Greinar (Vísindafélag Íslendinga) - 01.01.1976, Blaðsíða 33

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It is clear from Fig. 4, as well as from a comparison of Figs. 2
and 3, that magnetic anomalies are larger in the 1972 surveys than
in the 1973 survey. In fact, after making corrections due to the
slower survey speed in 1973, the anomalies in areas 9 and 7 aver-
age very nearly twice those in area 10 in the depth ranges between
55 and 300 m (Fig. 4). In the shallower of these depths, the dif-
ference between the areas may he partly explained by rough
topography in a zone of about 30 km width north and west of the
Snaefellsnes peninsula, but generally there must he a difference in
the magnetic character of the rocks in these areas.
Puranen et al. (8) point out that when the water depth becomes
smaller than the dominant horizontal wavelength of magnetization
contrasts, the deviations of field values should approach a corres-
ponding quantity for the rock magnetization variations (multi-
plied by 2jt if Gaussian units are used). Such a levelling off is not
seen in the data of Fig. 4, and may only occur at less than 20 m
above the basalt basement.
B. Causative bodies
The magnetic field undulations after smoothing are similar to
those at 900 m altitude over flood basalts in Southwestern Iceland
(1). Their width is commonly 5-20 km, and amplitudes are typi-
cally of order 1000y in the 1972 survey, 500y in the 1973 survey.
Sigurgeirsson (1) and Serson et al. (2) have shown that there is
much less linearity in the anomalies over Iceland and the shelf than
over the nearby active ocean-ridge areas. The reasons for this have
been discussed by several authors, including Kristjansson (5,10)
and Piper (11), who provided data on average magnetization values
in Icelandic rocks and carried out model calculations.
In the present surveys, anomalies can rarely be traced across
many adjacent survey lines, though in Figs. 2 and 3 a few ex-
amples have been emphasized by heavy broken lines. Their trend
is consistent with the general strike of the tilted lava pile in the
westem part of the Northwest peninsula. The largest of tliese linear
anomalies may be produced by a few hundred meters thickness of
uniform-polarity lavas, as found to correlate with aeromagnetic
anomalies on shore (11,12).