Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 31

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serve as 52000-gamma baselines for the plotting of magnetic field
strength, as done by Sigurgeirsson (1). The field values are plotted
at right angles to the track, positive being upwards or to the right.
In case of ambiguities in lines running SW-NE, it should be kept
in mind that the broken line is always “above” 52000v. The scale
used is that of Sigurgeirsson, with 1000 gamma being equivalent
to 2.5 km on the map. In order to visually distinguish positive and
negative anomalies, a 1965 sea-level field was computed from re-
gional results of the Dominion Observatory of Canada (quoted by
(1)) and updated by the addition of 340y in 1972, 380y in 1973,
using the annual total-field means of Leirvogur Observatory (7).
It fits the observed marine results quite well.
The regional field has been plotted on the magnetic maps as
broken lines. The smoothed observed field values were plotted manu-
ally at every 5 minutes of track and a smooth curve then drawn
through them (Figs. 2,3). The tracks are too far apart to allow
the construction of reliable contour maps of total-field intensity.
3. INTERPRETATION OF DATA
A. Amplitude-depth relation
The shelf has a smooth, gently sloping surface (Fig. 1), the
average westwards slope in areas 9 and 7 thus being about 0.1°,
and the mean numerical difference between 795 successive depth
readings at 3-km intervals is there around 10 m. Sparker data
(K. Thors and A.Bjömsson, pers. comm. 1974) though incomplete,
reveal that the thickness of shelf sediments is generally much less
than the respective water depth. If the basement surface is also
smooth, and the character of the strata is broadly uniform over the
survey areas, there should be some inverse correlation between
anomaly amplitudes and water depth (see (8)).
This question has been put to a quantitative test in the following
empirical way, arrived at after testing several alternatives: For all
10-minute survey intervals in areas 9 and 7, a straight average of
the 20 digitized field values was computed, as well as the standard
deviation of these values from the average. These resrdts were then
grouped by water depth into seven non-overlapping depth ranges