Jökull - 01.01.2013, Page 11

The Stardalur magnetic anomaly, SW-Iceland
Figure 3. Aerial photo of fields of the Stardalur
farm. The main farm buildings are in the north-
east part of the photo. Access to the farm is from
the Reykjavík – Þingvellir highway #36, seen near
the south edge of the figure. Smoothed total-field
contours in nT (1000 nT = 1 µT) are included,
based on ground measurements at the points shown
in Figure 4. – Loftmynd af Stardal, bæjarhús eru
norðaustantil. Afleggjari kemur að bænum frá veg-
inum yfir Mosfellsheiði, sem sést neðst. Jafnstyrks-
línur fyrir nokkur gildi heildarsviðsins í 2 m hæð
frá jörð hafa verið teiknaðar inn á myndina, eftir
minniháttar útjöfnun mæligilda.
Figure 4. Sketch of some of the roads and tracks (red),
streams (blue) and farm buildings in Figure 3, along with
points where total-field measurements were made. The
three squares show the dimensions of vertical uniformily
magnetized columns used for simulating the anomaly
peak; see text. However, it is more likely that the actual
body causing the anomaly has a smoothly sloping sur-
face. – Skissa af vegum, slóðum, ám og byggingum á 3.
mynd, ásamt mælipunktum fyrir sviðsstyrk í megin-toppi
segulfráviksins. Þverskurðarfletir lóðréttra bergsúlna sem
notaðar voru sem einfalt líkan til að reikna nálgun fyrir
frávikið, eru sýndir. Sjá nánar í megintexta.
topographic features in the fields of the farm were not
taken into account.
– The northernmost square column has its upper sur-
face at a depth of 35 m from the probe, the central one
at 90 m and the southern one at 150 m.
Gravity was measured at seven points along the
road towards the main Stardalur farm buildings (Fig-
ure 3), and at two points on the track towards west
from these buildings. A smooth regional trend is seen,
except for a point situated on the 65 µT contour 500 m
south of the main buildings where a positive anomaly
reaching 1–2 mgal above the trend occurred. It has
no clear relation to the magnetic anomaly. It could
be due to a small intrusive body of appreciably higher
density than the 2500 kg m−3 found for the magnetic
lava series.
GROUND SURVEY AT HVANNEYRI IN
2012
Þ. Sigurgeirsson made separate surveys at low alti-
tude, probably 300 m, over the magnetic anomaly
sites at Ferstikla in southwestern Iceland and at
Hvanneyri in western Iceland (Figure 1). Records
from the former survey seem to have been lost, but
a contour map based on Sigurgeirsson’s three flight
lines at Hvanneyri was published by Kristjánsson et
al. (1989). The contours of this map (Figure 5a) trend
30–40◦ east. The peak within the 56 µT contour in
the aeromagnetic results occurs over flat meadowland
which lies a few m above sea level. To the east and
southeast of these meadows, the ground slopes up
to about 20 m a.s.l. We have made total-field mea-
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