Jökull - 01.12.1989, Blaðsíða 49

Fig. 7. The flight lines in the aeromagnetic survey.
Mynd 7. Fluglínur segulmœlinganna.
900-1200 meters above sea level. Figure 7 shows the
aeromagnetic survey lines. Figure 8 shows the resi-
dual aeromagnetic field in SW-Iceland after the
removal of a linear trend, henceforth referred to as
the magnetic map.
Further enhancement of geologically interesting
magnetic anomalies depends on defining suitable
criteria for the anomaly separation. Inspection of the
map shows it to be dominated by NE-SW trending
lineaments and the geological significance of this
linearity is quite clear since it is known to be the
strike of the active rifting zone.
The directionality of the map is more rigorously
analyzed by calculating its two-dimensional power
spectrum, shown in Figure 94. If no lineation
4. The spectrum shown in Figure 9 includes 32x32 values and
the wavelength at the edge of the spectral plot is
approximately 8 kilometers. The average distance between
flight lines is about 4 km and the sampling distance along the
lines is about an order of magnitude less. The contour lines
plotted thus correspond to wavelengths longer than the
distance between data points.
occurred in the map the spectrum should consist of
concentric circles, but the strong NE-SW lineation
produces an elongation of the spectrum perpendicu-
lar to that direction. The NE-SW trend can be
separated from the rest of the magnetic field by
directional filtering (Þórarinsson and others, 1988),
and this tums out to be an effective enhancement
technique. The filtering is accomplished by cutting
out a pie slice from the spectrum containing only the
dominant trend and transforming it back to a regular
space domain map, leaving out the rest of the mag-
netic field (Þórarinsson and others, 1988). The posi-
tive magnetic anomalies5 thus separated out of the
magnetic map are shown in Figure 10, henceforth
referred to as the magnetic anomaly map.
In addition to the main trend discussed above, the
magnetic spectrum also contains a high-frequency
peak representing NW-SE striking lineaments. The
main source of this signal is a negative magnetic
anomaly which coincides with the Hafnarfjall-Fer-
stikla gravity high. These cross-grain anomalies,
oriented perpendicular to the rifting zone, are dis-
cussed in more detail below.
INTERPRETATION OF THE POSITIVE
MAGNETIC ANOMALIES
The magnetic anomaly map in Figure 10 is dom-
inated by two large anomalies. The first one starts at
the coast, encompasses the Hengill area and runs up
to the gravity low east of Þingvellir discussed ear-
lier. North of Þingvellir it appears to continue as a
much narrower anomaly up to Skjaldbreiður, but
this could be a filtering effect. The second anomaly
starts in Grímsnes, an area that was volcanically
active up until a few thousand years ago, runs to the
northeast and terminates near the gravity minimum
on the eastern edge of the Langjökull glacier dis-
cussed earlier. Taken together, the two anomalies
5. The directionally filtered map contains both positive and
negative anomalies parallel to the Hengill-Langjökull rifting
zone, but the strongest signal is.due to the positive anomalies
which reflect volcanism in the Brunhes epoch. This signal is
further emphasized in Figure 10 by applying a threshold filter
of +250 nanotesla to the directionally filtered map.
JÖKULL, No. 39, 1989 47