- ple approach in the creation of the final bedrock DEM (with 20×20 m cell size). Kriging interpolation was applied in Surfer 13 (©Golden Software, LLC), with input data (coordinate list of easting, northing, bedrock elevation) consisting of the filtered and re- vised bedrock traces from the 2D migration, bedrock traces from the 3D migration, elevation of nunataks at their edges, elevation of the previous bedrock DEM (Figure 2b) at the edges of the study area and a few manually created elevation contours, drawn to obtain realistic landforms where RES-data is lacking (Figure 3). The bedrock DEM obtained with this interpola- tion was then mosaicked with the previously existing bedrock DEM outside our survey area and lidar DEM (Jóhannesson et al., 2013, subsampled to 20× 20m cell size) of nunataks, resulting in the final product (Figure 6). The ice thickness map (Figure 7a) is calcu- lated as the difference between a glacier surface DEM, obtained from Pléiades images in 28 September 2019, and the presented bedrock DEM. Extraction of tephra layer data The 2D migrated data from May 2016 and February 2017 along with 3D migrated data from 2017, (around K6, K7, K10, K11 and K16) and 2018 (K1 and K2) was used to map the ice thickness above the 1918 tephra layer. To compensate for the time difference between observations, data from 2017 and 2018 were shifted upwards by 3 and 6 m, respectively, to rep- resent the year 2016. This is based on the crude as- sumption that the layer depth increases linearly with time; in 2016, almost a century after the eruption, the tephra layer was on average at ∼300 m depth. Given the variable depth of the layer and that the vertical mo- tion is expected to decrease with depth (see discussion JÖKULL No. 71, 2021 51
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