Jökull - 01.01.2010, Side 140

Agustsdottir et al.
2. Forward modelling, where details of dome form
are examined, including the possible existence
of a root extending into the crust, and whether
the assumption used in Nettleton’s method of a
single bulk density of a formation is valid.
Nettleton’s method (Nettleton, 1976; Kearey
et al., 2002) of density determination involves tak-
ing gravity observations over an isolated topographic
prominence. Field data is reduced using a series
of different densities for the Bouguer and terrain
corrections. The density that provides a Bouguer
anomaly with the least correlation with the topogra-
phy is taken to represent the mean bulk density of the
topographic formation (Kearey et al., 2002). The Net-
tleton method works where formations are not buried
or where formation and surroundings have the same
density. A Nettleton profile determines the mean bulk
density of an entire mountain, but can give a mislead-
ing result if the formation is partly buried in lava or
sediments.
It is common that the density obtained from Nett-
leton’s method is less than the mean density of rock
samples from the same formation, especially in frac-
tured and porous rock. This discrepancy arises be-
cause 5–10 cm diameter rock samples do not include
the volume occupied by large fractures or voids. Thus,
a systematic difference is to be expected between the
two methods. However, the overestimation of bulk
density found using the mean of the densities of rock
samples, is likely to be similar for porous and frac-
tured rocks, regardless of composition. This is sup-
ported by comparisons of Nettleton profiles and rock
samples (Agustsdottir, 2009). The accuracy of esti-
mating the bulk density in this study by Nettleton’s
method is considered to be ±100 kg m−3 (Agusts-
dottir, 2009). Figure 3 shows the Nettleton’s profile
over Hlíðarfjall. We note a slight local rise in Bouguer
anomaly immediately adjacent to the formation (Fig-
ure 2). Comparing Figure 2 with Figure 3 we con-
clude that this dome has insignificant roots.
Gravity forward modelling
Forward, 2.5-D gravity models are generated using
the GravMag sofware (Pedley et al., 1997), using the
background density 2500 kg m−3 (Johnsen, 1995) and
the density values obtained for each formation with
the Nettleton method. The model profiles are con-
structed by subtracting a regional field obtained as the
linear fit (since all the profiles are short) that best rep-
resents the mean trend of of the Bouguer anomaly
at the location of the profile. Bodies are assumed
to strike perpendicular to the survey line and a finite
strike length can be assigned, i.e. the true length of
the formation perpendicular to strike can be used as
the length of the modelled body.
RESULTS
Density values
The main results are that all the domes are of low den-
sity, reflecting both low grain-density and high poros-
ity (Table 1). Table 1 also shows that the dome’s den-
sity values are significantly smaller than those of the
surroundings. Table 2 shows the volume and mass of
the formations.
Table 1. Mean bulk densities (ρ̄) for each profile (Fig-
ure 1), determined by the Nettleton method (ρN ) and
rock samples (ρs). The density of the surroundings
are from Johnsen (1995). – Meðaleðlismassi ákvarð-
aður með aðferð Nettletons fyrir hvert snið á 1. mynd
og út frá grjótsýnum (ρs). Eðlismassi umhverfisins
(bakgrunns-eðlismassi) samkvæmt Johnsen (1995).
ρ [kg m−3]
Location profile ρN ρs ρ̄
Hlíðarfjall HF1 1600 2060 1700
HF2 1800 2060
Hrafntinnuhr. HR1 1575 1750 1692
HR2 1875 1750
HR3 1625 1750
Hraunbunga HB1 1775 1950 1763
HB2 1750 1950
Surroundings 2500
Gravity models
Gravity models for all the formations show consistent
results. Therefore only one selected profile from each
formation is presented. Three models are presented
for Hlíðarfjall and Hrafntinnuhryggur, four for Hraun-
bunga. The three models are defined as:
140 JÖKULL No. 60