Jökull - 01.12.1970, Page 40

Gravity Survey on Western Vatnajökull
SVEN Þ. SIGURDSSON
ABSTRACT
Results from a gravity survey carried out by
the author on Tungnárjökull in June 1967 are
presented. These are used along with previous-
ly unpublished results from J. P. Martin’s seis-
mic survey in 1955 to obtain estimates of the
ice-thickness. Similarly the already published
data of Martin’s seismic survey in the Grims-
vötn area are combined with gravity results ob-
iained by G. Pálmason in 1960 to add an argu-
ment to the discussion of the ice-thickness in
the Grímsvötn caldera. The Bouguer anomaly
is calculated for some gravity stations on Vatna-
jökull to supplement the map on Bouguer ano-
maly in Iceland published by T. Einarsson in
1954.
INTRODUCTION
During the Vatnajökull expedition of the
Icelandic Glaciological Society in June 1967 a
gravity survey was carried out by the author
on Tungnárjökull (Thorarinsson, 1967). The
purpose was to fill in the picture we have of
the thickness of this glacier from J. P. Martin’s
seismic survey in 1955 (Thorarinsson, 1955).
The results from that survey have not been
previously published but are shown in Fig. 1
(S. Thorarinsson, pers. comm.). It will become
clearer in the next section that the main value
of gravity survey for this type of investigation
does indeed lie in the filling-up of results from
the more cumbersome seismic methods.
For interpretation of the gravity observations
we are dependent on good elevation deter-
minations. Consequently the lines of survey
coincided with those of the profile survey
carried out simultaneously by S. Freysteinsson
(1968). These are shown in Fig. 1.
Location of individual stations is indicated
with dots in Figs. 2 and 3. The survey started
in station N2 and proceeded through NKP to
38 JÖKULL 20. ÁR
SNR. We refer to stations N2, N23, N41, NKP,
KIO, K18 and SNR as reference stations.
The instrument used was the Worden Geo-
detic Gravity Meter No. 68 of the National
Energy Authority, Reykjavík.
THEORETICAL NOTES
The Bouguer anomaly (in mgals (1 mgal zr
10—3cm/s2)) at a gravity station is defined to
be (Parasnis, 1962):
gobs-gteor + 0-3086 • h - 0.04191 • h • o + T (1)
where gobs is the observed value of gravitational
force (in mgals) at the station.
gteor is the theoretically expected value of the
gravitational force (in mgals) at the station
assuming it were at sea level. This is a value
depending on latitude.
h is the elevation of the station in meters.
The third term in (1) takes care of the fact
that the gravitational force decreases as the
distance from the earth increases. The fourth
term in (1) takes care of the gravitational
attraction of the bedrock mass between the
station and sea level assuming this mass to
form an infinite horizontal plane sheet.
q is the estimated density of this mass (in
g/cm3).
T is terrain correction because of irregularities
on the surface. The correction is made to
some given distance from the station.
In the case when the mass beneath the sta-
tion is made up of two layers with differing
densities as will be the case with a station on
the surface of a glacier the fourth term in (1)
becomes:
0.04191 (hioi + h202) (2)
where in the special case of the glacier, hi
would be the thickness of the ice, oi its density,