Greinar (Vísindafélag Íslendinga) - 01.01.1976, Page 30

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resolution to tíie nearest 10 gamma (1 gamma (y) = 1 nT). Posi-
tion data and depth were recorded manually by Hydrographic
Office staff every 10 minutes, and kindly made available to the
author. The magnetic records were digitized manually by the au-
thor at 30-sec intervals (except in a major part of area 10, where
they had been recorded digitally on tape by staff of the U.S. De-
fence Mapping Agency). All these data were punched on IBM
cards and thoroughly screened for spurious errors before proces-
sing.
As the surveys took place in shallow waters (20—330 m in 1972
and 40-720 m in 1973) the results are often much too detailed for
direct plotting. AU the digitized field values have therefore been
smooíhed before plotting by a triangular window of total width 10
minutes, i.e. generally 2.5 - 3.3 km, along the track. This is also
equivalent to upward continuation by some hundreds of meters
and therefore facilitates comparison with the aeromagnetic maps
of Sigurgeirsson ((1) and unpublished) on shore.
At the time of these surveys, 29 July — 10 Aug. 1972 and
18-24 July 1973, considerable geomagnetic disturbances occurred,
necessitating corrections to the data. Comparisons with analog and
digital magnetograms kindly fumished by staff of the Leirvogur
Magnetic Observatory (Fig. 1) at track intersection times in the
1972 survey showed that the best corrections were obtained from
manually smoothed Z-component analog records. At 15 cross-check
points where one or both legs were traversed during geomagnetic
activity, this corrections was found to reduce the average numerical
difference between these from 140 to 53 gamma. Subsequently,
the smoothed marine magnetic results have been corrected by
means of the Leirvogur Z-records whenever the field departed from
quiet-day conditions by more than 60 gamma.
In case of large (amplitude > 100y) disturbances of short periods
(< 15 min.) the Leirvogur corrections may not be rehable, and
the smoothed field has been plotted as a broken curve. It has also
been plotted as a broken curve in one line of area 9, where waters
are very shallow and anomalies consequently very sharp, and parts
of several lines in that area could not be processed at all due to the
sharpness of these anomalies.
The straight lines used to approximate the survey tracks also