Jökull - 01.01.2010, Side 139

A gravity study of silicic domes in the Krafla area, N-Iceland
Figure 2. Three cases of expected Bouguer anomaly
from a topographic formation. A1: Formation has no
crustal root and a density (ρ1), significantly less than
the reduction density (ρ0). A2: Formation has a topo-
graphic root and density ρ1 < ρ0. Note the difference
in the form of the anomaly flanking the formation for
A1 and A2. B: No root but formation density higher
than reduction density (ρ1 > ρ0). – Bouguer þyngd-
arsvið frá fjalli. A1: Fjallið hefur ekki rót og eðlis-
massi fjallsins (ρ1) er lægri en sá eðlismassi sem not-
aður er til að reikna Bouguer leiðréttingu (ρ0). A2:
Fjallið hefur lágan eðlismassa og rót sem nær niður
í jarðskorpuna. B: Engin rót en eðlismassinn (ρ1) er
hærri en viðmiðunar eðlismassinn (ρ0).
The gravity field was measured along profiles,
preferably perpendicular to the strike of the forma-
tion, and gravity stations were placed at 0.2–0.5 km
intervals. Gravity readings were made using a Lacoste
and Romberg (G-445) gravimeter. A kinematic GPS
Trimble 5700 was used for elevation measurements at
each station. All surveys were carried out on foot and
all equipment carried by two surveyors. A combined
Bouguer and topographic correction was calculated
by direct integration of a topographic model with a
pixel size of 25 m (Gudmundsson and Milsom, 1997;
Gudmundsson and Högnadóttir, 2004). A Bouguer
anomaly was then obtained by subtracting the correc-
tion from the free air anomaly.
The accuracy of a gravity survey is limited by
the gravimeter, terrain corrections and the accuracy
in measured elevation. The gravity meter has an accu-
racy of 0.01 mGal (LaCoste and Romberg Inc., 1979).
The elevation accuracy in this study is equal to or bet-
ter than 0.5 m which causes a 0.1 mGal error in the
Bouguer anomalies. The combined accuracy of the
random uncertainty due to terrain and Bouguer cor-
rections (resulting from integration of the elevation
model) are minor and is considered to be better than
0.1 mGal. Total accuracy for measurements and pro-
cessing is therefore 0.2 mGal (Agustsdottir, 2009).
Rock samples
Rock samples were gathered from each formation.
Usually five representative samples for density deter-
mination were collected at important gravity stations
along the survey profiles. A total of 95 samples were
collected at 15 sites. The dry and wet densities of
these samples were obtained in the laboratory from
the difference in their weight in air and when sub-
merged in water.
Nettleton’s method
A two-step approach is adopted here to determine the
best density and study the roots of the domes:
1. Nettleton’s method, used to determine the most
likely bulk density of a formation and/or to indi-
cate whether a dome is partly buried in younger
formations.
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