Jökull - 01.06.2000, Page 53

Geirfinnur Jónsson and Leó Kristjánsson
-580000 -570000 -560000 -550000 -540000 -530000
330000
340000
350000
360000
370000
-2000 nT
-1000 nT
0 nT
1000 nT
2000 nT
Figure 3. A contour map of the magnetic field over Mýrdalsjökull, based on results shown in Figure 2. Contour
interval is 200 nT. Because of the wide spacing of lines the original data should always be kept in mind when
inspecting this map and the following figure. Dotted line: topographic caldera cf. Figure 1. Coordinates: -x,y
values of the Hjörsey-datum conical projection for Iceland, in meters. – Jafnsviðslínukort af segulfrávikinu á 2.
mynd. Bil milli fluglína er mun meira en bil milli punkta eftir fluglínunni og verður því að skoða þessa mynd og
þá næstu með það í huga.
hy is reduced down to a common depth, in this case
1000 m below sea level. Individual columns are 200
m square and the working area is 251 x 251 poles, i.e.
50 x 50 km, which matches Figures 2-4. The magnetic
field is calculated at a flight altitude of 2100 m us-
ing an overall remanence intensity (Jrem) of 8 A/m,
which is an estimate for Quaternary volcanics in Ice-
land (Kristjánsson, 1970). Figure 5 shows the vertical
component of the modelled field as a shaded reli-
ef. Qualitatively its resemblance to Figure 4 is clear
indicating that the topographic caldera is at least in
part the cause of the magnetic bowl.
Two perpendicular cross-sections over the magne-
tic bowl are shown in Figure 6 along with the calcula-
ted field due to the terrain. We generate a model
incorporating a vertical cylindrical structure of elliptic
cross-section, elongated SE-NW beneath the caldera.
For best fit to the observations, the model was set to
52 JÖKULL No. 49