Jökull - 01.01.2021, Page 49

Bedrock and tephra layer topography within the Katla caldera
The RES processing closely follows Magnússon
et al., (2016) for 2D migrated data and Magnússon
et al. (2017) for 3D migrated data. The RES-data
were acquired by towing a low frequency transmit-
ter (5 MHz centre frequency) and a receiver unit on
sledges, separated by distance, a, varying from 30 to
40 m, with corresponding antennae in a single line
on a snowmobile, equipped with a DGNSS receiver
(Figure 1b). The receiver unit as well as the trans-
mitter used before 2016, in 2017 and partly in 2018
was developed by Blue System Integration Ltd. (see
Mingo and Flowers, 2010). The transmitter used in
2016 and again in 2018 and later was developed by
Sverrisson et al. (1980). The raw RES-data (series
of amplitude/time records) are processed as backscat-
ter images where the x-axis corresponds to the num-
ber of the RES-survey records (each record consists
of 256 or 512 stacked measurements). The y-axis
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