Jökull - 01.01.2015, Page 64

Sigurðardóttir et al.
55
21
36
4733
(60)
78
31
105
114
41
40
(14)
32
60 (77)
72
101
122
110
25 m
12 m
18-25 m
8 m
Álftaver
lava field
Ha
Hj
D
ýralæ
kir
63
º2
5’
N
18º45’W
0 4
18º30’W
63
º3
0’
N
C
A
B
Boreholes
VES
(128)
M
úlakvísl
D
km
Figure 3. Loose sediment thickness on
Mýrdalssandur from vertical electrical
soundings (VES, Thorarinsson and Gud-
mundsson, 1979) and research boreholes
on the Álftaver lava field (Vegagerðin,
2013). At each VES the numbers give
sandur thickness estimates while at loca-
tions A, B, C and D, lava thickness is
given. Ha: Hafursey, Hj: Hjörleifshöfði. –
Fyrri rannsóknir á þykkt Mýrdalssands.
Viðnámsmælingar eru merktar með svört-
um punkti með þykkt í metrum fyrir ofan
hvern punkt. Rannsóknarborholur gegn-
um Álftavershraunið eru merktar með
grænum punkti ásamt þykkt hraunsins
fyrir svæði A, B, C og D.
was based on one magnetic profile surveyed south of
Skálm in 1999 by students at the University of Iceland
on a field course in geophysics. The present work was
aimed at exploring this issue through surveying and
analysing a series of magnetic profiles.
METHODS
In total 14 magnetic profiles were measured across the
area where the western margin of the Álftaver lava
field was expected to be located (Figure 4). The lo-
cation of the profiles was pre-determined, with profile
length varying from 1 to 15 km. The profiles were
set approximately perpendicular to the expected edge
of the lava flow. The total length of the profiles is
∼75 km. A GSM-19T v7.0 automatic proton mag-
netometer was used in the survey. It has a <0.1 nT
sensitivity, 0.01 nT resolution and 1 nT absolute accu-
racy (GEM systems, 2010). Most of the profiles were
measured on foot. However, the western most parts of
profiles 4, 5 and 8, total length of ∼24 km, were mea-
sured over snow with the magnetometer mounted on a
wooden sledge pulled 4 m behind a car driven across
the plain. A handheld GPS instrument was used for
positioning of the profiles but the magnetometer has
an inbuilt GPS with differential correction yielding
position accuracy of 1 m. Measurements were done at
2 sec intervals with the total magnetic field in nT and
the GPS position measured. The sensor was placed 2
m above the surface with the GPS antenna one meter
higher, at 3 m height.
Measurements done on foot took a total of 8 days
in June, September and October of 2013. The part
measured by car was done in one day in March of
2014. Days for measurements were carefully chosen
to avoid any high amplitude variations due to mag-
netic storms (Geophysical Institute, 2014; Leirvogur
Magnetic Observatory, 2014). In addition, survey-
ing was only done during fairly dry weather since the
study area is an outwash plain and in very wet weather
rivers can be impossible to cross by foot. Corrections
for diurnal variations (<100 nT) were not considered
to be needed as the anomalies studied are orders of
magnitude larger. Topographic corrections were not
used as the survey area is flat and without appreciable
topography that can give rise to anomalies.
The principal aim of the study was to identify the
edge of the Álftaver lava field beneath Mýrdalssandur
as well as possible ledges within the flow. We de-
termine the maximum depth to magnetic sources, in
particular changes to source depth. After the profiles
had been plotted, sections of individual profiles, with
similar spacing of amplitudes (anomaly spatial fre-
64 JÖKULL No. 65, 2015