Jökull - 01.06.2000, Page 44

Foulger and Field
Figure 7. (a) The Bouguer gravity field for the Krafla area reduced with a density of 2,300 kg/m3. The
contour interval is 1 mGal. (b) The Bouguer gravity field de-trended by the removal of a regional gradient
of 0.5 mGal/km. The mean of the data is 0 mGal and the contour interval is 1 mGal. Data are from Karlsdótt-
ir et al. (1978). - (a) Bouguer þyngdarsvið á Kröflusvœði, skert við eðlisþyngdina 2,300 kg/m3. (b) Bouguer
þyngdarsvið á Kröflusvœði að frádregnum svœðisstigli sem nemur 0.5 mGal/km. Mæligögnin erufengin frá R.
Karlsdóttur ogfl. (1978).
(Spector and Grant, 1970). The natural logarithm of
the power spectrum is plotted against wavenumber in
Figure 9, and the spectral gradient is -2zg. This ana-
lysis yielded an estimate of Zg of 700 m below dat-
um, or 300 m above sea level, confirming the rough
estimate obtained using the half-widths of the anom-
alies.
An inversion was conducted to determine the
density distribution required in a 1 km thick slab with
its top surface at sea level (Figure 10). Density variati-
ons up to ±150 kg/m3 are required. Inversions using
thinner slabs required proportionately higher density
contrasts, but much thicker slabs, which would require
lower density contrasts, are ruled out by the power
spectrum analysis. Densities higher than average are
required beneath the NW, SW, SE and NE caldera
rims and beneath Leirhnjúkur in the centre of the
caldera. The latter is surrounded by material with
density lower than average, and low density material
is required south of the caldera.
42 JÖKULL No. 48