Jökull - 01.01.2021, Page 46

Magnússon et al.
emphasis has been placed on improving the bedrock
topography map of the caldera, with specific focus
on the ice cauldrons and their vicinity. In this paper
we present results from RES-field campaigns, carried
out in 2016–2021 on Mýrdalsjökull for this purpose,
as well as some pre-existing RES-data dating back to
2012. The outcome is a new bedrock DEM based on
much denser observations than in 1991 (Figure 2a–
d). The DEM is particularly elaborate around many
of the ice cauldrons where RES-profiles only 20 m
apart were surveyed, allowing for 3D migration of the
reflections (e.g. Moran et al., 2000), greatly enhanc-
ing details and accuracy of the bedrock topography.
The new RES-data was also used to map ice thickness
above the 1918 tephra layer as well as the ice thick-
ness above a tephra layer detected in the northern part
of the caldera and linked here with the 1755 eruption
of Katla using a full Stokes ice flow model. We dis-
cuss the subglacial topographic features revealed in
this study, and imprints of geothermal activity seen
in the topography of the tephra layers, including ev-
idence for previously unknown activity. Finally, we
discuss the possibility of further studies enabled by
the presented data sets.
DATA AND METHODS
RES-data
The RES-data used in this study were acquired in
2012–2021 and cover 116 km2, mostly within the
glacier-filled caldera of Katla central volcano in Mýr-
dalsjökull (Figures 2c and 3). The RES-data allows
construction of a detailed bedrock elevation DEM
and maps of ice thickness above two isochrone layers
marked by tephra fall on the glacier surface during
eruptions. The RES-data set includes approximately
760 km of 2D migrated RES-profiles, which partly
repeats the survey of 1991 in this area (Figure 2a)
but enhances the network with denser profiles. Most
of the area has been surveyed with north-south and
east-west profiles, where distance between profiles
is typically 400–500 m for both directions. For the
cauldron areas, profiles ∼200 m apart were surveyed
Figure 3. The data used for interpolating the new bedrock DEM. Blue lines indicate location of traced bedrock
reflections from 2D migrated RES-profiles used directly without modification. Red lines indicate location of
traced bed reflections in the 2D migrated profiles considered as cross track reflection and therefore, either omit-
ted or shifted cross track (yellow lines) to fit bedrock elevation from crossing RES-profile. Magenta clusters
indicate location of traced bed reflections from 3D migrated RES-data (Table 1) tagged with names of survey
areas (mostly cauldron names). The area outside the white dotted polygon, covered with transparent grey mask
indicates bedrock DEM not modified in this study. The elevation of the previous bedrock DEM at the border
(white dotted line) was used as additional input to the interpolation to secure smooth mosaic of the new and
the unchanged previous DEMs. The elevation at glacier margin and nunataks boundary (cyan line) within the
white dotted polygon were also added as input in the bedrock interpolation. Purple lines indicate manually
digitized elevation contours added in areas where interpolation without any further constraints resulted in odd
topographic features. – Gögn sem eru notuð til að brúa nýtt botnhæðarkort. Bláar línur sýna staðsetningar
botnendurkasta úr íssjársniðum, sem unnin eru með tvívíðri staðsetningarleiðréttingu á endurkastsflötum og
notuð við brúun botnhæðarkorts án frekari leiðréttinga. Rauðar línur sýna endurköst úr samskonar sniðum
sem talin voru vera hliðarendurköst og því annað hvort sleppt í brúunarreikningum eða hnikað til hliðar (gular
línur) þannig að botnhæðir falli að mælingum úr sniðum sem liggja þvert yfir viðkomandi íssjársnið. Pur-
purarauðar skellur, merktar með nafni mælisvæðis (aðallega nöfn katla), sýna staðsetningar botnendurkasta
úr íssjársniðum, unnar með þrívíðri staðsetningarleiðréttingu á endurkastsflötum (1. tafla). Hæðum úr eldra
botnhæðarkorti á hvítu punktalínunni var bætt við sem inntaksgögnum í brúunina til að tryggja samfellu við
það svæði sem er óbreytt frá eldra korti (gráskyggða svæðið utan hvítu punktalínunnar). Einnig voru landhæðir
við jökuljaðar og á útmörkum jökulskerja (innan hvítu punktalínunnar) nýttar við brúun nýja botnkortsins.
Fjólubláar línur sýna handteiknaðar hæðarlínur sem bætt var við brúunargögnin á svæðum þar sem brúun án
frekari skorða skilaði ónáttúrulegu botnlandslagi.
44 JÖKULL No. 71, 2021