Jökull - 01.01.2021, Síða 47

Bedrock and tephra layer topography within the Katla caldera
approximately along the overall surface slope direc-
tion of each cauldron area. Most of the 2D migrated
RES-profiles were surveyed on 14–19 May 2016,
2 February 2017, 19 May 2019 and 15 May 2021.
Other 2D migrated data used includes RES-profiles
that have been repeatedly surveyed across many of
the cauldrons since spring 2012, with the aim to de-
tect if substantial water accumulation were occurring
beneath the cauldrons (Magnússon et al., 2017).
A far more detailed survey was carried out for sev-
eral of the cauldron areas and the estimated location of
the 1918 eruption (Guðmundsson et al., 2021), where
profiles with 20 m interval were surveyed (Figure 4a),
enabling 3D migration of the survey data. Such data
sets are not commonly acquired from glaciers, even
though several studies applying this survey approach
have been reported (e.g. Moran et al., 2000; Schlegel
et al., 2020). This approach was adopted for several
subsections of our survey area (total of 14 km2, Figure
3), with most surveyed more than once. K1, K2, K6
and K7 and their vicinity were surveyed twice, and
K10, K11 and K16 five times (Table 1). Only a single
survey was carried out at K9, K17 and the estimated
location of 1918 eruption (survey area labelled E1918
in Figure 3). The focus of the repeated survey was to
study changes in location of bed reflections related to
water accumulation and depletion beneath the caul-
drons (Figure 4d,e). This subject is, however, mostly
beyond the scope of the current paper.
JÖKULL No. 71, 2021 45