Jökull - 01.01.2021, Page 56

Magnússon et al.
Again, results from 2D migrated data were not used
if the area was covered with data from 3D migration.
The result from both the 2D and 3D migrated RES-
data was exported as a list of coordinates (easting,
northing, tephra layer elevation). The tephra layer el-
evation tends to reflect the surface topography; hence
variation in depth down to the tephra layer relative to
the glacier surface is of more interest than the tephra
layer elevation. A map of tephra layer elevation ob-
tained by interpolating directly tephra layer elevation
from discrete profiles would lack correlation with the
surface elevation in areas where RES-data is miss-
ing. RES-profiles showing e.g. the tephra layer at
approximately the same elevation on each side of an
un-surveyed topographic surface high would result in
a flat plane cutting through this high if the tephra layer
elevation were interpolated. Consequently the depth
to the tephra layer at the surface high would be overes-
timated. By interpolating the depth to tephra layer rel-
ative to the glacier surface, such artefacts are avoided.
The tephra layer coordinate lists were therefore com-
pared with surface elevation DEM obtained from Pléi-
ades optical satellite images in 27 September 2016 to
replace the third column of the lists (tephra layer ele-
vation) with depth down to tephra layer relative to this
surface DEM. This coordinate list was used as input
into kriging interpolation to calculate an ice thickness
map (grids with 20×20 m cell size) above the corre-
sponding tephra layer (Figure 8a,b).
The detected tephra reflections are in most cases
from gentle sloping layers directly beneath the survey
profile, not due to cross-track reflections. Steep tephra
slopes can occur, in vicinity of prominent cauldrons,
where strong subglacial melting bends the glacier
isochrones towards the glacier bed. The 1918 tephra
layer is extractable from the 3D migrated data near
most of the ice cauldrons. The most prominent ex-
ception is in the vicinity of K13 and K14, where
only 2D migrated data was available and used uncor-
rected, since the difference in tephra layer depth at
the few crossing profiles was within ∼10 m. How-
ever, the width of the depression in the tephra layer
near these cauldrons may be underestimated by sev-
eral tens of metres. This shortcoming of the 2D mi-
grated RES-profile also explains a ∼10 m difference
between tephra layer depths for crossing profiles at the
centre of K19. The shallower trace was considered as
cross-track reflection and omitted, but the deeper one
a reflection from tephra layer directly below the radar
and therefore included in the record used as input into
the tephra layer interpolation.
RESULTS AND RELATED
DISCUSSIONS
Topographic features
Primary results lie in a revised DEM of the glacier
bed (Figure 6) and a map of ice thickness within the
caldera (Figure 7a), confirming the main features of
the older DEM (Figure 2b) and the thickest ice in the
northern part of the caldera, to be up to 740±40 m,
in accordance with previous studies (Björnsson et al.,
2000). Uncertainty of 5% is assumed, mostly due to
uncertain cgl (Lapazaran et al., 2016), which has not
been measured specifically for the study area. The
amount of ice within the caldera rim (as defined by
Björnsson et al., 2000) was 45±2 km3 in Septem-
ber 2019. The complex topography within the caldera
described in Björnsson et al. (2000) is further high-
8. mynd. – a,b) Kort af þykkt íss í september 2016 ofan 1918 gjóskulagsins (a) og eldra gjóskulags í norðurhluta
öskjunnar (b). Kortin eru brúuð út frá gjóskulagsendurköstum sem greinast í íssjármælingunum en staðsetning-
ar þeirra eru sýndar sem gráar línur (tvívíð staðsetningarleiðrétting) og skellur (þrívíð staðsetningarleiðrétt-
ing). c) Dæmi um tvívítt staðsetningarleiðrétt íssjársnið, frá A til B á mynd (a), þar sem greina má endurkast
frá báðum gjóskulögunum auk endurkasts frá botni. d) Snið á ísaskilum Entujökuls og Kötlujökuls (frá C til D á
b og á innskotsmynd) sem sýnir brúaða botnhæð og gjóskulög í september 2016. Til samanburðar er sýnd lega
jafnaldurslaga reiknuð miðað við æstætt hreyfisvið á þeim stöðum sem íssjársnið þvera ísaskilin (innskotsmynd
sýnir hvar gjóskulags og botnendurköst greinast í nágrenni þeirra). Yngstu reiknuðu lögin svara til gosa sem
gætu hafa skilið eftir sig gjóskulög sem enn greinast með íssjá í jökulísnum.
54 JÖKULL No. 71, 2021