Jökull - 01.12.1999, Page 94

DATA
Cameron et al. (1992) measured gravity at 41 stations
in Surtsey in July and August 1989 (Fig. 4) and pre-
sented a Bouguer anomaly map of the island using a
reduction density of 2000 kg/m3 for topography above
sea level. Cameron et al. (1992) estimate the error in
Bouguer anomaly values of data points as 0.3 mGal,
of which 0.26 mGal is attributed to an estimated un-
certainty of 200 kg/m3 in the reduction density. Any
error in density is systematic and is in a way corrected
for in modeling of the gravity field when density val-
ues are assigned to topographic units. Thus, the ran-
dom error of individual gravity points due to errors in
elevation, position, tides, terrain correction etc., is
smaller. We conservatively estimate this random error
as less than 0.2 mGal, assuming that any error due to
terrain corrections is 0.1-0.15 mGal.
The Bouguer anomaly map (Fig. 4) shows that the
greatest variation occurs along lines that run north-
south. We have therefore used these data to model the
Fig. 3. Forward gravity model of Surtsey presented by
Cameron et al. (1992). Densities are in 103 kg/m3 (g/cm3).
The model has no anomalous bodies above sea level. Below
sea level there are 5 bodies with anomalous density. - Líkan
Cameron et al. (1992) af Surtsey. Eðlismassi er gefinn í
g/cm3. í líkaninu er ekki gertfyrir neinum breytileika í eðlis-
massa ofan sjávarmáls en neðan sjávarmáls eru 5 skrokkar.
gravity field along a profile that runs north-south over
the island (Fig. 2, 4). It crosses the crater of Surtur I,
the tuff ring bordering it to the north, the lava field
making up the southern part of the island and the
northern peninsula. Seven data points coincided with
the line and an 8th point is obtained by using the aver-
age of two stations on each side of the profile.
A regional trend of gravity rising to the south (0.5
mGal/km) was removed from the profile. Topography
along the profile was extracted from Moore et al.
(1992) and Einarsson et al. (1994). For bathymetry
below sea level, maps by Norrman and Erlingsson
(1992) and Sjómælingar íslands (1977) were used.
In order to locate the likely transition from tephra to
pillow lava breccia on the southem part of the island,
Fig. 4. The Bouguer anomaly map of Surtsey (after Cameron
et al., 1992). The contour interval is 0.5 mGal. The profile
used for gravity modeling is marked AA', and the dots indicate
where data was collected. - Kort af Bouguer fráviki í Surtsey
(frá Cameron et al., 1992). Bil milli sviðslína er 0.5 mGal.
Þversniðið sem notað er íreikningunum er merkt með AA'.
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JÖKULL, No. 47, 1999