Jökull - 01.12.1976, Page 28

Fig. 2. Aeromagnetic anomalies (total-field)
near drill hole K1 (indicated by +). From a
survey in 1959, sponsered by the National En-
ergy Authority.
Mynd 2. Segulkort af Elliðaársvceðinu, teiknað
eftir mœlingum á vegum Orkustofnunar 1959.
Fig. 3. Magnetic field contours in the vicinity
of the Heimaey drill hole in 1973, in thousands
of gamma units (y).
Mynd 3. Segulmœlinganiðurstöður við djúpa
borholu á Heimaey.
from drill liole field measurements. The follow-
ing two examples from Iceland illustrate the
problems involved in the regional field deter-
mination.
Example 1.
The drill hole K1 (Fig. 1 a) in Reykjavík is
very suitable for magnetometer work, being
cooler than nearby holes (66° C at 650 m depth)
and having a diameter of 20 cm. However, in
the vicinity of the hole there are buildings and
power lines, making a comprehensive ground
magnetic survey impossible. A small survey was
carried out on the grid shown in Figs. 1 a and b.
The standard deviation of tlie 52 field values
about their mean is 540 y at 4 m above ground.
Fig. 2 shows total field anomalies, referred
to an arbitrary datum, at 150 m above ground
(G. Gudmundsson, pers. comm. 1973). The
lateral variations in the field on scale 1 km are
clearly of order ± 1000 y.
Example 2.
A deep drill hole is situated on Heimaey,
Westman Islands. A ground magnetic survey
was carried out here (Fig. 3) but is even more
limited in extent than that of Ex. 1 since the
hole is surrounded by buildings, cliffs and the
local harbour. The mean and standard devia-
tion of 69 irregularly spaced measurements were
50700 and 1000 y respectively.
A map of the island (Fig. 4a) shows total-
field contours interpolated from a USNOO
survey at 1800 m altitude in 1964, with total-
field variations of order 100 y. Fig. 4b plots
the total field on a single line flown at 1800 m
on Sept. 8, 1973; it appears to have increased
by some 600 y, due in part to the geomagnetic
secular variation. A parallel line at 300 m alti-
tude (Fig. 4) indicates variations of order
± 300 y.
SUSCEPTIBILITY LOGS
In the light of the previous examples, the
best drill holes to measure magnetically may be
those where thick (— 20 m) beds of relatively
non-magnetic rocks (sediments or clastics) occur
within the basalt sequence; here, one is most
likely to obtain reliable regional field estimates.
The quickest method of finding such beds is
to run a susceptibility log on the cuttings re-
26 JÖKULL 26. ÁR