Jökull - 01.01.2014, Page 28

B. A. Óladóttir et al.,
Figure 3. Photos of part of the selected soil sections displaying the basalt tephra layers chosen for volume esti-
mation at four different locations (Figure 1). The Óbrinnishólmi section is one of the most distal sections. Note
the „Blue layer“, an easily recognisable tephra marker layer east of the Katla volcano (Table 1). – Ljósmyndir
af hluta fjögurra jarðvegssniða með völdum gjóskulögum. Óbrinnishólmi er í hvað mestri fjarlægð frá Kötlu.
Takið eftir hinu svokallaða Bláa lagi (blue layer) sem er auðþekkt gjóskuleiðarlag austan Kötlu (sjá 1. töflu).
Isopach maps
Isopach maps were drawn manually from point data
based on secure correlation of the selected tephra
layers. Isopach lines were digitized and converted
to a grid using the kriging method in the Surfer©
Golden software map processing software (e.g. Odd-
sson, 2007).
Volume estimations
Volumes were calculated from isopach maps, by using
the Surfer© Golden software. The Surfer© software
defines volume under a function f(x, y) by a double
integral, a method explained in detail in Press et al.
(1988).
Mass =
∫ xmax
xmin
∫ ymax
ymin
f(x, y)dxdy
Volume calculations give compacted tephra vol-
ume and to convert that to freshly fallen tephra we
assume 40% compression as done by Thorarinsson
(1967). To test if the program gives comparable re-
sults to what has previously been published on Katla
volumes an isopach map of the K-1625 eruption,
previously used for volume estimation of the layer
(Larsen, 2000), was redrawn using the Surfer © soft-
ware and treated the same way as used for the prehis-
toric layers in this study. Both methods gave identical
results.
One of the most critical aspects when estimating
volumes is what maximum thickness (Tmax) should
be used. Here, we use Tmax values similar to those
of the historical data (40–140 cm) to obtain compara-
ble data between previously estimated historical vol-
umes and prehistoric volumes. These Tmax values are
based on proximal thickness measurements (∼10 km
28 JÖKULL No. 64, 2014